Glastonbury Gneiss and Mantling Rocks (a Modified 

Oliverian Dome) in South-central Massachusetts and 

North-central Connecticut; Geochemistry, 

Petrogenesis, and Isotopic Age

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1295

Glastonbury Gneiss and Mantling Rocks (a Modified 

Oliverian Dome) in South-central Massachusetts and 

North-central Connecticut: Geochemistry, 

Petrogenesis, and Isotopic Age

By GERHARD W. LEO, ROBERT E. ZARTMAN, and DOUGLAS G. BROOKINS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1295

UNITED STATES GOVERNMENT PRINTING OFFICE, W AS H I N GTO N : 1 9 8 4
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Leo, Gerhard W.

Glastonbury gneiss and mantling rocks (a modified 

Oliverian dome) in south-central Massachusetts and northcentral Connecticut.

(Geological Survey professional paper ; P-1295) 

Bibliography: p. 

Supt. of Docs. no. : 119.16:1295

1. Domes (Geology) Connecticut. 2. Domes (Geology) 

Massachusetts. 3. Gneiss Connecticut. 4. GneissMassachusetts. I. Zartmen, Robert E. II. Brookins, 

Douglas G. III. Geological Survey (U.S.) IV. Title.

V. Series: Geological Survey professional paper ; 1295. 

QE611.5.U6L46 1983 552'.4 83-600093

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Acknowledgments 

Regional geology 

Monson Gneiss 

Ammonoosuc Volcanics 

Partridge and Collins Hill Formations 

Glastonbury Gneiss 

Structural character 

Contact relationships 

Lithologic character 

Major elements 

Ammonoosuc Volanics and Monson Gneiss 

Glastonbury Gneiss 

Trace elements-_ _~"~_-_ __ _ ._.__

Trace elements other than rare earths 

Page

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- 9

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24

Analytical results Continued 

Trace elements Continued 

Rare-earth elements 

Monson Gneiss 

Northern Glastonbury Gneiss 

Southern Glastonbury Gneiss 

Isotopic ages 

"DiSCUSSiOn ________________________________________

Northern Glastonbury Gneiss 

Anatexis of Monson lithology 

Middle Ordovician to Early Silurian heat flow

and metamorphism -

Southern Glastonbury Gneiss 

Plate-tectonic setting of the Glastonbury dome 

References cited 

Appendix: Sample field numbers, descriptions,

and locations 43

24

24

32

32

32

37

38

38

39

40

40

41

ILLUSTRATIONS

FIGURE 1. Simplified geologic map of Bronson Hill anticlinorium - 

2. Geologic sketch map of Glastonbury dome 

3-5. Photographs of

3. Ammonoosuc Volcanics and Monson Gneiss 

4. Monson Gneiss showing partial anatexis and plastic flow -

5. Structures and textures in northern Glastonbury Gneiss 

6. Ternary diagram showing modal variations in Glastonbury Gneiss, Monson Gneiss, and felsic layers of 

Ammonoosuc Volcanics 

7. Photographs of structures and textures in southern Glastonbury Gneiss -

8. Normative An-Ab-Or diagram 

9. Normative Q-Ab-Or diagram 

10-12. Variation diagrams for

10. Major elements versus SiO2 -

11. Th, Co, and Sc versus SiO2 - --- - - 

12. K versus Rb/Sr - 

13. Plot of rare-earth-element patterns

14. Concordia diagram for zircon from Glastonbury and Monson Gneisses 

15. Rb-Sr whole-rock isochron diagram for Glastonbury Gneiss 

Page

- 2

- 5

10

12

13

20

21

22

29

30

31

35

37

in
IV CONTENTS

TABLES

Page

TABLE 1. Major-element compositions, norms and modes of Glastonbury Gneiss, Monson Gneiss, and Ammonoosuc Volcanics 15

2. Trace-element and Sr-isotope data for Glastonbury and Monson Gneisses 25

3. Uranium-thorium-lead isotope ages of zircon from Glastonbury and Monson Gneisses 34

4. Whole-rock rubidium-strontium isotope age of the Glastonbury Gneiss 36
GLASTONBURY GNEISS AND MANTLING ROCKS 

(A MODIFIED OLIVERIAN DOME) IN SOUTH-CENTRAL MASSACHUSETTS

AND NORTH-CENTRAL CONNECTICUT: 

GEOCHEMISTRY, PETROGENESIS, AND ISOTOPIC AGE

By GERHARD W. LEO, ROBERT E. ZARTMAN, and DOUGLAS G. BROOKINS'

ABSTRACT

The Glastonbury dome is a long, narrow structure trending approximately 

70 km north-northeast through Connecticut and Massachusetts along the 

west side of the Bronson Hill anticlinorium. Structurally and stratigraphically 

the dome is analogous to the Oliverian domes of New Hampshire. It is 

cored by Glastonbury Gneiss and is mantled by Ammonoosuc Volcanics 

and Partridge Formation (or their equivalents) of Ordovician age. The 

Glastonbury Gneiss intrudes the Ammonoosuc and, thereby, establishes the 

relative age of the two units. Monson Gneiss, which unconformably underlies 

the Ammonoosuc Volcanics in the Monson anticline to the east, is not in 

contact with Glastonbury Gneiss except near Stafford Springs, Conn., 

where the contact may be gradational. In some places, Monson Gneiss 

shows evidence of plastic flow and potential anatexis. The northern part of 

the Glastonbury Gneiss typically is leucocratic, granoblastic, relatively 

potassium-poor gneiss that appears homogeneous in outcrop, but proves to 

be chemically and modally inhomogeneous over short distances, as shown 

by variation diagrams and REE plots. The gneiss straddles the compositional 

fields of trondhjemite, tonalite, and granodiorite, and partly overlaps that 

of Monson Gneiss. The southern part of the Glastonbury Gneiss is consistently 

more potassic than the northern, having compositions ranging from granite 

to granodiorite. All of the Glastonbury Gneiss show pervasive, strong foliation, deformation, and local shearing related to the Acadian orogeny.

Field relations, textures, and chemistry of the northern part of the Glastonbury suggest an origin by anatexis of the premetamorphic Monson 

sequence at temperatures of about 690 °C to 750 °C and pressures of 

<3kbars. The southern part of the Glastonbury appears to have been 

generated contemporaneously but not comagmatically from calcalkaline crust. U-Pb zircon ages for both the northern and southern 

bodies are slightly discordant with 207Pb/208Pb ages of 445 to 467 m.y. 

At first these results seem to contradict the known stratigraphic 

position of the Glastonbury relative to the Monson, which yields distinctly younger zircon 207Pb/ao8Pb ages of 428 to 440 m.y. However, 

this apparent discrepancy in the radiometric ages younger Monson, 

older Glastonbury could be resolved by postulating either (1) a 

small component of old inherited zircon in the Glastonbury or (2) preferential metamorphic overprinting of the zircon in the Monson. In 

any case, the isotopic age discrepancy is not so large as to render the 

proposed Monson anatectic model implausible. Rb-Sr whole-rock 

data show a large amount of scatter on an isochron diagram and 

hence do not permit a reliable estimate of age. This condition may 

reflect inhomogeneities in the initial 87Sr/88Sr ratio or may have been 

also induced by later Acadian or Alleghanian metamorphism.

An early Silurian to Middle Ordovician age of the Glastonbury 

Gneiss gives evidence of higher heat flow and more extensive

plutonism in the Taconic than has generally been recognized. With 

certain qualifications, the Glastonbury and associated volcanic rocks 

are compatible with recent plate-tectonic models involving the 

Bronson Hill anticlinorium.

INTRODUCTION

The Glastonbury dome is a narrow, elongate structure 

that extends from just south of the Belchertown Quartz 

Monzodiorite pluton (Ashwal and others, 1979) in central Massachusetts to the vicinity of Middle Haddam, 

Connecticut. The term "Glastonbury dome" as used 

henceforth refers to the tectonic feature with constituent lithologies, whereas "Glastonbury Gneiss" refers 

only to the core gneiss itself. A parallel distinction 

applies to the terms "Oliverian dome(s)" and "Oliverian 

core gneiss(es)". The Glastonbury dome constitutes 

part of the Bronson Hill anticlinorium (Billings, 1956; 

Thompson and others, 1968), a complexly folded and 

deformed sequence of lower Paleozoic metasedimentary, 

metavolcanic, and plutonic rocks trending southsouthwest from northern New Hampshire to Long 

Island sound (fig. 1). The stratified rocks are intruded 

by a variety of granitic plutons that constitute several 

plutonic or plutonic-volcanic suites ("magma series" of 

Billings, 1937, 1956) and range in age from Middle 

Ordovician to Cretaceous. The age of the Oliverian 

Plutonic Suite in New Hampshire has been reliably 

established as Middle Ordovician (Naylor, 1969; Poland 

and Loiselle, 1980; Zartman and Leo, 1981). Oliverian 

plutons form the cores of a number of gneiss domes 

mantled by the Ammonoosuc Volcanics of the Middle 

Ordovician age (Naylor, 1968,1969; Leo, 1980a,b). The 

Ammonoosuc Volcanics are a mostly bimodal sequence 

of metamorphosed tholeiite and quartz keratophyre 

tuff, locally associated with sills and stocks of trondhjemite (Leo, 1980c; Leo and Gromet, 1981).

"University of New Mexico
GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

Southern part

43'

0 10 20 30 40 50KM

MAINE

73° 72°

FIGURE 1.-Simplified geologic map showing the central part of the domes (see explanation) as follows (generally north to south): J, JefferBronson Hill anticlinorium, emphasizing the Oliverian domes and son; SH, Sugar Hill pluton; OH, Owls Head; M, Mascoma; L, Lebanon; 

Ammonoosuc Volcanics. Geology east and west of the Ammonoosuc C, Croydon; U, Unity; A, Alstead; S, Swanzey; W, Warwick; P, Pelham; 

Volcanics mostly omitted. Adapted from Robinson and Hall (1980, T, Tully; Mo, Monson Gneiss; G, Glastonbury; Wi, Willimantic; K, Killfig. 1) and Billings (1955). Letter symbols for Oliverian and other ingworth.
INTRODUCTION

EXPLANATION

Layered rocks 

including volcanic rocks but 

excluding granitic gneisses

Intrusive rocks 

including orthogneisses

Portland Formation and related

Silurian and Devonian rocks 

undivided

Felsic to intermediate plutonic 

rocks, mostly gneissic

Cretaceous

to 

Triassic

Unconformity

Devonian

to 

Silurian

Unconformity

Ammonoosuc Volcanics and 

Partridge Formation 

(Middletown Gneiss and 

Collins Hill Formation, respectively, in southern Connecticut); also includes Orfordville Formation (southwest 

New Hampshire) and Brimfield Group (Middle Ordovician or older; Massachusetts 

and Connecticut)

Gneisses in cores of Oliverian 

and related domes. (See 

figure captions for names 

corresponding to letter 

symbols)

Highlandcroft Plutonic Suite 

(New Hampshire)

Albee Formation (New Hampshire)

Other Ordovician rocks, undivided

(southwest New Hampshire)

Ordovician

Metasedimentary and 

metavolcanic rocks

Gneiss in cores of domes

 Contact

 Fault

Unconformity

Proterozoic Z

FIGURE 1. Continued
GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

The Glastonbury Gneiss resembles Oliverian core 

gneisses in that (1) it also is mantled by Ammonoosuc 

Volcanics (and, at its southern end, by the Collins Hill 

Formation of Rodgers and others, 1959, hereby adopted 

for U.S. Geological Survey usage), which overlies the 

Ammonoosuc Volcanics or its correlative, the Middletown Gneiss in Connecticut, and intrudes these strata 

along much of the western side of the dome; and (2) the 

dome is pervasively metamorphosed at middle amphibolite facies grade and exhibits Acadian deformation.

The Glastonbury Gneiss (fig. 2) can be divided into 

two parts, northern and southern, on the basis of minerology and chemistry. The northern part of the gneiss 

(henceforth referred to informally as "northern Glastonbury Gneiss" or "northern gneiss") is typically low 

in potassium (compositionally trondhjemite to tonalite), 

although the composition is variable on a local scale. 

The southern part of the gneiss (henceforth referred to 

informally as "southern Glastonbury Gneiss" or "southern gneiss") is much more potassic and is mostly granite. Field relationships and chemical data suggest that 

the northern gneiss was produced by anatexis of Middle 

Ordovician volcanic rocks (the protolith of the Monson 

Gneiss).

In several of the northern domes, notably the Mascoma dome (Naylor, 1969; fig 1), K-poor, felsic, stratified rocks of volcaniclastic aspect mantle the intrusive 

core gneiss. Naylor applied the term "stratified core 

gneiss" to these rocks in the Mascoma dome and regarded them as an integral part of the Oliverian lithology, although he left open the possibility that they 

might represent a downward continuation of the Ammonoosuc Volcanics. Further study (Leo, unpub. data) has 

shown that somewhat similar rocks underlie the Ammosue in several, but by no means all, of the other Oliverian domes. It seems appropriate, therefore, to put such 

rocks in a special category, for example, an informally 

designated "pre-Ammonoosuc volcanic section," and to 

discontinue the use of the term "Oliverian" as part of 

their name.

In New Hampshire and Vermont, intrusive trondhjemite is associated with the Ammonoosuc Volcanics in 

several of the Oliverian domes, commonly as sills and 

other minor intrusions. In three small domes, trondhjemite constitutes the entire core gneiss (Leo and Gromet, 

1981). These trondhjemites differ somewhat from the 

northern Glastonbury Gneiss in that they contain relatively lower K2O and CaO and higher Na2O. Thus they 

define a different compositional field on normative diagrams (table 1; figs. 8 and 9). Trace element abundances, 

including rare earths, are also somewhat different and 

suggest a mantle origin for at least some of the New 

Hampshire and Vermont trondhjemites (Leo, unpub.

data). By contrast, field relationships as well as textural and chemical features of the northern Glastonbury 

Gneiss suggest that it originated in the upper crust by 

anatexis of a premetamorphic Monson lithology. Nevertheless, as will be discussed below, the possibility that 

the northern Glastonbury Gneiss represents a magmatic Ammonoosuc phase, analogous to the trondhjemites to the north, cannot be completely discounted.

If the mode of origin here proposed for the northern 

Glastonbury Gneiss is correct, it implies a higher heat 

flow related to the Taconic orogeny than has heretofore 

been proposed or documented.

ACKNOWLEDGMENTS

Thanks are due Richard S. Naylor for providing 

GWL with insights to the Oliverian domes and for reviewing an early draft of the manuscript. We also wish 

to thank Rosalind T. Helz, Michael W. Higgins, Joseph G. 

Arth, and especially Richard Goldsmith for reviews of 

later versions of the paper. The authors, however, take 

sole responsibility for the interpretations and conclusions herein.

REGIONAL GEOLOGY

The layered rock units described below are arranged 

in order from oldest to youngest based on field relationships and not on the somewhat anomalous radiometric 

dates for the Monson and Glastonbury Gneisses (to be 

described below). The Glastonbury Gneiss is listed last 

(youngest) because it cuts Ammonoosuc Volcanics; 

however, the Glastonbury has not been shown to cut 

the Partridge and (or) the equivalent Collins Hill Formations that conformably overlie Ammonoosuc Volcanics. It is therefore conceivable that the Glastonbury 

is actually older than the Partridge/Collins Hill. This 

relationship is not basic to the subject of this paper.

MONSON GNEISS

The Monson Gneiss (Monson granodiorite of Emerson, 

1917, p. 241-243) underlies the Ammonoosuc Volcanics 

from southern New Hampshire to Long Island Sound. 

The Monson is exposed in broad domes in northern 

Massachusetts ("main body" and "Tully dome" of Robinson and Hall, 1980); and in southern Connecticut (Killingworth dome) (fig. 1). Between these two areas it constitutes a stratigraphic layer, locally much faulted and 

displaced (fig. 2). Quartz-plagioclase gneisses in the 

Willimantic dome (fig. 1) and the Selden Neck dome to 

the south, traditionally correlated with Monson Gneiss 

to the west across the Chester syncline (not shown in 

fig. 1), are reported to show chemical and
72°30 42°15'

REGIONAL GEOLOGY

72°15'

41°45'

Jp

EXPLANATION

Portland Formation (Lower 

_____ Jurassic)

DSu Devonian and Silurian rocks, 

 I undivided includes Waits 

River, Erving, and Littleton 

Formations and Clough 

Quartzite

Glastonbury Gneiss (Middle Ordovician)

Ammonoosuc Volcanics

(Middletown Gneiss in central 

Connecticut; Middle 

Ordovician or older)-Locally 

includes Partridge Formation 

(Collins Hill Formation in 

central Connecticut); also 

includes Brimfield Group east 

of the Glastonbury dome

Monson Gneiss (Middle 

Ordovician)

Contact 

 Fault, dashed where inferred

' Sample location (see table 1 

for major-element analyses; 

see tables 2 and 3 for traceelement analyses)

31 Sample location of zircon age 

sample (see table 3)

Towns bearing names of related 

7V2-minute quadrangles

10 KILOMETERS

Middle Haddam

Figure 2. Geologic sketch map of the Glastonbury Gneiss dome and 

adjacent lithologic units. Compiled from published sources 

(Collins, 1954; Aitken, 1955; Herz, 1955; Snyder, 1970) and from 

various unpublished data, mainly those of M. H. Pease, Jr., and J.

D. Peper of the U.S. Geological Survey. Sample locations are keyed 

to tables 1-4. Locations within the Ludlow quadrangle, Massachusetts, are shown more accurately in Leo and others, 1977.
GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

mineralogical features which make such a correlation 

doubtful (Wintsch, 1980).

Contacts between the Monson Gneiss and overlying 

Ammonoosuc Volcanics are poorly exposed and are as 

yet somewhat problematical. Peper (1966) described 

the contact as gradational, although subsequent inspection of one of the better exposures (unnamed hill 0.8 km 

west of Palmer Center, Palmer 7V2-minute quadrangle, 

Massachusetts) suggested that the contact is sharp and 

unconformable. Robinson (1979, p. 137) has cited a 

quartz-pebble conglomerate on the contact. On the 

basis of isotopic ages determined for the Monson Gneiss 

(see below), it appears that any unconformity between 

the Monson and the Ammonoosuc must represent a relatively short interval of erosion.

The Monson Gneiss is commonly a light-gray quartzplagioclase rock with less than 15 percent mafic minerals 

(including biotite, hornblende, and epidote). K-feldspar 

is typically absent, but locally constitutes 10 percent or 

more. Ordinarily the gneiss shows some degree of 

lithologic banding that may or may not be related to 

primary layering (fig. 3B-D). Sharply bounded, 

tabular amphibolite layers are very probably sills (fig. 

3C, D). Where observed, such sills are pre-Acadian and 

thus could be Ammonoosuc feeders. Banding in Monson may be swirled, shadowy, and suggestive of incipient anatexis (fig. 4A, B). Locally, banded gneiss 

grades to nearly homogeneous, weakly foliated rock 

resembling some exposures of Glastonbury Gneiss 

(fig. 3E). Locally, as at the Flynt quarry north of Monson (loc. 34, 35, fig. 2), the gneiss lacks layering and 

shows a number of features suggestive of plastic flow 

(fig. 4C-.E). In these places, the rock is massive, faintly 

foliated, and compositionally inhomogeneous and is 

traversed by felsic veins and streaks, mafic schlieren, 

and localized sharp to shadowy contacts between more 

felsic and more mafic rock (fig. 4£). Discordant, typically ellipsoidal inclusions of amphibolite are abundant 

(fig. 4C, £>). Their contacts against enclosing felsic 

gneiss are sharp, and the gneissic foliation flows around 

the inclusions. Felsic layers within amphibolite have a 

blotchy texture suggestive of segregation in response 

to partial melting, but such layers are more or less 

sharply truncated against the enclosing gneiss. These 

features suggest that the felsic gneiss reached the condition of anatexis and plastic flow, disrupting mafic 

layers and carrying fragments from the site of 

anatexis.

Textures of the Monson Gneiss generally reflect thorough recrystallization. Textures are generally granoblastic, with more or less distinct foliation, which is 

parallel to original layering where the latter is recognizable. The variation in grain size within a thin section 

ranges from an estimated 1:5 to 1:25 or more. Large

grains of quartz and plagioclase in a much finer granoblastic matrix are characteristic. Quartz tends to form 

interstitial, elongate patches, locally virtual ribbons, 

with scalloped margins against adjacent minerals. 

These quartz patches are evidently the result of metamorphic recrystallization under stress and (or) cataclasis. Plagioclase forms large, undisturbed grains 

with scalloped margins which evidently are porphyroblasts (possible recrystallized clasts).

The mineralogy, textural features, megascopic appearance, layered character, and associated amphibolite 

tend to confirm the impression of earlier workers that 

the Monson is dominantly, if not entirely, of volcaniclastic origin. This view is reinforced by the bulk chemical compositions and norms of the analyzed samples 

(table 1).

AMMONOOSUC VOLCANICS

The Ammonoosuc Volcanics is a lithologically diverse 

unit of volcanic, volcaniclastic, and epiclastic origin of 

Middle Ordovician age that mantles Oliverian core 

gneisses from northern New Hampshire to Connecticut. 

Northwest of the Oliverian domes (northwestern New 

Hampshire and eastern Vermont), the Ammonoosuc 

overlies the quartzose to pelitic, largely nonvolcanic 

Albee Formation along a gradational contact. Southeast 

of a line corresponding roughly to the axis of the Bronson Hill anticlinorium, the Albee Formation is lacking 

and Ammonoosuc Volcanics is intruded by Oliverian 

core gneisses.

The Ammonoosuc Volcanics includes mafic hornblendeplagioclase amphibolite and felsic granofels consisting 

mostly of quartz and sodic plagioclase, thus quartz 

keratophyre, in addition to intrusive trondhjemite (see 

below). Rocks of other compositions are widespread, 

notably such that contain Ca-poor amphiboles, and in 

some cases these have an intermediate chemical character (dacite or andesite). Nevertheless, the dominant 

Ammonoosuc assemblage is bimodal as well as K-poor. 

At least some metabasaltic rocks correspond to oceanic 

tholeiite (Aleinikoff, 1977; Leo, unpub. data). Associated 

felsic rocks, especially where thinly banded (fig. 3A), 

are almost certainly tuffaceous. More massive layers of 

felsic Ammonoosuc could be flows, though primary textures, if any, have been obliterated. Geochemical characteristics of the Ammonoosuc Volcanics are compatible 

with the development of these rocks in an environment 

suggestive both of an island-arc and a continentalmargin regime (Leo and Gromet, 1981; Leo, unpub. 

data). Textures and mineralogy, especially of the felsic 

Ammonoosuc near the Glastonbury dome, have been 

described in more detail elsewhere (Leo and others, 

1977; Leo, 1977). Eleven analyses of felsic layers of 

Ammonoosuc Volcanics are given in table 1.
REGIONAL GEOLOGY

FIGURE 3. Ammonoosuc Volcanics and Monson Gneiss. A, Typical 

association of mafic and felsic layers in folded Ammonoosuc Volcanics. Southeast side of Minechoag Mountain, Ludlow 

7%-minute quadrangle, Massachusetts (fig. 2, loc. 47). B, Monson 

Gneiss showing vertical compositional banding (not necessarily 

primary) in north side of cut on Massachusetts Turnpike (1-90), 

2.5 km north-northeast of center of Palmer, directly east of 

Breckenridge Street overpass, Palmer 7%-minute quadrangle, 

Massachusetts (fig. 2, loc. 31). C, One-meter wide amphibolite sill

in Monson Gneiss. Note sharp contacts and septum of Monson in left 

half of sill. Same location as B. D, Folded amphibolite sill in banded 

Monson Gneiss. The fold is a north-plunging S-fold related to the 

Acadian deformation that involved both the amphibolite and the 

gneiss. One hinge of fold is under hammer (arrow along fold axis). 

Same location as B. E, Imperceptible transition from banded Monson 

Gneiss (right side of photo) to weakly foliated but unlayered rock 

(under hammer) similar to some northern Glastonbury Gneiss. Same 

location as B.
GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

FIGURE 4. Monson Gneiss showing partial anatexis and plastic flow. 

A, Gneiss showing swirled banding and local felsic segregations 

(incipient anatexis). Massachusetts Route 2, southwest ramp of 

Orange exit, 3.3 km south-southeast of center of Orange, Orange 

7%-minute quadrangle, Massachusetts. Adjacent gneiss is mostly 

finely banded, comparable to figure 3B. B, Detail of A. C, Ellipsoidal amphibolite inclusions evidently alined by flow of felsic 

Monson Gneiss. Flynt quarry, east side of upper Palmer Road,

about 2 km north-northwest of Monson, Mass. (fig. 2, loc. 34, 35). D, 

Detail of C. Note blotchy texture of felsic layer in amphibolite that is 

indicative of partial melting in this layer, apparently preceding 

detachment of amphibolite block. Note also sharp boundary of inclusion against gneiss, except for apparent felsic reaction rim along top 

and left margins. E, Swirled and diffuse banding, mafic schlieren 

(upper right), and felsic (anatectic?) veins (bottom) in gneiss that has 

apparently flowed plastically. Flynt quarry.
REGIONAL GEOLOGY

In several areas in western New Hampshire, locally 

abundant trondhjemite sills intrude the layered Ammonoosuc and core gneiss in three of the smaller domes is 

trondhjemite (Leo and Gromet, 1981; Leo, unpub. data). 

The chemistry and mineralogy of the trondhjemite are 

in most respects similar to that of the layered felsic 

Ammonoosuc, but there are some consistent differences 

in rare earth element (REE) patterns (Leo and Gromet, 

1981). The trondhjemites of New Hampshire will be 

discussed further in the context of the northern Glastonbury Gneiss.

The Ammonoosuc Volcanics has been correlated with 

the lithologically similar Middletown Gneiss in central 

Connecticut (Eaton and Rosenfeld, 1960, 1972). Herz 

(1955) mapped the Middletown along the west edge of 

the Glastonbury dome in the Glastonbury quadrangle 

(fig. 2); the southward extension of this formation into 

the Middle Haddam quadrangle, however, was regarded 

by Eaton and Rosenfeld (1972) not as Middletown but 

as a distinct unit of possible Permian age (amphibolite 

of Reservoir Brook of Eaton and Rosenfeld, 1972).

PARTRIDGE AND COLLINS HILL FORMATIONS

The Partridge and Collins Hill Formations of Middle 

Ordovician age are discussed here only in the context 

of the regional stratigraphy but are not otherwise involved in the study. The two formations are stratigraphically equivalent and conformably overlie with the 

Ammonoosuc Volcanics. The name Partridge Formation 

is applied principally to rocks north of the Connecticut 

boundary, whereas Collins Hill is the named used for 

stratigraphically equivalent rocks in the Middle Haddam quadrangle, Connecticut. Both units consist of 

micaceous schists containing graphite and sulfide 

(commonly pyrrhotite) with associated calc-silicate and 

siliceous granofels, coticule, and mafic and felsic volcanic layers (Thompson and others, 1968, p. 206; Eaton 

and Rosenfeld, 1960, 1972). The sulfide results in a 

characteristic yellow-brown weathering crust.

GLASTONBURY GNEISS

STRUCTURAL CHARACTER

The Glastonbury dome has only some of the structural 

attributes of a dome. For this reason it was previously 

(Leo, 1977) referred to as a body instead of a dome, but 

the more traditional term of dome is again used here. 

Flanking units (Ammonoosuc Volcanics in the north, 

Collins Hill Formation and Silurian and Devonian 

rocks in the south) wrap around the ends of the Glastonbury Gneiss (fig. 2), but foliation trend lines do not 

appear to close (Gordon Eaton, oral commun., 1975).

Foliation in the gneiss trends predominantly north to 

northeast with low to moderate northwest dips (fig. 5D\ 

and mineral lineations and minor fold axes mostly 

plunge north to northwest. These minor structures, 

which generally parallel those of the mantling rocks, 

indicate a moderate east to southeast overturn of the 

body and are assumed to be related to the Acadian 

orogeny.

CONTACT RELATIONSHIPS

Exposed contacts between the Glastonbury Gneiss and 

adjacent rocks are rare, but outcrop patterns are clear 

in a number of places. Glastonbury Gneiss intrudes 

Ammonoosuc Volcanics on the southeast side of Baptist 

Hill in the Palmer quadrangle (fig. 5A). The crosscutting relations are seen in several outcrops over a distance of about 50 m. The contact is sharp, without any 

evidence of reaction or other alteration. Glastonbury 

Gneiss from near the contact is among the least potassic encountered. This is the most clearly exposed intrusive contact known to us. Smaller dikes or sills of the 

Glastonbury in the Ammonoosuc are locally observed 

(fig. 5fi). Elongate lenses of Glastonbury within Ammonoosuc in the southern part of the Ludlow 7%-minute 

quadrangle (Leo and others, 1977) are probably intrusive, but contacts are not exposed.

Evidence for intrusion of the Ammonoosuc Volcanics 

by the Glastonbury Gneiss on a much larger scale is 

found along the western margin of the dome south of 

the Ludlow quadrangle. In the southern part of the 

Hampden quadrangle, the gneiss has breached the 

Ammonoosuc in several partly crosscutting sills, creating inliers and roof pendants ranging from a few meters 

to several kilometers in diameter; one such semiconcordant inlier rifted from the Ammonoosuc section 

trends more than 4 km south from the town of Hampden 

(Peper, 1977; fig. 2). Sharp contacts between the Ammonoosuc amphibolite and the Glastonbury Gneiss are 

locally exposed. Similar relationships are found in the 

Ellington 7%-minute quadrangle to the south; large 

blocks of amphibolite in that area, such as that at Soapstone Mountain, are interpreted as roof pendants of the 

Ammonoosuc in the Glastonbury (M. H. Pease, oral 

commun., 1974). South of the Ellington quadrangle the 

Glastonbury dome is mostly fault bounded on both 

sides. At the southern end of the dome in the Middle 

Haddam quadrangle, strongly foliated, hornblendebearing Glastonbury Gneiss crops out a few meters 

from the lithologically distinct Collins Hill Formation, 

but an intrusive contact cannot be proved at this locality. Large tabular mafic inclusions along the eastern 

border of the Glastonbury Gneiss (fig. IE) suggest an 

intrusive relationship with the Middletown/Ammonoosuc, but inasmuch as the latter is structurally separated
10 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

FIGURE 5. Structures and textures in the northern Glastonbury 

Gneiss. A, Intrusive contact between Glastonbury Gneiss (light 

gray) and Ammonoosuc amphibolite. Southeast side of Baptist 

Hill, Palmer 7%-minute quadrangle, Massachusetts (fig. 2). 

B, Folded sill of Glastonbury Gneiss (right center) in Ammonoosuc amphibolite. Hillside north of Root Road at origin of 

Schanade Brook, 2.4 km east-southeast of North Somers, southeastern part of Hampden quadrangle, Massachusetts. C, Massive,

well-foliated and lineated Glastonbury Gneiss in cut on north side of 

Massachusetts Turnpike (1-90) just east of Kelly Hill Street overpass, about 3 km west of center of Palmer, Mass. (fig. 2, loc. 2). Note 

flattened mafic inclusions parallel or subparallel to foliation. Lens cap 

(left) gives scale. D, Strongly foliated Glastonbury Gneiss outcrops on 

north side of Shenipsit Lake road, 3 km east of Ellington, Conn. (fig. 

2, loc. 10). View looking northeast.
REGIONAL GEOLOGY 11

from the Glastonbury in this area (Snyder, 1970), an 

intrusive contact also cannot be documented here.

The contact between the Glastonbury Gneiss and the 

Clough Quartzite of Early Silurian age, which flanks 

the dome along its southeast side, is not exposed. Snyder 

(1970) mapped the Glastonbury-Clough contact as a 

fault that was assumed to have obliterated an intrusive 

contact. In the light of the newly established Early 

Silurian to Middle Ordovician age of the Glastonbury 

the contact probably is an unconformity.

LITHOLOGIC CHARACTER

The northern Glastonbury Gneiss is weakly to conspicuously foliated and typically has a well-defined lineation 

(fig. 5C). Despite a superficially homogeneous appearance, the composition of the gneiss varies significantly 

from outcrop to outcrop, mainly in the relative proportions of quartz and feldspars (fig. 6). Mafic inclusions 

are common and locally abundant; they are generally 

small, disk-shaped, and subparallel to foliation (fig. 5C). 

Rare tabular amphibolite bodies (late-Ammonoosuc or 

post-Ammonoosuc feeders?) as much as 50 cm thick are 

partly concordant with foliation and partly crosscutting.

The northern gneiss consists dominantly of quartz 

and plagioclase with subordinate K-feldspar, biotite, 

and epidote (both as isolated grains and as idiomorphic 

granules scattered through plagioclase), and may or 

may not contain minor muscovite, hornblende, garnet, 

and various other accessories (table 1). A strong lineation is produced by alinement of biotite-epidote clusters 

and elongate aggregates of quartz grains. Textures 

range from equigranular to highly inequigranular; in 

some cases large porphyroblasts of quartz and plagioclase cut across a granoblastic matrix. Overall, the textures are metamorphic, giving no definite clues to a 

preexisting igneous fabric.

Modal K-feldspar in the northern gneiss ranges from 

0 to 10.4 percent, with a median value of 4.5 percent 

(table 1, fig. 6). The K-feldspar has the grillwork twinning of microcline and generally occurs in small interstitial grains, or as larger, locally crosscutting patches. 

Where samples are closely spaced, notably in the northern end of the main body and in the eastern outlier in 

the Monson and Stafford Springs quadrangles, rocks 

deficient in K-feldspar are near the margins of the 

mass, and more potassic rocks are near the center.

South of the Ellington quadrangle the Glastonbury 

Gneiss shows pervasive changes in texture and composition (Aitken, 1955; Herz, 1955; Snyder, 1970; Eaton 

and Rosenfeld, 1974). The most consistent compositional 

change is an increase in the proportions of K-feldspar 

and a roughly proportional decrease in quartz and 

plagioclase (fig. 6). The K-feldspar content for nine 

hornblende-free rocks is 12.4 to 33.0 modal percent,

with a median value of 22.1 percent (table 1). Along the 

southwest side of the gneiss in the Middle Haddam 

quadrangle, a progressive change in composition toward 

the margin from granite to granodiorite and tonalite is 

marked by the appearance of hornblende and increasing 

amounts of biotite and epidote (table 1). The southern 

gneiss is granodioritic to granitic and shows essentially 

no compositional overlap (fig. 6) with the tonalitic to 

granodioritic northern gneiss.

The range of textural variations in the southern 

gneiss likewise appears considerably greater than in 

the northern gneiss. In the northern part of the Rockville quadrangle (fig. 2, loc. 11) the rock is weakly lineated and foliated, medium-grained flaser gneiss with 

prominent blotchy biotite aggregates (fig. 7 A); 9 km to 

the south the rock is comparatively massive and contains prominent microcline porphyroblasts (fig. 2, loc. 

12,13; fig. IB). In the Glastonbury quadrangle, roadcuts 

along Connecticut Route 2 within the Glastonbury 

body show the following variations over less than 2.5 km 

from the western margin southeast towards the interior: 

closely foliated, biotite-rich gneiss; weakly foliated, 

porphyroblastic, fine-grained granitic gneiss; somewhat 

coarser grained, better foliated gneiss containing ovoid 

microcline prophyroblasts as much as 2 cm long (fig. 1C) 

that grades gradually to abruptly to much finer grained, 

nonporphyritic gneiss; and gneiss with ellipsoidal microcline augen (this is probably sheared porphyritic 

gneiss). Variations such as these led Aitken (1955), and 

to a lesser extent Herz (1955), working the Rockville 

and Glastonbury quadrangles, respectively, to regard 

the southern Glastonbury Gneiss as having originated 

by granitization of preexisting metasedimentary rocks. 

Eastwards into the Glastonbury body the rocks are 

generally more homogeneous and less strongly foliated; 

thin, continuous mafic bands in otherwise massive 

gneiss (fig. ID) are locally developed but are not typical. 

Three texturally distinct gneisses from the Glastonbury 

quadrangle have fairly similar compositions (nos. 16, 

19, and 20 in fig. 2 and table 1).

Textural variations at the south end of the Glastonbury dome in the Middle Haddam quadrangle are less 

extreme and appear to be controlled largely by the proportion of mafic minerals. Hornblende-free granitic 

gneiss in the center and southeastern part of the dome 

is relatively massive with faint but distinct regional 

foliation. Hornblende- and biotite-bearing gneiss is 

increasingly foliated towards the southwest margin.

Mafic inclusions of two general types are locally abundant: (a) an angular, irregular-shaped, sharply bounded 

type (fig. IE) and (b) a more or less ellipsoidal type, 

which is stretched parallel to foliation and is sharply 

bounded to shadowy and diffuse (fig. 7C). The latter 

type probably was produced by shearing of originally
12 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

quartz

plagioclase

10 35 65 90

K-feldspar

EXPLANATION

A Northern Glastonbury Gneiss (table 1, nos. 1 10)

O Southern Glastonbury Gneiss (table 1, nos. 11-13 

and 18-24)

 Hornblende-bearing southern Glastonbury Gneiss 

(table 1, nos. 25, 27, 28)

Felsic layers of Ammonoosuc Volcanics (table 1, 

nos. 38-48)

D Monson Gneiss (table 1, nos. 29-37)

FIGURE 6. Ternary diagram showing modal variations in the Glastonbury Gneiss, Monson Gneiss, and felsic Ammonoosuc 

Volcanics. Rock classification according to International Union of Geological Sciences (Geotimes, 1973).
REGIONAL GEOLOGY 13

FIGURE 7. Structures and textures in the southern Glastonbury 

Gneiss. A, Flaser gneiss containing blotchy biotite aggregates. 

Roadcut on north side of Connecticut Route 15-Route (1-86), 1.5 

km south of Rockville, Conn. (fig. 2, loc. 11). B, Massive Glastonbury Gneiss, approximate center of roadcut on north side of Connecticut Route 15 (1-84) directly east of Wyllys Street overpass 

and approximately 0.8 km east of Highland Street exit (fig. 2, loc. 

12, 13). Shows indistinct foliation, conspicuous microcline 

porphyroblasts, and angular amphibolite inclusions. C, Porphyroblastic, plastically sheared gneiss containing highly stretched, 

ellipsoidal mafic inclusions. West side of Connecticut Route 2,

^ * -»*dtvi *§,;;; -*-'-<»« ° J - -v-'" * A^ " *"% ;siiP^ ' --o.i^ »*?* 

, % ?TPSi«*«La^>-" 4<s -irv* . j«, i: ,---f -^ .^ "- . *i""t -'s. ;«*H5 5

--I ^2 ,? ^-.w - :j!-\-, .' .a^-a »» rv*s-*j8: -- . *- >*-- i

% r°SSi.*«L^f>-'- ««^-*''-.-». i: - « -^ - ? - *i'i rs.'-«*!~5 

^S'^^^^^P1^^?^' ^^^

i^.! * * »

0.7 km north of Quarry Street overpass, Glastonbury quadrangle, 

Connecticut (fig. 2, loc. 50). D, Faint, continuous mafic septa in otherwise homogeneous, equigranular gneiss. Tower Hill quarry, south of 

New London Turnpike, 0.9 km northwest of intersection with 

Quarry Street, Glastonbury quadrangle, Connecticut (fig. 2, loc. 51). 

E, Partly assimilated, plastically deformed mafic layers, assumed to 

be Middletown Formation, parallel to strong regional foliation in 

otherwise homogeneous granitic Glastonbury Gneiss. Pegmatite (foreground) cuts inclusions and Glastonbury gneiss. Hebron Avenue 

gravel pit south of Connecticut Route 94, west side of Marlborough 

quadrangle, Connecticut (fig. 2, loc. 18).
14 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

angular inclusions (of Ammonoosuc amphibolite?). 

Pegmatites are abundant (fig. IE), especially along an 

axial line of the dome in the Glastonbury quadrangle 

(Herz, 1955).

Microtextures in the southern Glastonbury Gneiss 

are generally comparable with those in the northern 

part, except that microcline is more abundant both as 

small interstitial grains and as large, ragged, crosscutting plates. The patchy microcline porphyroblasts replace plagioclase along margins and contain partly 

resorbed plagioclase remnants. Much of the quartz, 

too, occurs in isolated patches of strongly sutured and 

strained grains elongated parallel to the foliation and 

locally replaces plagioclase. These features suggest 

late redistribution, and possibly some late introduction, 

of silica and alkalies in the southern part of the Glastonbury. As in the northern part, epidote and biotite 

are ubiquitous and seemingly stable, suggesting that 

conditions of metamorphism were similar throughout 

the Glastonbury Gneiss.

ANALYTICAL RESULTS 

MAJOR ELEMENTS

Major-element compositions of Glastonbury Gneiss, 

Monson Gneiss, and felsic layers of Ammonoosuc Volcanics are listed in table 1 and graphically portrayed in 

figure 10; normative diagrams are shown in figures 8 

and 9. For the purpose of this study only felsic (no mafic) 

Ammonoosuc was analyzed in order to provide a comparison with Monson and northern Glastonbury 

compositions.

AMMONOOSUC VOLCANICS AND MONSON GNEISS

On a normative An-Ab-Or diagram (fig. 8) Ammonoosuc and Monson plot fairly close to the An-Ab join and 

all fall into the compositional field of trondhjemite and 

tonalite. Ammonoosuc compositions, moreover, are 

mostly encompassed by the field of oceanic plagiogranite as defined by Coleman and Donate (1979). Inasmuch 

as trondhjemites and plagiogranites are magmatic 

rocks and Ammonoosuc is dominantly tuffaceous, the 

significance of these comparisons should not be overdrawn; they are useful, however, in pointing up the 

typically low abundance of potassium in these Ordovician volcanic rocks.

On the normative Q-Ab-Or diagram (fig. 9), which 

does not reflect variations in the An content of plagioclase, the fields of Monson and felsic Ammonoosuc 

appear smaller but are still partly overlapping and 

more clearly depict the somewhat more potassic nature 

of Monson Gneiss relative to felsic Ammonoosuc. The 

bulk compositions of these two units are notably inhomogeneous. Variation diagrams of various oxides plotted against SiO2 (fig. 10A) show significant scatter, 

especially in regard to A12O3, Fe2O3, FeO, and CaO. j 

The alkalies show more constant abundances; K2O is 

conspicuously low throughout, especially in Ammonoosuc (0.34 percent average for 11 samples). The relatively 

large degree of variation that is also seen in Ti and P, 

elements that are normally stable during alteration or 

low-grade metamorphism (see Gottfried and others, 

1978, for discussion), suggests that the scatter in the 

abundances of other elements probably reflects a varied 

source for the Monson and the Ammonoosuc, rather 

than differential leaching or mobilization in the course 

of alteration. This observation is in accord with the 

apparent volcaniclastic nature especially of the felsic 

Ammonoosuc. Unsystematic variations in some stable 

trace elements (next section) support the idea of inhomogeneous source material. Nevertheless, some alteration following eruption or deposition may have occurred 

in old, deformed, and polymetamorphosed rocks such 

as these, although its effect cannot be quantitatively 

assessed with the available data.

GLASTONBURY GNEISS

Analyses of Glastonbury Gneiss fall into two distinct 

compositional fields with very little overlap (figs. 8 and 

9). The northern gneiss straddles the compositional 

boundary between tonalite, granodiorite, and trondhjemite (fig. 8), whereas the southern gneiss is granite 

gradational to granodiorite. The rather extensive compositional field of granitic Oliverian core gneisses (Leo, 

unpub. data) encompasses all but one of the southern 

Glastonbury Gneiss samples (fig. 8).

On both figures 8 and 9 the shaded field of Ammonoosucrelated trondhjemite from the northern Oliverian domes 

(Leo, unpub. data) is compositionally distinct from 

northern Glastonbury Gneiss, indicating relatively 

lower K2O and higher Na2O. This distinction is taken 

as partial evidence for the postulated different origin 

for the two units, as discussed below.

Variation diagrams of both northern and southern 

Glastonbury have generally smoother trends against 

SiO2 than do the Monson and the Ammonoosuc (fig. 

10JB). Of the two areas of Glastonbury Gneiss, greater 

irregularities are apparent in the northern gneiss, particularly for CaO, Na2O and K2O. K2O contents are relatively low (1.73 percent average for 10 samples), whereas 

Na2O contents are consistently higher than in the 

southern gneiss. These variations, which are clearly 

reflected in the modal analyses (table 1), are compatible 

with an origin of the northern gneiss by partial anatexis 

of K-poor volcanic rocks. Anatexis would be initiated 

by eutectic melting to relatively potassic liquid in
ANALYTICAL RESULTS 15

TABLE 1. Chemical compositions (major elements), norms and modes of Glastonbury Gneiss, Monson Gneiss, and felsic layers of

Ammonoosuc Volcanics (in percent)

[Rapid rock analyses (three significant figures) by Paul Elmore, Joseph Budinsky, Herbert Kirschenbaum, and Lowell Artis under direction of Leonard Shapiro. Standard rock analyses (four 

significant figures) by Elaine L. Brandt and Christel Parker under direction of Lee C. Peck, n.d., not determined; , absent or not calculated because inapplicable. Analysis numbers match

location numbers in fig. 2. See Appendix.]

Northern Glastonbury Gneiss

Analysis No.

SiO

TiO.

Al2Og 

Fe O

FeO

Mnf)

p o 

-1 2 V-'S 

CO

F

Cl

Subtotal 

Less O 

Total

Q

Or 

J"}^ ___________________________

Wo

En

ps _________________

Hv **j 

En 

Fs

11

Ab/An 

Flag. comp. calc. 

Flag. comp. observed 

Plagioclase 

K-Feldspar 

Muscovite - 

Epidote 

Other amph. 

Garnet 

1

74.0 

0.11 

13.7 

0.80 

1.3 

.0 

.51 

1.7 

4.1 

2.3 

.84 

.02 

0.07 

0.02 

0.03 

0.062 

99.

99.

37.04 

13.65 

34.40 

7.71 

1.75

2.83 

1.28 

1.55 

1.17 

.21 

.17 

.05 

4.5 

An18 

An10

43.5 

44.1 

5.5 

2.6 

3.8 

.3

.2

Remainder8 

See footnotes at end of table.

2

76. 

0.10 

13. 

3.3 

n.d. 

0.07 

0.58 

2.65 

3.30 

2.10 

n.d. 

n.d. 

<0.10 

n.d. 

n.d. 

<0.10 

101.

101.

41.34 

12.39 

27.87 

13.12 

.38

1.44 

1.44

2

_ 2

.15

2.1 

Ang2 

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.3. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

3

75.3 

.16 

13.2 

1.1 

1.4 

0.0 

0.54 

2.6 

4.1 

1.5 

0.43 

0.01 

0.06 

0.02 

0.01 

0.039 

100.

100.

38.96 

8.82 

34.24 

12.29 

.36

2.73 

1.34 

1.39 

1.59 

.30 

.14 

.05 

2.8 

An,,,

44.7 

41.5 

4.5 

4.9 

.2 

.2

.1

4 5

Major elements

76.5 74.9 

0.28 0.10 

12.5 14.3 

1.2 0.90 

1.2 0.92 

0.04 0.06 

0.25 0.28 

3.6 2.9 

3.1 3.8 

0.57 1.5 

0.64 0.69 

0.02 0.01 

0.08 0.08 

0.02 0.04 

n.d. n.d. 

n.d. n.d. 

100. 100.

100. 100. 

Norms

48.09 40.14 

3.37 8.82 

26.23 32.00 

17.23 13.58 

.47 1.42

1.45 

.62 

.82 

1.74 

.53 

.19 

.05 

1.5 

An40 

An2S_S7

Modes5

47.6 

42.6

3.1 

.3 

6.1 

0.2

-

1.58 

.69 

.89 

1.30 

.19 

.19 

.09 

2.4 

An30

40.1 

46.6 

3.7 

6.5 

.3 

2.5

-

-

6

75.2 

0.19 

13.9 

0.70 

1.3 

0.03 

0.57 

1.3 

4.2 

1.7 

0.61 

0.01 

0.07 

0.06 

n.d. 

n.d. 

100.

100.

40.29 

10.06 

35.58 

5.71 

3.06

2.97 

1.42 

1.55 

1.02 

.36 

.17 

.14 

6.2 

An14

34.4 

45.7 

4.7 

8.0 

6.0 

1.2

-

7

72. 

0.23 

14. 

1.4 

n.d. 

0.08 

0.65 

3.03 

3.30 

2.38 

n.d. 

n.d. 

<0.10 

n.d. 

n.d. 

<0.10 

99.

99.

36.41 

14.04 

27.89 

16.35 

.20

1.62 

1.62

2

_ 2

.17

1.7 

Ang7 

n.d.

34.2 

41.5 

10.4 

8.5 

1.7 

3.8

tr.4

8

72.4 

0.25 

14.8 

1.2 

2.6 

0.17 

0.84 

1.4 

3.2 

1.8 

0.97 

0.03 

0.04 

0.04 

n.d. 

n.d. 

100.

100.

41.24 

10.70 

27.13 

6.54 

5.21

5.79 

2.10 

3.69 

1.74 

.48 

.10 

.09 

4.1 

An19 

An,,,

39.9 

39.1

11.7 

8.7

.3 

.2

9

71.7 

0.22 

14.8 

1.2 

1.5 

0.09 

0.64 

3.5 

3.1 

2.4 

0.68 

0.01 

0.13 

0.06 

n.d. 

n.d. 

100.

100.

35.76 

14.14 

26.20 

16.27 

1.14

3.16 

1.59 

1.57 

1.74 

.42 

.31 

.14 

1.6 

An38

34.2 

44.3 

9.6 

8.1

3.7

10

69.3 

0.27 

15.9 

1.3 

1.0 

0.03 

0.80 

5.0 

4.3 

1.1 

0.73 

0.05 

0.22 

0.02 

n.d. 

n.d. 

101.

101.

29.19 

6.50 

36.38 

20.83

1.91 

1.01 

0.76 

0.12 

1.46 

1.23 

.23 

1.88 

.51 

.52 

.05 

1.7 

An44

"^25-82

26.8 

56.1 

.6 

12.6 

1.5 

5.4
16 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

TABLE 1. Chemical compositions (major elements), norms and modes of Glastonbury Gneiss, Monson Gneiss, and felsic layers of

Ammonoosuc Volcanics (in percent) Continued

Southern Glastonbury Gneiss

Analysis No.

SiO2 

TiO

Al O

F6* 3

MnO 

MgO

CaO 

Na O

If C\

H O+

H O~

P O

Cl

Subtotal 

Less O 

Total

Or

Ab 

En

 ps ________________ 

Hv iij 

En

Mt

11

 " F An

CC

Ab/An 

"PI a rf i^nm n f*fi 1 c*

Flag. comp. observed 

Plagioclase 

K-Feldspar 

Biotite 

Muscovite 

Opaque 

R.fim a i n A f>r3

11

75.1 

.12 

13.1 

.50 

.76 

.03 

.40 

4.0 

3.1 

3.5 

.54 

.04 

.09 

.04 

n.d. 

n.d. 

101.

101.

35.64 

20.41 

25.89 

11.34

3.68 

1.87 

.98 

.83

.72 

.23 

.21 

.09 

2.3 

An30 

n.d.

46.3 

29.5 

15.3 

7.0

.8 

.7 

.1

.1

9

12

68.1 

.37 

14.7 

1.9 

1.2 

.07 

1.0 

4.4 

2.8 

3.6 

.57 

.02 

.16 

.06 

n.d. 

n.d. 

99.

99.

28.72 

21.50 

23.94 

17.90

2.77 

1.47 

1.22 

.08 

1.38 

1.30 

.08 

2.78 

.71 

.38 

.14 

1.3 

An43 

An2S

32.8 

28.2 

22.1 

9.5

7.4

tv

13

66.3 

.40 

15.0 

2.3 

1.4 

.07 

1.2 

5.0 

2.7 

3.4 

.69 

.20 

.25 

.05 

n.d. 

n.d. 

99.

99.

27.02 

20.30 

23.10 

18.97

3.23 

1.73 

1.44 

.07 

1.66 

1.58 

.07 

3.37 

.77 

.60 

.12 

1.2 

An4S

24.1 

25.7 

33.0 

10.7

5.9

7

14

Major elements

73.56 

.17 

14.00 

.48 

1.08 

.06 

.46 

1.98 

3.24 

4.28 

.30 

.04 

.05 

.01 

.04 

.00 

99.75 

.02 

99.73

Norms

33.13 

25.32 

27.45 

9.18 

.68

2.46 

1.15 

1.31 

.89 

.32 

.12 

.02 

3.0 

An2s 

n.d.

Modes5

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

n rl

15

66.54 

.39 

15.00 

1.80 

2.25 

.10 

1.94 

4.16 

2.44 

4.10 

.87 

.07 

.10 

.02 

.05 

.00 

99.83 

.02 

99.81

24.73 

24.27 

20.68 

17.90

1.38 

.71 

.45 

.21 

6.37 

4.39 

1.98 

2.61 

.74 

.24 

.05 

1.2 

An46 

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

n r\

16

74.08 

.15 

13.68 

.61 

.88 

.03 

.45 

1.76 

2.90 

4.59 

.33 

.14 

.04 

.01 

.06 

.00 

99.71 

.03 

99.68

35.21 

27.21 

24.62 

8.02 

1.02

2.06 

1.12 

.94 

.86 

.29 

.10 

.02 

3.1 

An46 

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

n r\

17

76.30 

.09 

12.90 

.26 

.86 

.05 

.21 

.92 

3.15 

4.72 

.28 

.07 

.02 

0.3 

.04 

.00 

99.88 

.02 

99.86

37.46 

27.93 

26.52 

3.97 

1.19

1.83 

.52 

1.31 

.38 

.17 

.05 

.07 

6.7

n.d. 5

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

18

75. 

.05 

14. 

1.4 

n.d. 

.04 

.35 

1.61 

3.25 

4.30 

n.d. 

n.d.

n.d. 

n.d.

100.

36.57 

25.82 

27.95 

6.65

1.13 

.60 

.52

.36 

.36

_ 2 

_ 2

.09

4.2 

An19 

n.d.

30.0 

35.7 

27.3 

7.1

fr

19

71.9 

.24 

13.8 

.60 

.96 

.03 

1.1 

1.9 

2.8 

4.8 

.61 

.10 

.12 

.02 

n.d. 

n.d. 

99.

99.

31.83 

28.66 

23.94 

8.60 

.89

3.70 

2.77 

.94 

.88 

.46 

.29 

.05 

2.8 

An26 

n.d

30.5 

28.4 

26.8 

11.8 

1.7 

.6

-

9

See footnotes at end of table.
ANALYTICAL RESULTS 17

TABLE I. Chemical compositions (major elements), norms and modes of Glastonbury Gneiss, Monson Gneiss, and felsic layers of

Ammonoosuc Volcanics (in percent) Continued

Southern Glastonbury Gneiss Continued

Analysis No.

SiO

TiO ___ _ 

A1203 

Fe2 03 

FeO

MnO ________ 

MgO _____

PaO

Na O

K2O ___

H O+

H O~

P Jr 2Ov-'5 

CO

F

PJ __________________

Subtotal 

Less O 

Tntal

Or

Ab __________ 

An

c

Hi

Wo

En

JTS _______________ 

Hv iiy 

En

Fs

Mt

11

.rt.p An

PP

Ab/An -______ 

Flag. comp. calc. 

Flag. comp. observed 

Plagioclase 

K-Feldspar 

Biotite 

Muscovite 

Epidote 

Hornblende 

Other amph. 

Opaque 

Remainder3 

20

76.2 

.11 

12.9 

.29 

.92 

.02 

.23 

1.1 

3.0 

4.4 

.49 

.00 

.02 

<.05 

n.d. 

n.d. 

100.

100.

38.80 

26.08 

25.47 

5.34 

1.25

1.88 

.57 

1.31 

.42 

.21 

.05

4.8 

An17 

An13_32

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

21

66.7 

.29 

16.2 

2.0 

1.5 

.04 

1.1 

4.6 

2.6 

3.7 

.97 

.04 

.17 

<.05 

n.d. 

n.d. 

100.

100.

26.12 

21.88 

22.02 

21.62

.08 

.04 

.03 

.01 

3.40 

2.71 

.69 

2.90 

.55 

.40

1.0 

An50 

n.d.

41.1 

27.0 

12.4 

12.5

6.7 

tr.

.4

22

74.7 

.14 

14.1 

.16 

1.4 

.16 

.43 

2.0 

3.5 

2.6 

.64 

.00 

.06 

<.05 

n.d. 

n.d. 

100.

100.

38.54 

15.38 

29.65 

9.54 

2.04

3.58 

1.07 

2.51 

.23

.27

.14

3.3 

An24 

n.d.

38.0 

40.0 

14.2 

7.5

.3 

.1

23

Major elements

76.3 

.10 

12.8 

.9 

1.1 

.06 

.31 

1.5 

3.1 

3.8 

.82 

.02 

.03 

<.05 

n.d. 

n.d. 

100.

100. 

Norms

39.26 

22.45 

26.22

7.24 

.93

2.66

.77 

1.89 

.13 

.19 

.07

3.7 

An22 

n.d.

Modes5

29.7

44.8 

17.6

7.7

.1

.2

24

75.7 

.07 

13.7 

.14 

.76 

.06 

.22 

1.7 

3.0 

4.1 

.58 

.02 

.06 

<.05 

n.d. 

n.d. 

100.

100.

38.21 

24.45 

25.41 

8.05 

1.18

-

1.83 

.55 

1.28 

.20 

.13 

.14

3.2 

An24 

n.d.

39.6 

27.1 

28.9

4.4

tr.

-

25

63.8 

.42 

16.2 

2.3 

2.2 

.12 

1.7 

5.8 

2.6 

3.4 

.90 

.00 

.22 

<.05 

n.d. 

n.d. 

100.

100.

21.68 

24.25 

22.08 

22.57

3.91 

2.03 

1.35 

.53 

4.04 

2.90 

1.14 

3.35 

.80 

.52

.98 

An50 

n.d.

28.8 

30.2 

12.8 

14.6

10.0 

5.5

.2

26

56.8 

.47 

17.7 

2.8 

3.8 

.17 

2.8 

8.3 

2.6 

2.6 

1.1 

.03 

.31 

<.05 

n.d. 

n.d. 

99.

99.

10.99 

20.16 

22.12 

29.10

8.35 

4.28 

2.54 

1.53 

7.17 

4.74 

2.70 

4.08 

.90 

.74

.76 

An57 

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

27

56.0 

.55 

17.4 

3.2 

4.1 

.19 

3.3 

8.9 

2.1 

1.9 

1.2 

.04 

.27 

<.05 

n.d. 

n.d. 

99.

99.

13.58 

15.44 

17.92 

32.72

8.14 

4.19 

2.59 

1.36 

8.71 

5.70 

3.01 

4.68 

1.05 

.65

.55 

An65 

n.d.

24.0 

32.2 

1.1 

21.4

13.3

7.8

.2

28

57.4 

.55 

17.3 

3.3 

3.4 

.18 

2.8 

8.2 

2.5 

2.0 

1.2 

.04 

.38 

<.05 

n.d. 

n.d 

99.

99.

15.06 

11.91 

21.31 

30.30

6,56 

3.42 

2.94 

.94 

6.81 

4.79 

2.02 

4.82 

1.05 

.91

.70 

An59 

n.d.

18.5 

31.2 

0.5 

22.6

16.5 

10.1

.5

See footnotes at end of table.
18 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

TABLE I. Chemical compositions (major elements), norms and modes of Glastonbury Gneiss, Monson Gneiss, and felsic layers of

Ammonoosuc Volcanics (in percent) Continued

Monson Gneiss

Analysis No.

SiO

TiO

Al O

FeO 

MnO

MgO _______ 

CaO

Na.0 _________ 

K O

H2O+ ______________ 

H20~ 

P * 2 O^5

F 

Cl

Subtotal 

Less O

Total

Or _________________ 

Ab

An 

Wo

En

Fs

Hv

TTn

FS ___________

AP -

Ab/An 

Flag. comp. calc. 

Flag. comp. observed 

Biotite 

Epidote _ 

Hornblende 

Other amph. 

Garnet _ 

Opaque 

Remainder3 

See footnotes at end of ti

29

77.2 

.15 

12.9 

.61 

.72 

.00 

.16 

1.0 

4.0 

1.9 

.51 

.03 

.04 

.02 

n.d. 

n.d. 

99.

99.

44.47 

11.31 

34.09 

4.70 

2.57

.98 

.40 

.57 

.89 

.29 

.10 

.05 

7.3 

An12 

n.d.

43.4 

37.6 

11.4 

5.3 

2.1

able.

30

75.4 

.25 

14.5 

.40 

1.4 

.01 

.64 

2.8 

3.4 

.73 

.63 

.02 

.10 

.02 

n.d. 

n.d. 

100.

100.

45.21 

4.30 

28.68 

13.11 

3.29

3.43 

1.59 

1.34 

.58 

.47 

.24 

.05 

2.2 

An31 

An27

48.6 

41.0

10.0

.4

31

77. 

<.02 

12. 

1.5 

n.d. 

.06 

.10 

.65 

3.93 

1.68 

n.d. 

n.d.

n.d. 

n.d. 

n.d.

97.

48.23 

10.27 

34.09 

3.24 

2.36

.25 

.25

_ 2 

_ 2

10.5

Alia

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

32

Major elements

75.3 

.25 

12.7 

1.1 

2.3 

.03 

1.1 

1.2 

4.1 

.93 

.91 

.09 

.05 

.02 

n.d. 

n.d. 

100.

100. 

Norms

42.47 

5.49 

34.66 

5.53 

2.92

5.69 

2.74 

2.95 

1.59 

.47 

.12 

.05 

6.3 

An14 

n.d.

Modes"

36.0 

44.4

19.0

.2 

.4

33

76.2 

.11 

12.9 

.50 

1.4 

.05 

.32 

2.0 

4.1 

2.2 

.53 

.01 

.02 

.02 

n.d. 

n.d. 

100.

100.

38.17 

12.95 

34.56 

9.68 

.21

2.85 

.79 

2.06 

.72 

.21 

.05 

.05 

3.6 

An22

 " ^22-25

46.7 

35.3 

9.6 

5.6 

.8 

1.6

.5

34

73.9 

.03 

16.1 

.27 

.36 

.06 

.03 

4.0 

4.2 

.22 

.42 

.01 

.04 

.02 

n.d. 

n.d. 

100.

100.

40.01 

1.30 

35.65 

19.56 

1.81

.58 

.08 

.50 

.39 

.06 

.10 

.05 

1.9 

An35

40.0 

57.4

.9 

1.0 

.2

.5

35

65.9 

.36 

17.2 

1.7 

2.3 

.09 

1.4 

5.6 

3.4 

.57 

.71 

.00 

.21 

.06 

n.d. 

n.d. 

100.

100.

29.65 

3.39 

28.91 

26.19 

1.44

5.90 

3.50 

2.40 

2.48 

.69 

.50 

.18 

1.1 

An47

38.7 

47.5

7.0

.1 

6.3

.1 

.3

36

75.1 

.21 

13.7 

1.2 

1.3 

.02 

.28 

2.4 

3.8 

.70 

.73 

.11 

.05 

.02 

n.d. 

n.d. 

100.

100.

44.59 

4.15 

32.27 

11.55 

2.48

1.79 

1.70 

.09 

1.75 

.40 

.12 

.05 

2.8 

An2S 

An23_27

41.2 

49.4

7.2 

.6 

.3

.9

.4

37

67.4 

.26 

18.4 

1.1 

1.4 

.03 

1.0 

5.8 

3.4 

.54 

.75 

.06 

.14 

.02 

n.d. 

n.d. 

100.

100.

31.39 

3.18 

28.68 

27.68 

2.04

3.77 

2.48 

.28 

1.59 

.49 

.33 

.05 

1.0 

An49

37.9 

48.5

8.5

1.1 

3.9

.1
ANALYTICAL RESULTS 19

TABLE 1. Chemical compositions (major elements), norms and modes of Glastonbury Gneiss, Monson Gneiss, and felsic layers of

Ammonoosuc Volcanics (in percent) Continued

Ammonoosuc Volcanics

Analysis No.

TiO

Al O

Fe i ^2^3O

FeO 

MnO

MgO 

CaO

Na O

K O

H2O+ 

H O~

p o __ ____ 

CO

F

Less O _ 

Total

Or __ 

Q

Wo

Fs

Hv

TCn

Mt

 " P An

PP

Flag. comp. calc. 

Pig. comp. observed 

K-Feldspar 

Biotite 

fli i*n ^"fr

Opaque 

Remainder3 

38

77.63 

.11 

12.97 

.54 

.54 

.02 

.14 

.86 

5.82 

1.00 

.17 

.09 

.01 

.01 

.01 

.01 

99.93

99.93 

2.65

37.88 

5.91 

49.21 

4.07 

.84

-

.75 

.35 

.40 

.78 

.21 

.02 

.02 

12.1 

An8 

An4_6

39.0 

55.2 

2.7 

1.1 

.2

-

.7

39

73.3 

.24 

13.6 

1.5 

1.9 

.00 

.86 

3.3 

4.1 

.76 

.59 

.00 

.10 

.02 

.01 

.013 

100.

100.

37.63 

4.48 

34.52 

15.54 

.34

-

3.98 

2.13 

1.85 

2.17 

.45 

.24 

.05 

2.2 

An3l

50.5 

36.8

9.6

1.7

1.1 

.3

40

74.6 

.24 

12.7 

1.5 

1.6 

.00 

1.3 

3.4 

4.3 

.03 

.62 

.00 

.06 

.02 

.01 

.06 

100.

100.

40.58 

.18 

35.01 

15.81

.34 

.17 

.12 

.05 

4.35 

3.10 

1.25 

2.16 

.45 

.14 

.05 

2.2 

An3l 

An25_32

56.6 

33.8

8.8

.8

41

Maj

68.0 

.31 

14.9 

1.9 

3.6 

.00 

1.8 

5.9 

2.8 

.35 

.75 

.02 

.08 

.02 

.01 

.013 

100.

100.

33.50 

2.06 

23.51 

26.97

1.24 

.62 

.31 

.31 

8.36 

4.16 

4.20 

2.74 

.59 

.19 

.05 

.87 

An53

40.3 

37.2

3.6

17.2

1.2 

.6

42

or elements

66.7 

.29 

15.9 

2.0 

3.9 

.00 

1.8 

5.3 

3.5 

.13 

.54 

.02 

.08 

.02 

.01 

.015 

100.

100.

Norms

29.96 

.77 

29.52 

25.67 

.24

-

9.53 

4.49 

5.04 

2.91 

.55 

.19 

.05 

1.2 

An46

Modes5

35.5 

43.3

.4

19.0

1.6 

.2

43

63.2 

.35 

16.4 

2.9 

3.5 

.12 

1.8 

7.9 

2.9 

.28 

.80 

.04 

.09 

.02 

n.d. 

n.d. 

100.

100.

25.84 

1.65 

24.47 

30.81

6.21 

3.15 

1.68 

1.38 

5.08 

2.79 

2.29 

4.19 

.66 

.21 

.05 

.79 

An56 

Ana!!_60

29.0 

38.6

3.5

21.2

7.5

44

67.54 

.50 

14.20 

2.85 

4.59 

.20 

2.13 

2.76 

3.21 

.79 

.88 

.11 

.09 

.01 

.04 

.01 

99.91 

.02 

99.89 

2.79

34.66 

4.67 

27.11 

12.82 

3.39

-

0.94 

5.31 

5.63 

4.14 

.95 

.21 

.02 

2.1

48.3 

31.2

12.7

4.7 

2.8 

.3

45

75.18 

.22 

13.41 

.88 

1.19 

.03 

.66 

3.76 

4.01 

.12 

.23 

.08 

.05 

.01 

.02 

.01 

99.86 

.01 

99.85 

2.70

42.16 

.71 

33.91 

18.18 

.05

2.80 

1.65 

1.15 

1.28 

.42 

.12 

.02 

1.9

48.7 

44.2

2.1

.4 

3.9

.7 

.1

46

67.79 

.68 

13.91 

3.34 

3.20 

.12 

1.99 

2.79 

5.35 

.17 

.31 

.08 

.23 

.02 

.03 

.00 

99.99 

.01 

99.98 

2.76

27.53 

1.01 

44.30 

12.16 

.67

-

7.18 

4.96 

2.22 

4.85 

1.29 

.56 

.05 

3.6 

An22 

An25

33.2 

52.7

1.1

.3 

1.7 

"7.6

3.1 

.2

47

70.39 

.40 

13.08 

3.24 

2.52 

.08 

1.15 

2.70 

4.92 

.07 

.36 

.12 

.12 

.02 

.01 

.01 

99.19 

.00 

99.19 

2.74

33.80 

.41 

41.57 

12.49 

1.14

4.30 

2.86 

1.44 

4.70 

.76 

.28 

.05 

3.3 

An23 

An23

35.9 

58.4

.2

tr 

"6.9

2.0 

.2

48

79.87 

.22 

10.16 

.77 

1.62 

.07 

1.12 

1.84 

3.81 

.06 

.25 

.05 

.03 

.01 

.06 

.01 

99.95 

.03 

99.92 

2.71

51.23 

.36 

32.18 

8.45 

.75

4.90 

2.79 

2.11 

1.12 

.42 

.07 

.02 

3.8 

An.,! 

An25

73.2 

11.0

5.2

3.0 

"7.0

.6

'Partial analysis; SiO,, A1,OS, total iron as Fe2O,, MnO and P2OS by XKF, J. S. Wahlberg, USGS, analyst; MgO, CaO, NajO and K,O by AAS, Violet Merritt, USGS, analyst.

'Due to the absence of FeO determinations in analyses 2, 7, 18, and 31, Fs and Mt=0, and the norms contain, respectively, 3.29, 3.20, 1.42, and 1.37 percent hematite (hm).

"Remainder includes sphene, apatite, zircon, carbonate, and allanite.

4Trace.

"No modal analyses for Nos. 2, 15, 16, 17 20, 26, and 31 because thin sections unavailable.

"Col. 40: cummingtonite; col. 47: anthophyllite-cummingtonite; col. 48: tremolite.
20 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

An

Oceanic plagiogranite 

(Coleman and Donato, 1979)

54 Olivenan core gneisses 

(Leo, unpub. data)

Trondhjemites associated 

with Ammonoosuc 

Volcanics, New Hampshire 

(Leo, unpub. data)

EXPLANATION

A Northern Glastonbury Gneiss

O Southern Glastonbury Gneiss

 Hornblende-bearing southern Glastonbury Gneiss

 Felsic layers of Ammonoosuc Volcanics

O Monson Gneiss

0 Saipan dacite (Barker and others, 1976)

FIGURE 8. Normative An-Ab-Or diagram using rock classification of O'Connor (1965) as modified by Barker (1979).
ANALYTICAL RESULTS 

Q

21

felsic layers of 

Ammonoosuc Volcanics

northern

Glastonbury

Gneiss

Trondhjemites associated 

with Ammonoosuc 

Volcanics, New Hampshire 

(Leo, unpub. data)

i granitic rocks

I (Tuttle and Bowen, 1958)

EXPLANATION

A Northern Glastonbury Gneiss

O Southern Glastonbury Gneiss

 Hornblende-bearing southern Glastonbury Gneiss

FIGURE 9. Normative Q-Ab-Or diagram. The Q-Ab cotectic 

(Ab/An=oo) is shown for PHiO =2 Kb (Tuttle and Bowen, 

1958, p. 75). Dashed line above the Q-Ab cotectic is 

projection (based on von Platen, 1965) of the quartzplagioclase cotectic at Ab/An=3.1, the average normative plagioclase composition of the northern Glaston-

 Felsic layers of Ammonoosuc Volcanics

D Monson Gneiss

 Average graywacke of Pettijohn (1963, table 7, col. A)

0 Saipan dacite (Barker and others, 1976)

bury Gneiss. Field labeled "Granitic rocks" includes most 

of the analyzed rocks containing 80 percent or more 

Q+Ab+Or in Washington's tables (Tuttle and Bowen, 

1958, p. 128, fig. 63). Small field surrounded by short 

dashes near center of diagram is granite minimum of 

Tuttle and Bowen (1958).
22 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

o

DC

w 111 

o

X

20

15

10

Ov

u, 0.2

9, 0.1

Q. i i i i

55 60

n aL

D

on

2 O

CO

O

0.8 

0.4 5

65 70 75 80

EXPLANATION

 Felsic layers of Ammonoosuc 

Volcanics

D Monson Gneiss

SiO2, IN WEIGHT PERCENT

FIGURE 10. Variation diagrams for major elements versus SiO2 . A, are drawn on felsic Ammonoosuc and in B, on southern Glastonbury;

Felsic layers of Ammonoosuc Volcanics and Monson Gneiss; lines are fitted visually. Only 16 southern Glastonbury samples are

B, northern and southern Glastonbury Gneiss. Trend lines in A plotted because of overlap of two points.
ANALYTICAL RESULTS 23

20

CO 

LU 

Q

X

o

15

10

O 

o> u. 2

10

§ 5

0.6

0.2

I I I I

55 60 65 70 75

Si02, IN WEIGHT PERCENT

Figure 10 Continued

CD

10

O

4

3 z

0.8 

0-4 5

80

EXPLANATION

A Northern Glastonbury Gneiss 

O Southern Glastonbury Gneiss

9 Hornblende-bearing southern 

Glastonbury Gneiss

^/ Compositional fields of MonS son Gneiss (from fig. 10 A)
24 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

scattered pockets throughout the protolith. As argued 

below, the low K content of the model protolith (Monson 

lithology) would have limited the amount of melt, at 

geologically reasonable temperatures, to perhaps 20 

percent, producing a crystal mush instead of a mostly 

liquid magma. Such a mush should be capable of moving through the crust (Robertson and Wyllie, 1971). 

The northern Glastonbury Gneiss is thought to reflect 

the inhomogeneous character of this mush, notably 

variable K content, that characterized the partialmelting process.

The southern Glastonbury Gneiss shows the smoothest variation trends that can be extended to include 

the hornblende-bearing marginal phase in the Middle 

Haddam quadrangle (figs. 2 and 10B). These trends 

appear to reflect moderate differentiation of a relatively 

calc-alkaline magma very likely unrelated to the 

northern Glastonbury. K2O values correspond to the 

granite-granodiorite range (4.28 percent average for 14 

hornblende-free samples, 3.66 percent average for 18 

samples including hornblende-bearing rocks). In contrast to the Monson and the Ammonoosuc, Ti and P for 

both northern and southern Glastonbury have relatively smooth variation trends, in accord with a magmatic origin.

TRACE ELEMENTS

Trace elements for Monson Gneiss and Glastonbury 

Gneiss are listed in table 2, but trace elements were 

not analyzed for Ammonoosuc samples. Elements are 

listed along the general lines recommended by Taylor 

and White (1966) on the basis of their geochemical 

association.

TRACE ELEMENTS OTHER THAN RARE EARTHS

Trace elements, even nominally stable ones such as 

Th, Zr, Hf, Co and Cr, show some unsystematic variations. Th, Co, and Sc plotted against SiO2 (fig. 11) display little regularity in the Monson and the northern 

Glastonbury samples, rather, the data points occupy 

irregular but partly overlapping fields. By contrast, 

the southern Glastonbury shows distinctly systematic 

variations of Co and Sc as well as linear variation in 

Th/SiO2 at lower SiO2 values. Similar plots for Zr, Cr, 

and Hf (not shown here) are analogous. The traceelement variations further point up the inhomogeneous 

character of the Monson and the northern Glastonbury, 

as compared to the southern Glastonbury. A plot of 

K vs. Rb/Sr (fig. 12) shows reasonably linear correlations analogous to those of other variation diagrams. 

Two southern Glastonbury points, however, fall far out 

of the field encompassing the remaining points. Inasmuch as these elements are among the most mobile

during alteration or metamorphism, the significance of 

this distribution cannot be assessed.

RARE-EARTH ELEMENTS

Rare-earth element (REE) patterns for the analyzed 

rocks are shown in figure 13. The patterns for the Monson and the northern Glastonbury show major variations 

and consequently are hard to interpret in a coherent 

way. To test the possibility that some of the irregularities could be related to the relatively high margin of 

error (a maximum of 10 percent for any determination) 

inherent in the instrumental neutron-activation technique, all the samples were resubmitted for a second 

run; however, in spite of some relatively minor and systematic variations, the form of the patterns changed 

little. Moreover, the much more uniform patterns for 

the southern Glastonbury Gneiss increase the likelihood that the variations for other samples are real.

Monson Gneiss

The Monson patterns define two more or less distinct 

groups: slightly light-REE-enriched but otherwise 

fairly flat patterns with overall high REE abundances 

and pronounced negative Eu anomalies (solid lines, 

fig. 13A); and much more fractionated, heavy-REEdepleted patterns with negligible Eu anomalies (dashed 

lines, fig. 13A). The first and second groups are generally similar to patterns for low-alumina (< 15 percent 

A12O3) trondhjemites and high-alumina (> 15 percent 

A12O3) trondhjemites, respectively (Barker and others, 

1976). The A12O3 contents of the Monson samples are 

mostly in accord with these categories (fig. 13A). Although the Monson Gneiss is not a trondhjemite in the 

strict sense, its overall composition is trondhjemitictonalitic (fig. 8), and the REE patterns confirm the 

high/low-alumina trondhjemite relationship. The A12O3- 

poor Monson patterns, moreover, are similar to patterns 

for felsic Ammonoosuc Volcanics from New Hampshire 

(Leo, unpub. data).

These two distinctive REE patterns are most readily 

explained by assuming at least two different sources. 

The low-alumina type could have been derived from a 

source in which plagioclase was residual, producing 

the negative Eu anomalies and the relative enrichment 

in heavy REE. The high-alumina type, by contrast, 

could be derived from a source in which hornblende or, 

less likely, garnet was a residual phase. It may be noted 

that the modal analyses of the samples in question 

(table 1) show only small differences in plagioclase content. In any case, the distinct gap in heavy REE contents makes it unlikely that the two Monson types are 

related by differentiation.
ANALYTICAL RESULTS 

TABLE 2. Minor and trace elements in northern and southern Glastonbury Gneiss and Monson Gneiss

25

[Data for all elements except Rb with asterisks and all Sr obtained by instrumental neutron-activation analysis carried out by Louis J. Schwarz under general direction of Jack J. Rowe and 

Philip A. Baedecker. Values are averages of 2 to 3 replicate runs and may be considered accurate to 10 percent. Sr data without asterisks obtained by atomic absorption spectroscopy (limit 

of error approx. 10 percent) by Robin Moore and Violet Merritt. Rb and Sr data with asterisks and Rb-Sr isotopic data (partly repeated in table 4) obtained by D. G. Brookins by isotope 

dilution analysis (see Lee and Brookins, 1978, for details of analytical procedure), n.d., not determined; , (1) result inconclusive, (2) calculation not made because data lacking, or (3) not 

applicable. Field numbers, sample descriptions and locations in Appendix.]

Northern Glastonbury Gneiss

Analysis No.

Rh

Sr

jja _________ 

Cs 

K/Rb 

K/Ba _ 

Rb/Sr 

Ba/Sr

87Sr/"8Sr 

87Rb/86Sr 

Th 

Zr

Hf 

Zr/Hf 

Ta 

Co

La _________ 

Ce 

Nd 

Sm 

TT.i

Gd

Tb

Yh

LU 

La/Yb 

1

19,100. 

83. 

95. 

445. 

2.1 

230. 

42.9 

0.87 

4.7

6.6 

113. 

2.8 

40.3 

0.31 

1.8 

4. 

7.3

15. 

36. 

14. 

2.6 

0.38 

2.0 

0.34 

2.2 

0.32 

6.8

2

17,400. 

76. 

83. 

387. 

2.6 

229. 

45.0 

0.91 

4.7

8.3 

74. 

2.3 

32.2 

0.36 

5.0 

5. 

11.7

19. 

32. 

15. 

3.3 

0.47 

2.3 

0.56 

2.5 

0.38 

7.6

2a 4 5

Large cations

n.d. 4,700. 14,100. 

83.5 20. 48.2* 

80.0* 150. 128.* 

n.d. 150. 388. 

n.d. 1.5 1.3 

- 235. 293. 

- 31.3 36.3 

1.04 0.13 0.38 

- 1.0 3.0 

0.7297 - 0.7124 

3.02 - 1.09

High-valence and ferromagnesian elements

n.d. 1.3 4.1 

n.d. 56. n.d. 

n.d. 1.7 5.4 

n.d. 32.9 43.9 

n.d. <0.20 0.10 

n.d. 1.6 1.6 

n.d. 6. 9. 

n.d. 12.9 12.0

Rare-earth elements

n.d. 5. 14. 

n.d. 11. 25. 

n.d. n.d. 10. 

n.d. 1.6 2.9 

n.d. 0.84 0.80 

n.d. 1.4 2.1 

n.d. 0.19 0.41 

n.d. 1.2 1.9 

n.d. 0.16 0.27 

n.d. 4.2 7.4

7

19,700. 

82.0* 

136.* 

840. 

1.0 

240. 

23.4 

0.60 

6.2 

0.7205 

1.76

13.3 

142. 

3.8 

37.4 

0.45 

4.0 

18. 

11.7

42. 

79. 

30. 

6.5 

1.11 

4.3 

0.77 

3.0 

0.42 

14.0

7a

n.d. 

56.0* 

196.* 

n.d. 

n.d. 

n.d. 

n.d. 

0.29

0.7148 

.83

n.d. 

n.d. 

nid. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

10

9,100. 

52.1* 

281.* 

1,040. 

1.5 

175. 

8.8 

0.19 

3.7 

0.7107 

0.54

13.2 

393. 

7.9 

49.7 

0.30 

2.9 

4. 

4.5

50. 

59. 

20. 

3.2 

1.10 

2.0 

0.23 

1.0 

0.17 

50.
26 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

TABLE 2. Minor and trace elements in northern and southern Glastonbury Gneiss and Monson Gneiss Continued

Southern Glastonbury Gneiss

Analysis No.

 

Rb ________

Sr OI 

BaCs 

K/Rh

K/Ba 

Rb/Sr 

Ba/Sr 

87Sr/86Sr 

87Rb/"6Sr 

Th _________

Zr ______

Hf ______

Zr/Hf 

Ta _________

Co __ 

Cr _

Sc - 

11

29,000.

144.*

m *. 

1,130.

5.5

9H1 &i\JL.

25.7

1.27

10.0

0.7263

3.68

24.3

259.

5.8

44.7

0.41

1.3

1.3

7.4

11A

n.d.

128.*

167.*

n.d.

n.d.

n.d.

n.d.

0.77

_

0.7228

2.22

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

12

29,900.

115.

OCA zou. 

1,360.

14.6

Cl£*f\ 2bU.

22.0

0.46

5.4

High-valence

28.7

304.

7.2

42.2

1.41

5.4

5.4

16.1

ISA

Large cations

n.d.

43.6*

OCA *

^uu. 

n.d.

n.d.

_

0.17

0.7163

1.66

13B

n.d.

168.*

9R4. * £t\J^t

n.d.

n.d.

_

0.64

0.7195

1.18

13C

n.d.

140.*

970 * £t 1 \J.

n.d.

n.d.

_

0.52

0.7191

1.50

13D

n.d.

142.*

91ft £iL\J. *

n.d.

n.d.

_

0.68

0.7185

1.53

18

35,700.

103.*

1QQ *

J.3t7.

1,820.

5.4

347.

19.6

0.74

13.

0.7205

2.14

and ferromagnesian elements

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

15.5

95.

2.6

35.4

0.42

1.6

3.

4.7

Rare-earth elements

La ______

Ce 

Nd 

Sm 

Eu 

Gd 

Tb -

Yb _________

Lu _________

La/Yb 

53.

89.

31.

5.2

0.72

4.0

0.69

3.3

0.55

16.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

__ J

n.d.

n.d.

n.d.

n.d.

53.

88.

35.

6.8

1.07

3.9

n 73 U. i G

2.5

0.42

21.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

38.

57.

17.

3.3

0.52

2.0

ft u.oa ^Q

1.8

0.28

21.
ANALYTICAL RESULTS 

TABLE 2. Minor and trace elements in northern and southern Glastonbury Gneiss and Monson Gneiss Continued

27

Southern Glastonbury Gneiss

ISA

n.d.

119.*

302.*

n.d.

n.d.

_

0.39

_

0.7161

1.14

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

19

39,800.

129.

160.

1,190.

6.0

309.

33.4

0.81

7.4

-

45.3

240.

5.2

46.2

2.0

3.5

10.

8.7

49

n.d.

112.

424.*

n.d.

n.d.

0.26

0.7142

0.77

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

50

n.d.

140.*

254.*

n.d.

n.d.

0.55

0.7193

1.60

High-valence

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

51

Large cations

n.d.

138.*

309.*

n.d.

n.d.

0.45

0.7187

1.29

25

28,200.

93.

300.

1,340.

1.8

303.

21.0

0.31

4.5

n.d.

n.d.

26

21,600.

70.

360.

1,180

7.7

309.

18.3

0.19

3.3

n.d.

n.d.

28

16,600.

60.

110.

1,090.

3.2

277.

15.2

0.55

9.9

n.d.

n.d.

Chondrite 

normalizing 

values1

_

_

_

_

_

-

and ferromagnesian elements

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

14.9

203.

5.6

36.3

0.84

9.7

27.

23.0

11.0

195.

4.5

43.3

0.66

16.7

32.

35.0

9.4

162.

3.9

41.5

0.38

16.7

18.

33.6

_

_

_

_

_

_

-

Rare-earth elements

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

41.

85.

30.

6.3

0.88

4.8

0.55

3.5

0.59

11.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

42.

74.

36.

7.1

1.21

4.3

0.77

2.6

0.40

16.

46.

81.

40.

7.9

1.35

5.4

0.84

2.6

0.38

17.

37.

67.

30.

6.5

1.25

3.8

0.67

2.4

0.37

15.

0.330

0.880

0.600

0.181

0.069

0.249

0.047

0.200

0.034

'Chondrite normalizing values from Haskin and others (1968).
28 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

TABLE 2. Minor and trace elements in northern and southern Glastonbury Gneiss and Monson Gneiss Continued

Monson Gneiss

Analysis No. 30 31 32 33 34 35 36

Chondrite 

normalizing 

37 values1

Large cations

j£ _________

Rb 

Sr

Ra

Cs _________

K/Rb - 

K/Ba 

Rb/Sr 

Ba/Sr 

"Sr^Sr 

87Rb/86Sr 

6,100.

32.

1 en IbU.

OCQ 3OO.

0.6

191.

17.0

0.20

2.2

n.d.

n.d.

13,900.

39.

£io. 98

AQQ too.

0.2

356.

28.5

1.4

17.4

n.d.

n.d.

7,700.

52.

7C1 O.

9<jn

^3U.

0.4

148.

33.5

0.69

3.1

n.d.

n.d.

18,300

70.

enou.

O A O

343.

1.2

261.

53.3

0.88

4.3

n.d.

n.d.

1,800.

4.4

400.

170.

0.1

409.

10.6

0.01

0.43

n.d.

n.d.

4,700.

16.

400.

QCQ obo.

0.8

294.

12.8

0.04

0.92

n.d.

n.d.

5,800.

32.

1 en IbU.

424.

0.4

181.

13.7

0.20

0.65

n.d.

n.d.

4,500.

28.

440 A J.V. 

9CA xiOU.

0.4

161.

18.0

0.06

0.57

n.d.

n.d. -

High-valence and ferromagnesian elements

rrvu In 

Zr 

Uf ill 

Zr/Hf

Ta 

Co 

Cr 

gc _________

La

Ce

Nd

Sm 

EU 

Gd 

1<k _________

Yb _____ _ 

LU 

La/Yb 

9 3 ^.3 

204.

c Q 0.3

OO K. 3O.5 

0.10

3.7

3.

2.9

q3.

.

0.6

0.53

0.6

0.09

0.5

0.09

6.0

6.4

145.

4.5

09 9 &£.£

0.51

0.4

_

11.5

90^3.

46.

^ 97* .

7.8

0.71

6.2

1.49

7.1

1.05

3.2

c 9 O.xi

278.

7

/j .b 

oc c 3D.D

0.27

2.1

10.

9.7

19.LC/. 

OQ3y.

17.

6.0

1.08

5.6

1.10

4.8

0.73

4.0

5

1. i 

165.

4.8

O A A 34.4 

0.35

0.9

8.0

Rare-earth

. 

oc £i<O.

7.

1.7

0.25

1.5

0.28

4.8

0.77

1.9

1.4

72.

1.7

AJ) 0

4t-4.O

0.6

9.8

0.58

elements

11.

19J-i/.

4.

0.9

0.43

0.5

_

4.1

75.

1.4

CO C

O3.D

0.20

8.5

8.6

8.4

1 8

lo. 

093^.

1 ^

13.

2.1

0.52

1.2

0.16

0.5

0.08

36.

7.1

185.

4.7

39 4 OC/.^

0.20

5.4

2.2

q& 

£y. 9Q

. 

1.4

0.53

0.8

0.11

0.5

0.08

18.

1 ^ x.o 

123.

2

0

.0

43 *±O*tfQ

0.12

16.9

8.5

8.3

11.

19AC/.

.

1.7

0.53

1.1

0.16

0.5

0.07

22.

n U.33U 330.

ft U.OOU 88ft 

0.600

0.181

0.069

0.249

0.047

0.200

0.034

'Chondrite normalizing values from Haskin and others (1968)
E

Q. 

Q.

50

40

30

20

10

i i f r

ANALYTICAL RESULTS 

50

29

D

A A

A A

D

9 |Pl A i

55 60 65 70 75

Si02 in weight percent

80

E

Q. 

Q.

40

30

20

10

55 60 65 70 75

SiOj in weight percent

80

co

50

40

30

20

10

EXPLANATION

A Northern Glastonbury Gneiss 

O Southern Glastonbury Gneiss

 Hornblende bearing southern 

Glastonbury Gneiss

D Monson Gneiss

55 60 65 70 75 80 

Si62 in weight percent

FIGURE 11. Variation diagrams for Th, Co, and Sc versus SiO2 for Monson Gneiss and Glastonbury Gneiss. Trend lines are drawn on

southern Glastonbury Gneiss.
30 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

t.u

3.5

3.0

2.5

ILU

o

oc

LU

a.

Io 2'°

LU

Z

:*£

1.5

1.0

0.5

0 

1 ' i ' i ' /r>\ ' / /

/ /

/ /

/ /

/ / x /

/ /

°/ / /

/ i

/ / X / X '

/ /

/ /

/ /

-

X

/* Q'

/ xx

/

/ //

- / /x

/ ^xx

/ /

/ /x

/ /

/_ /x

l*-^

~ /S ^

/ ^\ / \ X^O-N / \ /

/ A/ y

/ ^ ^.-^ /

/ .-'" /

 / -^ /

X --" '

XA ^-^ /

/ ^-' /

/ ^-^ S

~

/ -^ ^-^ ^ - / -^ ^-^ ^.--^ / /x ^-^ ^^-^ / / .^ .-'-

 / X ^^ .-^

1 1

_

_

o

-

_

\

\

\ \

\

\ _

D\

-^-

.-'

/A / ^ ^/ EXPLANATION / / .-^ .-^ / / ^- ^-^

/ / ^^-^ a ^^^^ A Northern Glastonbury Gneiss

/ / x ** x^ 1^

/ / ^- -^ ^^^ O Southern Glastonbury Gneiss

/ L^"r~( -*-~^~^

/~~-"l 1 ^--^^ Hornblende-bearing southern 

/Q] ^-A^ ^--^"^ Glastonbury Gneiss

/ ^-^

/ x- O Monson Gneiss

t X"

/

1 1 1 1 1 1 1

0 0.2 0.4 0.6 0.8 1.0

_

1

1.2 }.'

Rb/Sr

FIGURE 12. Variation diagram for K versus Rb/Sr. Dashed lines enclose most samples of each lithology, but two

points fall far outside their fields.
ANALYTICAL RESULTS 

B

31

200 i 2001 

100

La Ce Sm Eu Gd Tb Yb Lu La Ce Sm Eu Gd Tb Yb Lu

D

200

100

- 18

10

1.0

12

200

100

10

1.0

La Ce Nd Sm Eu Gd Tb Yb Lu La Ce Nd Sm Eu Gd Tb Yb Lu

FIGURE 13. Rare-earth-element patterns. Analysis numbers (tables 

1 and 2) are shown near left-hand margins. A, Monson Gneiss. 

See text for discussion. Values are given for SiOj content (upper 

numbers) and for AljO3 content (lower numbers). B, Northern

Glastonbury Gneiss. C, Hornblende-free southern Glastonbury. D, 

Hornblende-bearing southern Glastonbury. Chondrite normalizing 

values are from Haskin and others (1968).
32 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

The solitary pattern (no. 30, dash-dot line, fig. 13A) 

showing overall REE depletion and a strong positive 

Eu anomaly fits neither of the two foregoing categories 

and thus suggests yet a third source. The pattern suggests a cumulate of K-feldspar or plagioclase, but again 

the modal analysis does not particularly bear this out. 

In the absence of a stratigraphy of the Monson Gneiss 

and of any knowledge of source areas, such hypothetical 

sources must remain speculative. It is presently unknown what further variations in REE patterns might 

be shown by additional samples. It is certain only that 

the Monson patterns cannot be accounted for by a single 

homogeneous source.

Northern Glastonbury Gneiss

Patterns for the northern Glastonbury Gneiss (fig. 

135) are moderately to strongly fractionated with respect to light REE, are flat across the heavy REE, and 

show variable Eu anomalies ranging from moderate 

negative to moderate positive. The lack of consistency 

probably reflects both different sources (see above) and 

inhomogeneity of the partly melted crystal mush prior 

to its solidification. Because of these irregularities, correlation of these patterns with those of the Monson 

Gneiss, the hypothetical protolith, cannot be attempted 

in a rigorous manner. In a general way it can be stated 

that differentiation of high-alumina source rocks (nos. 

35, 36, and 37, fig. 13A) could yield rocks with the 

northern Glastonbury patterns, whereas the already 

highly differentiated Monson (nos. 31, 32, fig. 13A) 

could not. On the other hand, addition of a melt phase 

rich in K-feldspar component to the more differentiated Monson type should result in an overall slight 

depletion in all REE's except Eu, thus producing patterns something like numbers 2 and 5 (fig. 135). Such 

addition also should produce relative enrichment in 

K-feldspar in the resulting northern Glastonbury. 

There is, indeed, some indication of such correlation 

(tables 1 and 2). However, because geological control is 

effectively absent, this is almost tantamount to saying 

that the REE patterns reflect the major-element chemistry. At best, the REE data are ambiguous regarding 

the relationship between the Monson and northern 

Glastonbury Gneisses.

Southern Glastonbury Gneiss

The REE patterns for hornblende-free southern 

Glastonbury Gneiss (fig. 13Q show relatively high 

abundances, strongly fractionated light REE, slightly 

fractionated heavy REE, and negative Eu anomalies. 

Patterns for the hornblende-bearing marginal phase 

(fig. 13Z))are closely similar to patterns of hornblendefree rocks except for a smaller Eu anomaly, a difference that is consistent with the presence of hornblende, 

other things being equal (Arth and Barker, 1976). The 

negative Eu anomalies in all the patterns could be 

related to residual plagioclase; alternately, they could 

be due to predominantly divalent Eu in the magma 

(Nagasawa and Schnetzler, 1971).

The greater uniformity and consistency of these patterns reinforce the idea that the southern Glastonbury 

is a moderately differentiated intrusion with an origin 

distinct from that of the northern Glastonbury or Monson Gneiss.

ISOTOPIC AGES

The age of emplacement of the Glastonbury Gneiss 

has remained problematic despite considerable previous 

efforts to establish a chronology for the rocks of the 

Bronson Hill anticlinorium. Difficulties associated 

with sample selection in a polydeformed terrane make 

some of the earlier work obsolete, and we summarize 

here only the more significant results directly bearing 

on this study. Throughout this paper all radiometric 

ages are given in terms of the decay constants recommended by the International Union of Geological Sciences Subcommission on Geochronology (Steiger and 

Jager, 1977). This adoption of the new constants has 

necessitated the conversion of much of the published 

literature but permits a more straightforward comparison between dating methods.

A minimum age for the Glastonbury is provided by a 

350 m.y. Rb-Sr whole-rock isochron age on folded pegmatite dikes that cut across the gneiss and its adjacent 

country rock in the Middle Haddam and Glastonbury 

quadrangles in Connecticut (Brookins and Methot, 

1971). One can argue from structural evidence to the 

north in central Massachusetts that the Belchertown 

pluton must also postdate the Glastonbury, that is, the 

Belchertown pluton deforms the north end of the Glastonbury dome (Leo and others, 1977). Here U-Pb zircon 

data have been used by Ashwal and others (1979) to 

assign the Belchertown a 380 ±5 m.y. (Middle Devonian) 

intrusion age. A tighter constraint on the younger age 

limit can only be made by inferences based on the state 

of deformation of the gneiss dome. Even so, although 

the Glastonbury would seem to have fully participated 

in the dynamothermal metamorphism of the Acadian 

orogeny fixed at 380 m.y. to 405 m.y. ago (Naylor, 

1971; Lyons and Livingston, 1977), field relationships 

do not unequivocally resolve between an early Acadian 

or a Taconic time of origin for this igneous rock.

As an older age limit, the Glastonbury Gneiss intrudes 

the Ammonoosuc Volcanics, which has been dated at 

450 ±15 m.y. by the Rb-Sr whole-rock isochron method
ISOTOPIC AGES 33

on samples from several localities along the Bronson 

Hill anticlinorium (Brookins, 1968). Brookins and Hurley (1965) earlier had reported a 430 ±15 m.y. age for 

samples of the Middletown Gneiss (Ammonoosuc 

equivalent) from the Middle Haddam and Glastonbury 

quadrangles, but Brookins and Methot (1971) later implied that the Ammonoosuc results should be used for 

the Middletown Gneiss, too. Brookins and Methot 

(1971) also report an Rb-Sr whole-rock isochron age of 

470+15 m.y. for the Monson Gneiss, supporting our 

contention that no significant time gap separates this 

unit from the overlying Ammonoosuc Volcanics. A 

similar conclusion is reached based on new U-Pb zircon 

data included in this paper.

Brief mention should be made of a previously published age of 348 ±10 m.y. for the southern Glastonbury 

Gneiss (Brookins and Hurley, 1965), which was slightly 

revised to 354±10 m.y. by Brookins and Methot (1971). 

The former result is considered suspect primarily because it includes a probable pegmatite-gneiss mixed 

sample (R3372, see table 4). At the Spinelli quarry the 

country rock of the pegmatite has in part been contaminated by reaction between the pegmatite and wall 

rock, the uncontaminated wall rock is a foliated, partially chloritized biotite-quartz-feldspar rock now known 

to be chemically quite different from sample R3372. 

The revised isochron age of Brookins and Methot 

(1971) is still weighted heavily by three samples 

(R4792a, b, and c, see table 4), which are also probably 

not truly representative of southern Glastonbury Gneiss 

as they contain higher Rb/Sr and 87Sr/86Sr ratios than 

other samples included in the present study. Reexamination of the collection site reveals that the three 

anomalous samples were obtained close to granite dikes 

possibly produced by Acadian anatexis.

A major emphasis of this paper is the chemical and 

mineralogical distinction between the northern and 

southern Glastonbury Gneiss, which may imply fundamental differences in modes of origin. Thus, the possibility of different ages for the two parts of the Glastonbury 

also presents itself. In order to examine this possibility 

and to shed additional light on the Monson anatectic 

model, we undertook a further geochronologic investigation by the U-Th-Pb zircon and Rb-Sr whole-rock isochron methods.

The U-Th-Pb zircon analytical procedure is identical 

to that reported recently by Ashwal and others (1979), 

and it need not be repeated here. Difficulty in determining the concentration of thorium mentioned in that 

paper has been resolved, however, and we consider all 

concentration data for U, Th, and Pb in the present 

study to be accurate to ±1 percent (two sigma). Rb-Sr 

analytical methods are described in detail by Lee and 

Brookins (1978).

Representative samples of the northern Glastonbury 

Gneiss (GWL357A), the southern Glastonbury Gneiss 

(GWL368A), and two localities of the Monson Gneiss 

(GWL358A and Pec657) were collected for zircon age 

determination. Petrographic descriptions together with 

location information for these samples are given in the 

Appendix. The U-Th-Pb analytical data determined for 

two size fractions of each sample four size fractions in 

the case of GWL368A are presented in table 3 and are 

shown on a concordia diagram in figure 14. Also included 

in table 3 and figure 14 are data for two size fractions 

of a third sample of Monson Gneiss from near Orange 

in northern Massachusetts taken from Zartman and 

Naylor (in press).

The U-Pb analyses for the Glastonbury display minor 

discordance and, the 207Pb/206Pb ages permit no distinction between the two samples. Commonly employed 

models for interpreting such isotopic systematics would 

lead to a time of intrusion essentially equal to the average 07Ph/M6Pb age of 456 ±10 m.y. The scatter in plotted 

error envelopes on the concordia diagram, however, 

somewhat exceeds that expected solely from analytical 

uncertainty for a precisely colinear array. This scatter 

could indicate a mixed zircon population, in which case 

a more complicated interpretation of the primary crystallization age may be required. Because the average 

207Pb/206Pb age lies near the oldest limit permitted by 

the intrusive relationship of the Glastonbury into the 

Ammonoosuc Volcanics, it is appropriate to consider 

whether a component of older inherited zircon was 

present in the magma at the time of crystallization. In 

this regard the four closely clustered points with 206Pb/ 

238U ages of about 430 m.y. do place a restriction on the 

position of the end members of possible mixed zircon 

populations. If for example, we appeal to an inherited 

zircon as might be contributed by the extensive Proterozoic Z terrain to the south and east (fig. 1), the minimum age of crystallization allowed by a two-component 

mixing line would be about 420 m.y. Without definitive 

evidence either to support or to reject this hypothesis of 

inherited zircon in the Glastonbury, we proceed to consider the Monson Gneiss data, which provide additional 

insight into the chronology of both units.

In general appearance, the U-Pb analyses for the 

Monson closely resemble those of the Glastonbury in 

that they also display only minor discordance with similar ^'Pb/^'Pb ages among the three samples. The 

obvious difference between the two sets of data is that 

the averge 207Pb/206Pb age of 435 ±6 m.y. for the Monson 

is younger than that of the Glastonbury, a situation at 

odds with the field relationships that require the reverse order of emplacement. The possibility mentioned 

in the preceding paragraph of older inherited zircon in 

the Glastonbury remains as one means of resolving this
34 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT 

TABLE 3. Uranium-thorium-lead isotopic ages of zircon from the Glastonbury and Monson Gneisses

[Field numbers, sample descriptions, and locations in Appendix.]

Concentration (ppm)

Isotopic composition of lead 

(atomic percent) Age, in millions of years

Mesh size U Th Pb 4Pb 6Pb 8Pb 6Pb

8U

M7Pb

235JJ

7Pb 8Pb

6Pb

Northern Glastonbury Gneiss [2]

-100+150-

-200+270-

754

978

261

376

54.0

67.0

0.0670

.0873

83.68

82.25

5.671

5.901

10.58

11.76

428

401

432

410

456

463

374

339

Southern Glastonbury Gneiss [51]

-100+150 

-200+270 

-270+325 

-325+400 

539

781

839

1042

289

459

532

663

39.8

57.2

61.6

74.1

.0323

.0118

.0125

.0148

80.70

81.46

81.05

81.14

4.975

4.764

4.724

4.755

14.29

13.76

14.21

14.10

429

431

430

417

431

436

433

422

445

467

453

451

405

374

358

341

Monson Gneiss [31]

-150+200 

-250+325 

-1304 

-1365

396 

370

81.1 

87.8

.0108 

.0043

88.32 

88.76

5.062 

4.996

6.602 

6.240

398 

414

404 

417

434 

436

287 

325

Monson Gneiss [52]

-100+150 

-200+250 

1241

1069

485

368

80.2

70.4

.0030

.0023

86.65

88.01

4.843

4.920

8.501

7.066

407

420

409

422

428

433

314

302

Monson Gneiss from near Orange, Mass.2

-100+150 -

97A_i qoc

-383 

- 455

137 

156

24.3 

30.3

.0079 

.0167

85.29 

84.81

4.861 

4.967

9.843 

10.20

393 

409

399

414

438 

440

381 

419

'Decay constants: XM8U =1.55125xlO-"> yr'1 ; XjMTj=9.8485xlO-10 yr1 ; X2MTb =4.9475x10-" yr1 ; as8U/235U=137.88. Isotopic composition of common lead assumed to be

M4Pb:M8Pb:""Pb:!M8Pb=l:18.20:15.60:38.10. 

2Sampe from Zartman and Naylor (in press).

enigma. However, we are faced with an apparent age of 

the Monson that lies at the youngest limit permitted 

by its stratigraphic position underneath the Ammonoosuc Volcanics. While the Monson data show less scatter 

in "'Fb/^Pb ages than the Glastonbury data, the hint 

of a trend does emerge with the three Monson localities 

becoming progressively younger southward. The age 

discrepancy between the Monson and Glastonbury 

might thus be alternatively explained by preferential 

metamorphic overprinting of the Monson zircons, in 

the form of either episodic disturbance of preexisting 

grains or new growth. The weakness in this hypothesis 

is that the reason for such greater response of the Monson zircons to subsequent metamorphism as compared 

to the Glastonbury is not apparent. Can we simply 

appeal to variations in physiochemical conditions when 

even adjacent localities, such as numbers 2 and 31 (fig.

2), maintain the age distinction? Likewise, different 

susceptibilities related to different uranium contents 

are not obviously supported by the analyses. Again, 

further speculation is not warranted without additional 

work except to point out that zircon with 207Pb/206Pb 

ages considerably older than Ordovician has been found 

at a number of localities flanking the southern Bronson 

Hill anticlinorium (Naylor, 1975; Zartman, unpub. 

data). The survival of such ages makes it unlikely that 

the Monson is much older than indicated by existing 

U-Pb zircon and Rb-Sr whole-rock results.

For our purposes, the anomalous age relationship 

revealed by the present study can, most likely, be resolved by postulating either (1) a small component of 

old inherited zircon in the Glastonbury or (2) preferential Acadian or Alleghanian metamorphic overprinting 

of the zircon in the Monson. Despite the ambiguity that
ISOTOPIC AGES 35

JD 

O_

0.076

0.074 

0.072 

0.070 -

0.068 

Glastonbury Gneiss 

Monson Gneiss

0.066 

0.064 

0.062 

0.58

FIGURE 14. Concordia diagram for zircon from the Glastonbury and Monson Gneisses. Linear regression lines 

constrained to pass through origin are shown for each unit.

exists around making precise age assignments within 

the 470 to 420 m.y. (Middle Ordovician to Early Silurian) interval, we feel confident to interpret the Glastonbury Gneiss as a Taconic intrusion not recognizably 

younger than the mantling strata into which it was 

emplaced.

We turn now to the considerably more complex Rb-Sr 

whole-rock results. The Rb-Sr data for the Glastonbury 

are presented in table 4 and plotted as either northern 

or southern gneiss on an isochron diagram in figure 15. 

We include on the isochron diagram results on normal 

varieties of the gneiss from previous studies (Brookins 

and Hurley, 1965; Brookins and Methot, 1971; Brookins 

1980) and on seven new determinations of material 

especially collected for the present study. Excluded from 

the diagram are the four samples (R3372, R4792a,b,c) 

discussed above that largely controlled earlier attempts 

to date the Glastonbury but probably represent chemically altered or even foreign rock. By inspection, it is

obvious that there is too much scatter even in this selected suite of samples to attempt any age interpretation.

The southern Glastonbury is represented by 19 samples (11, lla, 13a, 13b, 13c, 13d, 18, 18a, 49, 50, 51, 

R1132b, R1132d, R1136a, R1066a, R1066b, R1066c, 

R4998, R4999), for which a York (1966) regression 

yields an apparent age of 309 ±84 m.y. with an initial 

87Sr/86Sr ratio of 0.7108±0.0022. Apart from the dubious worth of attempting a linear regression through 

such scattered data, the resultant age is too low for a 

rock that has been involved only in Acadian deformation and metamorphism.

A regression of the five samples from the northern 

Glastonbury (2a, 5, 7, 7a, and 10) yield an apparent age 

of 536 ±130 m.y. with an initial 87Sr/86Sr ratio of 0.7066 

±0.0034. Clearly, such a result is also uninterpretable 

in terms of an emplacement age because the gneiss 

intrudes the Middle Ordovician Ammonoosuc Volcanics. Although the limited sampling of the northern
36 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT 

TABLE 4. Rubidium-strontium isochron ages of whole rocks from the Glastonbury Gneiss

[Field numbers, sample descriptions and locations of samples 2A to 51 in Appendix.]

Sample No.

Rb 

(ppm)

Sr 

(ppm)

87Rb

6Sr

Age1 

(m.y.)

Northern Glastonbury Gneiss

Oo

g

7 

7a

10

R1 *>

AQ 0

 82.0

KG A

 *>9 1

80.0 

128. 

136. 

196. 

281.

3.02 

1.09 

1.76 

.83

.54

0.7297 ^ 

.7124 

.7205 

.7148 

.7107 .

536 ±130 

0.7066 ±0.0034

Southern Glastonbury Gneiss

11

Ha _____ 

13a 

13b 

13c _______ 

nd

1 Q

Iftfl

49 

50

51 ______ 

R1132b 

Rll32d 

R1136a 

R1066a 

"D -I f\£*G"LR1066c 

R4998 

R4999 _____

144

 128. 

 43.6 

 168. 

 140.

14.9

IftQ

1 1Q

 112. 

 14.0

 138.

 

113. 

167. 

250. 

264. 

270. 

210. 

139. 

302. 

424. 

254. 

309.

3.68 

2.22 

1.66 

1.18 

1.50 

1.53 

2.14 

1.14 

.77 

1.60 

1.29 

1.41 

1.55 

2.90 

1.84 

1.47 

1.77 

1.30 

.56

.7263 - 

.7228 

.7163 

.7195 

.7191 

.7185 

.7205 

.7161 

.7142 

.7193 

.7187 

.7164 

.7178 

.7230 

.7170 

.7159 

.7167 

.7169 

.7095 .,

309 ±84 

0.7108±0.0022

375 ±80 

0.7093 ±0.0020

Questionable Southern Glastonbury Gneiss (see text)

R3372 - 

R4792a 

R4792b 

R4792c27.90

13.26

17.43

6.80

.8458

.7743

.7975

.7440

'Decay constant: Xa=1.42xlO"11 yrl; ages calculated from least squares regression method of York (1966). See graphical representation of these data in figure 15. 

Sample numbers with an R prefix refer to data previously published by Brookins and Hurley, 1965; Brookins and Methot, 1971; and Brookins, 1980. Other samples were especially 

collected for the present study.

Glastonbury makes it difficult to compare the isotopic 

systematics with the more extensively studied southern 

Glastonbury, the data do largely overlap in Rb/Sr ratio 

and strontium isotopic composition. The big difference 

in calculated ages would seem to merely reflect the 

strong control of the most radiogenic samples on the 

regressions.

Finally, a regression involving all 24 samples from both 

varieties of Glastonbury yields an apparent age of 375 

±80 m.y. with an initial "Sr/^Sr ratio of 0.7093 ±0.0020. 

While this age could be accommodated within the geologically imposed constraints, the large uncertainty 

caused by the badly scattered and rather unradiogenic 

data casts serious doubt upon such a combined isochron 

plot as well. Especially in the light of the more precise

U-Pb zircon results, which suggest that the Glastonbury 

was emplaced sometime between 420 and 470 m.y. ago, 

we believe that the Rb-Sr whole-rock systems violate 

one or more of the basic assumptions of the isochron 

method.

An Early Silurian to Middle Ordovician age for the 

Glastonbury translates into an initial 87Sr/86Sr ratio 

range of 0.7046 to 0.7128 if the scatter on the isochron 

diagram is caused solely by variability in initial isotopic 

composition. Although the parent magma may well 

have originated by rapid, high-level generation from a 

diverse upper crustal terrane, this extremely heterogeneous strontium probably would not be available from 

only slightly older island arc rocks, such as represented 

by the immediately adjacent Ammonoosuc Volcanics
DISCUSSION 37

C/5

I 

C/5

0.735

0.730 -

0.725

0.720 

0.715 -

0.710 -^

0.705 -

0.700

EXPLANATION

A Northern Glastonbury Gneiss 

O Southern Glastonbury Gneiss

0.0 1.0 2.0

87Rb/86Sr

3.0 4.0

FIGURE 15. Rb-Sr whole-rock isochron diagram for the Glastonbury Gneiss. Linear regression lines are shown for the

northern (N), southern (S), and composite (NS) gneiss.

and Monson Gneiss. Initial isotopic variability is usually associated with substantially older and, therefore, 

isotopically evolved source material. The identification 

of more ancient basement rock in the vicinity would 

strengthen the case for this interpretation.

The scatter in the data may also result from opensystem conditions superimposed at a later time, and 

this hypothesis is especially attractive where the geology reveals subsequent metamorphism, as along the 

Bronson Hill anticlinorium. Here the local redistribution or even metasomatic addition and (or) removal of 

rubidium and strontium in the rocks during the Acadian orogeny is a distinct possibility. Mounting evidence that southern New England was also involved in 

the late Paleozoic Alleghanian orogeny cannot be ignored in the interpretation of radiometric systematics. 

In fact, a valid objection to the Acadian orogeny as the 

only disturbing event is the short time interval thereby 

implied between the emplacement and the metamorphic ages. If a redistribution of Rb and Sr initiated in 

the Acadian were further accentuated during the Alleghanian, however, the scatter observed on the isochron 

diagram seems entirely plausible.

DISCUSSION

As stated in the introduction, the Glastonbury dome 

is stratigraphically and structurally analogous to the 

Oliverian domes of New Hampshire. The one respect in 

which the Glastonbury dome differs most markedly 

from other Oliverian domes is in the petrologic character and apparent origin of the northern, potassium-poor 

gneiss.

A gray gneiss with relatively low K2O content (2 to 

3.5 percent) forms a border phase to granitic gneiss in 

several domes (for example, Jefferson dome, Leo, unpub. 

data; Lebanon dome, Lyons, 1955; fig. 1). Relatively 

homogeneous trondhjemitic gneiss intrudes Ammonoosuc Volcanics as dikes and small plugs in several Oliverian domes and constitutes the entire core of at least 

three of the small domes in southwestern New Hampshire and adjacent Vermont (Leo and Gromet, 1981
38 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

and Leo, unpub. data). Such trondhjemitic rocks, which 

are very similar to felsic Ammonoosuc Volcanics (Leo, 

unpub. data), may constitute part of a primitive islandarc assemblage of mantle origin. By contrast, the northern Glastonbury Gneiss with its odd, unsystematic 

variations in texture, in bulk composition, and in traceelement abundances; its compositional overlap with 

Monson Gneiss and relative lack of overlap with the 

northern trondhjemites; and field relations that locally 

suggest a transition from plastically deformed Monson 

Gneiss, implies a mode of origin distinct from that of 

the other Oliverian core gneisses. In the present section 

some of the peculiar problems related to the genesis of 

the northern Glastonbury are discussed.

NORTHERN GLASTONBURY GNEISS

The proposed origin of the northern Glastonbury by 

anatexis of a Monson-like lithology is supported by 

field relations, textures, and, to some extent, by majorand trace-element chemistry. Other aspects that must 

be considered include the following: (1) what is known 

about the possible attainment of pressures and temperatures adequate to initiate melting of Monson lithology?, and (2) what does this scenario imply regarding 

heat flow and metamorphism along the southern part of 

the Bronson Hill anticlinorium in Middle Ordovician 

to Early Silurian time? These questions are considered 

in turn below.

ANATEXIS OF MONSON LITHOLOGY

In addressing this question, it should be noted that 

the Monson sequence is not unique as a potential protolith for the northern Glastonbury Gneiss, inasmuch as 

rocks stratigraphically underlying the Monson are 

lithologically comparable. The pre-Monson section in 

southern Connecticut includes the New London Gneiss 

and the Mamacoke Formation (Goldsmith, 1966); in 

the Haddam area, the Monson is underlain by the Haddam Gneiss (Eaton and Rosenfeld, 1972). However, 

there are difficulties in relating these older rocks to the 

northern Glastonbury Gneiss, to wit: (1) such rocks are 

known only in central and southern Connecticut, well 

to the south of the latitude of the northern Glastonbury 

(where the bottom of the Monson section is not exposed); 

and (2) no transition from pre-Monson rocks to Glastonbury has been demonstrated, and existence of such a 

transition can only be speculative.

In the absence of evidence to the contrary, we believe 

it unlikely that the Monson or underlying rocks already 

were recrystallized gneiss prior to Middle Ordovician 

to Early Silurian (that is, Taconic) metamorphism. The 

point is significant because of the water needed for 

anatexis to proceed at geologically reasonable temperatures, in particular in view of the low potassium content of the Monson rocks. The water content of the felsic 

tuffs and volcaniclastic sediments constituting the primary Monson sequence might have been of the order of 

2 to 3 percent, whereas that of the Monson Gneiss 

averages about 0.5 percent (table 1).

The depth of burial of the base of the Monson section 

at the onset of Middle Ordovician metamorphism can 

only be approximated. Apparent thicknesses of the 

units, based on present exposures, are unlikely to be 

equivalent to original thicknesses, in view of changes 

related to original compaction followed by intense deformation including tectonic thinning, notable in the 

Acadian (Robinson and Hall, 1980). In the aggregate, 

these processes probably have led to a net reduction of 

thicknesses. Estimated thicknesses of presently exposed 

sections at the approximate latitudes of Palmer, Mass. 

and Ellington, Conn. (fig. 2), are as follows (data from 

Leo and others, 1977; Geologic Map of Massachusetts, 

in press; and M. H. Pease, Jr., written commun., 1980):

Palmer, Mass. Ellington, Conn.

Monson Gneiss (minimum 

thickness; base not exposed)-

Ammonoosuc Volcanics - 

Partridge Formation -

Northern Glastonbury 

Gneiss -

Total - 

Average 

4.0 km 1.5 km

1.5 km 1.0 km

0.05 km -

3.5 km 9.0 km

9.1 km 11.5km 

10.3 km

If a conservative 20 percent is added to these totals to 

compensate for tectonic and compactional thinning, 

the minimum estimated thickness to the base of the 

exposed section is 12 to 13 km. Such a depth corresponds 

to approximately 3 kbar.

The experimental studies of H. G. F. Winkler and his 

coworkers (summarized in Winkler, 1974) and P. J. 

Wyllie and his coworkers (e.g., Robertson and Wyllie, 

1971) provide much insight on the anatexis of K-poor 

quartzo-feldspathic rocks under varying conditions of 

water pressure. Winkler and his coworkers demonstrated that in a water-saturated system with PH O =P 

total=2kbar, micacecous quartz-plagioclase gneisses 

lacking K-feldspar began to melt below 700 °C (Winkler, 

1974, p. 300). Breakdown of micas (muscovite or biotite) 

contributed an Or component to the melt. Somewhat 

comparable results were reported by Piwinskii and 

Wyllie (1968) working with water-saturated systems 

between 1 kbar and 3 kbar. The rock most closely comparable to Monson Gneiss was a tonalite (their sample 

1213) with 63 percent SiO2 ,12 percent modal quartz, 16 

percent biotite, and 15 percent hornblende. In a 2-kbar 

system this rock began to melt at 730 °C and yielded 8
DISCUSSION 39

percent liquid at 750 °C. The melt fraction is referred 

to as "granitic," but no specific composition is given. 

At 3 kbar, melting began at 700 °C, and 20 percent liquid had formed at 750 °C. A temperature of 750 °C is 

not unreasonable for the upper amphibolite facies, 

although attainment of such a temperature at 13 km 

depth implies a higher than normal heat flow (approximately 60 °C per km).

It appears, then, that anatectic development of the 

northern Glastonbury magma is compatible with P-T 

conditions at the base of the Monson section in Middle 

Ordovician time, assuming an essentially closed and 

water-saturated system with PHZO =P total (type IV 

system of Robertson & Wyllie, 1971). Such a system, 

however, in which available water can range between 

6 and 15 percent, may have only limited application to 

anatexis in the crust where the water content is 

unknown, but likely lower. A possibly more realistic 

model is the type III system of Robertson and Wyllie 

(1971). This system is defined as an assemblage of silicate minerals with or without hydrous phases plus a 

vapor phase but without sufficient water to saturate 

the completely melted assemblage at the existing pressure. Under these conditions, melting, which begins a 

few degrees above the solidus, is limited by the amount 

of available pore water that is dissolved in the melt 

(about 13 percent melt per 1 percent of pore water; Robertson and Wyllie, 1971, p. 270). Subsequent melting 

proceeds along a saturation boundary, defined by the 

amount of water required to saturate the combined 

crystals and liquid at any given temperature. The result 

is a much higher liquidus temperature than that of a 

water-saturated system, especially so in the case of 

K-poor rocks. However, assuming a pore fluid of about 

2 percent and a limited amount of melt formed during 

anatexis (20 to 30 percent), the situation would differ 

little from the water-saturated system (type IV of 

Robertson and Wyllie, 1971). In either case, the melt 

thus formed should be sufficient to produce a crystal 

mush consisting of the K-enriched liquid plus crystals 

which for the rocks under consideration would be dominantly quartz and plagioclase. Such a mush could rise 

in the crust prior to consolidation, provided the liquid 

fraction was either water unsaturated or significantly 

hotter than the solidus temperature, or both (Robertson 

and Wyllie, 1971, p. 271; Winkler, 1974, p. 306-307). 

Assuming incomplete homogenization of the mush, the 

resulting intrusive rock could be expected to resemble 

the northern Glastonbury Gneiss prior to its subsequent 

recrystallization in Acadian time.

MIDDLE ORDOVICIAN TO EARLY SILURIAN 

HEAT FLOW AND METAMORPHISM

Temperatures in the range of 700 to 750 °C at a depth

of 12 to 13 km imply a thermal gradient of the order of 

60°C/km. Such a thermal gradient is considerably 

higher than average values (20 to 30 °C/km), but might 

be expected during a regime of active subduction with 

associated island-arc plutonism and volcanism such as 

is now considered to characterize the early development 

of the Bronson Hill anticlinorium (see below).

Devonian (Acadian) metamorphism of this general 

intensity has been widely documented. Progressive 

regional metamorphism in southwestern New Hampshire, summarized by Turner (1968, fig. 8.4 and p. 362), 

involved temperatures in excess of 700 °C at pressures 

of about 3 kbar. Similar temperatures and somewhat 

higher pressures (> 5 kbar) have been documented in 

the Quabbin reservoir area of central Massachusetts 

(Tracy and others, 1976; Tracy, 1978) and in southeastern Connecticut (Lundgren, 1966). Localized to widespread anatexis has occurred in each of these areas.

Direct evidence of high-grade Middle Ordovician to 

Early Silurian (Taconic) metamorphism is harder to 

find, in large part because such evidence has been 

mostly obliterated by Acadian metamorphism. Nevertheless, occasional relics of sillimanite-orthoclase or 

sillimanite-muscovite grade found within kyanite-grade 

rocks may bear evidence of a Taconic metamorphism 

(Leo and others, 1977; Tracy and Robinson, 1980). 

Sillimanite-grade metamorphism of Taconic age in 

northern Vermont has been confirmed by 40Ar/39Ar dating of muscovite (Lanphere and Albee, 1974).

The discussion thus far has centered on an anatectic 

model of origin for the northern Glastonbury Gneiss, 

but some alternative hypotheses must be considered. 

One possibility is that the gneiss crystallized from a 

trondhjemitic magma derived by partial melting or 

fractional crystallization of basaltic source rocks in the 

oceanic crust or the upper mantle. Such an origin has 

the advantage of being less circumscribed by limitations on pressure and temperature. Trondhjemites, 

tonalites, and genetically related low-K dacites have 

received much study in recent years (Barker, 1979), 

and the distinctive mode of genesis of these rocks is by 

now fairly well understood. Trondhjemites as parts of 

island-arc assemblages have been documented by 

Barker and others, 1976; Payne and Strong, 1979; 

Barker and others, 1979; Bryan, 1979; Gill and Stork, 

1979. The northern Glastonbury Gneiss would appear 

to fit into this context, inasmuch as trondhjemite in the 

northern Oliverian domes is regarded as part of a 

bimodal Ammonoosuc assemblage with some characteristics of an oceanic setting and others developed in a 

complex volcanic arc in a continental-margin environment (Leo and Gromet, 1981; Leo, unpub. data).

The principal arguments against considering the 

northern Glastonbury of deep-seated magmatic origin
40 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

are based on the same characteristics that seem to support the crystal-mush hypothesis: the unsystematic 

variations in major- and trace-element abundances are 

REE patterns, plus the relatively high K and Rb contents compared to most mantle-derived trondhjemites. 

These features are incompatible with an origin involving separation of an essentially homogeneous and 

largely liquid magma from a mantle source. The trondhjemites of New Hampshire are distinct in this regard 

from the northern Glastonbury in that they appear to 

be more homogeneous, are consistently lower in K and 

Rb, and have more uniform REE patterns (Leo, unpub. 

data).

Admittedly, these criteria are somewhat subjective 

as a basis for a major genetic distinction. Moreover, the 

possibility cannot be ruled out that some of the chemical 

variation in the northern Glastonbury may be due to 

post-intrusion alteration and (or) mobilization of unstable cations. A potentially more definitive criterion 

for a mantle origin could be provided by 87Sr/86Sr initial 

ratios. Bona fide trondhjemites consistently show low 

initial ratios characteristic of most mantle-derived 

rocks; although Sr evolution through geologic time 

gradually raises the 87Sr/86Sr0, trondhjemites of early 

to middle Paleozoic age should show initial ratios of 

less than about 0.704 (Peterman, 1979). Unfortunately, 

the Sr-isotopic data for the Glastonbury Gneiss are too 

ambiguous to permit a meaningful evaluation of the 

initial strontium composition.

Collins (1954) proposed a metasomatic origin involving large-scale conversion of Ammonoosuc Volcanics 

and the overlying Silurian-Devonian sedimentary 

rocks in the Ellington quadrangle by "fluids" bearing 

silica and alkalies. However, our own field observations, supported by recent work in the Ellington quadrangle (M. H. Pease, oral commun., 1978) and in the 

Hampden quadrangle to the north (Peper, 1977; fig. 2), 

lend little support to this idea. In particular, large-scale 

metasomatism of the kind envisaged by Collins, which 

reflected then-current ideas of granite petrogenesis, is 

incompatible with the intrusive character of the gneiss, 

its lack of gradation to adjacent rocks, and the total 

absence of relict compositional layering.

SOUTHERN GLASTONBURY GNEISS

The southern Glastonbury Gneiss, with its relatively 

smooth variation trends and uniform REE patterns 

(figs. 10-13) and associated mafic border phase, appears 

to be a relatively uncomplicated calc-alkaline granite 

intrusion. The only incongruous aspect of the southern 

gneiss is the notable variation in grain size and textures 

throughout the pluton (fig. 7). This is further discussed 

below.

The compositional fields of the southern and northern 

Glastonbury, although rather broad, show no appreciable overlap (fig. 6). Considering the similar ages of the 

two gneiss bodies it must be concluded that the calcalkaline magma that crystallized to form the southern 

Glastonbury was generated concurrently but spatially 

separate from the crystal mush thought to have produced the northern Glastonbury.

The source of protolith for the southern Glastonbury 

Gneiss is even more speculative than in the case of the 

northern gneiss. The composition of the southern gneiss 

virtually precludes its being derived from a Monson 

lithology, unless the latter grades to more calc-alkaline 

compositions towards the south. However, there is no 

indication that this is the case; rather, Monson Gneiss 

and underlying units continue as K-poor, volcanigenic 

gneisses along the length of the Bronson Hill anticlinorium and into southern Connecticut (Herz, 1955; Goldsmith, 1966; Snyder, 1970; Lundgren and others, 1971).

The most plausible kind of calc-alkaline source for 

the southern Glastonbury would be granite of preMiddle Ordovician age, that is, part of the basement 

upon which the Monson assemblage was deposited. 

K-rich metasedimentary rocks could also have been 

involved. Granites that are potential source rocks are 

exemplified by the Sterling Plutonic Group which appears to be older than the Monson although its exact 

age is unknown (Goldsmith, 1966; Dixon and Lundgren, 

1968). The nearest mapped exposures of these granitic 

rocks are in the core and on the west flank of the Willimantic dome (Goldsmith, 1963), some 15 to 20 km east 

of the southern part of the Glastonbury dome (fig. 1). Although the region is structurally complex and shows no 

evidence of older granite at depth beneath the Glastonbury dome, it appears likely that suitable rocks were 

available. Regarding the conditions of melting of such 

source rocks, it may be assumed that, relative to the 

Monson sequence, they were (1) more potassic, (2) 

deeper seated (a pressure range of perhaps 3 to 5 kbar), 

and (3) crystalline and not unconsolidated like the Monson sequence, hence containing less water. The first 

two conditions would lower the temperature at which a 

given amount of melt would form, whereas the third 

would raise it. Experimental evidence (summarized by 

Wyllie, 1979, p. 511-513) suggests that in an undersaturated granite system at 3 to 5 kbar and 700 to 

750 °C, anatectic melting would proceed quite readily.

PLATE-TECTONIC SETTING OF THE GLASTONBURY DOME

Recent attempts to place early to middle Paleozoic 

development of the northern Appalachians in a platetectonic framework consider the Bronson Hill anticlinorium the locus of a Middle Ordovician island arc
REFERENCES 41

over a subduction zone. In the earliest comprehensive 

regional analysis (Bird and Dewey, 1970), the subduction zone was considered to dip westward. More recent 

models (Osberg, 1978; Robinson and Hall, 1980) involve 

an eastward-dipping subduction zone, which more logically fits the distribution of lithologic and tectonic features and which is also in accord with more extensively 

documented plate-tectonic regimes in Newfoundland 

(Strong and others, 1974).

Tectonic and geometric considerations aside, the 

rationale for the island-arc model is found in the widely 

distributed Ammonoosuc Volcanics and the associated 

plutons (Oliverian core gneisses). Recent work on this 

assemblage in New Hampshire and northern Massachusetts (Aleinikoff, 1977; Leo, 1980a,b, and unpub. 

data) indicates that the mafic component of the Ammonoosuc is low-K, oceanic tholeiite, and contains negligible andesite. Thus the Ammonoosuc has moderate 

analogs in relatively primitive, essentially oceanic arcs 

characteristic of the early period of arc construction 

(Kuno, 1966). The source of the felsic Ammonoosuc, 

mostly K-poor keratophyres, is less clear. Available 

data suggest that these rocks were not derived by differentiation of the tholeiites but reflect a different 

(mantle?) source. The same is presumably true of the 

trondhjemitic plutons of southwestern New Hampshire 

and trondhjemite associated with Ammonoosuc Volcanics elsewhere along the Bronson Hill anticlinorium. 

The calc-alkaline Oliverian plutons, of which the southern Glastonbury Gneiss is reasonably representative, 

show evidence of being derived from sialic crust. A 

problem as yet incompletely understood is the apparent 

juxtaposition of oceanic and continental crust, with virtually simultaneous generation of magma from both 

sources, in Middle Ordovician time (Leo, 1980a).

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APPENDIX

Sample field numbers, descriptions, and locations for tables 1-4 

[Except as noted on table 4; analysis no. as used in tables 1-4. See figure 2.]

NORTHERN GLASTONBURY GNEISS

[Inasmuch as Glastonbury Gneiss is invariably quartzo-feldspathic and contains biotite, 

lesser muscovite, and epidote, these features are not repeated in the remaining descriptions. 

Specimens of Monson Gneiss and felsic layers of Ammonoosuc Volcanics likewise are 

quartzo-feldspathic. Other distinguishing features of these rocks are included in descriptions 

as appropriate.]

1. 73-GWL-34-1. Lineated, faintly foliated quartzofeldspathic gneiss with blotchy, elongated biotite-epidote 

aggregates; scattered small garnets. Chilson Road, 

65 m south of intersection with Three Rivers Road, 

Ludlow quadrangle.

2. 74-GWL-357-1. Equigranular gneiss with crenulated 

foliation. Large cut on Massachusetts Turnpike (1-90) 

just east of Kelly Hill Street overpass, about 3 km west 

of center of Palmer, Palmer quadrangle.

2A. 74-GWL-357-2. Similar to no. 2 except somewhat less 

foliated and richer in epidote. Same location as no. 2.

3. 71-GWL-35-1/2. Similar to no. 1, garnet free. Southwest 

corner of Pulpit Rock Pond, southeast corner of Ludlow 

quadrangle.

4. 367. Inequigranular, nearly unfoliated felsic gneiss. About 

1 km northeast of Hampden, Hampden quadrangle.

5. 620. Fine-grained, pin-striped, and delicately foliated 

gneiss. East side of Chapin Road, 0.8 km east southeast 

of summit of Pine Mountain, east-central part of Hampden quadrangle.

6. H7-A. Fine-grained, pin-striped gneiss. East side of Crow 

Hill, 2.5 km north northeast of West Stafford, Stafford 

Springs quadrangle.

7. 74-GWL-361. Fine-grained, delicately foliated gneiss; 

center of eastern gneiss body. Hillock south of airstrip, 

1.7 km east southeast of West Stafford, Stafford Springs 

quadrangle.

7A. 74-GWL-361-2. Generally similar to, and from same 

location as no. 7.

8. P9-32. Delicately foliated gneiss with scattered garnets. 

Knob at 209 m level, east side of unnamed ridge, 0.7 km 

north northeast of north end of Bradway Pond and 

1.3 km southeast of West Stafford.
44 GLASTONBURY GNEISS AND MANTLING ROCKS, MASSACHUSETTS AND CONNECTICUT

9. P9-26. Fine- to medium-grained, well-foliated gneiss. 

Quarry on south side of Cooper Road, 1.5 km southeast 

of West Stafford.

10. 73-GWL-329-2. Strongly lineated, micaceous gneiss, 

somewhat darker than average. Outcrops on north side 

of Shenipsit Lake Road about 3 km east of Ellington, 

Conn.

SOUTHERN GLASTONBURY GNEISS

11. 73-GWL-330. Medium-grained, strongly lineated and 

weakly foliated rock with blotchy biotite aggregates 

and porphyroblasts of quartz and K-feldspar. Roadcut 

north side of Connecticut Route 15-Route 1-86, 1.5 km 

south of Rockville, Rockville quadrangle.

11A. 73-GWL-330-3. Generally similar to no. 11. Location 

50 m west along roadcut from no. 11.

12. 73-GWL-331-1. Rather massive gneiss with crenulated 

foliation, prominent biotite and K-feldspar porphyroblasts up to 1.5 cm long. Near east end of roadcut, north 

side of Connecticut Route 15 (1-84) directly east of Wyllys Street overpass and approximately 0.8 km east of 

Highland Street exit, Rockville quadrangle.

13. 73-GWL-331-2. Somewhat more mafic appearing gneiss 

near center of same roadcut as no. 12. About 160 m west 

of no. 12.

13A-13D. 73-GWL-331-3 to 73-GWL-331-6. From same location 

as no. 13, samples showing a range of slight compositional variations, but all generally similar to nos. 12 

and 13.

14. 2309. 1.2 km southwest of Birch Mountain, northwest 

corner of Marlborough quadrangle. Descriptive data not 

available.2

15. 2392. 0.8 km east of Buckingham Reservoir, northwest 

part of Marlborough quadrangle. Descriptive data not 

available.2

16. 2379. "Schistose facies" of Herz (1955); 0.6 km southwest 

of Buckingham village, Glastonbury quadrangle. Descriptive data not available.2

17. 2424. Northwest side of Goodale Hill Road, 0.6 km southwest of Diamond Lake, west edge of Marlborough quadrangle. Descriptive data not available.2

18. 74-GWL-359. Inequigranular, poorly foliated, quartzmicrocline-plagioclase-biotite gneiss. Washed outcrop 

at site of Hebron Ave. gravel pit, about 0.5 km south of 

Connecticut Route 94, west edge of Marlborough 

quadrangle.

ISA. 74-GWL-359-2. Generally similar sample as no. 18, 

same location.

'Collected by G. L. Snyder.

19. 73-GWL-334. Light-gray, fine-grained, delicately laminated gneiss; "flaser facies" of Herz (1955). Goodale 

Hill Road, about 1.6 km east of East Glastonbury, Glastonbury quadrangle.

20. 796B. "Eastern border facies" of Herz (1955); roadcut on 

Connecticut Route 2, eastbound (south) side, about 

0.2 km west of Hollow Brook crossing, southeast part of 

Glastonbury quadrangle.3

21. 1680. Biotite-epidote granodiorite gneiss; 0.5 km east 

northeast of intersection of Thompson Hill and Cotton 

Hill Roads, north-central part of Middle Haddam 

quadrangle.3

22. 1702. Biotite granodiorite gneiss; 0.3 km northeast of 

Raccoon Hill, northeast part of Middle Haddam 

quadrangle.3

23. 1704. Biotite granodiorite gneiss; 0.3 km east of Raccoon 

Hill, 0.5 km south southwest of preceding location.3

24. 1594. Biotite granite gneiss; quarry, east slope of Larson 

Hill, 0.5 km southeast of intersection of Stewart Hill 

and Great Hill Roads, central part of Middle Haddam 

quadrangle.3

25. 1728. Fine-grained, moderately foliated quartz-plagioclaseK-felspar-biotite-hornblende-epidote gneiss. 0.4 km 

east of South Road and 1 km north northeast of intersection of South Road and Cox's Road, north-central 

part of Middle Haddam quadrangle.3

26. 804. Gneiss similar to no. 25, but with better foliation 

and somewhat higher color index. Slope west of South 

Road, approx. 310-ft level, about 0.5 km west of no. 25.3

27. 809. Generally similar rock to no. 26. Ridge 0.3 km west 

southwest of no. 26.3

28. 1716. Fine-grained gneiss with crenulated foliation 

generally similar to nos. 26 and 27. 0.5 km east of summit of Strickland Hill and 0.3 km north of Cox's Road, 

about 250-ft level, Middle Haddam quadrangle.

49. 74-GWL-367-3. Fine- to medium-grained, gray to pink, 

slightly porphyritic, biotite-rich gneiss. South side of 

Connecticut Route 2, approx. 5 km southeast of Glastonbury, point where powerline crosses road, Glastonbury quadrangle.

50. 74-GWL-368-2. Gray porphyritic gneiss with ovoid, 

abraded K-feldspar porphyroblasts. West side of Connecticut Route 2, 1.0 km southeast of no. 49, Glastonbury quadrangle.

51. 74-GWL-371-2. Fine- and even-grained, faintly foliated 

gneissic granite (fig. ID). Tower Hill quarry, 300m 

south of New London Turnpike, approx. 5 km southeast 

of Glastonbury, Glastonbury quadrangle.

'Collected by G. P. Eaton.
APPENDIX 45

MONSON GNEISS

29. 212. Delicately foliated and crenuljated gneiss with cataclastic texture. Large overhanging outcrop 0.75 km 

south southwest of summit of Pattaquattic Hill, northeast part of Palmer quadrangle.

30. 768. More homogeneous and evenly foliated gneiss than 

no. 24, base of cliff east of jeep trail, 0.6 km northeast of 

intersection of Warren and Gates Streets, central part 

of Palmer quadrangle.

31. 74-GWL-358-1. Weakly foliated and compositionally 

laminated felsic gneiss with blotchy mica aggregates on 

foliation plane. North side of cut on Massachusetts 

Turnpike (1-90) just east of Breckenridge St. overpass, 

2.5 km north northeast of center of Palmer, Palmer 

quadrangle.

32. 807. Granular, sugary-textured rock with scattered feldspar megacrysts; possible relict tuffaceous textures. 

West slope of small hill about 0.4 km north northwest of 

intersection of Smith and Mason Streets, 0.7 km west of 

Thompson Lake, east-central part of Palmer quadrangle.

33. M-CC. Fine-grained, delicately foliated gneiss. Roadcut, 

west side of access road to Children's Colony, Monson 

State Hospital, 0.9 km south southwest of intersection 

of Hospital and upper Palmer Roads, Palmer quadrangle.

34. A-14. Very leucocratic, sugary-textured, faintly foliated 

rock. Flynt quarry, east side of upper Palmer Road, 

about 2.0 km north northwest of center of Monson, 

Monson quadrangle.

35. MQ-2. Relatively mafic, even-grained, and nearly homogeneous rock. Same locality as no. 34.

36. P8-270. Fine- to medium-grained, well-foliated gneiss. 

Headwaters of Bonemill Brook, 0.8 km southeast of Tolland Ave., Stafford Springs quadrangle.

37. P8-272. Finely foliated, hornblende-bearing 

100 m upstream from no. 36.

gneiss,

52. (Table 3 only). Pec 657. Homogeneous, fine-grained gneiss 

with moderate foliation and faint compositional banding. South exit ramps from 1-86 to Connecticut Route 

195; northwest corner of South Coventry quadrangle.

FELSIC LAYERS OF AMMONOOSUC VOLCANICS

38. 71-GWL-17-1. Fine-grained, sugary-textured, leucocratic 

granofels, 0.6 km northeast of intersection of Glendale 

and Ridge Roads, about 2 km east southeast of North 

Wilbraham, Ludlow quadrangle.

39. 71-GWL-35-4/6. Fine-grained, finely laminated biotitic 

granofels. South side of Massachusetts Turnpike, 1.1 km 

east of Chicopee River, Ludlow quadrangle.

40. 71-GWL-43-1/4. 2 cm felsic layer interbedded with 

hornblende-plagioclase amphibolite. 0.4 km east of fire 

lookout tower, Minechoag Mountain, approx. 490-ft elevation, Ludlow quadrangle.

41. 71-GWL-41-4/1. Finely foliated, crenulated hornblendebiotite-garnet-bearing gneiss. 30 m east of Glendale 

Road-Crane Hill Road intersection, 2.6 km southeast of 

North Wilbraham, Ludlow quadrangle.

42. 71-GWL-41-4/3. Gneiss generally similar to no. 41. 

About 10 m north of Glendale Road-Crane Hill Road 

intersection.

43. 357. Fine-grained, closely foliated, hornblende-bearing 

gneiss. Same unit as nos. 41 and 42.

44. 71-GWL-45-2/4. Medium-grained, garnetiferous grano 

fels interlayered with amphibolite. Peak west of Ridge 

Road, 0.4 km southwest of Crane Hill Road-Glendale 

Road intersection.

45. 71-GWL-46-4/1. Fine-grained, leucocratic, speckled 

granofels associated with amphibolite. Eastern slope of 

small peak north of no. 44,0.2 km northwest of GlendaleCrane Hill Road intersection.

46. 71-GWL-43-1/1. Fine-grained, thin-bedded, gray-brown 

cummingtonite-hornblende-bearing granofels. Same as 

no. 40.

47. 71-GWL-66-2/4. Felsic layer in Ammonoosuc generally 

similar to no. 46, but contains cummingtoniteanthophyllite. About 300 m east southeast from fire 

lookout tower, Minechoag Mountain, Ludlow 

quadrangle.

48. H7-B. Fine-grained,sugary-textured, finely striped 

leucocratic tremolite-actinolite-bearing granofels. Just 

west of no. 6, Stafford Springs quadrangle.