**APPENDIX B**

**CLASSIFICATION OF METAMORPHIC AND OTHER COMPOSITE-GENESIS ROCKS,
INCLUDING HYDROTHERMALLY ALTERED, IMPACT-METAMORPHIC, MYLONITIC, AND
CATACLASTIC ROCKS**

Version 1.0

North American Geologic-map Data Model Steering Committee

**Science Language Technical Team (SLTT)**

Composite-Genesis Subgroup

This document should be cited as:

North American Geologic-Map Data Model Science Language Technical Team,
2004, Report on progress to develop a North American science-language
standard for digital geologic-map databases; Appendix B --
Classification of metamorphic and other composite-genesis rocks,
including hydrothermally altered, impact-metamorphic, mylonitic, and
cataclastic rocks, Version 1.0 (12/18/2004), *in* Soller, D.R., ed.,
Digital Mapping Techniques '04---Workshop Proceedings: U.S. Geological
Survey Open-File Report 2004-1451, 56 p. Appendix B accessed at
http://pubs.usgs.gov/of/2004/1451/sltt/appendixB/.

\[This report was formerly available at the North American Geologic Map
Data Model Steering Committee website as
http://nadm-geo.org/sltt/products/sltt_composite_genesis_12_18_04.doc.\]

TABLE OF CONTENTS

[1 EXECUTIVE SUMMARY 4](#executive-summary)

[2 INTRODUCTION 5](#introduction)

[2.1 Composite-genesis rocks 5](#composite-genesis-rocks)

[2.2 Background and purpose 6](#background-and-purpose)

[2.3 Definitions: classification, naming, and description
6](#definitions-classification-naming-and-description)

[2.4 Conceptual framework 6](#conceptual-framework)

[2.5 Guides and precedents 8](#guides-and-precedents)

[2.6 Philosophy of classification (SLTTM_1.0)
8](#philosophy-of-classification-slttm_1.0)

[2.7 Who developed this document, and how?
9](#who-developed-this-document-and-how)

[3 COMPOSITE-GENESIS ROCKS 11](#composite-genesis-rocks-1)

[3.1 Metamorphic rock definition and limits (including hydrothermally
altered rock)
11](#metamorphic-rock-definition-and-limits-including-hydrothermally-altered-rock)

[3.1.1 Definition 11](#definition-1)

[3.1.2 Lower limit of metamorphism 11](#lower-limit-of-metamorphism)

[3.1.3 Upper limit of metamorphism 12](#upper-limit-of-metamorphism)

[3.1.4 Hydrothermally altered rock 12](#hydrothermally-altered-rock)

[3.2 Cataclastic rock, composite-genesis melt rock, impact-metamorphic
rock
12](#cataclastic-rock-composite-genesis-melt-rock-impact-metamorphic-rock)

[4 CLASSIFICATION COMPONENTS 13](#classification-components)

[5 CLASSIFICATION BY ROCK FABRIC 14](#classification-by-rock-fabric)

[5.1 Terminology---phaneritic and aphanitic
14](#terminologyphaneritic-and-aphanitic)

[5.2 Fabric Prototypes 14](#fabric-prototypes)

[5.3 Metamorphic (Including Hydrothermally Altered) Rock
22](#metamorphic-including-hydrothermally-altered-rock)

[5.3.1 Granoblastic Rock 22](#granoblastic-rock)

[5.3.1.1 Granofels 22](#granofels)

[5.3.1.2 Hornfels 22](#hornfels)

[5.3.2 Non-mylonitic Foliated Metamorphic Rock
23](#non-mylonitic-foliated-metamorphic-rock)

[5.3.2.1 Historical usage of schist and gneiss
23](#historical-usage-of-schist-and-gneiss)

[5.3.2.2 Schistose rock (schist, phyllite, and slate)
26](#schistose-rock-schist-phyllite-and-slate)

[5.3.2.3 Layered gneiss and non-layered gneiss
27](#layered-gneiss-and-non-layered-gneiss)

[5.3.2.4 Migmatitic rocks as subclasses
27](#migmatitic-rocks-as-subclasses)

[5.3.3 Mylonite-series rock 27](#mylonite-series-rock)

[5.4 Cataclastic rock 28](#cataclastic-rock)

[5.5 Composite-genesis melt rock 29](#composite-genesis-melt-rock)

[5.5.1 Definition 29](#definition-2)

[5.6 Impact-metamorphic rock 29](#impact-metamorphic-rock)

[5.7 Term of last resort in absence of fabric class
30](#term-of-last-resort-in-absence-of-fabric-class)

[6 CLASSIFICATION BY COMPOSITION (QUALIFIER TERMS)
31](#classification-by-composition-qualifier-terms)

[6.1 Definition of rock-forming mineral groups in this classification
31](#definition-of-rock-forming-mineral-groups-in-this-classification)

[6.2 Quartz-feldspar-pelitic rocks 34](#quartz-feldspar-pelitic-rocks)

[6.3 Monomineralic rocks 34](#monomineralic-rocks)

[6.4 Calcsilicate and metacarbonate rocks
35](#calcsilicate-and-metacarbonate-rocks)

[6.5 Ferromagnesian rocks 35](#ferromagnesian-rocks)

[6.6 Special-composition rocks 36](#special-composition-rocks)

[7 CLASSIFICATION BY PROTOLITH 37](#classification-by-protolith)

[8 REFERENCES CITED 38](#references-cited)

[9 APPENDIX 1. PRELIMINARY REPORTS OF THE IUGS SUBCOMMISSION ON THE
SYSTEMATICS OF METAMORPHIC ROCKS (SCMR)
42](#appendix-1.-preliminary-reports-of-the-iugs-subcommission-on-the-systematics-of-metamorphic-rocks-scmr)

[10 APPENDIX 2. SELECTED FABRIC TERMS AND DEFINITIONS
44](#appendix-2.-selected-fabric-terms-and-definitions)

[11 APPENDIX 3. METAMORPHIC FACIES AND TYPES
46](#appendix-3.-metamorphic-facies-and-types)

[12 APPENDIX 4. GLOSSARY OF METAMORPHIC ROCK TERMS AND PARENT-CHILD
RELATIONSHIPS (DATABASE)
48](#appendix-4.-glossary-of-metamorphic-rock-terms-and-parent-child-relationships-database)

# EXECUTIVE SUMMARY

A provisional draft classification system for metamorphic rocks and
other composite-genesis rocks is proposed as part of a comprehensive
scheme for use in digital geologic maps and computer-based geoscience
information systems in North America. The purpose of the rock
classification is to identify lithologic constituents of map units or to
search digital geologic-map databases for specific rock types.

The domain of this classification system includes not only metamorphic
rocks as commonly understood, but also a variety of hydrothermally
altered, mylonite-series, cataclastic, and impact-metamorphic rocks. We
classify these composite-genesis rocks according to descriptive
properties that reflect the multiple processes that make the rock
composite. This classification is fundamentally descriptive, based
mainly on rock composition and fabric; therefore, different observers
should be able to classify a rock in the same way. Assignment to a
lithologic class implies that certain descriptive criteria are met, and
these criteria must be defined in the database. This classification also
attempts to follow common usage of terms in the geoscience community,
reconciling that usage and database requirements wherever necessary.

This report summarizes the logic and rationale for the classification
system, which is presented in a series of four diagrammatic decision
trees, two class hierarchy diagrams, and a glossary in the form of a
Microsoft Access database that has definitions and parent-child
relationships for terms in the classification system. Comments from the
geoscience community on this proposed classification are encouraged.

# INTRODUCTION

This document (hereafter referred to as SLTTM_1.0) proposes a
provisional classification system for metamorphic rocks and other
composite-genesis rocks as part of a comprehensive scheme for use in
digital geologic maps and computer-based geoscience information systems
in North America (http://geology.usgs.gov/dm/). The purpose of an Earth
material terminology system (or controlled vocabulary, see introduction
to NADM science language) is to provide a basis for identifying and
describing materials that are the substances of the Earth by classifying
them using standard terms and definitions. Classification links the
normative description (definition) for a term to the material being
described. Rock-lithology classification has two important functions in
geoscience information systems. First, in the case of rock descriptions,
a lithologic constituent of a geologic-map unit may be identified using
a standard rock classification in cases for which a detailed description
of the rock is unavailable or unnecessary. Second, in cases for which
lithologic constituents are described in detail, assignment of the
constituent to a standard rock classification allows users to search for
standard kinds of rock without having to design queries that analyze the
complete description structure. A lithology field in a geoscience
database is a place for classification of the constituents used to
describe bodies of rock.

This report describes the rationale for the accompanying classification
system, which is presented in a series of four linked decision trees
(flow charts), two class hierarchy diagrams, and an accompanying
glossary in the form of a Microsoft Access database that has definitions
and parent-child relationships for terms in the classification system
for composite-genesis rocks.

The domain of this classification system includes composite-genesis
rocks, defined as rocks that are the product of more than one
rock-forming process. They are not purely igneous or sedimentary. This
domain includes metamorphic rocks as commonly understood, as well as
impact metamorphic rocks, hydrothermally altered rocks, mylonite-series
rocks, and cataclastic rocks. These composite-genesis rocks are
classified strictly according to descriptive properties related to the
changes that made them 'composite.'

## Composite-genesis rocks

### Definition

*Composite-genesis rock*---Any rock having observable features that
document mineralogical, chemical, or structural change of a preexisting
earth material essentially in the solid state. It includes metamorphic
(*sensu strictu*), hydrothermally altered, cataclastic, and
impact-metamorphic rock, but not weathering products or soils.

A metamorphic rock has observable features that document change after
the original formation of the rock, under physical or chemical
conditions that differ from those normally occurring at the surface of
the Earth and in zones of cementation and diagenesis below the surface
(Smulikowski and others, 1997). Hydrothermally altered rock has fabric
and composition indicating solid-state mineralogical and chemical
changes in response to hot, mineral-rich waters, and is included as
'metamorphic rock' in this classification. Cataclastic rock,
impact-metamorphic rock, and composite-genesis melt rocks are treated as
special classes. Where possible, metamorphic classification schemes
proposed by the British Geological Survey (Robertson, 1999) and
preliminary recommendations of the IUGS Subcommission on the Systematics
of Metamorphic Rocks (SCMR) (Schmid and others, 2002) were adapted to
meet SLTT database requirements.

The composite-genesis rocks are classified along two orthogonal
dimensions, fabric and composition (Figs. 1 and 2). Class hierarchy is a
directed acyclic graph rather than a tree. Classes that have composition
and fabric criteria are 'children' of both a 'composition and fabric'
lithology parent, and of a generic composition and a generic fabric
parent. Class names have a fabric component (such as granofels) and a
compositional component (modifier such as marble or quartzofeldspathic).

## Background and purpose

A standard North American database model for the input, storage,
manipulation, retrieval, and analysis of digital geologic-map
information is being developed as a cooperative effort by a consortium
of interests including the Association of American State Geologists
(AASG), the U.S. Geological Survey (USGS), the Geological Survey of
Canada (GSC), and the Canadian Provincial Surveys. The standards for
this data model are being developed under the auspices of the
multi-constituency North American Data Model Steering Committee (NADMSC,
[http://nadm-geo.org)](). The NADMSC has commissioned technical teams,
including the Science Language Technical Team
(<http://nadm-geo.org/sltt>), to develop aspects of the data model.
Thus, the purpose of the Science Language Technical Team (SLTT) is to
develop standard terminology for classifying and describing geologic
materials for digital geologic-map databases produced throughout North
America. The Science Language Technical Team includes four subgroups:
the Subgroup on Plutonic Rocks, the Subgroup on Volcanic Rocks, the
Subgroup on Sedimentary Materials, and the Subgroup on Metamorphic
Rocks. This draft report was produced by the Subgroup on Metamorphic
Rocks.

## Definitions: classification, naming, and description

**Classification** is the assignment of an instance (individual object
from some point of view) to a group (class) that is defined based on a
shared set of properties. It answers the question "what kind of X is Y?"
where X represents the domain of the classification. A class of things
is defined by the set of the properties shared by members of the class.
Boundaries of the class are defined by threshold property values.

**Naming** is assignment of an identifier to an instance (object within
a class). Ideally, every instance would have a unique name, but of
course in the realm of earth and planetary materials, this is not the
case. Formalized stratigraphic nomenclature is an example of naming
mappable units. Lithologic naming provides an identifier for a
particular rock or Earth (and planetary) material that allows geologists
to communicate when formal nomenclature is not defined, or when they
need to subdivide in more detail than the formal nomenclature allows.
Names are also assigned to identify individual classes in a
classification system (each class is an instance of a class!).

**Description** is a set of statements that characterize the nature of a
thing (a class or instance) such that the thing may be identified. The
set of shared properties that define a class constitute a description.

## Conceptual framework

Lithologic classification is the assignment of a rock or unconsolidated
material to a named class defined based on its physical properties. The
scope of the classification, 'rock or unconsolidated material',
corresponds to the concept 'CompoundMaterial' as defined by the North
American Geologic Map Data Model Steering Committee Data Model Design
Team (NADMSC, 2003). In this report, the term 'material' is used in
place of the more precise, but less familiar, term 'CompoundMaterial' to
mean 'rock or unconsolidated material'. Note also that this concept may
be extended to include not only materials found on Earth (as defined by
NADMSC, 2003), but also any planetary material. Lithologic
classification is used in a number of contexts. The field geologist
classifies rocks as part of the process of defining mappable bodies of
rock within a geographic area. A map compiler uses the classification of
rocks that compose geologic map units to determine similarity between
units on different maps, or as criteria to define composite map units
that combine features of more detailed maps. A non-geologist uses the
classification system to identify rocks of interest without having to
study their descriptions in detail. Earth scientists use standard
classification systems to characterize rocks as part of the process of
describing them. These applications highlight two sorts of
classification process---one aimed at identifying particular bodies of
rock in a particular region, and one aimed at grouping similar kinds of
rock that may be present in many places.

Lithologic classification is scale dependent. Hand-sample-dimension
lithologic classification systems are designed to group kinds of
material into named classes based on a 1-cm to 30-cm diameter
representative volume. A particular rock name, based on a naming scheme
such as that proposed by the British Geological Survey (e.g. Robertson,
1999) or by Travis (1955), corresponds to a lithologic class meant to
identify a particular kind of rock described on the basis of hand-sample
observations. The dimension of the representative volume used for
lithologic classification may vary from hand-sample to kilometer scale.

An ideal lithologic classification system would assign every material in
its scope (domain) to a unique class. However, there are many examples
of materials that can be recognized as belonging to more than one class,
depending on the criteria used for classification. Examples include
low-grade metasedimentary rocks that may be described as metamorphic
rocks and as sedimentary rocks, saprolites that may be described as an
unconsolidated material and as their bedrock parent, and calc-lithic
sandstone that may be classified as both a sandstone and a limestone.
Any classification system that attempts to define disjoint
(non-overlapping) classes over the entire domain of rocks and
unconsolidated materials either must define ad hoc rules that allow
ambiguous instances to be assigned to unique classes, or must add
numerous new classes that include composite kinds of materials. A better
solution is to allow separate classification schemes. Earth scientists
who have different geologic interests may use different classification
schemes. Individual materials may be classified differently using
different schemes. Different lithologic classification schemes have
different classification criteria, and may have different domains of
classification. The domains of classification for different schemes may
overlap, but classes in any particular scheme are disjoint. Lithologic
classification over the whole domain of rock and unconsolidated
materials is thus overlapping (i.e. not disjoint).

To produce a lithologic classification system that allows different
observers to classify a given material in the same way, the system must
be based on physical properties recognizable by all observers. Strict
adherence to this rule would not allow use of genetic interpretations in
the classification of a material unless they could be couched in purely
descriptive terms (see discussion in Travis, 1955, p. 1). The properties
used for field lithologic classification include:

-   modal mineralogy

-   grain size

-   grain shape

-   rock fabric (the arrangement of grains in an aggregate to form the
    rock)

-   structures in the rock (bedding, layering, etc.)

Distinct bodies of rock may be defined based on other physical
properties, such as magnetic susceptibility or density, but these are
not generally used as field criteria.

The approach to a lithologic classification proposed here is
fundamentally descriptive \[unavoidable genetic classes such as
impact-metamorphic rock are subdivided based on descriptive
attributes\]. Classification of a particular material is based on
observable features, and assignment of a material to a lithologic class
implies that certain descriptive criteria are met. These criteria must
be archived in the database in order to document the classification
system. The descriptions that define the lithologic classes also serve
to provide default values for properties when a material is assigned to
the class but not described in greater detail. The definition of a
lithologic class must be part of a classification scheme (or terminology
system, or controlled vocabulary) that defines the domain of
classification and classification criteria. The definition must state
the dimension of the representative volume for the class, the criteria
that are sufficient to assign membership in the class, and to the extent
possible, a default description of other aspects of materials that are
assigned to the class.

In order to gain acceptance in the geoscience community, any lithologic
classification system needs to be consistent with common usage. This may
require some relaxation of the strict adherence to observable physical
properties as criteria for classification, because traditional rock
classification has always involved some genetic interpretation (e.g.,
igneous, sedimentary, metamorphic are fundamentally genetic). The
operational rule for consistency is that existing terms may be redefined
to narrow their meaning, but may not be redefined to include rocks that
are not included as part of that class in common usage.

## Guides and precedents

Pioneering efforts by the British Geological Survey to develop a
comprehensive, systematic classification of metamorphic rocks
(Robertson, 1999) have proven to be extremely useful as a precedent. In
addition, the International Union of Geological Sciences (IUGS)
Subcommission on the Systematics of Metamorphic Rocks (SCMR) is
following a precedent for igneous rocks (LeMaitre and others, 1989,
2002) in developing standards for the classification and nomenclature of
metamorphic rocks (Schmid and others, 2002). Recent SCMR proposals
(Appendix 1) include a systematic nomenclature that combines rock
composition (mineral components) and structural root terms (gneiss,
schist, granofels), as well as standards for non-systematic
metamorphic-rock names in common use. Where possible, we have attempted
to follow or adapt the SCMR proposals available at this time to meet the
SLTT need for a rigorous hierarchical classification compatible with
digital databases.

Useful glossaries of terms related to metamorphic rocks are available in
Miyashiro (1994, Appendix II), Passchier and Trouw (1996), Jackson
(1997), Barker (1998), and the IUGS proposals listed in Appendix 1. The
IUGS SCMR is currently preparing an authoritative glossary of about 1500
terms for metamorphic rocks. That glossary will be similar to Le
Maitre's (1989, 2002) glossary for igneous rocks. When completed, it
will be released online at <http://www.bgs.ac.uk/SCMR> The final IUGS
glossary will eventually provide authoritative definitions of common
metamorphic terms for international use, recommendations for use of
uncommon terms, and terms that should be abandoned and replaced by
common rock names.

## Philosophy of classification (SLTTM_1.0)

The classification system proposed here is designed for database
applications. This makes it distinct from a system for naming rocks,
which provides guidelines for giving a descriptive name to a particular
rock or kind of rock for use in discourse. Two naming schemes have
recently been proposed (Robertson, 1999; Schmid and others, 2002), and
the reader is referred to those documents for more information.

The lithologic classification needed for a database is designed to
answer the question \'what kind of rock is this?' The flexibility of the
database allows a more complete answer to this question that can be
encapsulated in a single rock name, and several equally valid answers
that depend on the context of the 'kind' question can be provided.
Ideally, the database system would also provide a 'free-form rock name\'
field that allows the geoscientist to give a rock being described
whatever name they feel best characterizes the rock.

The philosophy of this classification is based (to the degree possible)
on features that reflect the transformation of a rock from one state
(the protolith) to another (the composite-genesis rock). Features from
the original state are not the basis for classification as a metamorphic
rock. If such features are still apparent, they are the basis for
classification of the lithology of the metamorphic rock protolith, which
will also be part of the database description of the rock.

Classification Rules:

-   The domain of classification is hand sample (1 cm to 30 cm) scale
    metamorphic rocks.

-   The scheme should be as descriptive as possible. Genetic
    considerations are used only to the extent that they can be defined
    based on features observable in the rock at hand-sample scale.

-   Classification must be complete---if a rock is determined to be a
    composite-genesis rock, it must have a place in the classification
    scheme.

-   A rock may be classified in more than one way based on different
    criteria (fabric, composition), but given a particular set of
    criteria, classification must be unique.

-   The scheme is for *classifying rocks* *as metamorphic rocks*, thus
    interpretation of the protolith of a rock is not used to determine
    the classification of the rock. \[However, where protoliths are
    known, the combined use of a metamorphic rock classification (as
    proposed here) and an independent protolith (sedimentary or igneous)
    rock classification should be permissible.\]

-   Modal mineralogy (mineral volume percentage) is described in
    database fields other than the field containing material names.

-   Geologists can give a rock any name they feel is most appropriate
    using an informal 'rock name' field in the database that is separate
    from this lithologic classification.

-   Classification must be hierarchical. Superclasses must be included
    that group the lithologic classes based on fabric or composition
    criteria. These should include a small number of sub-classes. Each
    sub-class may be further broken down into smaller subordinate
    classes, etc.

-   Classification must be useful. The scheme should be sufficiently
    simple and flexible to facilitate use by workers of varying
    experience and expertise (Robertson, 1999), both in producing and
    using digital geologic maps and map databases.

The basic elements of the classification are rock fabric and composition
(modal mineralogy). If both are known, both the fabric and composition
parts of the classification decision tree must be navigated to determine
the classification. If only one is known, only one of the two parts will
need to be navigated. Fabric may be unknown, in which case the fabric
term will just have to be \'rock\'. The sorts of fabric that can be
defined for aphanitic or very fine-grained rocks include massive,
laminated, and glassy. A wider variety of fabrics is defined for
phaneritic rocks, including schistose, gneissic, and granoblastic.
Composition may be unknown, either because it is unspecified, in which
case the fabric classification tree is used, or because the grain size
is too small to determine mineralogy beyond silicic, calcsilicate,
argillic, or calcareous.

In the class hierarchy, classes that have a composition and a fabric
criteria are considered 'children' of both a 'composition and fabric'
lithology parent, and of a generic composition and a generic fabric
parent. The class hierarchy is thus a directed acyclic graph, not a
tree. Ramifications of this knowledge representation in database
implementations should be carefully considered.

## Who developed this document, and how?

This document was developed by geoscientists from a variety of American
and Canadian geoscience agencies (Table 2.7.1). The group was assembled
in early 2000 as the Metamorphic Science Language Technical Team (SLTTM)
of the North American Geologic-map Data Model Steering Committee,
although the panel soon came to be called the Composite-genesis
Technical Team in recognition of the fact that earth materials they were
classifying included more than just metamorphic rocks as traditionally
envisioned. Panel members were appointed in the following ways:

(1) Most participants from the U.S. Geological Survey (USGS) were
    identified by Regional Geologic Executives from the USGS Western,
    Central, and Eastern Regions. Some USGS scientists were appointed by
    Coordinators of USGS line-item science programs;

(2) Scientists from the Geological Survey of Canada (GSC) were
    identified by Canadian members of the North American Geologic-map
    Data Model Steering Committee;

(3) Scientists from Canadian Provincial geological surveys were
    identified by Canadian members of the North American Geologic-map
    Data Model Steering Committee;

(4) Scientists from academic institutions were selected by the committee
    co-chairs.

The document was written by co-chairs Horton and Richard in consultation
with members of the panel.

+------------------------------+---------------------------------------+
| **Committee Members**        |                                       |
|                              |                                       |
| **Composite-genesis Science  |                                       |
| Language Technical Team      |                                       |
| (SLTTM)**                    |                                       |
|                              |                                       |
| **J. Wright Horton, Jr., and |                                       |
| Stephen M. Richard,          |                                       |
| co-chairs**                  |                                       |
+------------------------------+---------------------------------------+
| William F. Cannon            | U.S. Geological Survey, Reston,       |
|                              | Virginia                              |
+------------------------------+---------------------------------------+
| Stephen P. Colman-Sadd       | Geological Survey of Newfoundland and |
|                              | Labrador, St. John's, Newfoundland    |
+------------------------------+---------------------------------------+
| Thomas D. Hoisch             | Northern Arizona University,          |
|                              | Flagstaff, Arizona                    |
+------------------------------+---------------------------------------+
| J. Wright Horton, Jr.        | U.S. Geological Survey, Reston,       |
| (co-chair)                   | Virginia                              |
+------------------------------+---------------------------------------+
| Bruce R. Johnson             | U.S. Geological Survey, Reston,       |
|                              | Virginia                              |
+------------------------------+---------------------------------------+
| Alison Klingbyle             | Geological Survey of Canada, Ottawa,  |
|                              | Ontario                               |
+------------------------------+---------------------------------------+
| Diane E. Lane                | U.S. Geological Survey, Denver,       |
|                              | Colorado                              |
+------------------------------+---------------------------------------+
| Jonathan C. Matti (ex        | U.S. Geological Survey, Tucson,       |
| officio)                     | Arizona                               |
+------------------------------+---------------------------------------+
| Stephen M. Richard           | Arizona Geological Survey, Tucson,    |
| (co-chair)                   | Arizona                               |
+------------------------------+---------------------------------------+
| Lambertus C. Struik,         | Geological Survey of Canada,          |
|                              | Vancouver, British Columbia           |
+------------------------------+---------------------------------------+
| Robert J. Tracy              | Virginia Polytechnic Institute and    |
|                              | State University, Blacksburg,         |
|                              | Virginia                              |
+------------------------------+---------------------------------------+
| Michael L. Williams          | University of Massachusetts, Amherst, |
|                              | Massachusetts                         |
+------------------------------+---------------------------------------+
| Robert P. Wintsch            | University of Indiana, Bloomington,   |
|                              | Indiana                               |
+------------------------------+---------------------------------------+
| Michael L. Zientek           | U.S. Geological Survey, Spokane,      |
|                              | Washington                            |
+------------------------------+---------------------------------------+

Table 2.7.1

# COMPOSITE-GENESIS ROCKS

The domain of composite-genesis rocks includes rocks that record the
effects of more than one sequential rock-forming process. The primary
rock-forming processes are considered to be those related to the
formation of sedimentary and igneous rocks. Sedimentary rocks are formed
by the accumulation of sediment subsequent lithification by diagenetic
processes to form rock. Igneous rocks are formed by the cooling of
melted rock (magma) within or derived from the Earth's (or another
planetary body's) interior. Composite-genesis rocks are those that
record the effects of one or more geologic events subsequent to the
primary rock-forming process. These may include burial, heating, ductile
or brittle deformation, and open-system chemical changes. A purely
descriptive definition of composite-genesis rock is problematic because
the definition requires interpreting genetic process, which depends to
some extent on the knowledge and skill of the observer.

The major classes of composite-genesis rock, based on genetic as well as
descriptive criteria, are metamorphic (including hydrothermally altered)
rock, cataclastic rock, composite-genesis melt rock, and
impact-metamorphic rock. These classes are subdivided and organized
based on descriptive criteria into separate hierarchies for rock fabric
and composition. The application of this classification to particular
rock units requires the dual use of both hierarchies as outlined later
in this report.

## Metamorphic rock definition and limits (including hydrothermally altered rock)

### Definition

*Metamorphic rock*---any rock derived from pre-existing rock by
mineralogical, chemical, or structural changes, essentially in the solid
state (based on the *Glossary of Geology*, Jackson, 1997).

In order to be considered a metamorphic rock, there must be observable
features in the rock that document change subsequent to the original
formation of the rock, under physical or chemical conditions that differ
from those normally occurring at the surface of the Earth (or other
planetary bodies) or in zones of cementation and diagenesis below the
surface (Smulikowski and others, 1997). Robertson (1999) stipulates that
the process of metamorphism encompasses all of the solid-state changes
that occur between the upper and lower limits of metamorphism.
'Metamorphic rock' thus includes hydrothermally altered rock and
mylonitic rock, but not melt rock. Although 'cataclastic rock' and
'impact metamorphic rock' could be considered 'metamorphic' in the most
general sense, they are here distinguished from conventional
'metamorphic rock' because their formative processes can be outside the
lower and upper limits of metamorphism discussed below. Metamorphism
does not include rock weathering or soil formation.

### Lower limit of metamorphism

Because identification of a rock as a metamorphic rock is predicated on
observation of features not resulting from the original rock-forming
process, identification as a metamorphic rock depends on the skill and
interests of the observer, and the kinds of features observed.
Distinction of the 'original rock-forming process' as distinct from
subsequent 'metamorphic processes' is not always clear. Sediment
undergoes a continuous progression of changes from deposition through
lithification to form sedimentary rock; this process is known as
diagenesis. Changes in a deeply buried sedimentary rock may be
continuous from diagenesis into recrystallization to form a metamorphic
rock.

Robertson (1999) defines the boundary between diagenesis and
metamorphism in sedimentary rocks as follows:

> "...the boundary between diagenesis and metamorphism is somewhat
> arbitrary and strongly dependent on the lithologies involved. For
> example changes take place in organic materials at lower temperatures
> than in rocks dominated by silicate minerals. In mudrocks, a white
> mica (illite) crystallinity value of \< 0.42D.2U obtained by X-ray
> diffraction analysis, is used to define the onset of metamorphism
> (Kisch, 1991). In this scheme, the first appearance of glaucophane,
> lawsonite, paragonite, prehnite, pumpellyite or stilpnomelane is taken
> to indicate the lower limit of metamorphism (Frey and Kisch, 1987;
> Bucher and Frey, 1994; Frey and Robinson, 1998). Most workers agree
> that such mineral growth starts at 150 ± 50° C in silicate rocks. Many
> lithologies may show no change in mineralogy under these conditions
> and hence the recognition of the onset of metamorphism will vary with
> bulk composition."

### Upper limit of metamorphism

At the highest grades of metamorphism, rocks begin to melt. The
temperatures and pressures of the onset of melting range from
approximately 650° C to more than 1100° C depending on bulk composition
and the proportion of water in the fluid phase. Migmatitic rocks grade
into igneous rocks as the proportion of granitic melt increases. The
upper limit of metamorphism is defined somewhat arbitrarily. Here, a
rock mass that consists \>=50% (by volume) of rock having igneous
composition and texture (crystallized from a melt) is classified as an
igneous rock.

### Hydrothermally altered rock

Hydrothermally altered rock has fabric and composition indicating
solid-state mineralogical and chemical changes in response to hot,
mineral-rich waters. Igneous rocks may continue to undergo mineralogical
and chemical changes during cooling even after most or all of the magma
has crystallized to produce a 'solid' rock. Hydrothermal activity due to
residual heat and mineral-rich water from a pluton may change igneous
rocks significantly following their crystallization. ***Hydrothermally
altered (including metasomatic) rock is treated as 'metamorphic rock' in
this classification.*** We propose the following criteria to distinguish
hydrothermally altered or metasomatic rock from igneous rock. The rock
is classified as metamorphic if (1) the texture has been modified such
that it can no longer be considered igneous, (2) the bulk composition of
the rock is inconsistent with compositions that can be derived purely
from a magma and associated processes such as assimilation and
differentiation, or (3) minerals inconsistent with magmatic
crystallization are present.

## Cataclastic rock, composite-genesis melt rock, impact-metamorphic rock

The classes of composite-genesis rock, in addition to metamorphic rock,
include cataclastic rock, impact-metamorphic rock, and composite-genesis
melt rock.

*Cataclastic rock*---A rock having more than 10% of its volume
consisting of fragments bounded by fractures related to brittle
deformation that occurred after the formation of the protolith.
Cataclastic rocks are commonly associated with faults, although that
genesis is not required by the descriptive definition, and they can form
below the limits of metamorphism discussed above.

*Composite-genesis melt rock*---A rock that solidified from melt
(liquid) of a preexisting rock under conditions (such as contact
metamorphism, friction along faults, or meteorite impact) outside the
domain of typical igneous rocks.

*Impact-metamorphic rock*---A rock formed by the impact of a planetary
body (projectile) on a planetary surface (target). While this criterion
is unavoidably genetic by definition, it is based on the interpretation
of observable features such as shock-induced planar deformation
features, high-pressure minerals, shatter cones, and crater structure,
and subdivisions are based on descriptive features as discussed in a
later section.

# CLASSIFICATION COMPONENTS

It is important to remember that the proposed classification of
metamorphic rocks is for use in a computer database. This classification
system is presented in five diagrams, which include a class hierarchy
diagram in two parts (fabric and composition) and four decision trees
(metamorphic rock fabric, metamorphic rock composition, mylonitic rock,
cataclastic rock). The classification system includes a glossary of
definitions, parent-child relationships, and references for terms in the
form of a Microsoft Access database. This glossary should be expanded in
the future to encompass terms presently missing from the classification,
including those to be discouraged on new maps, and recommended usage and
suggestions for integrating them into the classification. Future
expansion of the glossary for this classification should strive for
agreement with the glossary of metamorphic terms under development by
the IUGS SCMR \[<http://www.bgs.ac.uk/SCMR>\] except where precluded by
database requirements or common North American usage.

The system proposed here classifies a given composite-genesis rock along
two orthogonal dimensions, fabric (including texture) and composition.
Class names thus have a fabric component (e.g., granofels, schist,
gneiss) and a compositional component (e.g., marble, calcsilicate,
quartzite, quartzofeldspathic, pelitic, amphibolite, serpentinite). The
alternative classification rationale of Robertson (1999, Fig. 3)
requires that a rock name is first assigned based on mineralogy, and in
the absence of mineralogical information, the classification uses a
textural term. In our view, this results in loss of information.

# CLASSIFICATION BY ROCK FABRIC

Rock fabric (including texture), as defined in Appendix 2, is the
complete spatial and geometrical configuration of all those components
that are contained in a rock and that are penetratively and repeatedly
developed throughout the volume of rock under consideration. It includes
the shapes and characters of individual parts of a rock mass and the
manner in which these parts are distributed and oriented in space.

## Terminology---phaneritic and aphanitic

The IUGS Subcommission on the Systematics of Metamorphic Rocks (SCMR)
extended the use of igneous grain-size terms 'phaneritic' (large enough
to be distinguished by the unaided eye) and 'aphanitic' (too fine
grained to be distinguished by the unaided eye) to metamorphic rocks,
noting that these qualitative definitions correspond approximately to
"aphanitic" (ca. \<0.1 mm) and "phaneritic" (ca. \>0.1 mm) (Schmid and
others, 2002, Table 5). The British Geological Survey rock
classification (Gillespie and Styles, 1999, section 3.2, p. 6) applies
"phaneritic" and "aphanitic" to metamorphic rocks but interprets the
actual grain sizes differently:

> "Placing the boundary between 'medium grained' and 'fine grained'
> crystals in crystalline igneous and metamorphic rocks at 0.25 mm
> essentially divides aphanitic rocks (in which individual crystals are
> too fine grained to be distinguished by the naked eye) from phaneritic
> rocks (in which individual crystals can be distinguished by the naked
> eye)...This is also the boundary between medium and fine sand for
> sedimentary clasts."

Others interpret the phaneritic-aphanitic (visible-invisible)
distinction as approximately equivalent to the sand-silt grain size
distinction for sediments, which is variously considered to be in the
range of 0.032 mm (Robertson, 1999, BGS grain size scheme) to 0.062 mm
(Jackson, 1997) to 0.074 mm (Engineering grain-size scale, ASTM standard
D422-63; D643-78). This is smaller than the phaneritic-aphanitic
boundary of 0.1 mm proposed by Schmid and others (2002). However, in
practical terms, such detailed grain size distinctions are impossible in
the field, and very difficult under any conditions. The IUGS usage is
endorsed and applied in this classification.

## Fabric prototypes

Several basic types of rock fabric are recognized in virtually all
approaches to composite-origin rocks. For classification purposes, the
problem is defining the boundaries between the types based on
descriptive criteria. There are terms in common usage (e.g., schist,
gneiss, granofels, mylonite, cataclasite) that provide basic prototypes,
but there is a great deal of variation in the definition of boundaries
between the classes.

The basic level of classification is the definition of seven fabric
prototypes. These are presented at this stage without assigning names
(although the names may be obvious) to emphasize that they represent
fundamental *kinds* of rock.

The following list of basic fabric classes is meant to partition the
domain of composite-origin rocks based on descriptive criteria in a
manner that different observers can agree upon. All quantitative
boundaries (percentages, ratios, and dimensions) should be considered
open to discussion---the qualitative distinctions are more important
here. In general, 'phaneritic' (having visible grains) is implied except
where 'aphanitic' is specified.

(1) Rocks that consist of angular fragments bounded by fractures. \[A
    threshold for assignment of rocks to this category is proposed where
    ≥ 10% of the rock consists of fragments bounded by fractures.\]

(2) Aphanitic metamorphic rocks that are too fine-grained to determine
    mineralogy. \[This criterion is meant to separate rocks that can be
    classified based on modal mineralogy from rocks too fine grained to
    distinguish mineralogy.\]

(3) Phaneritic metamorphic rocks that have granoblastic fabric and very
    little or no foliation. \[Very little foliation means some foliation
    may be present, but does not meet the criteria for foliated.
    Foliated means that ≥ 10% of the mineral grains in the rock are
    fabric elements in the foliation. To be a foliation-defining fabric
    element, a mineral grain must have an inequant crystal habit, or an
    inequant shape due to deformation and an aspect ratios ≥ 1.5:1
    between the long and short axis of the deformed grain.\]

(4) Well-foliated rocks characterized by tectonic reduction in grain
    size and having a foliation defined by the shapes of oriented
    mineral grains or grain aggregates. \[Criteria proposed here for
    implementation are a foliation defined by the shape of deformed
    mineral grains or grain aggregates having an aspect ratio \> 1.5:1,
    and \>10% \'matrix\' showing evidence of tectonic reduction in grain
    size without loss of material continuity. The matrix consists of new
    or recrystallized mineral grains that are interpreted to be smaller
    than the mineral grains in the original, undeformed protolith. The
    definition is meant to identify a fabric in the rock due to crystal
    plastic and/or other types of non-cataclastic deformation.\]

(5) Phaneritic rocks that have a well-developed schistosity. \[The
    sticking point is the definition of "well developed" schistosity. We
    propose that \>50% of rock consists of mineral grains having a
    tabular, lamellar, or prismatic crystallographic habit that are
    oriented in a continuous planar or linear fabric (following Jackson,
    1997). Continuous is defined on a hand sample-scale, to mean that
    domains lacking the fabric are \<1 cm thick if they are layers, and
    \<5 cm in diameter if they are irregular patches, and constitute \<
    25% of the rock. The IUGS SCMR (Brodie and others, 2002) suggests
    using criteria that rock splits on scale \<1 cm, but this criteria
    depends on the tool used to do the splitting, the skill of the
    operator doing the splitting, and the degree of weathering or
    alteration of the rock being split, and is thus not objective and
    universally applicable. The IUGS criteria could not be used to
    classify a rock in thin section. The IUGS (Brodie and others, 2002)
    also proposes to define schistosity as due to inequant mineral
    grains or grain aggregates, without specifying that their shape is
    due to crystallographic habit.\]

(6) Foliated, layered phaneritic rocks that lack well developed
    schistosity and have laterally continuous compositional layering.
    \[We here propose a definition of "laterally continuous
    compositional layering" as having layers \> 5 mm thick that can be
    traced laterally \> 10 cm (length of lateral continuity).\]

(7) Foliated non-layered phaneritic rocks that do not have well
    developed, continuous schistosity or continuous compositional
    layering.

Table 1. Fabric classification terms

+-----------+------------------------------------------+---------------+
| **Name**  | **Definition**                           | **Source**    |
+===========+==========================================+===============+
| breccia   | Generic classification for rock in which | Sibson        |
|           | penetrative, through going fractures     | (1977), Snoke |
|           | separate visible fragments (\>0.1 mm     | and Tullis    |
|           | diameter) that form \> 30% of rock, and  | (1998)        |
|           | fragments are rotated relative to each   |               |
|           | other.                                   |               |
+-----------+------------------------------------------+---------------+
| broken    | Rock consisting of \<30% fragments, or   | Robertson     |
| rock      | rock that consists of \>30% fragments,   | (1999)        |
|           | but fragments are not rotated with       |               |
|           | respect to each other. Because protolith |               |
|           | is assumed to be recognizable, do not    |               |
|           | apply a composition modifier.            |               |
|           | Composition is indicated by protolith    |               |
|           | link.                                    |               |
+-----------+------------------------------------------+---------------+
| ca        | cataclastic rock that maintained primary | Sibson        |
| taclasite | cohesion during deformation, and         | (1977),       |
|           | consists of 50-90% matrix. Matrix is     | Scholz        |
|           | broken mineral grains or rock fragments  | (1990), Snoke |
|           | that are too small to be discernible     | and Tullis    |
|           | (0.1 mm following definition of          | (1998),       |
|           | aphanitic). Equivalent to                | Barker (1998, |
|           | "mesocataclasite" of Brodie and others   | Appendix II)  |
|           | (2002)                                   |               |
+-----------+------------------------------------------+---------------+
| cataclasi | Collective term for cataclastic rocks    | Sibson        |
| te-series | that maintained primary cohesion during  | (1977),       |
| rock      | deformation; includes protocataclasite,  | Scholz        |
|           | cataclasite, and ultracataclasite        | (1990), Snoke |
|           | (Sibson, 1977). These subtypes are       | and Tullis    |
|           | differentiated based on the fraction of  | (1998)        |
|           | the rock considered 'matrix'. Matrix is  |               |
|           | broken mineral grains or rock fragments  |               |
|           | that are too small to be discernible     |               |
|           | (0.1 mm following definition of          |               |
|           | aphanitic).                              |               |
+-----------+------------------------------------------+---------------+
| ca        | "A rock, such as tectonic breccia,       | Jackson       |
| taclastic | containing angular fragments that have   | (1997)        |
| rock      | been produced by the crushing and        |               |
|           | fracturing of preexisting rocks as a     |               |
|           | result of mechanical forces in the       |               |
|           | crust." (Jackson, 1997). Criterion added |               |
|           | here for database precision is \>10% of  |               |
|           | rock volume composed of fragments        |               |
|           | bounded by fractures related to a        |               |
|           | secondary tectonic event. Use of the     |               |
|           | term 'cataclastic' denotes deformation   |               |
|           | in the absence of crystal plastic        |               |
|           | processes. \[Mylonite-series rock is not |               |
|           | classified here as a cataclastic rock,   |               |
|           | although it may contain subordinate      |               |
|           | cataclastic fabric.\]                    |               |
+-----------+------------------------------------------+---------------+
| Composi   | Rock with unspecified fabric, for which  | this report   |
| te-origin | insufficient information is available to |               |
| rock      | determine if it is cataclastic,          |               |
|           | impact-related, glassy, or metamorphic.  |               |
+-----------+------------------------------------------+---------------+
| contact   | Rock in which framework of rock is       | this report   |
| me        | glassy mineral material that encloses    |               |
| tamorphic | rock and mineral fragments, and there is |               |
| melt rock | evidence that melting is related to      |               |
|           | contact metamorphism.                    |               |
+-----------+------------------------------------------+---------------+
| fault     | Cataclastic rock lacking evidence for    | Snoke and     |
| breccia   | primary cohesion during deformation, in  | Tullis        |
|           | which fractures separate visible         | (1998),       |
|           | fragments (\>= 0.1mm in diameter) that   | Barker (1998, |
|           | form \>30% of rock mass, and are rotated | Appendix II)  |
|           | relative to each other, and no evidence  |               |
|           | of impact metamorphism is observed.      |               |
|           | Composition modifiers refer to character |               |
|           | of comminuted rock between fragments;    |               |
|           | composition of fragment protolith is     |               |
|           | assumed identifiable, and composition of |               |
|           | that is indicated via protolith          |               |
|           | relationship.                            |               |
+-----------+------------------------------------------+---------------+
| foliated  | Cataclastic rock that maintained primary | Snoke and     |
| ca        | cohesion during deformation, and         | Tullis        |
| taclasite | consists of 50-90% matrix, and has a     | (1998);       |
|           | foliation. Matrix is broken mineral      | Sibson (1977) |
|           | grains or rock fragments.                | modified by   |
|           |                                          | Scholz (1990) |
+-----------+------------------------------------------+---------------+
| foliated  | Cataclastic rock lacking evidence for    | Snoke and     |
| gouge     | primary cohesion during deformation, in  | Tullis        |
|           | which visible fragments (\> 2 0.1 mm in  | (1998);       |
|           | diameter) constitute \<30% of the rock   | Sibson (1977) |
|           | mass, and having a foliation.            | modified by   |
|           | Composition modifier refers to general   | Scholz (1990) |
|           | character of comminuted matrix.          |               |
|           | Protolith of fragments assumed to be     |               |
|           | recognizable.                            |               |
+-----------+------------------------------------------+---------------+
| foliated  | Any metamorphic rock that contains a     | this report   |
| me        | foliation.                               |               |
| tamorphic |                                          |               |
| rock      |                                          |               |
+-----------+------------------------------------------+---------------+
| foliated  | Cataclastic rock that maintained primary | Snoke and     |
| protoca   | cohesion during deformation, and         | Tullis        |
| taclasite | consists of 10-50% matrix, and has a     | (1998);       |
|           | foliation. Matrix is broken mineral      | Sibson (1977) |
|           | grains or rock fragments.                | modified by   |
|           |                                          | Scholz (1990) |
+-----------+------------------------------------------+---------------+
| foliated  | Cataclastic rock that maintained primary | Snoke and     |
| ultraca   | cohesion during deformation, and         | Tullis        |
| taclasite | consists of 90-100% matrix, and has a    | (1998);       |
|           | foliation. Matrix is broken mineral      | Sibson (1977) |
|           | grains or rock fragments.                | modified by   |
|           |                                          | Scholz (1990) |
+-----------+------------------------------------------+---------------+
| gneiss    | General term for a foliated, phaneritic  | Schmid and    |
|           | rock without well developed, continuous  | others        |
|           | schistosity                              | (2002),       |
|           |                                          | Jackson       |
|           |                                          | (1997),       |
|           |                                          | Barker (1998, |
|           |                                          | Appendix II)  |
+-----------+------------------------------------------+---------------+
| gneissic  | Mylonite that has continuous             | revised from  |
| mylonite  | compositional layering, \>5 mm thick.    | IUGS (Brodie  |
|           | Continuous means that layers defining    | and others,   |
|           | the foliation can be traced for \>10 cm  | 2002) and     |
|           | (length of lateral continuity), and are  | Robertson     |
|           | spaced at a distance ≤ the average       | (1999)        |
|           | length of lateral continuity.            |               |
|           | Monomineralic compositional modifiers do |               |
|           | not apply, because if monomineralic,     |               |
|           | cannot develop gneissoid banding.        |               |
+-----------+------------------------------------------+---------------+
| gneissic  | Protomylonite that lacks well developed  | this report   |
| prot      | continuous schistosity, but displays     |               |
| omylonite | continuous compositional banding \>5 mm  |               |
|           | thick.                                   |               |
+-----------+------------------------------------------+---------------+
| gouge     | Cataclastic rock lacking evidence for    | Sibson        |
|           | primary cohesion during deformation, in  | (1977), Snoke |
|           | which visible fragments (\> 0.1 mm in    | and Tullis    |
|           | diameter) constitute \<30% of the rock   | (1998),       |
|           | mass. Composition modifier refers to     | Barker (1998, |
|           | general character of comminuted matrix.  | Appendix II)  |
|           | Protolith of fragments assumed to be     |               |
|           | recognizable.                            |               |
+-----------+------------------------------------------+---------------+
| gra       | Phaneritic metamorphic rock having       |               |
| noblastic | granoblastic fabric and very little or   |               |
| rock      | no foliation or lineation. Specifically, |               |
|           | \<10% of the particles in the rock are   |               |
|           | planar or linear fabric elements with an |               |
|           | aspect ratio  1.5:1. \[Use of generic   |               |
|           | term granoblastic rock denotes that      |               |
|           | nothing is known about the rock fabric   |               |
|           | beyond that it is granoblastic.\]        |               |
+-----------+------------------------------------------+---------------+
| granofels | Phaneritic metamorphic rock that has     | adapted from  |
|           | little or no foliation or lineation.     | Goldsmith     |
|           | Specifically, \<10% of the particles in  | (1959),       |
|           | the rock are planar or linear fabric     | Robertson     |
|           | elements with an aspect ratio ≥ 1.5:1.   | (1999),       |
|           | In this sense, granofels is the          | Barker (1998, |
|           | phaneritic equivalent of hornfels.       | Appendix II), |
|           |                                          | and IUGS      |
|           |                                          | (Schmid and   |
|           |                                          | others, 2002; |
|           |                                          | Brodie and    |
|           |                                          | others, 2002) |
+-----------+------------------------------------------+---------------+
| hornfels  | Aphanitic metamorphic rock that has      | Winkler       |
|           | little or no foliation or lineation      | (1967) in     |
|           | Although hornfels is typically a product | Jackson       |
|           | of contact metamorphism, that genesis is | (1997),       |
|           | not a requirement of this descriptive    | Winkler       |
|           | definition. Hornfels, in this sense, is  | (1979)        |
|           | the aphanitic equivalent of granofels.   |               |
+-----------+------------------------------------------+---------------+
| impact    | Breccia (as generally defined)           | Stöffler      |
| breccia   | containing fragments that show           | (2001c),      |
|           | unequivocal evidence of shock            | Stöffler and  |
|           | metamorphism; typically occurring        | Grieve(2001)  |
|           | "around, inside, and below impact        |               |
|           | craters" (Stöffler, 2001c); subclasses   |               |
|           | are monomict impact breccia, polymict    |               |
|           | impact breccia, and suevite.             |               |
+-----------+------------------------------------------+---------------+
| impact    | Crystalline, semi-glassy, or glassy rock | Stöffler      |
| melt rock | in which \>=50% of the rock volume is    | (2001c),      |
|           | solidified from impact melt (implying    | Stöffler and  |
|           | \<50% non-melt inclusions). Framework of | Grieve(2001). |
|           | rock is glassy mineral material and      |               |
|           | evidence of impact is observed.          |               |
+-----------+------------------------------------------+---------------+
| impactite | Rock that shows evidence of impact       | Stöffler      |
| (impact   | metamorphism. General term for rocks     | (2001c),      |
| me        | affected by impact resulting from the    | Stöffler and  |
| tamorphic | collision of planetary bodies.           | Grieve        |
| rock)     |                                          | (2001).       |
+-----------+------------------------------------------+---------------+
| i         | Cataclastic rock for which evidence for  | Snoke and     |
| ncohesive | primary cohesion during deformation is   | Tullis        |
| ca        | lacking or not specified, and evidence   | (1998);       |
| taclastic | for impact metamorphism is lacking or    | Sibson (1977) |
| rock      | not specified.                           | modified by   |
|           |                                          | Scholz (1990) |
+-----------+------------------------------------------+---------------+
| layered   | Foliated, phaneritic rock that lacks     | modified from |
| gneiss    | well developed, continuous schistosity   | IUGS (Schmid  |
|           | and has laterally continuous             | and others,   |
|           | compositional layering \> 5 mm thick.    | 2002; Brodie  |
|           | Laterally continuous means that layers   | and others,   |
|           | defining the foliation can be traced for | 2002), and    |
|           | \>10 cm (length of lateral continuity),  | Robertson     |
|           | and are spaced at a distance ≤ the       | (1999)        |
|           | average length of lateral continuity.    |               |
+-----------+------------------------------------------+---------------+
| me        | Rock with framework of glassy material   | this report   |
| tamorphic | that is the product of metamorphic       |               |
| glass     | event.                                   |               |
+-----------+------------------------------------------+---------------+
| me        | A metamorphic rock is any rock derived   | Jackson       |
| tamorphic | from pre-existing rock by mineralogical, | (1997),       |
| rock      | chemical, or structural changes,         | Smulikowski   |
|           | essentially in the solid state (based on | and others    |
|           | Jackson, 1997). In order to be           | (1997)        |
|           | considered a metamorphic rock, there     |               |
|           | must be observable features in the rock  |               |
|           | that document change after the original  |               |
|           | formation of the rock as a rock, under   |               |
|           | physical or chemical conditions that     |               |
|           | differ from conditions normally          |               |
|           | occurring at the surface of the Earth    |               |
|           | and in zones of cementation and          |               |
|           | diagenesis below the surface             |               |
|           | (Smulikowski and others, 1997).          |               |
+-----------+------------------------------------------+---------------+
| migmatite | A heterogeneous composite rock mass      | paraphrase of |
|           | consisting of irregular and              | Barker (1998, |
| \[re      | discontinuous interleaving of            | Appendix II), |
| commended | leucocratic granitoid material           | (Wimmenauer   |
| for text  | (leucosome) and residual high-grade      | and Bryhni,   |
| field     | metamorphic material (restite) (Barker,  | 2002)         |
| only\]    | 1998, Appendix II: Glossary). *Migmatite |               |
|           | is not used as a root name in this       |               |
|           | classification for reasons discussed in  |               |
|           | the text*. Here, the adjective           |               |
|           | "migmatitic" is applied to a fabric      |               |
|           | term, as in migmatitic gneiss, where the |               |
|           | granitoid part makes up \>10% of the     |               |
|           | rock volume.                             |               |
+-----------+------------------------------------------+---------------+
| m         | Foliated, phaneritic rocks without well  | composite     |
| igmatitic | developed, continuous schistosity that   | from Jackson  |
| gneiss    | is megascopically composite, consisting  | (1997) and    |
|           | of two or more petrographically          | Robertson     |
|           | different parts, one of which is the     | (1999)        |
|           | country rock in a more or less           |               |
|           | metamorphic state, the other is of       |               |
|           | pegmatitic, aplitic, or granitic         |               |
|           | appearance. The granitoid phase makes up |               |
|           | \>10% of the rock volume.                |               |
+-----------+------------------------------------------+---------------+
| m         | Non-foliated, phaneritic rock that is    | composite     |
| igmatitic | megascopically composite, consisting of  | from Jackson  |
| granofels | two or more petrographically different   | (1997) and    |
|           | parts, one of which is the country rock  | Robertson     |
|           | in a more or less metamorphic state, the | (1999)        |
|           | other is of pegmatitic, aplitic, or      |               |
|           | granitic appearance. The granitoid phase |               |
|           | makes up \>10% of the rock volume.       |               |
|           | Example \-- \'opthalmic\' or patch       |               |
|           | migmatite of Mehnert (1968).             |               |
+-----------+------------------------------------------+---------------+
| m         | Foliated, phaneritic rock without well   | composite     |
| igmatitic | developed, continuous schistosity that   | from Jackson  |
| layered   | has continuous compositional layering,   | (1997) and    |
| gneiss    | \> 5 mm thick, and is megascopically     | Robertson     |
|           | composite, consisting of two or more     | (1999)        |
|           | petrographically different parts, one of |               |
|           | which is the country rock in a more or   |               |
|           | less metamorphic state, the other is of  |               |
|           | pegmatitic, aplitic, or granitic         |               |
|           | appearance. The granitoid phase makes up |               |
|           | \>10% of the rock volume.                |               |
+-----------+------------------------------------------+---------------+
| m         | Foliated, phaneritic rock without well   | composite     |
| igmatitic | developed, continuous schistosity or     | from Jackson  |
| no        | continuous compositional layering that   | (1997) and    |
| n-layered | is megascopically composite, consisting  | Robertson     |
| gneiss    | of two or more petrographically          | (1999)        |
|           | different parts, one of which is the     |               |
|           | country rock in a more or less           |               |
|           | metamorphic state, the other is of       |               |
|           | pegmatitic, aplitic, or granitic         |               |
|           | appearance. The granitoid phase makes up |               |
|           | \>10% of the rock volume.                |               |
+-----------+------------------------------------------+---------------+
| m         | Foliated, phaneritic rock that has well  | composite     |
| igmatitic | developed, continuous schistosity that   | from Jackson  |
| schist    | is megascopically composite, consisting  | (1997) and    |
|           | of two or more petrographically          | Robertson     |
|           | different parts, one of which is the     | (1999)        |
|           | country rock in a more or less           |               |
|           | metamorphic state, the other is of       |               |
|           | pegmatitic, aplitic, or granitic         |               |
|           | appearance. The granitoid phase makes up |               |
|           | \>10% of the rock volume.                |               |
+-----------+------------------------------------------+---------------+
| monomict  | Impact breccia free of impact melt in    | Stöffler      |
| impact    | which all of the fragments (100%) have   | (2001c),      |
| breccia   | essentially the same composition.)       | Stöffler and  |
|           |                                          | Grieve(2001). |
+-----------+------------------------------------------+---------------+
| mylonite  | Mylonite-series rock consisting of       | Sibson        |
|           | 50-90% matrix showing evidence of        | (1977),       |
|           | tectonic grain size reduction.           | Hanmer        |
|           | Equivalent to "orthomylonite" of Wise    | (1987),       |
|           | and others (1984) and "mesomylonite" of  | Passchier and |
|           | Brodie and others (2002)                 | Trouw (1996), |
|           |                                          | Snoke and     |
|           |                                          | Tullis (1998) |
+-----------+------------------------------------------+---------------+
| myloni    | A collective term for the "mylonite      | Sibson        |
| te-series | series" rocks of Sibson (1977),          | (1977), Snoke |
| rock      | including protomylonite, mylonite,       | and Tullis    |
|           | ultramylonite, and phyllonite.           | (1998),       |
|           | Equivalent to Brodie and others' (2002)  | Brodie and    |
|           | broader usage of "mylonite" for "A fault | others (2002) |
|           | rock which is cohesive and characterized |               |
|           | by a well developed \[foliation\]        |               |
|           | resulting from tectonic reduction of     |               |
|           | grain size, and commonly containing      |               |
|           | rounded porphyroclasts and lithic        |               |
|           | fragments of similar composition to      |               |
|           | minerals in the matrix." Additional      |               |
|           | criteria introduced here for precision   |               |
|           | in databases include a foliation defined |               |
|           | by deformed mineral grains or grain      |               |
|           | aggregates having aspect ratio \> 1.5:1, |               |
|           | and \>10% of rock consisting of          |               |
|           | \'matrix\' . Matrix is an aggregate of   |               |
|           | new mineral grains (not present in the   |               |
|           | protolith, but may be same mineral       |               |
|           | species) that are significantly smaller  |               |
|           | (1% of diameter) than the original size  |               |
|           | of mineral grains, and are the product   |               |
|           | of non-cataclastic deformation.          |               |
|           | Non-cataclastic deformation is           |               |
|           | deformation in which the material        |               |
|           | continuity of the deformed volume is     |               |
|           | maintained on the scale of observation,  |               |
|           | indicated by the absence of              |               |
|           | thoroughgoing fractures in the volume.   |               |
|           | The definition is meant to identify a    |               |
|           | fabric in the rock due to crystal        |               |
|           | plastic and/or other types of            |               |
|           | non-cataclastic deformation.             |               |
+-----------+------------------------------------------+---------------+
| no        | A foliated, phaneritic rock that lacks   | IUGS (Schmid  |
| n-layered | well developed, continuous schistosity   | and others    |
| gneiss    | and laterally continuous compositional   | (2002; Brodie |
|           | layering \> 5 mm thick. "Laterally       | and others,   |
|           | continuous" here means that layers       | 2002)         |
|           | defining the foliation can be traced \>  |               |
|           | 10 cm (length of lateral continuity).    |               |
|           | Foliated means that ≥ 10% of mineral     |               |
|           | grains in the rock are fabric elements.  |               |
+-----------+------------------------------------------+---------------+
| non-      | Metamorphic rock that is not             | this report   |
| mylonitic | granoblastic and does not have a         |               |
| foliated  | mylonitic fabric.                        |               |
| me        |                                          |               |
| tamorphic |                                          |               |
| rock      |                                          |               |
+-----------+------------------------------------------+---------------+
| phyllite  | Rock that has a well developed,          | Jackson       |
|           | continuous schistosity, an average grain | (1997),       |
|           | size (excluding porphyroblasts) \<0.25   | Barker (1998, |
|           | mm and \>0.1 mm, and a silvery sheen on  | Appendix II)  |
|           | cleavage surfaces.                       |               |
+-----------+------------------------------------------+---------------+
| p         | Phyllosilicate-rich mylonite-series rock | Sibson        |
| hyllonite | of phyllitic appearance (having a        | (1977),       |
|           | silvery sheen on cleavage surfaces);     | Passchier and |
|           | "sometimes (erroneously) used for        | Trouw (1996), |
|           | ultramylonite" (Passchier and Trouw,     | Snoke and     |
|           | 1996). Criterion added here for          | Tullis        |
|           | precision is having \>40% phyllosilicate | (1998),       |
|           | minerals (e.g., mica, chlorite).         | Robertson     |
|           |                                          | (1999),       |
|           |                                          | Brodie and    |
|           |                                          | others (2002) |
+-----------+------------------------------------------+---------------+
| polymict  | Impact breccia that contains fragments   | Stöffler      |
| impact    | of different composition and is free of  | (2001c),      |
| breccia   | impact melt particles.                   | Stöffler and  |
|           |                                          | Grieve(2001). |
+-----------+------------------------------------------+---------------+
| protoca   | Cataclastic rock that maintained primary | Snoke and     |
| taclasite | cohesion during deformation, and         | Tullis        |
|           | consists of 10-50% matrix. Matrix is     | (1998):       |
|           | broken mineral grains or rock fragments. | Sibson (1977) |
|           |                                          | modified by   |
|           |                                          | Scholz (1990) |
+-----------+------------------------------------------+---------------+
| prot      | Mylonite-series rock consisting of       | composite     |
| omylonite | 10-50% matrix showing evidence of        | from          |
|           | tectonic grain size reduction.           | Robertson     |
|           |                                          | (1999),       |
|           |                                          | Jackson       |
|           |                                          | (1997), Snoke |
|           |                                          | and Tullis    |
|           |                                          | (1998),       |
|           |                                          | Barker (1998, |
|           |                                          | Appendix II), |
|           |                                          | Passchier and |
|           |                                          | Trouw (1996)  |
+-----------+------------------------------------------+---------------+
| pseudo    | "Ultrafine-grained vitreous-looking      | Spray,(1995), |
| tachylite | material, usually black and flinty in    | Passchier and |
|           | appearance, occurring as thin planar     | Trouw (1996), |
|           | veins, injection veins or as a matrix to | Snoke and     |
|           | pseudo-conglomerates or breccias, which  | Tullis        |
|           | infills dilation fractures in the host   | (1998),       |
|           | rock" (Brodie and others, 2002). \[see   | Brodie and    |
|           | "impact pseudotachylite"\]               | others (2002) |
+-----------+------------------------------------------+---------------+
| schist    | Phaneritic metamorphic rock having a     | Jackson       |
|           | well developed schistosity. Well         | (1997),       |
|           | developed schistosity is defined to mean | Barker (1998, |
|           | that \>50% of rock consists of mineral   | Appendix II)  |
|           | grains having a tabular, lamellar, or    |               |
|           | prismatic crystallographic habit that    |               |
|           | are oriented in a continuous planar or   |               |
|           | linear fabric (following Jackson, 1997). |               |
|           | Continuous is defined on a hand          |               |
|           | sample-scale, and in quantitative terms  |               |
|           | to mean that domains lacking the fabric  |               |
|           | are \<1 cm thick if they are layers, and |               |
|           | \<5 cm in diameter if they are irregular |               |
|           | patches, and constitute \< 25% of the    |               |
|           | rock.                                    |               |
+-----------+------------------------------------------+---------------+
| schistose | Mylonite that has a well developed       | modified from |
| mylonite  | continuous schistosity                   | Snoke and     |
|           |                                          | Tullis        |
|           |                                          | (1998),       |
|           |                                          | Sibson (1977) |
+-----------+------------------------------------------+---------------+
| schistose | Protomylonite that has a well developed  | modified from |
| prot      | schistosity                              | Snoke and     |
| omylonite |                                          | Tullis        |
|           |                                          | (1998),       |
|           |                                          | Sibson (1977) |
+-----------+------------------------------------------+---------------+
| shocked   | non-brecciated rock which shows          | Stöffler      |
| rock      | unequivocal effects of shock             | (2001c),      |
|           | metamorphism exclusive of whole-rock     | Stöffler and  |
|           | melting (Stöffler (2001c). Specifically, | Grieve (2001) |
|           | \<= 30% of the rock is fracture-bounded  |               |
|           | fragments, or if fragments form \>30% of |               |
|           | rock, they are not rotated relative to   |               |
|           | each other.                              |               |
+-----------+------------------------------------------+---------------+
| slate     | Aphanitic rock that has well developed   | Jackson       |
|           | schistosity An average grain size        | (1997),       |
|           | \<0.1mm (excluding porphyroblasts) is    | Robertson     |
|           | specified here for precision.            | (1999),       |
|           |                                          | Brodie and    |
|           |                                          | others (2002) |
+-----------+------------------------------------------+---------------+
| suevite   | Impact breccia that contains impact melt | Stöffler      |
|           | particles (\>0 and \<50%))               | (2001c),      |
|           |                                          | Stöffler and  |
|           |                                          | Grieve (2001) |
+-----------+------------------------------------------+---------------+
| ultraca   | Cataclastic rock that maintained primary | Snoke and     |
| taclasite | cohesion during deformation, and         | Tullis        |
|           | consists of 90-100% matrix. Matrix is    | (1998):       |
|           | broken mineral grains or rock fragments. | Sibson (1977) |
|           |                                          | modified by   |
|           |                                          | Scholz (1990) |
+-----------+------------------------------------------+---------------+
| ultr      | Mylonite-series rock consisting of       | composite     |
| amylonite | 90-100% matrix showing evidence of       | from          |
|           | tectonic grain size reduction.           | Robertson     |
|           |                                          | (1999);       |
|           |                                          | Jackson       |
|           |                                          | (1997), Snoke |
|           |                                          | and Tullis    |
|           |                                          | (1998),       |
|           |                                          | Passchier and |
|           |                                          | Trouw (1996), |
|           |                                          | Barker (1998, |
|           |                                          | Appendix II)  |
+-----------+------------------------------------------+---------------+

##  Metamorphic (including hydrothermally altered) rock

SLTTM_1.0 recognizes the following classes of metamorphic rock
(including hydrothermally altered rock) based on fabric: (1)
granoblastic rock including granofels and hornfels, and (2)
non-mylonitic foliated metamorphic rock including schistose rock
(schist, phyllite, slate) and gneiss (layered gneiss and non-layered
gneiss), and (3) mylonitic rock.

![](media/image1.wmf){width="6.495138888888889in"
height="4.2034722222222225in"}

### Granoblastic rock

#### Granofels

Granofels was introduced by Goldsmith (1959) for "medium- to
coarse-grained granoblastic metamorphic rock with little or no foliation
or lineation" (Jackson, 1997). The British Geological Survey (Robertson,
1999) and provisional IUGS nomenclature (Brodie and others, 2002; Schmid
and others, 2002) use the term regardless of grain size.

SLTTM_1.0 defines **granofels** as *a phaneritic metamorphic rock that
has little or no foliation or lineation,* implying that that \<10% of
the particles in the rock are fabric elements. To be a fabric element, a
particle must have an inequant shape with an aspect ratio ≥ 1.5:1, and
be aligned with other particles as part of a fabric. In this sense,
granofels is the phaneritic equivalent of hornfels.

#### Hornfels

The *Glossary of Geology* (Jackson, 1997) cites an earlier edition
(1967) of Winkler (1979) in defining hornfels as "A fine-grained rock
composed of a mosaic of equidimensional grains without preferred
orientation and typically formed by contact metamorphism. Porphyroblasts
or relict phenocrysts may be present in the characteristically
granoblastic (or decussate) matrix." A genetic connotation of contact
metamorphism is required by the British Geological Survey definition
(Robertson, 1999, p. 12) of hornfels as "a specific variant of
granofels" to be used as a root name where "features of the original
rock have been modified by contact metamorphism...Robertson (1999)
states that \"a massive, compact, fine-grained rock should be classified
as a fine-grained granofels (Goldsmith, 1959) and not a hornfels if
there is no direct evidence for contact metamorphism.\" Alternatively,
Winkler (1976, p. 327) states that "Hornfelses are typically produced by
contact metamorphism...and *occasionally by regional metamorphism*
\[italics added\]."

SLTTM_1.0 defines hornfels as a non-foliated aphanitic rock having
granoblastic fabric. The term hornfels does not necessarily denote a
contact metamorphic origin (although that is most commonly the case).
This is consistent with the goal of keeping this terminology as
descriptive as possible. Hornfels, in this sense, is the aphanitic
equivalent of granofels.

### Non-mylonitic foliated metamorphic rock

#### Historical usage of schist and gneiss

The term \"gneiss\" is used inconsistently in the geological literature.
Some reports distinguish gneiss and schist based on layering whereas
others use mineral percentages (e.g., mica vs. feldspar + quartz). Note
the etymology of 'gneiss': "alteration of Middle High German *gneiste*
spark" \[Merriam-Webster's Dictionary, 2004,
http://www.m-w.com/cgi-bin/dictionary\], suggesting the term originally
had more to do with composition than with foliation.

Definitions of gneiss:

1.  Medium- to coarse-grained rock having a gneissic fabric, i.e. "it
    splits parallel to 's' generally along mica or hornblende layers,
    into plates and angular blocks, a few centimeter to tens of
    centimeters in thickness, or parallel to 'B\' into cylindrical
    bodies (pencil gneiss). The prevalent light-colored constituents
    (quartz+feldspar) have interlocking boundaries and provided, as
    compared to schists, a better coherence and a coarser fissility to
    the rock; nevertheless the fissility in many cases creates an almost
    perfect plane" (Wenk, 1963, quoted in Winkler, 1979)

2.  Rock having recognizable parallel structure consisting predominantly
    of quartz and feldspar (feldspar \>20%, and mica \>10%) Fristch and
    others, 1967, quoted in Winkler, 5^th^ edition, 1979, p. 342).

3.  A medium- to coarse-grained irregularly "banded" rock that has
    fairly poor schistosity because of the preponderance of quartz and
    feldspar; equivalent or higher regional metamorphic grade than a
    schist (Hyndman, 1972).

4.  Rock composed chiefly of quartz and feldspar; medium- to
    coarse-grained phaneritic, granoblastic to lepidoblastic;
    compositional layering expressed by varying modal proportions of
    quartz, feldspar, mica and hornblende usually evident, but in some
    granitic gneisses layering is quite subtle or even absent and a weak
    foliation is expressed by preferred orientation of inequant mineral
    grains or grain aggregates. (Best, 2002).

5.  Banded rock that does not break along a preferred plane (Blatt and
    Tracy, 1996). A gneiss has large grains and is foliated i.e. quartz
    and feldspar rich layers separated from micaceous or mafic layers
    (Yardley, 1989). Types of Occurrences: Silicic: light colored
    minerals such as quartz and feldspar make up the majority of the
    rock; Intermediate: equal amounts of light and dark minerals; Mafic:
    ferromagnesian minerals make up most of the rock; Undifferentiated
    (<http://www.geol.lsu.edu/rkd_dir/gneiss.html>[)]{.underline}

6.  A foliated rock formed by regional metamorphism, in which bands or
    lenticles of granular minerals alternate with bands or lenticles in
    which minerals having flaky or elongate prismatic habits
    predominate; generally \<50% of minerals show preferred parallel
    orientation; although commonly feldspar+quartz rich, mineral
    composition is not an essential factor in its definition (Jackson,
    1997).

7.  A rock (regardless of protolith and modal composition) that is
    medium- to coarse-grained, inhomogeneous, and characterized by a
    coarse foliation or layering and some layers \>5 mm thick
    (Robertson, 1999).

8.  Gneiss is a roughly foliated or banded metamorphic rock consisting
    largely of granular minerals such as quartz (
    <http://www.cst.cmich.edu/users/dietr1rv/gemrxD-K.htm> , gem rock
    definitions)

9.  A rock that has alternating bands of granular and flaky (or
    elongate) minerals. Generally less than 1/2 the minerals show a
    preferred parallel orientation.
    (<http://forestry.about.com/library/glossary/blforgll.htm>[)]{.underline}
    forestry terms.

10. A metamorphic rock displaying a gneissose structure (A type of
    foliation on the hand specimen scale, defined by (a) irregular or
    ill-defined layering, or (b) augen and/or lenticular aggregates of
    mineral grains (augen structure, flaser structure), or (c) inequant
    mineral grains which are present, however, only in small amounts or
    which display only a weak preferred orientation, thus defining only
    a poorly developed schistosity.). The term gneiss may also be
    applied to rocks displaying a dominant linear fabric rather than a
    gneissose structure, in which case the expression \"lineated
    gneiss\" is applied. The term gneiss is used almost exclusively for
    rocks containing abundant feldspar +/- quartz, but may be used in
    exceptional cases for other compositions, in which case the exact
    mineralogy should be given (e.g. feldspar and quartz- free
    cordierite-anthophyllite-gneiss).
    (<http://web.met.unimelb.edu.au/TeachingSupport/625-224/224practerms.pdf>
    [,]{.underline} definitions of technical terms for Roger Powell's
    Metamorphism class).

Definitions of schist:

1.  A strongly foliated crystalline rock, formed by dynamic
    metamorphism, that can be readily split into thin flakes or slabs
    due to the well developed parallelism of more than 50% of the
    minerals present, particularly those of lamellar or platy habit,
    e.g. mica and hornblende. The mineral composition is not an
    essential factor in its definition unless specifically included in
    the rock name (Jackson, 1997).

2.  Metamorphic rock "Characterised by parallel alignment of moderately
    coarse grains, usually clearly visible with the naked eye...This
    type of fabric is known as schistosity..." (Yardley, 1991, p. 22).

3.  "Metamorphic rock commonly of pelitic composition, with a well
    developed schistosity." (Barker, 1998, p. 242).

4.  "A metamorphic rock displaying schistose structure. For
    phyllosilicate-rich rocks the term schist is reserved for medium- to
    coarse-grained varieties, whereas finer-grained rocks are termed
    slates or phyllites." (Schmid and others, 2002).

5.  The main characteristic of "schist" is a well-developed
    *schistosity*. Schistosity has been defined as:

6.  "The foliation in schist or other coarse-grained, crystalline rock
    due to the parallel alignment of platy mineral grains (mica) or
    inequant crystals of other minerals\" (Jackson, 1997). \[This
    definition is preferred here because the distinction between
    foliation related to crystallographic orientation of tabular or
    elongate crystals (schistosity) and grain-shape fabric is important
    in interpreting the fabric genesis.\]

7.  "A preferred orientation of inequant mineral grains or grain
    aggregates produced by metamorphic processes." (Schmid and others,
    2002).

8.  "A foliation or lineation which allows the rock to be split easily
    along planes. Constituent minerals can be seen with the unaided
    eye." (Robertson, 1999, p. 13)

9.  A "secondary foliation defined by preferred orientation of inequant
    fabric elements in a medium to coarse grained rock. Individual
    foliation-defining elements are visible with the naked eye"
    (Passchier and Trouw, 1996). \[This definition allows a schistosity
    to be defined by the grains that are inequant due to plastic
    deformation---thus a pure quartz tectonite that has a grain shape
    fabric defined by tectonically flattened quartz grains would have a
    schistosity under this definition.\]

Based on the various definitions of schist and gneiss, the criteria used
to distinguish these rock types are:

-   Presence of compositional banding

-   Thickness of compositional banding

-   Homogeneity of the rock

-   Modal abundance of mica

-   Modal abundance of quartz +feldspar

-   Degree of fissility (how thinly can the rock be parted along the
    'schistosity')

-   Grain size

In the IUGS proposal of Brodie and others (2002), a "schistose
structure" is characterized by a "schistosity which is well developed,
either uniformly through the rock or in narrowly spaced repetitive zones
such that the rock will [split on a scale of one cm or
less]{.underline}" and a "gneissose structure" is characterized by a
"schistosity which is either poorly developed throughout the rock or, if
well developed, occurs in broadly spaced zones such that the rock will
[split on a scale of more than one cm]{.underline}."

In practical terms, spitting may be influenced by extraneous variables
such as later foliation-parallel joints and weathering. The thickness of
sheets split along the foliation in unweathered rocks cannot be used to
classify the weathered rocks commonly encountered in the field.
Classification would thus be a function of weathering---an undesirable
side effect. Criteria based on the thickness of visible layering would
not suffer from dependence on the degree of weathering.

#### Schistose rock (schist, phyllite, and slate)

*Schistose rock*---Any metamorphic rock that has a well developed,
continuous schistosity. "Well developed" schistosity is defined to mean
that \>50% of the rock consists of mineral grains having a tabular,
lamellar, or prismatic crystallographic habit that are oriented in a
continuous planar or linear fabric (following Jackson, 1997), and
"continuous" schistosity is defined on a hand-sample scale, and in
quantitative terms, to mean that domains lacking the fabric are \<1 cm
thick if they are layers, and \<5 cm in diameter if they are irregular
patches, and constitute \<25% of the rock.

SLTTM_1.0 subdivides rocks that have well developed continuous
schistosity (schistose rocks) into the following:

*Schist*---phaneritic rock having average grain size \>.25 mm). Schist
may have compositional layering at any scale.

*Phyllite*---phaneritic rock but finer-grained than schist, having
average grain size \<0.25 mm but \>0.1 mm and typically a silvery sheen
on cleavage surfaces.

The term phyllite is widely used and generally understood to describe a
fine-grained schistose micaceous rock. Robertson (1999) discusses the
term as follows:

> "The term phyllite has previously been used for rocks possessing a
> silky or lustrous sheen on foliation surfaces imparted by fine-grained
> (\< 0.1 mm) white mica (including muscovite, paragonite and phengite)
> orientated parallel to the foliation in the rock. Individual mica
> flakes can be seen with the naked eye in contrast to slates where they
> cannot be distinguished. Most are probably derived by the low- to
> medium-grade metamorphism of mudstones although some rocks that have
> been described as phyllites may have been confused with
> phyllonites...Here 'phyllite' is classified as a specific variant of
> schist. It is therefore not permissible as a root name but may be used
> as a specific qualifier, namely *phylliti*c, for example *phyllitic*
> semipelite." (Robertson, 1999, p. 12).

*Slate*---aphanitic rock, implying average grain size \<0.1 mm. In the
system proposed here, the term slate is restricted to schistose rocks
that are aphanitic except for porphyroblasts. This definition is
essentially equivalent to that of Brodie and others \[2002\].

Because "phyllite" and "slate" are widely used on geologic maps in North
America, SLTTM_1.0 differs from Robertson (1999) and advocates retaining
them as rock names. In order for the terms to be useful and unambiguous,
criteria for uniquely distinguishing phyllite from schist and slate must
be established.

The definition of slate as an aphanitic rock displaying a well developed
continuous schistosity is consistent with the provisional IUGS
definition (Brodie and others, 2002). In addition, it requires that
\>50% of the rock consists of mineral grains having a tabular, lamellar,
or prismatic crystallographic habit that are oriented in a continuous
planar or linear fabric. In an aphanitic rock this determination is
generally based on indirect evidence, which is typically the presence of
slaty cleavage, and the sheen observed on parting surfaces due to
alignment of tiny phyllosilicate grains. An average grain size that is
aphanitic (\<0.1 mm), except for porphyroblasts, is specified here for
precision, and the North American spelling of *slaty* (rather than
*slatey*; Jackson, 199*7*) is preferred for use in North America.

#### Layered gneiss and non-layered gneiss

Gneiss does not have continuous schistosity on a hand sample scale as
defined above, but it may have schistose layers separated by
non-schistose layers. The boundary of gneiss and schist in this logic is
placed where the mineral grains that define the schistosity are
distributed such that the schistosity is deemed "continuous.". SLTTM_1.0
defines gneissose rocks in the following way:

*Gneiss*---a foliated phaneritic metamorphic rock that does not have
"well developed" continuous schistosity. This follows the spirit of the
proposed IUGS classification scheme (Schmid and others, 2002; Brodie and
others, 2002). Classified into **layered gneiss** and **non-layered
gneiss**.

*Layered gneiss*---a foliated, phaneritic rock that lacks well
developed, continuous schistosity and has laterally continuous
compositional layering \> 5 mm thick.

*Non-layered gneiss*---

#### Migmatitic rocks as subclasses

The IUGS proposed definition of migmatite (Wimmenauer and Brynghi, 2002,
p. 2) is "A composite silicate rock, pervasively heterogeneous on a
meso- to megascopic scale. It typically consists of darker and lighter
parts. The darker parts usually exhibit features of metamorphic rocks
while the lighter parts are of plutonic appearance..." Wimmenauer and
Brynghi (2002) use "migmatite" as a superclass for any rock that has
these characteristics.

Numerous terms have been employed for varieties of migmatite (Mehnert,
1968), but most are either genetic or difficult to apply on a
hand-specimen scale. In the BGS classification, \"\'Migmatite\' is not
permissible as a root name...as it is not a single rock type. However,
migmatitic may be used as a specific textural modifier\" (Robertson,
1999, p. 11). Another problem is that "the scale of migmatitic
structures is such that they mainly require definitions, which refer to
rock masses greater than the preferred hand specimen size" (Wimmenauer
and Bryngi, 2002). The IUGS (Wimmenauer and Brynghi, 2002) recommends
'migmatite' as the "main term" for these rocks as well as "special
terms" (equated to recommended rock names) for parts of a migmatite
(such as leucosome and restite), genetic types (such as anatexite and
veinite), and three descriptive types (agmatite = breccia-like; phlebite
= veined, nebulite = having diffuse relics of pre-existing rock). The
terms for parts of a migmatite represent roles in a relationship, and
are thus not suitable as kinds of rock.

The system proposed here specifically avoids use of genetic terminology
where possible, and the genetic classification of migmatitic rocks is
thus outside the scope of the system. The descriptive terms are
logically equivalent to other terms in the classification e.g. agmatite
= migmatitic granofels (in most cases), and are thus not included as
separate terms. Any of these names could be used in an uncontrolled rock
name field in a database.

Rather than using "migmatite" as a root term or superclass as proposed
by Wimmenauer and Brynghi (2002) for IUGS, the classification here
follows the British Geological Survey rationale and precedent
(Robertson, 1999), which applies "migmatitic" as an adjective modifier
to terms such as gneiss and schist. For precision in database
applications, this classification requires that the lighter colored,
granitoid phase must form \>10% of the rock volume.

### Mylonite-series rock

A rock is here classified as a **mylonite-series rock** if the rock
displays a foliation defined by the shapes of deformed mineral grains or
grain aggregates having aspect ratios \> 1.5:1, \>10% of the rock is
composed of "matrix" showing evidence of tectonic reduction in grain
size, and the foliation and matrix are interpreted to be the product of
continuous, crystal-plastic deformation processes. Deformation in faults
and shear zones occurs in a continuum of environmental conditions from
near the Earth's surface, where discontinuous/brittle processes
dominate, to high temperature conditions deep in the Earth where
continuous deformation and crystal-plastic processes dominate (Sibson,
1977; Passchier and Trouw, 1996). Foliated, deformed rocks formed in the
crystal-plastic regime are included in the mylonite-series.

Mylonite-series rocks are commonly lineated. They typically contain
fabric elements such as quartz ribbons, mica fish, asymmetric
porphyroclasts, S-C composite planar fabrics, and microstructures
indicative of crystal-plastic deformation (e.g. recovery and
recrystallization, see Sibson, 1977; Passchier and Trouw, 1996; Snoke
and Tullis, 1998). The matrix consists of new or recrystallized mineral
grains that are interpreted to be smaller than the mineral grains in the
original, undeformed protolith. In cases where the protolith was very
fine-grained or is unknown, problems in applying this definition may be
overcome to some extent by observing the progressive development of
fabric and grain-size reduction along the margins of high-strain zones.
The distinction between relatively high-temperature cataclastic rocks
and mylonite-series rocks is based on the presence of a foliation
produced by aligned mineral grains whose shape has been modified by
crystal-plastic deformation in mylonite-series rocks.

Standard classification schemes for mylonite-series rocks are based on
the degree of deformation-related grain-size reduction, which is
quantified as the percentage of matrix produced by tectonic reduction in
grain size (Sibson, 1977; Wise and others, 1984; Scholz, 1990; Passchier
and Trouw, 1996; Snoke and Tullis, 1998). Snoke and Tullis (1998, Table
0.1) endorsed Scholz's (1990, Table 3.1) slightly modified version of
Sibson's (1977) classification of fault rocks. We adopt that
classification here with minor modifications for database applications.
The mylonite-series rocks, as used here, are equivalent to the "mylonite
series" of Sibson (1977), which is subdivided into **protomylonite**
(having 10-50% matrix), **mylonite** (having 50-90% matrix), and
**ultramylonite** (having 90-100% matrix).

**Phyllonite** is a common term for phyllosilicate-rich mylonite-series
rock of phyllitic appearance, i.e. having a silvery sheen on cleavage
surfaces (Sibson, 1977; Passchier and Trouw, 1996; Snoke and Tullis,
1998; Robertson, 1999, Brodie and others, 2002). SLTTM-1.0 also
specifies \>40% phyllosilicate minerals (e.g., mica, chlorite) and a
*well developed continuous schistosity* as defined in Appendix 2.

Under some conditions, recrystallization during deformation may lead to
an increase in grain size relative to the protolith of a highly strained
rock. Such rocks commonly have equant polygonal mineral grains and a
foliation defined by composition or grain-size variations. These rocks
are sometimes called "blastomylonite," but SLTTM_1.0 would classify them
as "layered gneiss" or possibly as "non-layered gneiss" if the foliation
is defined by flat mineral grains without layering.

## Cataclastic rock

A rock is here classified as a cataclastic rock if \>10% of the volume
consists of fragments bounded by fractures related to deformation that
occurred after the formation of the protolith. Cataclastic rocks
typically include fault rocks formed in the brittle regime, although
that origin is not required by the descriptive definition. Deformation
in faults and shear zones occurs in a continuum of environmental
conditions from near the Earth's surface, where discontinuous brittle
processes dominate, to high temperature conditions deep in the Earth
where continuous deformation and crystal-plastic processes dominate
(Sibson, 1977; Passchier and Trouw, 1996). Cataclastic rocks are further
classified based on the presence or absence of primary cohesion, the
percentage of broken fragments large enough to be visible, and the
amount of fragmental cataclastic matrix (Sibson, 1977; Snoke and Tullis,
1998).

Some cataclastic rocks are foliated, but where this is the case, the
foliation is defined by aligned aggregates of rock fragments, rather
than oriented mineral grains. Minerals in the same rock may deform by
different mechanisms, and loss of material continuity across slip
surfaces may vary, resulting some rocks that are transitional between
cataclastic rocks and mylonite-series rocks.

SLTTM_1.0 adapts Sibson's (1977) widely used classification of
cataclastic rocks, as slightly modified by Scholz (1990, Table 3.1) and
endorsed by Snoke and Tullis (1998, Table 0.1). The cataclastic rocks
are initially subdivided based on the presence or absence of primary
cohesion (cohesion during the deformation). Those having evidence of
primary cohesion ('**cataclasite series**' of Sibson) are subdivided
into **protocataclasite** (having 10-50% matrix), **cataclasite**
(having 50-90% matrix), and **ultracataclasite** (having 90-100%
matrix). Fluid flow along fault zones commonly re-cements rocks that
have lost cohesion during deformation, obscuring evidence for the
presence or absence of primary cohesion during deformation. **Incohesive
cataclastic rocks** are those that lack evidence of primary cohesion.
These include **fault breccia** (having visible fragments make up \>30%
of the rock) and **gouge** (having visible fragments \<30% of the rock).
Cataclastic rocks are further subdivided based on the presence or
absence of foliation. **Pseudotachylite**, as discussed below under
composite-genesis melt rock, is also regarded as a special type of
cataclastic rock. Distinctions between cataclastic rock and **broken
rock** related to mass wasting (e.g., landslides, rock avalanche,
caldera collapse) are based on the geologic setting.

## Composite-genesis melt rock

### Definition

*Composite-genesis melt rock*---a rock that solidified from melt
(liquid) of a preexisting rock under conditions outside the domain of
typical igneous rocks.

Examples of composite-genesis melt rock include contact-metamorphic melt
rock (melt under contact metamorphic conditions), impact melt rock (also
classified as impact-metamorphic rock and discussed under that heading),
and pseudotachylite.

Pseudotachylite (also spelled pseudotachylyte) is typically a dark,
glassy-looking rock that occurs as veins and dike-like injections in
fault systems, and it is commonly interpreted as a product of frictional
melting along faults (Spray, 1995; Snoke and Tullis, 1998). The IUGS
provisional definition for **pseudotachylite** is "Ultrafine-grained
vitreous-looking material, usually black and flinty in appearance,
occurring as thin planar veins, injection veins or as a matrix to
pseudo-conglomerates or breccias, which infills dilation fractures in
the host rock" (Brodie and others, 2002). SLTTM_1.0 modifies this
definition slightly for internal consistency, and to avoid introduction
of new terminology:

*Pseudotachylite*---an aphanitic, vitreous or flinty material that
occupies dilatent fractures associated with fault zones or impact
craters, and is interpreted to be related to frictional melting
associated with seismic or shock events. It typically contains abundant
fractured-rock inclusions and occurs within larger bodies of broken or
brecciated rock.

"Impact pseudotachylite" is regarded here as a special occurrence of
pseudotachylite as discussed below under "impact-metamorphic rock."

## Impact-metamorphic rock

Impact-metamorphic rock is defined (unavoidably) by genesis and
subdivided based on descriptive fabric criteria. In the strictest sense,
i**mpact metamorphism** is "caused by the passage of a shock wave due to
impact of a planetary body (projectile or impactor) on a planetary
surface (target)" whereas "shock metamorphism" is "caused by shock wave
compression due to impact of a solid body or due to the detonation of
high-energy chemical or nuclear explosives" (Stöffler, 2001c). Impact
metamorphic rocks display evidence of shock metamorphism, such as
microscopic planar deformation features within grains or shatter cones
(Stöffler and Grieve, 2001). Many rocks classified as impact metamorphic
rocks are interpreted as such based on field relationships (such as a
crater), and such interpretations can be subject to debate.

The field classification of impact-metamorphic rocks can be problematic,
because of their similarity to some sedimentary or fault-related
breccias, and to various fragmental volcanic or pyroclastic rocks in
other situations. Where impact metamorphic rocks would meet the
descriptive criteria for cataclastic rocks, the distinction of an impact
metamorphic rock is based on observable features attributed to shock
metamorphism, or by interpretation of field relations at the sample
location. The essential feature that identifies an impactite (impact
metamorphic rock) is the presence of microscopic planar deformation
features that are unequivocally the result of shock metamorphism
(Stöffler and Grieve, 2001).

Stöffler and Grieve (2001) and Stöffler (2001a, 2001b, 2001c) proposed a
threefold IUGS classification of impactites resulting from a single
impact event as (i) shocked rock, (ii) impact melt rock, and (iii)
impact breccia. The definitions, as slightly modified and adapted for
use in this classification, are:

**shocked rock**---non-brecciated rock which shows unequivocal effects
of shock metamorphism exclusive of whole rock melting.

**impact melt rock**---crystalline, semi-glassy, or glassy rock in which
\>=50% of the rock volume is solidified from impact melt (implying \<50%
non-melt inclusions).

**impact breccia**---breccia, as generally defined, that has unequivocal
evidence of shock metamorphism.

"Impact breccia" is further divided into three subclasses, slightly
modified from Stöffler and Grieve (2001) and Stöffler (2001a, 2001b,
2001c), which are: (i) **suevite** \[impact breccia that contains impact
melt particles (\>0 and \<50%)\]; (ii) **polymict impact breccia**
\[impact breccia that contains fragments of different composition and is
free of impact melt particles, and (iii) **monomict impact breccia**
\[impact breccia free of impact melt in which all of the fragments
(100%) have essentially the same composition."\].

Pseudotachylite, in addition to being associated with regional faults
(see Cataclastic Rocks), occurs in impact structures such as the
Vredefort structure in South Africa, where pseudotachylite was first
described (discussion and references in Snoke and Tullis, 1998).
Pseudotachylite is here classified as a cataclastic rock, whether or not
the associated fault or fracture zone happens to be part of an impact
structure. "Impact pseudotachylite" as used by Stöffler (2001c) is not
distinguished in this classification and, instead, is regarded as a
special occurrence of pseudotachylite.

Further subdivisions of impact metamorphic rock, including those from
multiple impacts as known from the Moon (Stöffler and Grieve, 2001), are
not considered here. More specific terms can be used in a text field
where desirable. In the case of "shocked rock," the dual use of a
protolith classification (sedimentary, igneous, or metamorphic) can be
applied. In a similar fashion, the dual use of an igneous rock
classification can provide more information on the composition and
texture of "impact melt rock" at places such as Sudbury, Ontario.

## Term of last resort in absence of fabric class

'Rock' is used as a root name of last resort only where the fabric of
the rock is unknown and therefore unspecified. Potential applications
may be necessary where geologic map units are inferred from geophysical
data or compiled from sources based on other criteria such as age or
formation name. If the fabric is known, a more informative term such as
schist, gneiss, or granofels should be used. A composition qualifier
could be added to rock (e.g., calc-silicate rock, quartzofeldspathic
rock, ultramafic rock).

# CLASSIFICATION BY COMPOSITION (QUALIFIER TERMS)

Selection of composition qualifiers is based on the modal mineralogy of
a metamorphic rock. Compositional classification is based on
classification of mineral species into groups based on general chemical
similarity. Definitions of the mineral groups that serve to identify the
various composition types are thus fundamental.

## Definition of rock-forming mineral groups in this classification

**Ferromagnesian minerals**. Omphacite (jadeitic pyroxene), chlorite,
dark-colored amphibole, dark-colored pyroxene, biotite, serpentine,
pyrope garnet, talc.

**Quartz-feldspar (quartzofeldspathic) minerals**: quartz, plagioclase,
K-feldspar.

**Calcsilicate minerals**: minerals that contain significant amounts of
Ca ± Mg and Si and include diopside, epidote, grossularite and uvarovite
garnet, calcic-amphiboles, titanite, wollastonite, vesuvianite and
calcic plagioclase. Mg-rich minerals such as forsterite and phlogopite
are also common constituents of calcsilicate-rocks. As a general rule,
plagioclase may be considered a calcsilicate mineral if it has \>50%
anorthite content (Robertson, 1999).

**Carbonate minerals**: calcite, dolomite, siderite.

**Aluminous minerals**: aluminosilicates, muscovite, kaolinite, garnet
(associated with feldspar), corundum, pyrophyllite.

**Phyllosilicate minerals**: mica group, chlorite group

Garnets occur in aluminous, ferromagnesian, and calcsilicate rocks.
Cordierite, staurolite, brucite, and periclase are not useful for
compositional rock classification without more detailed knowledge of
their mineral associations or compositions.

Table 2. Summary of composition terms

  ---------------------------------------------------------------------------------------------
  **Qualifier**             **definition**                      **source**      **kind**
  ------------------------- ----------------------------------- --------------- ---------------
  amphibolite               Rock that consists of \>75% green,  Coutinho and    common
                            brown, or black amphibole plus      others (2002)   
                            plagioclase (including albite) and                  
                            amphibole \>30% (modal) of whole                    
                            rock, and amphibole \>50% of total                  
                            mafic constituents.                                 

  argillic                  use for apparently clay-rich        based on        standard
                            aphanitic rocks                     Jackson, 1997   chemical

  calcareous                When mineralogy cannot be           Jackson (1997); standard
                            identified (aphanitic rocks), means criteria        chemical
                            that rock reacts to form bubbles    proposed by     
                            when hydrochloric acid is applied.  this report     

  calcareous                rock consists of 10%-50% carbonate  Robertson       standard
  quartzo-feldspathic       or calcsilicate minerals, and       (1999)          chemical
                            micaceous or aluminous minerals                     
                            form \<40% of non-(carbonate or                     
                            calcsilicate) minerals and quartz                   
                            forms \<60% of the non-(carbonate                   
                            or calcsilicate) minerals                           

  calcareous-pelitic        rock consists of 10%-50% carbonate  Robertson       standard
                            or calcsilicate minerals, and       (1999)          chemical
                            micaceous or aluminous minerals                     
                            form ≥ 40% of non-(carbonate or                     
                            calcsilicate) minerals                              

  calcareous-quartzite      rock consists of 10%-50% carbonate  Robertson       standard
                            or calcsilicate minerals, and       (1999)          chemical
                            micaceous or aluminous minerals                     
                            form \<40% of non-(carbonate or                     
                            calcsilicate) minerals, and quartz                  
                            forms ≥ 75% of the non-(carbonate +                 
                            calcsilicate) minerals                              

  calcareous-semi-pelitic   Rock for which the sum of modal     Proposed, this  standard
                            quartz+feldspar+ mica + aluminous   report          chemical
                            mineral is ≥ 70%, and quartz+                       
                            feldspar \< 60% and carbonate +                     
                            calcsilicate minerals \> 10%                        

  calcite marble            carbonate minerals form \> 75% of   modified from   common
                            rock, and calcite forms \>75% of    Robertson       
                            carbonate minerals.                 (1999)          

  calcsilicate              rock consists of ≥ 50% calcsilicate Barker (1998,   standard
                            or carbonate minerals and carbonate Appendix II),   chemical
                            minerals ≤ calcsilicate minerals in Robertson       
                            mineral mode. When used for         (1999)          
                            aphanitic rocks, indicates quartz                   
                            or feldspar is significant in the                   
                            mineral mode, and the rock does not                 
                            meet calcareous.                                    

  dark-colored              use for dark-colored aphanitic      descriptive     color
                            rocks about which other information                 
                            not available                                       

  dolomite marble           carbonate minerals form \> 75% of   modified from   common
                            rock, and dolomite forms \>75% of   Robertson       
                            carbonate minerals.                 (1999)          

  eclogite                  Rock composed of \>75% garnet       Carswell        common
                            (almandine-pyrope) and sodic        (1990), Barker  
                            pyroxene (omphacite).               (1998, Appendix 
                                                                II), modified   
                                                                from Jackson    
                                                                (1997)          

  epidosite                 Rock consisting of \>75% epidote,   modified from   monomineralic
                            and \>50% of non-epidote mineral is Jackson (1997)  
                            quartz.                                             

  ferromagnesian            rock having \>40% dark              Usage proposed, superclass
                            ferromagnesian minerals. Standard   this report     
                            term defined by Bates and Jackson                   
                            (1987) to mean \"containing iron                    
                            and magnesium\"                                     

  greisen                   Hydrothermal rock consisting of     modified from   common
                            \>70% quartz + muscovite or         Jackson (1997)  
                            lepidolite, and having accessory                    
                            (\>1%) topaz or tourmaline (or                      
                            other fluorine-bearing phase). In a                 
                            hierarchy based purely on                           
                            description, would be a kind of                     
                            semi-pelitic rock (10-40% aluminous                 
                            minerals; \<60% quartz + feldspar)                  
                            or quartz-feldspathic (\<40%                        
                            aluminous and \>60% quartz +                        
                            feldspar)                                           

  impure marble             rock consists of \>50% calcsilicate Usage proposed, superclass
                            or carbonate minerals and relative  this report     
                            proportion of calcsilicate and                      
                            carbonate minerals is unknown or                    
                            not specified                                       

  light-colored             use for light-colored aphanitic     descriptive     color
                            rocks about which other information                 
                            not available                                       

  mafic                     rock consists of ≥ 40% and \<90%    modified from   standard
                            ferromagnesian minerals.            Robertson       chemical
                                                                (1999)          

  magnesian                 No definition at present., but may                  
                            be useful?                                          

  marble                    rock in which carbonate minerals    Barker (1998,   common
                            form \> 75% of rock                 Appendix II),   
                                                                Robertson       
                                                                (1999)          

  metacarbonate             rock consists of \>50% calcsilicate Robertson       standard
                            or carbonate minerals and carbonate (1999)          chemical
                            minerals \> calcsilicate minerals                   
                            in mineral mode.                                    

  monomineralic             Rock that consists of \>75% of a    for consistency superclass
                            single mineral species and does not with Robertson  
                            meet any of the other composition   (1999)          
                            terms (e.g. quartzite, calcite                      
                            marble, dolomite marble,                            
                            serpentinite...)                                    

  None                      Composition not specified           this scheme     superclass

  pelitic                   Rock for which the sum of modal     modified from   standard
                            quartz+feldspar+ mica + aluminous   Robertson       chemical
                            mineral is ≥ 70%, and aluminous     (1999)          
                            mineral + mica content is ≥ 40%.                    

  quartz-feldspar-pelitic   Rock for which the sum of modal     Usage proposed, superclass
                            quartz+feldspar+ mica + aluminous   this report     
                            mineral is ≥ 70%                                    

  quartzite                 Rock that consists of ≥ 75% quartz  Robertson       common
                                                                (1999), revised 
                                                                to be           
                                                                consistent with 
                                                                other           
                                                                monomineralic   
                                                                rocks.          

  quartzo-feldspathic       Rock for which the sum of modal     revised from    standard
                            quartz+feldspar+ mica + aluminous   Robertson       chemical
                            mineral is ≥ 70%, and quartz +      (1999)          
                            feldspar \>60%.                                     

  semi-pelitic              Rock for which the sum of modal     revised from    standard
                            quartz+feldspar+ mica + aluminous   Robertson       chemical
                            mineral is ≥ 70%, and quartz+       (1999)          
                            feldspar \< 60%.                                    

  serpentinite              Rock that consists of \>75%         Barker (1998,   monomineralic
                            serpentine.                         Appendix II),   
                                                                Jackson (1997), 
                                                                revised to be   
                                                                consistent with 
                                                                other           
                                                                monomineralic   
                                                                rocks.          

  silicic                   Use for apparently siliceous        Usage proposed, standard
                            aphanitic rocks. Bates & Jackson    this report     chemical
                            (1987) include denotation of                        
                            igneous origin. For this                            
                            classification, should be                           
                            considered to mean \"appears to                     
                            consist largely of quartz and                       
                            feldspar\", generally is aphanitic                  
                            with hardness ≥ 6, and the rock                     
                            does not meet calcareous or                         
                            calcsilicate.                                       

  skarn                     Not recommended. Usage too          Einaudi (1982), common
                            problematic; distinction from       Einaudi and     
                            calcsilicate hornfels or            Burt (1982),    
                            calc-silicate granofels not clear.  Barker (1998,   
                                                                Appendix II)    

  special composition       Rock that has a mineral composition Usage proposed, superclass
                            that doesn\'t fit in any defined    this report     
                            composition class. A modal mineral                  
                            description is essential. The rock                  
                            consists of \<40% ferromagnesian                    
                            minerals and \<50% carbonate +                      
                            calcsilicate minerals and \<70%                     
                            Q+Fs + mica + aluminous minerals.                   

  ultramafic                Rock that consists of \>90%         Robertson       standard
                            ferromagnesian minerals.            (1999)          chemical

  whiteschist               Not recommended. Use pelitic rock   Schreyer (1973, common
                            if kyanite + garnet + white mica +  1977), Meyre    
                            quartz \>70%, and ky + garnet +     and others      
                            white mica must be ≥ 40%, otherwise (1999); Desmons 
                            whiteschist is a kind of special    and others      
                            composition metamorphic rock.       (2002)          
                            Whiteschist is defined as a rock                    
                            consisting of talc, kyanite, white                  
                            mica (paragonite), garnet, or                       
                            (sodic whiteschist) jadeite,                        
                            Mg-glaucophane, kyanite, quartz,                    
                            and garnet. Desmons and others'                     
                            (2002) IUGS proposal notes that                     
                            "Whiteschist is defined in a loose                  
                            descriptive manner, in a similar                    
                            way to blueschist...More precise                    
                            rock terms should be used wherever                  
                            possible (e.g.,                                     
                            kyanite-talc-phengite schist)."                     
  ---------------------------------------------------------------------------------------------

## Quartz-feldspar-pelitic rocks

Pelite has three definitions in the *Glossary of Geology* (Jackson,
1997): (1) "a sediment or sedimentary rock composed of clay- or
mud-sized particles," (2) "a fine-grained sedimentary rock composed of
more or less hydrated aluminum silicates with which are mingled small
particles of various other minerals; an aluminous sediment," and (3)
"the metamorphic derivative of a lutite, such as the metamorphosed
product of a siltstone or mudstone (as commonly used a pelite means an
aluminous sediment metamorphosed, but if used systematically, it means a
fine-grained sediment metamorphosed)." Since Al-rich clays are a major
component of most clay-sized sediment, the distinction between the
meaning of "pelite" as a grain size and the compositional connotation
has become blurred. The term "pelite" has a strong connotation of
aluminous composition when used to describe metamorphic rocks, and the
British Geological Survey classification of metamorphic rocks uses it in
that sense (Robertson, 1999).

SLTTM_1.0 follows the British Geological Survey's lead in emphasizing
the aluminous composition of pelite, but differs in using the adjective
form "pelitic-" as a compositional qualifier prefix to a fabric term as
in "pelitic-schist." We use the following classification categories
(Table 2):

**Pelitic**---a rock composition having modal percentages of quartz +
feldspar + mica + aluminous minerals ≥ 70% is here classified as if
aluminous minerals + mica ≥ 40%.

**Semipelitic**---quartz +feldspar \<60%

**Quartzofeldspathic**---quartz + feldspar \>60%.

SLTTM_1.0 does not consider biotite mica to be an aluminous or "pelitic"
mineral.

## Monomineralic rocks

Rocks are considered to be monomineralic if they consist predominantly
of a single mineral or closely related mineral group. Common
monomineralic rocks such as marble (\>75% carbonate), quartzite (\>75%
quartz), and serpentinite (\>75% serpentine) are defined individually in
the classification. Otherwise, "monomineralic" in the SLTTM_1.0
classification system is used as a general prefix to avoid including
separate "-ite" names for nearly every mineral. Metamorphic rocks
consisting predominantly (\>75%) of a single mineral are classified as
monomineralic-granofels, monomineralic-hornfels, monomineralic-schist,
etc., depending on the fabric. If someone is interested in a specific
monomineralic rock, the query would search the modal-mineralogy
description for \>75% of mineral Z. The user interface could have rock
names such as hornblendite, biotitite, and tourmalinite stored in the
database as 'monomineralic metamorphic rock (or granofels, schist,
hornfels)' and specify modal mineralogy \>75% of the appropriate
mineral.

## Calcsilicate and metacarbonate rocks

Calcsilicate and metacarbonate rocks are rocks that are composed of ≥50%
calcsilicate or carbonate minerals. The term **marble** implies that a
significant percentage of the rock consists of carbonate mineral. This
follows the definition in Jackson (1997): "a metamorphic rock consisting
predominantly of fine- to coarse-grained recrystallized calcite or
dolomite, usually with a granoblastic, saccharoidal texture".

SLTTM_1.0 defines **marble** as a monomineralic rock in which carbonate
minerals form \> 75% of rock. For rocks that meet the mineralogical
definition of marble, but do not have a granoblastic fabric, the term
marble becomes a strictly compositional modifier. Although a marble may
consist of several different minerals (calcite, dolomite, siderite...),
it is considered monomineralic because they all belong to the samel ,
closely related mineral carbonate minerals group. The name **impure
marble** is suggested as a general term for rocks that consist of \>50%
but ≤75% carbonate and calc-silicate minerals, and **metacarbonate
rock** for an **impure marble** in which carbonate minerals are more
abundant than calc-silicate minerals.

## Ferromagnesian rocks

Ferromagnesian rocks are those that consist of \>40% dark ferromagnesian
minerals. Rocks included in this group include eclogite, mafic and
ultramafic metamorphic rocks, and amphibolite.

The *Glossary of Geology* (Jackson, 1997) defines **amphibolite** as "A
crystalloblastic rock consisting mainly of amphibole and plagioclase
with little or no quartz. As the quartz content increases, the rock
grades into hornblende-plagioclase gneiss." A wide range of accessory
minerals can be present.

Most amphibolites are metamorphosed mafic igneous rocks
(ortho-amphibolites) but some may be metamorphosed calcareous sediments
(para-amphibolites) (Yardley, 1991). Coutinho and others (2002) compiled
the modal mineralogy of numerous rocks described as "amphibolite" from
different localities, noting that they fit the common definition of
amphibolite as a hornblende + plagioclase rock. Their data show that
most amphibolites contain \>50% amphibole although 30% to 50% is not
unusual, that the amphibole + plagioclase is mostly \>90% and less
commonly as low as 75%, and that most of the rocks described as
amphibolite have \<10% quartz. Based on these and other data, Coutinho
and others (2002) proposed the following IUGS definition:

> "Amphibolite is a gneissose or granofelsic metamorphic rock mainly
> consisting of green, brown, or black amphibole and plagioclase
> (including albite), which combined constitute more than 75% of the
> rock. The amphibole constitutes more than 50% of the total mafic
> constituents and is present in an amount more than 30% \[italics
> added\]. Other common minerals include quartz, clinoproxene, garnet,
> epidote-group minerals, biotite, titanite, and scapolite.
> Orthopyroxene is absent."

Members of the SLTT Subgroup on Metamorphic Rocks object to the
exclusion of orthopyroxene, which can be present in these rocks at
uppermost amphibolite facies, and to the inclusion of scapolite as a
\"common\" mineral. Scapolite may occur in amphibolites of calc-silicate
affinity but it is not \"common.\" The main part of Coutinho and others'
(2002) proposed IUGS definition shown in italics is adopted here,
without requiring the absence of orthopyroxene.

SLTTM_1.0 defines **amphibolite** as a metamorphic rock composed mostly
of green, brown, or black amphibole and plagioclase (including albite)
so that amphibole + plagioclase \>75% and amphibole \>30% (modal
percent) of the whole rock, and amphibole \>50% of the total mafic
constituents.

## Special-composition rocks

Special non-systematic rock names such as whiteschist, rodingite,
fenite, skarn, gondite, coticule, and greisen are commonly used in some
settings. Their definitions typically are based on modal mineralogy, but
not fabric, and as such may generally be considered as composition
qualifier terms. This classification leans towards a minimum of special
rock names for rocks of unusual composition, and relies on the presence
of a modal-mineralogy description included in the database to provide a
mechanism to describe and search for such rocks. Thus, many of these
special rocks would be assigned a composition qualifier 'special
composition'. The uncontrolled rock name field in the database is
available to assign any special rock name the geologist may prefer.

Some of these special-composition rock names may be added as subtypes of
special-composition rock, which are identified by decision switches near
the top of the classification decision tree. The problem posed by many
of these special names is that their definitions overlap other
composition-qualifier classes. For example, in the composition-qualifier
hierarchy, greisen is a subclass of quartz-feldspar-semipelitic rock but
may overlap semipelitic rock, quartzofeldspathic rock, or quartzite,
depending on the specific modal mineralogy. Thus, placement of these
special terms in the same class hierarchy results in ambiguous
classification (should the rock be classified as greisen or as
semi-pelitic granofels?), unless a modal mineral analysis description is
included. Use of special rock names must be constrained by requiring
that a modal-mineral description be provided for any rock for which
classification using the special name results in ambiguous placement in
the composition qualifier hierarchy. Again, this classification scheme
avoids such special terms as much as possible. Such names are better
used in the uncontrolled lithology name field of the database.

# CLASSIFICATION BY PROTOLITH

Two groups that have recently developed or are in the process of
developing terminology for metamorphic rocks (Robertson, 1999; Schmid
and others, 2002) include protolith as an important classification
criterion in cases where protolith is clearly identifiable. In many
metamorphic terranes the protolith for metamorphic rocks is obvious, and
in such terranes (e.g. Canadian Shield), rocks are commonly mapped based
on protolith, and named by adding the prefix 'meta' to the protolith
name to indicate that the rock has been metamorphosed. For many users,
such as mineral explorationists and resource appraisers, protolith is
the most important (commonly the only important) feature of these rocks,
and any useful geologic map database must represent the protolith
identification. Why not include protolith-based names (e.g., metabasalt,
metagranite, metaconglomerate) in this classification?

We acknowledge that, for rock-naming purposes, use of the 'meta' prefix
has been and no doubt will continue to be very useful as a way for
geologists to (informally) communicate information about metamorphic
rocks. However, we are concerned that, for incorporation into geoscience
databases where fabric and composition are clearly key attributes of
rock description, the tri-fold naming scheme that includes protolith in
the rock name (e.g., metabasalt, metaquartzite, metalimestone) clogs up
the works and introduces a highly subjective element of genetic
interpretation into an exercise that we are trying to keep as
descriptive as possible. The classification system proposed here is
designed for databases (automated knowledge representation), and in this
context, use of the 'meta' prefix is equivalent to dual
classification\--a protolith classification using the appropriate system
for the determined protolith, and classification as \'metamorphic rock\'
in our proposed composite-genesis classification. The usage tells
nothing about what kind of metamorphic rock the material is, beyond
whatever connotations the protolith rock name has for composition.
Accordingly, SLTTM_1.0 recommends that rock-name terms in the form
'meta(s*ome rock name*)' are to be placed in an uncontrolled rock name
field, designed to allow the geologist to assign the name that is most
meaningful for local or traditional use. Such terms also can appear in a
user interface for classifying rocks by having underlying software that
maps the name assignment to the implied dual classification. To keep the
meaning of the classification clear, a metamorphic rock must be
classified based on the aspects of the rock that make it a metamorphic
rock, i.e., fabric and composition.

#  REFERENCES CITED

Árkai, Peter, Sassi, F.P., and Desmons, Jacqueline, 2002, Towards a
unified nomenclature in metamorphic petrology: 4. Very low- to low-grade
metamorphic rocks, A proposal on behalf of the IUGS Subcommission on the
Systematics of Metamorphic Rocks, Web version of 31.10.2002: 12 p.,
available online at <http://www.bgs.ac.uk/SCMR>

Barker, A.J., 1998, Introduction to Metamorphic Textures and
Microstructures (second edition): Cheltenham, UK, Stanley Thomas
(Publishers), 264 p., including Appendix II. Glossary, p. 229-245.

Best, M.G., 2002, Igneous and Metamorphic Petrology: Boston, Blackwell
Science, 756 p. \[supersedes 1982 edition\]

Blatt, H., and Tracy, R.J., 1996, Petrology: Igneous, Sedimentary, and
Metamorphic (2^nd^ edition): New York, W.H. Freeman, 529 p.

Brodie, Kate, Fettes, Douglas, Harte, Ben, and Schmid, Rolf, 2002,
Towards a unified nomenclature of metamorphic petrology: 5. Structural
terms including fault rock terms, A proposal on behalf of the IUGS
Subcommission on the Systematics of Metamorphic Rocks, Web version of
31.10.2002: 8 p., available online at <http://www.bgs.ac.uk/SCMR>

Bucher, K., and Frey, M., 1994, Petrogenesis of Metamorphic Rocks, 6^th^
Edition Complete Revision of Winkler's Textbook: Springer-Verlag, 318 p.

Carswell, D.A., 1990, Eclogites and eclogite facies, *in* Carswell,
D.A., ed., Eclogite Facies Rocks: Glasgow, Blackie, p. 1-13.

Compton, R.R., 1985, Geology in the Field: New York, John Wiley & Sons,
398 p.

Coutinho, Jose, Desmons, Jacqueline, Kräutner, Hans, Peinado, Mercedes,
Sassi, Francesco, Schmid, Rolf, and Sen, Siser, 2002, Towards a unified
nomenclature in metamorphic petrology: 8. Amphibolite and granulite, A
proposal on behalf of the IUGS Subcommission on the Systematics of
Metamorphic Rocks, Web version of 31.10.2002: 11 p., available online at
<http://www.bgs.ac.uk/SCMR>

Desmons, Jacqueline, Smulikowski, Witold, and Schmid, Rolf, 2002,
Towards a unified nomenclature in metamorphic petrology: 3. High-P/T
rock terms, A proposal on behalf of the IUGS Subcommission on the
Systematics of Metamorphic Rocks, Web version of 31.07.2002: 7 p.,
available online at <http://www.bgs.ac.uk/SCMR>

Einaudi, M.T., 1982, Description of skarns associated with porphyry
copper plutons, southwestern North America, *in* Titley, S.R., ed.,
Advances in Geology of Porphyry Copper Deposits, Southwestern North
America: Tucson, University of Arizona Press, p. 139-183.

Einaudi, M.T., and Burt, D.T., 1982, Introduction; terminology,
classification, and composition of skarn deposits: Economic Geology, v.
77, no. 4, p. 745-754.

Frey, M. and Kisch, H.J., 1987, Chapter 1. Scope of subject, *in* Frey,
M., ed., Low Temperature Metamorphism: London, Blackie, p. 1-8.

Frey, M., and Robinson, D., eds., 1998, Low-Grade Metamorphism: Oxford,
Blackwell Science, 313 p.

Fristch, W., Meinner, H., and Weisendener, H., 1967: Neues Jahrbuch fuer
Mineralogie. Monatshefte, p. 364-376.

Gillespie, M.R., and Styles, M.T., 1999, BGS Rock Classification Scheme,
Volume 1, Classification of igneous rocks: British Geological Survey,
Research Report RR 99-06, 52 p.

Goldsmith, Richard, 1959, Granofels, a new metamorphic rock name:
Journal of Geology, v. 67, p. 109-110.

Hanmer, S., 1987, Textural map units in quartzo-feldspathic mylonitic
rocks: Canadian Journal of Earth Sciences, v. 24, p. 2065-2073.

Hobbs, B.E., Means, W.D., and Williams, P.F., 1976, An Outline of
Structural Geology: New York, John Wiley and Sons, 571 p.

Hyndman, D.W., 1985, Petrology of Igneous and Metamorphic Rocks: New
York, McGraw-Hill, 786 p.

Jackson, J.A., 1997, Glossary of Geology, Fourth Edition: Alexandria,
Virginia, American Geological Institute, 769 p.

Kisch, H.J., 1991, Illite crystallinity: recommendations on sample
preparation, X-ray diffraction settings and interlaboratory standards:
Journal of Metamorphic Geology, v. 9, p. 665-670.

Le Maitre, R.W., 1989, A Classification of Igneous Rocks and Glossary of
Terms, Recommendations of the International Union of Geological
Sciences, Subcommission on the Systematics of Igneous Rocks: Oxford,
Blackwell, 193 p.

Le Maitre, R.W., 2002, Igneous Rocks: A Classification and Glossary of
Terms, Recommendations of the International Union of Geological Sciences
Subcommission on the Systematics of Igneous Rocks, 2^nd^ Edition: New
York, Cambridge University Press, 252 p.

Mehnert, K.R., 1968, Migmatites and the origin of granite rocks:
Amsterdam, Elsevier, 393 p.

Meyre, C., De Capitani, C., Zack, T., and Frey, M., 1999, Petrology of
high-pressure metapelites from the Adula Nappe (Central Alps,
Switzerland: Journal of Petrology, v. 40, no. 1, p. 199-213.

Miyashiro, A., 1994, Metamorphic Petrology: New York, Oxford University
Press, 404 p., including Appendix II. Glossary of metamorphic
petrogenesis, p. 347-354.

Myers, J.S., 1978, Formation of banded gneiss by deformation of igneous
rocks: Precambrian Research, v. 6, p. 43-64.

North American Geologic Map Data Model Steering Committee, 2004, NADM
Conceptual Model 1.0---A conceptual model for geologic map information:
U.S. Geological Survey Open-File Report 2004-1334, 58 p., accessed
online at URL <http://pubs.usgs.gov/of/2004/1334>.

Passchier, C.W., and Trouw, R.A.J., 1996, Microtectonics: New York,
Springer, 289 p.

Passchier, C.W., Myers, J.S., and Kröner, A., 1990, Field Geology of
High-Grade Gneiss Terrains: New York, Springer, 150 p.

Paterson, M.S., and Weiss, L.E., 1961, Symmetry concepts on the
structural analysis of deformed rocks: Geological Society of America
Bulletin, v. 72, p. 841-882.

Robertson, S., 1999, BGS Rock Classification Scheme, Volume 2,
Classification of metamorphic rocks: British Geological Survey, Research
Report RR 99-02, 24 p.

Rosen, O.M., 2001, Metamorphic rocks in metamorphic systematics: What
are the calc-silicate rocks? \[indexed as: Towards a unified
nomenclature of metamorphism: Calcsilicate rocks. A proposal on behalf
of the IUGS Subcommission on the Systematics of Metamorphic Rocks.
Study, web version of 14.01.2001\]: 17 p. available online at
<http://www.bgs.ac.uk/SCMR>

Sander, Bruno, 1930, Gefugekunde der Gesteine: Vienna, Springer, 352 p.

Schmid, Rolf, Fettes, Douglas, Harte, Ben, Davis, Eleutheria, Desmons,
Jacqueline, and Siivola, Jaakko, 2002, Towards a unified nomenclature in
metamorphic petrology: 1. How to name a metamorphic rock, A proposal on
behalf of the IUGS Subcommission on the Systematics of Metamorphic
Rocks, Web version of 31.07.2002: 13 p., available online at
<http://www.bgs.ac.uk/SCMR>

Scholz, C.H., 1990, The mechanics of earthquakes and faulting: New York,
Cambridge University Press, 439 p.

Schreyer, W., 1973, Whiteschist: a high pressure rock and its geological
significance: Journal of Geology, v. 81, p. 735-739.

Schreyer, W., 1977, Whiteschists, their compositions and
pressure-temperature regimes based on experimental, field, and
petrographic evidence: Tectonophysics, v. 43, no. 1-2, p. 127-144.

Sibson, R.H., 1977, Fault rocks and fault mechanisms: Journal of the
Geological Society of London, v. 133, p. 191-213.

Smulikowski, Witold, Desmons, Jacqueline, Harte, Ben, Sassi, F.P., and
Schmid, Rolf, 1997, Towards a unified nomenclature of metamorphism: 3.
Types, grade and facies \[A proposal on behalf of the IUGS Subcommission
on the Systematics of Metamorphic Rocks\], compiled 1 September 1997 and
corrected 19 November 1997: 13 p., available online at
<http://www.bgs.ac.uk/SCMR>

Snoke, A.W., and Tullis, J., 1998, An overview of fault rocks, *in*
Snoke, A.W., Tullis, J., and Todd, V.R., Fault-related Rocks, A
Photographic Atlas: Princeton, N.J., Princeton University Press, p.
3-18.

Spear, F.S., 1993, Metamorphic phase equilibria and
pressure-temperature-time paths: Mineralogical Society of America,
Monograph 1, 799 p.

Spray, J.G., 1995, Pseudotachylyte controversy: Fact or fiction?:
Geology, v. 23, no. 2, p. 1119-1122.

Stöffler, D., 2001a, Towards a unified nomenclature of metamorphism:
Classification of impactites from single and multiple impacts. A
proposal on behalf of the IUGS Subcommission on the Systematics of
Metamorphic Rocks, Web version of 14.01.2001: 1 p., available online at
<http://www.bgs.ac.uk/SCMR>

Stöffler, D., 2001b, Towards a unified nomenclature of metamorphism: I.
Impactites from single impacts. A proposal on behalf of the IUGS
Subcommission on the Systematics of Metamorphic Rocks, Web version of
14.01.2001: 1 p., available online at <http://www.bgs.ac.uk/SCMR>

Stöffler, D., 2001c, IUGS Subcommission on the Systematics of
Metamorphic Rocks, Study Group for Impactites (D. Stöffler, Chairman),
Classification and nomenclature of impact metamorphic rocks (impactites)
\[indexed as: Towards a unified nomenclature of metamorphism: Impact
metamorphic rocks: Glossary of terms. A proposal on behalf of the IUGS
Subcommission on the Systematics of Metamorphic Rocks, Web version of
14.01.2001\]: 6 p., available online at <http://www.bgs.ac.uk/SCMR>

Stöffler, D., and Grieve, R.A.F., 2001, IUGS classification and
nomenclature of impact metamorphic rocks: Towards a final proposal
\[indexed as: Towards a unified nomenclature of metamorphism: IUGS
classification and nomenclature of impact metamorphic rocks. A proposal
on behalf of the IUGS Subcommission on the Systematics of Metamorphic
Rocks, Web version of 14.01.2001\]: 3 p., available online at
<http://www.bgs.ac.uk/SCMR>

Travis, R.B., 1955, Classification of Rocks: Quarterly of the Colorado
School of Mines, v. 50, no. 1, 98 p.

Turner, F.J., and Weiss, L.E., 1963, Structural Analysis of Metamorphic
Tectonites: New York, McGraw-Hill, 545 p.

Wenk, C., 1963: Neues Jahrbuch fuer Mineralogie. Monatshefte, p. 97-107.

Winkler, H.G.F., 1979, Petrogenesis of Metamorphic Rocks (5^th^
Edition): New York, Springer-Verlag, 348 p. \[superseded by completely
revised 6^th^ edition by Bucher and Frey, 1994\].

Wimmenauer, Wolfhard, and Bryhni, Inge, 2002, Towards a unified
nomenclature in metamorphic petrology: 6. Migmatites and related rocks,
A proposal on behalf of the IUGS Subcommission on the Systematics of
Metamorphic Rocks, Web version of 31.07.2002: 7 p., available online at
<http://www.bgs.ac.uk/SCMR>

Wise, D.U., Dunn, D.E., Engelder, J.T., Geiser, P.A., Hatcher, R.D.,
Kish, S.A., Odom, A.L., and Schamel, S., 1984, Fault-related rocks:
Suggestions for terminology: Geology, v.12, no. 7, p. 391-394.

Yardley, B.W.D., 1989, An Introduction to Metamorphic Petrology: New
York, John Wiley and Sons, Longman Earth Science Series, 248 p.

# APPENDIX 1. PRELIMINARY REPORTS OF THE IUGS SUBCOMMISSION ON THE SYSTEMATICS OF METAMORPHIC ROCKS (SCMR)

Árkai, Peter, Sassi, F.P., and Desmons, Jacqueline, 2002, Towards a
unified nomenclature in metamorphic petrology: 4. Very low- to low-grade
metamorphic rocks, A proposal on behalf of the IUGS Subcommission on the
Systematics of Metamorphic Rocks, Web version of 31.10.2002: 12 p.,
available online at <http://www.bgs.ac.uk/SCMR>

Brodie, Kate, Fettes, Douglas, Harte, Ben, and Schmid, Rolf, 2002,
Towards a unified nomenclature of metamorphic petrology: 5. Structural
terms including fault rock terms, A proposal on behalf of the IUGS
Subcommission on the Systematics of Metamorphic Rocks, Web version of
31.10.2002: 8 p., available online at <http://www.bgs.ac.uk/SCMR>

Coutinho, Jose, Desmons, Jacqueline, Kräutner, Hans, Peinado, Mercedes,
Sassi, Francesco, Schmid, Rolf, and Sen, Siser, 2002, Towards a unified
nomenclature in metamorphic petrology: 8. Amphibolite and granulite, A
proposal on behalf of the IUGS Subcommission on the Systematics of
Metamorphic Rocks, Web version of 31.10.2002: 11 p., available online at
<http://www.bgs.ac.uk/SCMR>

Desmons, Jacqueline, Smulikowski, Witold, and Schmid, Rolf, 2002,
Towards a unified nomenclature in metamorphic petrology: 3. High-P/T
rock terms, A proposal on behalf of the IUGS Subcommission on the
Systematics of Metamorphic Rocks, Web version of 31.07.2002: 7 p.,
available online at <http://www.bgs.ac.uk/SCMR>

NADMSC (North American Geologic-map Data Model Steering Committee),
2003, NADM Conceptual Model 1.0, A Conceptual Model for Geologic Map
Information: U.S. Geological Survey Open-File Report 04-XXX, 61 p.
(Draft available online at
http://geology.usgs.gov/dm/steering/teams/design/NADM-C1.0/NADMC1_0.pdf
, accessed 5/25/04)

Rosen, O.M., 2001, Metamorphic rocks in metamorphic systematics: What
are the calc-silicate rocks? \[indexed as: Towards a unified
nomenclature of metamorphism: Calcsilicate rocks. A proposal on behalf
of the IUGS Subcommission on the Systematics of Metamorphic Rocks.
Study, web version of 14.01.2001\]: 17 p. available online at
<http://www.bgs.ac.uk/SCMR>

Schmid, Rolf, Fettes, Douglas, Harte, Ben, Davis, Eleutheria, Desmons,
Jacqueline, and Siivola, Jaakko, 2002, Towards a unified nomenclature in
metamorphic petrology: 1. How to name a metamorphic rock, A proposal on
behalf of the IUGS Subcommission on the Systematics of Metamorphic
Rocks, Web version of 31.07.2002: 13 p., available online at
<http://www.bgs.ac.uk/SCMR>

Smulikowski, Witold, Desmons, Jacqueline, Harte, Ben, Sassi, F.P., and
Schmid, Rolf, 1997, Towards a unified nomenclature of metamorphism: 3.
Types, grade and facies \[A proposal on behalf of the IUGS Subcommission
on the Systematics of Metamorphic Rocks\], compiled 1 September 1997 and
corrected 19 November 1997: 13 p., available online at
<http://www.bgs.ac.uk/SCMR>

Stöffler, D., 2001a, Towards a unified nomenclature of metamorphism:
Classification of impactites from single and multiple impacts. A
proposal on behalf of the IUGS Subcommission on the Systematics of
Metamorphic Rocks, Web version of 14.01.2001: 1 p., available online at
<http://www.bgs.ac.uk/SCMR>

Stöffler, D., 2001b, Towards a unified nomenclature of metamorphism: I.
Impactites from single impacts. A proposal on behalf of the IUGS
Subcommission on the Systematics of Metamorphic Rocks, Web version of
14.01.2001: 1 p., available online at <http://www.bgs.ac.uk/SCMR>

Stöffler, D., 2001c, IUGS Subcommission on the Systematics of
Metamorphic Rocks, Study Group for Impactites (D. Stöffler, Chairman),
Classification and nomenclature of impact metamorphic rocks (impactites)
\[indexed as: Towards a unified nomenclature of metamorphism: Impact
metamorphic rocks: Glossary of terms. A proposal on behalf of the IUGS
Subcommission on the Systematics of Metamorphic Rocks, Web version of
14.01.2001\]: 6 p., available online at <http://www.bgs.ac.uk/SCMR>

Stöffler, D., and Grieve, R.A.F., 2001, IUGS classification and
nomenclature of impact metamorphic rocks: Towards a final proposal
\[indexed as: Towards a unified nomenclature of metamorphism: IUGS
classification and nomenclature of impact metamorphic rocks. A proposal
on behalf of the IUGS Subcommission on the Systematics of Metamorphic
Rocks, Web version of 14.01.2001\]: 3 p., available online at
<http://www.bgs.ac.uk/SCMR>

Wimmenauer, Wolfhard, and Bryhni, Inge, 2002, Towards a unified
nomenclature in metamorphic petrology: 6. Migmatites and related rocks,
A proposal on behalf of the IUGS Subcommission on the Systematics of
Metamorphic Rocks, Web version of 31.07.2002: 7 p., available online at
<http://www.bgs.ac.uk/SCMR>

# APPENDIX 2. SELECTED FABRIC TERMS AND DEFINITIONS 

**Aphanitic---**"Individual grains not visible with the unaided eye (ca.
\<0.1 mm)" (Schmid and others, 2002) \["Said of the texture of an
igneous rock in which the grains are too small to distinguish with the
unaided eye..." (Jackson, 1997) and extended to metamorphic rocks by
Schmid and others (2002).\]

**Cataclastic**---"Pertaining to the structure produced in a rock by the
action of severe mechanical stress during dynamic metamorphism;
characteristic features include the bending, breaking, and granulation
of minerals. Also said of the rocks exhibiting such structures."
(Jackson, 1997)

**Cataclastic rock** \--"A rock, such as tectonic breccia, containing
angular fragments that have been produced buy the crushing and
fracturing of preexisting rocks as a result of mechanical forces in the
crust." (Jackson, 1997). Mylonite-series rock is not classified here as
a cataclastic rock, although it may contain subordinate cataclastic
fabric.

**Cleavage**---"The property of a rock to split along a regular set of
parallel or sub-parallel closely spaced surfaces" (Brodie and others,
2002).

**Continuous foliation** --Foliation in which the fabric elements are
uniformly distributed. For precision, *continuous* is here defined on a
hand sample-scale, and in quantitative terms to mean that domains
lacking the fabric are \<1 cm thick if they are layers, and \<5 cm in
diameter if they are irregular patches, and constitute \<25% of the
rock. Distinct from spaced foliation (Passchier and Trouw, 1996, p.
64-65).

**Compositional layering**\--Non-genetic term for foliation defined by
layers of different composition (Passchier and Trouw, 1996).
Compositional layering is here considered to be *laterally continuous*
if layers can be traced \>10 cm.

**Fabric element**\-- Part of a rock fabric such as a foliation,
lineation, etc. (Jackson, 1997; Passchier and Trouw, 1996). Any of the
elementary parts of an aggregate whose orientation and geometry can be
described; only structures that are penetrative on the scale of the
domain contribute to the fabric (Turner and Weiss, 1963).

**Fabric**\--The complete spatial and geometrical configuration of all
those components that are contained in a rock and that are penetratively
and repeatedly developed throughout the volume of rock under
consideration; the shapes and characters of individual parts of a rock
mass and the manner in which these parts are distributed and oriented in
space (paraphrase of Passchier and Trouw, 1996, and Jackson, 1997), both
of which cite Hobbs and others (1976, p. 73). \[English translation of
German word *Gefüge* used by Sander (1930) to denote the internal
ordering of both geometric and physical spatial data in an aggregate
(Turner and Weiss, 1963). In a non-crystal, the internal geometric
configuration of the elementary parts and of any characteristic features
to which the arrangement of these parts gives rise \[paraphrase from,
Paterson & Weiss (1961, p. 854) quoted in Turner & Weiss (1963, p.
19).\]

**Foliation**---Planar fabric in a rock that is penetrative on a
mesoscopic scale (Turner and Weiss, 1963; Passchier and Trouw, 1996;
Brodie and others, 2002).

**Grain-shape fabric**\--Fabric defined by shape-preferred orientation
of mineral grains or mineral grain aggregates (Passchier and others,
1990; Passchier and Trouw, 1996, p.74).

**Gneissosity**---"General term for foliation in a gneiss. Use of this
term is discouraged because of its vague connotation; several types of
foliation (layering, schistosity) may occur in the same gneiss."
(Passchier and Trouw, 1996, Glossary)

**Granoblastic**\--Fabric dominantly formed by equidimensional crystals.
An example is a foam structure...in which platy or acicular mineral
shapes are absent, and most grains have equant shape. (Passchier and
Trouw, 1996, Glossary)

**Migmatitic**\--Having the characteristics of migmatite, which is "A
coarse-grained heterogeneous rock type characteristically with irregular
and discontinuous interleaving of leucocratic granitoid material
(leucosome) and residual high-grade metamorphic material (restite).
(Barker, 1998, Appendix II: Glossary)

**Mylonite-series rock**\--A collective term for the "mylonite series"
rocks of Sibson (1977), including protomylonite, mylonite,
ultramylonite, and phyllonite. Equivalent to Brodie and others' (2002)
broader usage of "mylonite" for "A fault rock which is cohesive and
characterized by a well developed \[foliation\] resulting from tectonic
reduction of grain size, and commonly containing rounded porphyroclasts
and lithic fragments of similar composition to minerals in the matrix."
By this usage, mylonite-series rock is not a cataclastic rock;
cataclastic fabrics, if present, are subordinate.

**Penetrative**---Repeated at distances so small, compared with the
scale of the whole...that they can be considered to pervade it uniformly
and be present at every point (paraphrase of Turner and Weiss, 1963, p.
21).

**Penetrative fabric element**\--A fabric element that occurs
penetratively throughout a rock at the scale of observation (Passchier
and Trouw, 1996).

**Phaneritic---**"Individual grains visible with the unaided eye (ca.
\>0.1 mm)" (Schmid and others, 2002). \["Said of the texture of an
igneous rock in which the grains are large enough to distinguish with
the unaided eye, i.e. megascopically crystalline" (Jackson, 1997) and
extended to metamorphic rocks by Schmid and others (2002).\]

**Schistosity**\--Foliation in a rock due to the parallel, planar
arrangement of mineral grains of the platy, prismatic or ellipsoidal
types, usually mica (Jackson, 1997). *Well developed schistosity* is
here defined to mean that \>50% of the rock consists of mineral grains
having a platy, lamellar, tabular, or prismatic crystallographic habit
that are oriented in a continuous planar or linear fabric (following
Jackson, 1997). *Continuous* is here defined on a hand sample-scale, and
in quantitative terms to mean that domains lacking the fabric are \<1 cm
thick if they are layers, and \<5 cm in diameter if they are irregular
patches, and constitute \<25% of the rock.

**Slaty cleavage**---A "well developed planar schistosity in a rock in
which the individual grains are too small to be seen by the unaided eye
and the schistosity is developed on the grain scale." (Brodie and
others, 2002)

**Spaced foliation**---Foliation in which the fabric elements are
separated by domains that lack the foliation; not "continuous" as
defined above. Discussion in Passchier and Trouw (1996, p. 65).

# APPENDIX 3. METAMORPHIC FACIES AND TYPES

This appendix presents recommendations (initially proposed by Subgroup
members Hoisch and Williams) for the classification of metamorphic
facies and types of metamorphism. These recommendations assume as a
prerequisite that any rock should fit into only one facies and only one
type of metamorphism. We exclude the hornfels and sanidinite facies, as
done by Spear (1993) because the mineral assemblages are no different
than in the higher-pressure facies. The only distinction appears to be
textural, and that will appear in other attribute lists (rocks called
hornfels or hornfelsic). \"Sanidinite facies\" is essentially synonymous
with pyrometamorphism and consequently is not considered a true facies.
We suggest including it in the granulite facies. Common usage for
pyrometamorphism includes ultra-high temperature contact metamorphism,
which poses the problem of a non-unique definition, unless it is
confined to metamorphism by burning coal seams. That seems appropriate,
since the root \"pyro\" means fire. It is an uncommon but spectacular
type of metamorphism. The term \"ultrametamorphism\" means partially
melted, but this overlaps both the upper amphibolite and granulite
facies, and so we exclude it here. Lithologic terminology for partially
melted rocks should follow the IUGS recommendations of Wimmenauer and
Bryhni (2002) as much as possible.

We do not define the metamorphic facies, at this stage, because that
would involve numerous debates on their boundaries. If we absolutely
must define them, then some general statements can be added.

The lists below are recommendations for consideration by others. There
is quite a range of published opinion. We tried not to make arbitrary
decisions, but it is unlikely that any set of recommendations will
please everyone.

> **[Types of metamorphism]{.underline}:**

1\) **Contact metamorphism**\--Metamorphism of country rock at the
contact of an igneous body.

2\) **Regional metamorphism**\--Metamorphism not obviously localized
along contacts of igneous bodies; includes burial metamorphism and ocean
ridge metamorphism.

3\) **Hydrothermal metamorphism (metasomatism)**\--Metamorphism
involving significant changes in a rock\'s bulk chemistry as a result of
interaction with chemically reactive fluids.

4\) **Shock (impact) metamorphism**---Shock metamorphism is "caused by
shock wave compression due to impact of a solid body or due to the
detonation of high-energy chemical or nuclear explosives" (Stöffler,
2001c). Impact metamorphism is "caused by the passage of a shock wave
due to impact of a planetary body (projectile or impactor) on a
planetary surface (target). It includes melting and vaporization of the
target rock(s)" (Stöffler, 2001c).

5\) **Pyrometamorphism**\--Ultra-high temperature metamorphism at
shallow depths caused by burning coal seams.

> Metamorphic facies:

1\) Zeolite

2\) Prehnite-pumpellyite

3\) Greenschist

4\) Epidote amphibolite

5\) Amphibolite

6\) Granulite

7\) Blueschist (glaucophane schist)

8\) Eclogite

9\) Ultra-high pressure

This proposed list of metamorphic facies differs from the IUGS
recommendations of Smulikowski and others (1997), who also include the
sanidinite facies (here included in granulite facies) and pyroxene
hornfels facies. The glaucophane schist facies as used by Smulikowski
and others (1997) is another name for the blueschist facies as listed
here.

# APPENDIX 4. GLOSSARY OF METAMORPHIC ROCK TERMS AND PARENT-CHILD RELATIONSHIPS (DATABASE)

Glossary is included in accompanying Microsoft Access database.