Tectonic Studies in the Brazilian Shield

The Serra de Gakfes Window, Goias

Precambrian Plate Tectonics in the Braziliari Shield 

Evidence from the Pre-Mirias Rocte

of the Quadrilatero Ferrifero, Minas Gerafe

GEOLOGICAL SURVEY PROFESSIONAL PAPER lli9-A,B

m cooperation witfythe 

DtiparMmenio Nacional de Agfias§ 

JOeffflrtamento N^monalde^Produgao Miner(d^ afiA 

CompanMa de P^fqitisa de fiecursoss Minerals oj, Braml
Tectonic Studies in the Brazilian Shield

The Serra de Galdas Window, Goias

By Avery Ala Drake, Jr.

Precambrian Plate Tectonics in the Brazilian Shield 

Evidence from the Pre-Minas Rocks

of the Quadrilatero Ferrifero, Minas Gerais

By Avery Ala Drake, Jr., and Benjamin A. Morgan

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1119-A, B

Prepared in cooperation with the 

Departamento Nacional de Aguas e Energia Eletrica, 

Departamento Nacional de Produqao Mineral, and the 

Companhia de Pesquisa de Recursos Minerals of Brazil

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Tectonic studies in the Brazilian shield.

(Geological Survey professional paper ; 1119)

Includes bibliographical references.

CONTENTS: Drake, A. A., Jr. The Serra de Caldas window, Goias.—Drake, A. A., and Morgan, B. A. Precambrian plate tectonics in the Brazilian shield.

1. Geology—Brazil—Goias (State) 2. Geology, Stratigraphic—Pre-Cambrian. 3. Geology—Brazil—Quadilatero 

Ferrifero. I. Drake, Avery, Ala, Jr., 1927- The Serra de Caldas window, Goias. 1979. II. Drake, Avery, 

Ala., Jr., 1927- Precambrian plate tectonics in the Brazilian shield. 1979. III. Morgan, Benjamin A. IV. 

Series: United States. Geological Survey. Professional paper ; 1119.

QE235.T42 551.8'0981'7 78-31617

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03205-8
The Serra de Caldas Window, Goias

By Avery Ala Drake, Jr.

TECTONIC STUDIES IN THE BRAZILIAN SHIELD

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1119-A

Prepared in cooperation with the 

Departamento Nacional de Aquas e Energia Electrica, 

Departamento Nacional de Produfdo Mineral, and the 

Companhia de Pesquisa de Recursos Minerals of Brazil

Precambrian oceanic rocks, including ophiolitic 

melange, were obducted onto shelf deposits on the 

west flank of the Sdo Francisco craton during the 

Eocambrian or earliest Paleozoic time

CONTENTS

Page

Abstract ___________________________________________ Al

Introduction __________________________________ 1

Regional geology _________________________________________ 1

Geology of th$ Serra de Caldas area _____________________________ 3

Araxa Group ________________________________________ 3

Canastra Group _____________________________________ 3

Bambui Group(?) _____________________________________ 3

Structural geology _____________________________________ 5

The window _________________________________ 6

The east frame ______________________________________ 6

The west frame ______________________________________ 8

Tectonic interpretation ________________________________ 8

Regional implications _________________________________ 10

References cited _______________________________________ 11

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of part of Goias and adjacent areas _________—__—————_——_—_—— A2

2. Generalized geologic map of the Serra de Caldas area_.____— ——— _—_———————————————————— 4

3. Photographs of Araxa Group ________________.—___ — — ___ —— — — __ — — — _ — __ — - — _— 5

4. Photographs of Bambui Group(?) ___________._________________————____—————— 6

5. Geologic map and sections, areas A and B, east flank, Serra de Caldas —_________________ — _— 7

6. Equal-area plots of small fold axes in rocks of the Bambui (?) and Araxa Groups, east flank, Serra

de Caldas __________________________—__________________________-__ 8 

7, 8. Geologic maps and sections, west flank, Serra de Caldas—

7. Areas C and D _____________________. —_——_________ —— _————___——————_ 9

8. Areas E and F _____________________.—______________________________ 10 

9. Equal-area plots of small fold axes in rocks of Bambuf, Canastra(?), and Araxa Groups, west flank,

Serra de Caldas ___________________._ —— _ — _ — _—__ — _ — _______ — _ — ____ ——— — 11

ill

TECTONIC STUDIES IN THE BRAZILIAN SHIELD

THE SERRA DE CALDAS WINDOW, GOIAS

By AVERY ALA DRAKE, JR.

ABSTRACT

The Serra de Caldas is a large topographic and structural 

high in southern Goias. The Serra is underlain by shelf 

deposits of quartzite, siltstone, and lesser shale that have been 

weakly metamorphosed and deformed into upright concentric 

folds. These rocks are tentatively correlated with the Bambui 

Group of Eocambrian age.

The shelf deposits are surrounded on all sides by higher 

grade polyphase deformed schist and micaceous quartzite 

(quartz-grain turbidite) of the Araxa and Canastra Groups 

of Precambrian age. The Araxa contains abundant bodies of 

ultramafic and related mafic rocks, discontinuous bodies of 

marble, and blocks of quartzofeldspathic material and can 

be considered an ophiolitic melange.

The contact between these different rocks suites is a tectonic and metamorphic discontinuity, marked by a zone of 

cataclastically deformed rock. These relations clearly show 

that the rocks are in fault contact and that the Serra is an 

antiformal window. Transport of the thrust sheet was probably from west to east, perhaps for a distance of as much 

as 200 km.

The thrusting of ophiolitic melange onto shelf deposits is 

of great importance because such occurrences commonly are 

the result of the obduction of oceanic rocks across a continental margin. The zone of thrusting described herein 

marks the western margin of the Sao Francisco craton. This 

obduction must date from the Eocambrian or earliest 

Paleozoic.

INTRODUCTION

The Serra de Caldas is a large topographic and 

structural dome or brachyanticline in southern Goias 

about 130 km south-southeast of Goiania, the capital 

of that State (fig. 1). The Serra and adjacent area 

served as a field-training area for a course in advanced geologic techniques for experienced geologists of the Companhia de Pesquisa de Recursos 

(CPRM) and the Departamento Nacional da Produgao Mineral (DNPM). This course, given in the 

austral winter of 1973, was taught jointly by Professor V. J. Hurst of the University of Georgia and 

me. While doing reconnaissance in the area to prepare field-mapping exercises in advanced structural

geology, I found evidence suggesting that the Serra 

de Caldas "dome" was a tectonic window. Subsequent detailed mapping by my students confirmed 

this idea. The purpose of this paper is to present 

the evidence for the window and to point out the 

regional implications of the structure.

I am indebted to my teaching colleague, V. J. 

Hurst, for field observations and for sharpening the 

talents of our students. Thanks are also due Hurst, 

as well as C. H. Thorman, Jr., of the U.S. Geological Survey for the photographs used herein, as 

I was plagued by the complete malfunction of two 

cameras during the course. Finally, I am in debt to 

my 22 students for their mapping and interest in 

this research. Special mention must be made of 

Lauri Bez, then of DNPM, and Gilberto Schudeller 

Pena, then of CPRM, for taking care of the logistical details during the course.

REGIONAL GEOLOGY

The area discussed herein is within the Araxa 

folded belt, which lies between the miogeoclinal deposits of the Paraguay-Araguria belt to the west 

and similar rocks of the Braziliano folded belt to 

the east (Almeida and others, 1973). This belt is 

underlain by the Araxa Group (Barbosa, 1955), a 

eugeosyclinal sequence of pelite, semipelite, psammite, arid minor carbonate rock of 900-1,300-m.y. 

(million year) age (Brazil Dept. Nac. Produgao Mineral, 1971). The Araxa contains much ultramafic 

and related mafic rock in both relatively small Alpine-type bodies and large complexes of gabbropyroxenite-peridotite (Berbert, 1970).

The Araxa is overlain to the east by the Canastra Group (Barbosa, 1955), which contains abundant quartzite interbedded with pelitic phyllite. This 

group has a flyschlike character and is considered 

to be 620-900 m.y. old (Brazil Dept. Nac. Produ<jao Mineral, 1971). These rocks pass upward into

Al
TECTONIC STUDIES IN THE BRAZILIAN SHIELD

EXPLANATION

250 KILOMETERS

Ultramafic and associated rocks. 

Includes Alpine-type material in 

Araxa ophiolitic melange as well

Older, granulite facies rocks as larger bodies of possible 

different age

Contact

Thrust fault 

Sawteeth on upper plate

FIGURE 1. Generalized geologic map of part of Goias and adjacent areas (modified from Brazil Dept. Nac. Produgao

Mineral, 1971).
THE SERRA DE CALDAS WINDOW, GOIAS A3

the molasse and shelf deposits of the Bambui Group 

of Eocambrian age (Brazil Dept. Nac. Producao 

Mineral, 1971).

The rocks of the Araxa folded belt have a strong 

tectonic vergence toward the Sao Francisco platform to the east (Ferreira, 1972). Rocks of the 

Araxa Group have been thrust over both the Canastra and Bambui Groups, and Canastra rocks have 

been thrust over rocks of the Bambui Group (fig. 1), 

marking strong west-to-east tectonic transport during the Eocambrian or earliest Paleozoic.

GEOLOGY OF THE SERRA DE CALDAS AREA

The geology of the Serra de Caldas area is known 

mostly from the work of Braun (1970b). Three 

rock sequences can be recognized—the Araxa Group, 

the Canastra Group, and a sequence of quartzite, siltstone, and lesser shale that can probably be 

correlated with the Bambui Group (fig. 2). These 

rocks form an extremely complex tectonic terrane, 

evident even at the small scale of figure 2. Structural 

studies have been, and will continue to be, hindered 

by the generally poor exposure, deep weathering, 

and complicated stratigraphy.

ARAXA GROUP

In the Serra de Caldas area, the Araxa Group 

was divided into three units by Braun (1970b). The 

lowermost unit consists primarily of muscovite-biotite-plagioclase-quartz schist that contains some intercalations of micaceous quartzite. The unit is in 

amphibolite fades (Braun, 1970b) and at several 

places is a gneiss.

The middle unit is more psammitic, consisting of 

interbedded quartzite (quartz sand turbidites) and 

muscovite-quartz schist. Bedding in this unit is 

transposed, and the primary planar element is flow 

cleavage.

The upper unit of the Araxa is the most important to this study because it surrounds the Serra 

de Caldas. It is quite pelitic, the most common 

rock type being muscovite-biotite-plagioclase-quartz 

schist, although garnet-chlorite-biotite-quartz schist 

is fairly abundant. The unit was metamorphosed to 

at least high greenschist facies, but at places it 

contains megascopic kyanite and andalusite, suggesting higher rank metamorphism.

The upper Araxa contains numerous small bodies 

of ultramafic and related mafic rock in the form of 

serpentinite, actinolite schist (probably metapyroxenite), talc schist, and chlorite schist (Braun, 1970a). 

These bodies are far more abundant than those

shown in figure 2 because most are too small to 

show at that scale. This part of the Araxa, then, can 

be considered an ophilitic melange in the sense of 

Gansser (1974) and Williams (1977).

The melangelike nature of the upper unit is further documented by many discontinuous bodies of 

marble (Braun, 1970b), which are probably exotic 

blocks within this complex schist terrane. In addition, polymictic melange has been noted elsewhere 

within the upper part of the Araxa (fig. 3A).

Bedding is completely transposed within this unit, 

and transposition foliation is the principal planar 

element. Most exposures are characterized by disrupted fold hinges swimming like tectonic fish within a sea of pelite (fig. SB). In many places, the 

transposition foliation is transposed, and strain-slip 

foliation is the dominant planar feature.

CANASTRA GROUP

The Araxa passes up into a sequence of quartzite, 

quartz-rich phyllite, and pelitic schist that have been 

assigned to the Canastra Group by Braun (1970b). 

The pelitic parts of the unit consist of muscovitebiotite-quartz schist and a quite alumina- and ironrich interval characterized by chloritoid-muscovitequartz schist. The Canastra, though strongly deformed, is less so than the Araxa, and bedding can 

be seen in many exposures. The unit is at biotite 

grade of regional metamorphism.

BAMBUI GROUP (?)

A sequence of orthoquartzite and slitstone containing interbeds of shale underlies the Serra de 

Caldas. The quartzite, which is quite pure, forms 

beds a meter or more thick and is responsible for 

the topographic expression of the Serra. This rock 

is overlain by quartz siltstone and interbedded shale. 

The shale has been called phyllite, but, although at 

chlorite grade, it lacks a penetrative foliation. Bedding is the principal parting surface and is well preserved in most exposures, as are other primary sedimentary structures such as ripple marks (fig. 4A), 

crossbeds, convolute folds, and sedimentary boudinage. In addition, probable grazing trails on bedding 

surfaces are only slightly deformed, if at all (fig. 

45).

These rocks were once confused with the middle 

unit of the Araxa Group because of their quartzose 

nature and their position beneath the upper part of 

the Araxa. Clearly, however, these rocks, probably 

deposited in a transitional (continental marine) environment, have nothing in common with the deep 

marine rocks of the middle part of the Araxa. The
TECTONIC STUDIES IN THE BRAZILIAN SHIELD

48°30'

- 17°30'

18°00'

FIGURE 2.—Generalized geologic map of the Serra de Caldas area (modified from an unpublished map by O. P. G. Braun, 

1968). Areas A and B are enlarged in figure 5, areas C and D in figure 7, and areas E and F in figure 8.

lack of a tectonic fabric is also strong evidence that 

these rocks cannot be Araxa Group. It seems inescapable then that the quartzite-siltstone sequence 

contact with the Araxa rocks is tectonic, not 

sedimentary.

The only rocks known in Goias that resemble 

those underlying the Serra de Caldas are in the 

Bambui Group. In the Brasilia area, Dyer (1970) 

described a sequence of quartzite, siltstone, and 

"slaty phyllites" at the -base of the Bambui Group.
THE SERRA DE CALDAS WINDOW, GOIAS A5

EXPLANATION

p€ac, upper unit; um, ultramafic and related rocks 

p-Cab, middle unit; p€aa, lower unit

Contact

Thrust fault 

Sawteeth on upper plate

Wrench fault 

Showing relative horizontal movement

Antiform 

Showing direction of plunge

— Ft

Overturned antiform 

Showing direction of dip of limbs

Synform 

Showing direction of plunge

Overturned synform 

Showing direction of dip of limbs

FIGURE 2.—Continued.

These rocks are predominantly sandy and are assigned to the Paranoa Formation. The description of 

these rocks is perfect for the rocks at the Serra de 

Caldas, which, therefore, are tentatively correlated 

with the Bambui Group.

B

FIGURE 3.—Araxa Group. A, Saprolite exposure of polymictic 

melange. B, Polyphase deformed schist. Photographs by 

C. H. Thorman, Jr.

STRUCTURAL GEOLOGY

Only a casual glance at figure 2 is needed to note 

the extreme structural complexity of the Serra de 

Caldas area. Strong west-to-east transport is shown 

by the overturned folds and imbricate thrust faults, 

which are also folded. Large type 1 and type 2 fold 

interference patterns (Ramsay, 1967) are immediately identifiable. Type 3 patterns are not apparent 

at the scale of figure 2, but such interference struc-
A6

FIGURE 4.—Bambui Group (?) A, Ripple-marked quartzite. B, Grazing trails (?) on bedding surface. Photographs by V. J. Hurst.

tures are abundant in outcrop (fig. 35). At many 

places, the Araxa rocks have three sets of coaxial 

folds. The axes of early recumbent folds have been 

recumbently folded, and both of these axes have 

been deformed toy steep, more or less symmetric 

folds. The axes of these three sets of folds have been 

folded by a later oblique set.

Two sets of wrench faults have been mapped (fig. 

2). The large east-trending fault is probably a tear 

related to the east-directed thrusting. The northwest-trending faults result from later deformation, 

but I have no specific data with which to interpret 

them.

THE WINDOW

During reconnaissance in the Serra de Caldas area, 

I discovered a marked difference in tectonic style 

between the rocks surrounding the Serra and those 

within it. In addition, cataclastically deformed rock 

was found at the contact of the Araxa and Bambui 

rocks at one place. It seemed likely that the Serra 

was a window. To test this hypothesis, students 

were assigned areas in which to make detailed strip 

maps. This mapping was done on aerial photographs 

at a scale of 1:60,000, as there are no topographic 

maps of the area. Six of these maps (outlinesi shown 

on fig. 2) are presented here as part of the evidence 

for the window.

THE EAST FRAME

Figure 5 shows geologic maps of areas A and B 

on the east flank of the Serra de Caldas. In both 

areas, rocks of the Bambui Group (?) are separated 

from more complexly deformed rocks at higher 

metamorphic grade by a zone of cataclastic deformed rock that dips gently to the east. In area A 

(fig. 5), the Bambui Group (?) is in fault contact 

with interbedded micaceous quartzite and schist like 

those of the Canastra Group. The quartzite and 

schist are correlated with the Canastra Group 

though they could conceivably be interbedded in the 

Araxa. Small, mostly upright, concentric folds in the 

Bambui Group (?) plunge gently west of north (fig. 

6), although folds in the southern part of area B 

(fig. 5) plunge east of south, reflecting the doubly 

plunging nature of the Serra de Caldas brachyanticline. Late small folds in the Araxa Group (fig. 6) 

about parallel those in Bambui (?). Most of the early 

folds plunge either southeast or northwest (fig. 6), 

depending on their location on the limb of a late synform (area B, fig. 5). These folds are in both transportation foliation and second-generation strain-slip 

foliation and are characteristically flattened similar
Area A

THE SERRA DE CALDAS WINDOW, GOIAS

Area B

p€ac

A'

0 600 1200 METERS

A'

100- pCac

e€bs

EXPLANATION

Bambuf Group (?) 

eCbs, siltstone; eCbq, quartzite —'

Araxa Group, upper unit

Contact

1000-

< 500 

o

e€b

0 600 1200 METERS

pCac

20 

Strike and dip of beds

Strike and dip of cleavage

Strike and dip of foliation

A7

A'

Thrust fault 

Sawteeth on upper plate

Synform

Showing trace of axial surface and direction 

of dip of limbs and plunge

Bearing and plunge of lineation

/ 30 

Bearing and plunge of early fold axis

Bearing and plunge of late fold axis

FIGURE 5.—Geologic maps and sections of areas A and B (fig. 2). Geology by H. de 0. Godoi and J. T.

Takanohashi.

folds. Few first folds can be seen, and their axes are 

virtually impossible to measure. On the east flank of 

the Serra, the obvious late folds, those in which the 

strain-slip foliation defines the form surface, parallel those in the Bambui( ?) rocks. These folds have

a spaced axial surface cleavage that parallels the 

cleavage in younger rocks. Clearly, the late fabric 

in the older rocks is the same as the fabric in the 

younger rocks.
A8 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

Araxa Group

Bambui Group (?)

FIGURE 6.—Equal-area plots of 32 small fold axes in 

rocks of the Bambui Group (?) and of 65 small fold 

axes in rocks of the Araxa Group on the east flank 

of the Serra de Caldas. Contours at 5, 20, 35, and 50 

percent per 1 percent area for the Bambui Group and 

at 10, 20, 30, and 40 percent per 1 percent area for the 

Araxa Group.

THE WEST FRAME

Figures 7 and 8 are geologic maps of areas C-F 

on the west flank of the Serra de Caldas. On this 

flank as well, a zone of cataclastically deformed 

rocks separates the Bambui Group (?) from the 

older polyphase terrane. In areas E and F (fig. 8), 

interbedded micaceous quartzite and schist like those 

of the Canastra Group lie between rocks of the 

Araxa Group and the thrust fault. Rocks of the 

Canastra Group crop out in the northwesternmost 

corner of area F (fig. 8) in a synform. It seems 

likely, therefore, that the micaceous quartz schist 

adjacent to the thrust fault shown in area A (fig. 5) 

and areas E and F (fig. 8) is Canastra folded into 

the Araxa.

Axes of small folds in bedding in rocks of the 

Bambui Group (?) plunge gently north-northwest 

(fig. 9), whereas those in bedding in the Canastra 

Group (?) plunge both west-northwest and northnorthwest. The late folds, those that plunge northnorthwest, also fold the schistosity in the Canastra (?) rocks.

Three sets of folds can be recognized in rocks of 

the Araxa Group (fig. 9). All three sets fold the 

transposition foliation, and the second-generation 

strain-slip foliation is folded by both the west-northwest- and north-northwest-plunging sets. First folds 

are difficult to find and impossible to measure. The 

earliest west-plunging folds are characteristically 

flattened similar folds. Many are so severely flattened that opposite limbs are completely appressed, 

and no hinge zone can be recognized. The westnorthwest-plunging folds are characteristically 

similar, whereas the late north-northwest-plunging 

folds are concentric and have a spaced axial surface 

cleavage that parallels the cleavage in the Bambui 

rocks. On this flank as well, the late fabric in the 

older rocks parallels the fabric in the Bambui (?) 

rocks, and earlier fabrics have been folded over the 

Serra de Caldas.

TECTONIC INTERPRETATION

Fieldwork has shown that a low-grade sequence 

of quartzite, siltstone, and lesser shale is surrounded 

by high-grade polyphase deformed rocks. This tectonic and metamorphic discontinuity is everywhere 

marked by a zone of cataclastically deformed rock. 

The Serra de Caldas, then, is a large antiformal 

window, about 250 km2 in extent, trending just west 

of north. Rocks of the Araxa and Canastra Groups 

were metamorphosed and deformed prior to their 

emplacement, as shown by the folded early foliations
THE SERRA DE CALDAS WINDOW, GOIAS A9

Area C Area D

0 600 1200 METERS 

I____i____I

-300

EXPLANATION

Bambui Group (?) 

eCbs, siltstone; eCbq, quartzite

Araxa Group, upper unit 

um, ultramafic rock

0 600 1200 METERS 

l___i___i

pCac 1100 

h 600

Antiform

Showing trace of axial surface 

and direction of plunge and dip of limbs 

30,

Strike and dip of beds

Strike and dip of first foliation 

45,,

Strike and dip of second foliation 

20,

Strike and dip of third foliation

14-«——

Bearing and plunge of early lineation 

10^

Bearing and plunge of late lineation

Contact

Thrust fault 

Sawteeth on upper plate

FIGURE 7. Geologic maps and sections of areas C and D (fig. 2). Geology by G. S. Pena, Paulo Frassinette,

and M. M. Sugahara.
TECTONIC STUDIES IN THE BRAZILIAN SHIELD

Area F

25^ 8^ 45^ 35/

p€ac

p€>

\

e€b

0 600 1200 METERS

A'

0 600 1200 METERS 

I____i____i

p€ac p€c? .. - p€c

p€ac / --****-

EXPLANATION Contact

Araxa Group, upper unit

Thrust fault 

Sawteeth on upper plate

3?_L_

Strike and dip of bedding

2LStrike and dip of foliation 

10

Bearing and plunge of lineation

FIGURE 8.—Geologic maps and sections of areas E and F (fig. 2). Geology by Amilcar Adamy and Lauri Bez.

in these rocks and by the low rank of metamorphism 

in the Bambui(?) rocks. Regional relations (figs. 

1 and 2) suggest that thrusting was directed nearly 

due east, but that suggestion has not been proved. 

The amount of transport is difficult if not impossible 

to determine. The main outcrop belt of Bambui rock 

is about 200 km east of the Serra de Caldas. Unless 

an area of Bambui rock in normal stratigraphic 

position is present west of that shown on the part 

of the geologic map of Brazil shown in figure 1,

transport of about 200 km is required. I should 

point out that rocks of the Canastra Group crop out 

in the Serra de Caldas area (fig. 2) that were not 

shown in figure 1; areas of unrecognized Bambui 

may also exist.

REGIONAL IMPLICATIONS

Shelf-facies rocks of Bambui aspect occur in a 

window beneath transported deep marine sediments,
THE SERRA DE CALDAS WINDOW, GOIAS All

Bambui Group Canastra Group Araxa Group

FIGURE 9.—Equal-area plots of 42 small fold axes in rocks of the Bambui Group, 30 in rocks of the Canastra Group(?), 

and 60 in rocks of the Araxa Group—all on the west flank of the Serra de Caldas. Contours at 15, 30, and 45 percent per 1 percent area for the Bambui Group; at 10, 20, and 30 percent per 1 percent area for the Canastra Group; 

and at 1, 15, and 30 percent per 1 percent area for the Araxa Group.

which are, in part, an ophiolitic melange. Such rocks 

are of extreme importance, as throughout the world 

they commonly mark the boundaries of suture zones 

(Coleman and Irwin, 1974), which in most places 

are destroyed continental margins (Williams, 1977). 

Most known examples are the result of obduction, 

that is, the overthrusting of oceanic rocks across 

continental margins (Gansser, 1974). As prePaleozoic occurrences are rare, this Brazilian example is important.

In most areas of ophiolitic melange, such as Newfoundland, Oman, Urals, Himalayas, and Zagros, the 

melange is overlain by an ophiolite sheet, the 

melange resulting from the mass wasting of the 

advancing nappe. I would like to suggest, therefore, 

that some of the larger bodies of ultramafic and related rock in the area mapped, such as those at and 

southwest of Niquelandia (fig. 1), may be ophiolite 

nappes and may have a much more complex origin 

than is commonly believed. In any case, oceanic 

material has been thrust upon cratonic shelf deposits, transport perhaps being as great as 200 km 

during the Eocambrian or earliest Paleozoic.

REFERENCES CITED

Almeida, F. F. M. de, Amaral, G., Cordani, U. G., and 

Kawashita, K., 1973, The Precambrian evolution of the 

South America cratonic margin south of the Amazon 

River, in Nairn, A. E. M., and Stehli, F. G., eds., The 

ocean basins and margins, v. 1, The South Atlantic: 

New York, Plenum Press, p. 411-446.

Barbosa, Octavio, 1955, Guia das excursoes do IX Congresso 

da Socidade Brasileira de Geologia: Soc. Brasileira Geologia Noticiario 3.

Berbert, C. 0., 1970, General geology of the basic-ultrabasic 

complexes of Goias: Cong. Brasileiro Geologia, 24th, 

Brasilia, 1970, Bol. Especial 1, p. 24-28.

Braun, 0. P. G., 1970a, The economic significance of the 

Caldas Novas greenschist: Cong. Brasileiro Geologia, 

24th, Brasilia, 1970, Bol. Especial 1, p. 134-135.

———— 1970b, Geologia da area de Caldas Novas, Ipamerie 

Arredores, no Estado de Goias: Cong. Brasileiro Geologia, 24th, Brasilia, 1970, Bol. Especial 1, p. 205-207.

Brazil Departmento Nacional da Produc.ao Mineral, 1971, 

Mapa geologico do Brasil: Rio de Janeiro, scale 

1:5000000.

Coleman, R. G., and Irwin, W. P., 1974, Ophiolites and ancient continental margins, in Burk, C. A., and Drake, 

C. L., eds., The geology of continental margins: New 

York, Springer-Verlag, p. 921-931.

Dyer, R. C., 1970, Apresentacao preliminar da geologia da 

folha "Brasilia": Cong. Brasileiro Geologia, 24th, 

Brasilia, 1970, Bol. Especial 1, p. 214-216.

Ferreira, E. 0., 1972, Carta tectonica do Brasil [Tectonic 

map of Brazil]: Brazil Dept. Nac. Produc.ao Mineral Bol. 

1, 19 p. (Portuguese), 14 p. (English), col. geol. map, 

scale 1:5 000 000.

Gansser, Augusto, 1974, The ophiolitic melange, a world-wide 

problem on Tethyan examples: Eclogae Geol. Helvetiae, 

v. 67, no. 3, p. 479-507.

Ramsay, J. G., 1967, Folding and fracturing of rocks: New 

York, McGraw-Hill, 568 p.

Williams, Harold, 1977, Ophiolitic melange and its significance in the Fleur de Lys Supergroup, northern Appalachians: Canadian Jour. Earth Sci., v. 14, no. 5, p. 

987-1003.

Precambrian Plate Tectonics in the Brazilian 

Shield—Evidence from the Pre-Minas Rocks 

of the Quadrilatero Ferrifero, Minas Gerais

By Avery Ala Drake, Jr., and Benjamin A. Morgan

TECTONIC STUDIES IN THE BRAZILIAN SHIELD

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1119-B

Prepared in cooperation with the 

Departamento Nacional de Aguas e Energia Electrica, 

Departamento Nacional de Produfdo Mineral, and the 

Companhia de Pesquisa de Recursos Minerals of Brazil

Oceanic rocks, including ultramafic and mafic rocks 

that are parts of a disrupted ophiolite assemblage, 

were obducted onto the east flank of the granitoid 

Sdo Francisco craton during Archean time

CONTENTS

Abstract ___________ 

Introduction _________.

Acknowledgments _____

The granitoid era ton _____

The Rio das Velhas Series 

Conclusion ___________ 

References cited ________

Page

Bl 

1 

3 

3

13 

18 

18

ILLUSTRATIONS

Page

FIGURE 1. Stratigraphic section in the Quadrilatero Ferrifero, Minas Gerais __________________ — ____ B-2

2. Generalized geologic map of the Quadrilatero Ferrifero.____________________—————__— 4

3. Photomicrographs of cataclastic granitoid rocks, Quadrilatero Ferrifero ________________ — ____ — _ 7

4. Photographs of polyphase folds in the granitoid terrane along the north front of the Quadrilatero

Ferrifero __________________________.________________________———___ 9

5. Diagram showing Al.Os-MgO-FeO-ILO projection for mineral assemblages for the staurolite schist._ 10

6. Photographs of schists from the Quadrilatero Ferrifero.--___________________________ 11

7. Photographs showing unconformable contact between staurolite-garnet schist and Moeda Quartzite on

southeast flank of Bagao dome ____________.___________________ — — ___ 12

8. Geologic map showing distribution of ultramafic and mafic rocks in the Rio Acima and Macacos quadrangles ____________________________________——————————————————————————— 15

TABLES

TABLE 1. Summary of isotopic age dates for the Ba?ao and Bonfim Complexes, Quadrilatero Ferrifero, Minas

Gerais _____________________________.________________________________ B6

2. Comparison of the chemistry of gneiss with graywacke, pelite, and calcareous phyllite of the Nova

Lima Group ___________________.____________________________ 8

3. Chemical analyses and normative minerals of gabbro and ultramafic rocks, Quadrilatero Ferrifero— 16

ill

TECTONIC STUDIES IN THE BRAZILIAN SHIELD

PRECAMBRIAN PLATE TECTONICS IN THE BRAZILIAN SHIELDEVIDENCE FROM THE PRE-MINAS ROCKS OF THE 

QUADRILATERO FERRIFERO, MINAS GERAIS

By AVERY ALA DRAKE, JRV and BENJAMIN A. MORGAN

ABSTRACT

The Precambrian pre-Minas rocks of the Quadrilatero 

Ferrifero in Minas Gerais were restudied and reinterpreted 

using a simplified plate-tectonic model. The principal elements of this interpretation are as follows: (1) Many of 

the granitoid rocks and gneisses of the area are part of an 

ancient cratonic massif (the Sao Francisco craton) and are 

not later intrusions into metasedimentary and metavolcanic 

rocks of the region. (2) The cratonic rocks are overthrust 

by a sequence of flyschoid rocks of the Nova Lima Group. 

This sequence includes ultramafic and related mafic rocks 

that are parts of a disrupted ophiolite assemblage. Metamorphic muscovite from the cratonic basement and from 

schists of the Nova Lima Group has an age of 2.6 or 2.7 

b.y.; therefore, the allochthon was in place by that time. 

The Nova Lima allochthon, which includes this ophiolitic 

material, is therefore a very early example of plate tectonics, 

and was obducted perhaps from the southeast onto the Sao 

Francisco craton during Archean time.

After emplacement of the allochthon, rocks of the Minas 

and Itacolomi Series were deposited. The style of deformation and the pattern of distribution of rock units are largely 

the result of later major orogeny in the Quadrilatero. During this orogeny, the basement was remobilized and deformed 

upward, producing gneiss domes. The allochthon and later 

sedimentary sequences were deformed into tight, pinched-in 

polyoriented synclines surrounding the domes.

INTRODUCTION

"The picture of the Atltantic Ocean segment 

crowding the eastern side of the South American 

Continent, squeezing it and forcing it upwards like 

a triangular wedge and slicing the edge comes very 

appealingly to mind" (Harder and Chamberlin, 

1915, p. 370).

The concept of plate tectonics seems well established for the Phanerozoic Eon; however, extrapolation of this concept backward in time into the Precambrian involves much uncertainty. Many students of Precambrian geology (for example, Anhaeusser and others, 1969) are reluctant or unable 

to interpret the older rocks of southern Africa in 

terms of plate-tectonic events. Anhaeusser and 

others (1969) and Coward and others (1976) have 

interpreted the Precambrian tectonics of southern 

Africa in terms of intracratonic block tectonics; this 

process consisted of relatively small movements of 

older microcontinents or cratons with respect to 

each other and a resulting high degree of deformation and metamorphism of the intervening younger 

sedimentary and volcanic rocks that had been deposited on a cratonic basement. Goodwin (1973) 

has stated quite correctly that evaluation of a platetectonic model for the Precambrian must rest principally on geological analogy to more recent models. 

He examined two supracrustal elements of Precambrian crust—the chemistry of the Archean volcanic 

belts (with ages of 2.7 and 3.1 billion years (b.y.) 

and the distribution of lower Proterozoic banded 

iron formations (ages between 1.9 and 2.1 b.y.) — 

and concluded that compositions, origins, and distribution patterns are compatible with a global pattern of plate boundaries and some form of intraplate motion. Burke and Dewey (1973) have suggested that plate-tectonic patterns may be extrapolated back into the Proterozoic Eon and that modern 

continental-collision (Himalayan) orogenies may be 

models for later Precambrian tectonics. They regard 

plate tectonics as a secular process involving an increasing rigidity of lithospheric blocks. Before about 

2.5 b.y. ago, a radically different tectonic regime 

(the Permabile Phase) prevailed. This phase was 

the expression of a thin lithosphere characterized by 

high heat generation and the absence of extensive 

areas of rigidity.

Bl
B2 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

The purpose of this paper is to reinterpret a large 

segment of the older Precambrian rocks of the Brazilian shield in rather simplified terms of plate tectonics. The elements of our interpretation involve 

the recognition of an ancient craton (age greater 

than 2.6 b.y.) and the obduction onto this craton of 

a structural block containing a thick accumulation 

of little-metamorphosed flyschoid sedimentary and 

volcanic rocks. Rather than containing only the reworked material of the older cratonic basement, this 

thrust block contains disrupted peridotite-gabbrovolcanic-chert units, suggesting an ophiolitic association, and thick marine sediments, which include 

widely scattered iron-formations of carbonate facies. 

By analogy with modern examples of plate tectonics, 

these rocks would seem to represent a continentaloceanic-plate convergent margin as opposed to the 

continental-plate intersections proposed, for the 

areas of the Precambrian, which were discussed by 

Burke and Dewey (1973).

Our area of investigation is the Quadrilatero Ferrifero, one of the great mining areas in the world, a 

region of about 7,000 km2 in central Minas Gerais, 

Brazil. Between 1946 and 1962, a joint team of 

geologists from the Departmento Nacional da Produgao Mineral of Brazil (D.N.P.M.) and the U.S. 

Geological Survey (U.S.G.S.) carried out a program 

of geologic mapping of the Quadrilatero under the 

auspices of the Agency for International Development (A.I.D.) of the U.S. Department of State. The 

objectives of this work were to evaluate the economic potential of the vast iron-formations of the 

region and to set up a stratigraphic and structural 

framework for the iron-formations and surrounding 

rocks. In this, the team was very successful; their 

work resulted in about 40 detailed geologic maps 

at 1:25,000, many topical papers, and a final synthesis by Dorr (1969), the program director. As a 

result, this is one of the geologically best known 

regions of Precambrian rocks in South America.

The Quadrilatero Ferrifero is about on the boundary between the old basement of the Sao Francisco 

platform (2.6 b.y.) and the remobilized zones to 

the east (Ferreira, 1972). The stratigraphy and 

general geology as summarized by Dorr (1969) are 

shown in figures 1 and 2. In addition, Herz has reported at some length on the gneisses and granitoid 

rocks (1970), and on the metamorphism of the sedimentary rocks (1978).

The area is underlain by gneiss and foliated 

granitoid rocks and three major series of sedimentary and subordinate volcanic rocks set off from each 

other by angular unconformities. The oldest series,

00

Cretaceous(?), Tertiary(?), Holocene and 

5

%

a. 

o 

a

Formation

|

c

:r

Precambrian

Itacolomi

5

•S

.2

•a

1

>

o

^^ 

, — -

•3•o

C

E

=

S

^^-'

.1

1

J3 

GO

0 

GO

m

c 

3 

u_

-8

u_

0

c "•5

o

~=^

•o 

a

CJ

DO

•o

i-<

(Unnamed)

Cambotas

Casa Forte

Palmital

-<^~

i

•o

c

Lithology

Canga, lacustrine and stream 

sand, clay, gravel, alluvium, 

and colluvium

Lake and stream sediments, 

minor lignite

Profound angular and erosional unconform

Type area' orthoquartzite, and 

protoquartzite, conglomerate, grit 

Santo Antonio facies, protoquartzite, 

phyllite, phyllitic quartzite, 

conglomerate

Sedimentary 

environment

Continental

Continental

ty]

Paralic 

Molasse?

Angular and profound erosional unconformity]

Chlonte schist and phyllite, metatuff, 

graywacke, tilloid, conglomerate, 

quartzite, minor iron-formation

Local erosional unconformity

Phyllite and graphitic phyllite

Orthoquartzite

Quartzose phyllite, 

dolomitic phyllite, 

siliceous dolomite

Ferruginous quartzite, quartzite, 

grit, phyllite, ferruginous phyllite, 

minor conglomerate and dolomite

Local erosional unconformity

Dolomite and minor limestone, dolomitic 

itabirite, itabirite, dolomitic phyllite

Itabirite (oxide facies iron-formation, 

dolomittc itabirite, minor phyllite 

and dolomite

Phyllite, slightly graphitic phyllite, 

minor metachert and oxide-facies 

iron-formation

Paralic facies: orthoquartzite and 

grit, conglomerate, phyllite 

Blanket facies: Sericitic quartzite, 

quartzose phyllite, protoquartzite

Local erosional unconformity

Dolomitic phyllite, dolomitic 

iron-formation (oxide-facies), 

quartzose phyllite

Orthoquartzite, conglomerate, grit, 

conglomeratic quartzite, minor 

quartzose phyllite

Erosional and angular unconformity

Protoquartzite, grit, conglomerate, 

minor phyllite and subgraywacke

Phyllite, quartzose phyllite, 

protoquartzite, graywacke, 

subgraywacke, minor basal 

conglomerate

Eugeosynclinal 

flysch

Stable 

shelf 

(blanket)

Stable 

shelf 

(blanket)

Stable 

shelf 

(blanket)

Stable 

shelf 

(blanket)

Stable 

shelf 

(blanket)

Stable 

shelf 

(blanket)

Stable 

shelf 

(blanket)

Stable 

shelf

Stable 

shelf

Paralic 

prismatic

Eugeosynclinal 

Molasse

Eugeosynclinal 

Molasse

Local erosional and possibly angular unconformity

Phyllite, largely chlontic, graywacke, 

carbonate-facies iron-formation, metavolcanics, minor quartzite, tilloid, 

conglomerate, and dolomite Rb — Sr 

age on muscovite 2800 million years 

before present

No basement rocks exposed in the 

Quadrilatero Ferrffero; all 

granitic rocks are intrusive

Eugeosynclinal 

(flysch)

Maximum 

thickness, 

in meters 

(approximate

100

100

2,000? 

1,000?

3,000 +

150

125

410

600

600

350 +

250

1,000 

150

300

1,000

400 +

1,400

4,000 +

FIGURE 1.—Stratigraphic section in the Quadrilatero Ferrifero, Minas Gerais (modified from Dorr, 1969).

the Rio das Velhas, is a eugeosyndinal suite that is 

interpreted to pass up into a molasse, at least locally. These rocks are overlain by the Minas Series
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS B3

of largely shallow-water sediments, including- the 

great deposits of iron-formations (oxide facies). 

The upperment rocks of this sequence are thought 

to be flysch. The youngest series, the Itacolomi, consists of rocks thought to represent molasse and 

paralic deposits. The sedimentary and lesser volcanic rocks of these three series have been regionally 

metamorphosed, primarily in greenschist facies, although staurolite-bearing schists of the amphibolite 

facies are present in some places.

Rocks of the Rio das Velhas Series are considered 

by Dorr (1969) to be the oldest in the Quadrilatero. 

All the gneiss and foliated granitoid rocks were interpreted to be intrusive into the sedimentary-volcanic pile and to be post-Rio das Velhas in age. The 

rocks in the amphibolite facies were thought to result from contact metamorphism and to be in thermal aureoles around granitoid intrusions. The 

Quadrilatero has, therefore, been interpreted as an 

island of metasedimentary rock floating in a sea of 

granite (Dorr, 1969).

The geologic map of the Quadrilatero (fig. 2) 

shows the area to be one of extraordinary structural 

complexity. The outcrop distribution is that of a 

Type I (eggbox) fold-interference pattern (Ramsey, 1967), very similar to those patterns formed in 

granitic gneiss-metesedimentary terranes in the 

Canadian shield (see particularly Pearson and 

Lewry, 1974, fig. 5).

Because of extensive geologic research in the area, 

the Quadrilatero was chosen to serve as a field area 

for training experienced geologists of the various 

agencies of the D.N.P.M. in advanced geologic field 

techniques. Such courses were held during the 

austral winters of 1974 and 1975. Through an 

A.I.D.-sponsored cooperative program between the 

U.S.G.S., D.N.P.M., Department Nacional de Aguas 

e Energia Eletrica (D.N.A.E.E.), and Companhia 

de Pesquisa de Recursos Minerais (C.P.R.M.), 

Drake served as an instructor for the 1974 and 1975 

courses, and Morgan, for the 1975 course.

The tectonic model for the Quadrilatero Ferrifero 

presented here is contrary to some conclusions made 

by earlier workers. Our paper is organized into two 

major sections: (1) the granitoid craton, in which 

is presented our arguments for the presence of an 

ancient continental massif and (2) the Rio das 

Velhas Series, in which is presented our arguments 

for the presence of a major thrust sheet containing 

blocks of disrupted ophiolite.

ACKNOWLDGMENTS

This paper could never have been conceived without the excellent geologic maps of Dorr and his coworkers. As Dorr has written (1969, p. 5), "As 

the knowledge of the region grew and evolved, interpretations made early in the work, although 

seemingly well based at the time, became obsolescent, 

just as any interpretations made in this report will 

also become obsolescent after more detailed work is 

done in the future."

Our reinterpretation of the older rocks of the 

Quadrilatero is made in the same spirit. We hope 

that the possibility of a major plate-tectonic event 

in the Archean in this area will spur other geologists 

to work in the Quadrilatero Ferrifero and that the 

understanding of this fascinating area will continue 

to expand.

We are indebted to our teaching colleagues in the 

Brazilian field courses, J. M. Aaron and C. H. 

Thorman of the Geological Survey and V. J. Hurst 

of the University of Georgia, for many observations 

in the field. We are indebted to Samir Nahass and 

Pedro Gervasio Ferrari of the Companhia de Pesquisa de Recursos Minerais for help with logistical 

details. Finally, we express our thanks to the 48 

students with whom we worked, both for their interest in our studies and their problem-solving strip 

mapping.

THE GRANITOID CRATON

A wide variety of granitoid rocks and gneiss, including paragneiss and foliated granidiorite, is 

shown as an undivided unit in figure 2. These rocks 

almost completely surround the Quadrilatero Ferrifero and also form a prominent massif, the Bac.ao 

Complex, within the district. The extreme weathering of these rocks has prevented detailed mapping 

by the D.N.P.M.-U.S.G.S. workers; structures and 

lithologies were impossible to extrapolate over large 

areas devoid of all data. With the exception of Simmons' (1968a) mapping in the western part of the 

area, all the detailed quadrangle maps at a scale of 

1:25,000 show granitoid rocks and feldspathic 

gneiss as an undivided unit; however, many of these 

workers have given different names to granitoid 

massifs separated by metasedimentary septa, and 

many also have mapped out minor bodies of chert 

and amphibolite within the granitoid terrane where 

it proved practicable. Simmons (1968, pis. 1 and 2), 

working in the Bonfim dome, and Wallace (1965, 

fig. 8), in the Bac.ao Complex were able to map large 

areas underlain by foliated rocks of probable igneous
B4 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

43°07'30"

FIGURE 2.—Generalized geologic map of the Quadrilatero Ferrifero, Brazil. Some places on map that are mentioned in the 

text: SN, Souza Nocheso; B, Brumadinho; q, areas of cataclasite mapped as quartzite; I, Ibirits; PP, Piedade do Parapeba; X, contact of Moeda with sheared granite; CC, contact of cataclastic granodiorite with phyllonized staurolite 

schist; Ba, Barreiro; um, ultramaflc and maflc rocks in Rio Acima and Macacos quadrangles; Co, Congonhas; BH, 

Belo Horizonte; NL, Nova Lima; S, Sahara; Ca, Caete; RC, railroad cut exposing contact of staurolite schist and 

Moeda Quartzite; SB, Santa Barbara; Ib, Itabira; PS, Piedade syncline; Igarape is off map about 15 km northwest of 

Souza Nocheso.

origin as well as areas underlain by gneissic rocks 

of probable sedimentary origin. Herz (1970), in a 

survey of the granitoid rocks of the Quadilatero, 

extended the mapped contacts of Simmons and Wallace in the Bonfmi and Bagao Complexes and 

showed that these massifs consist of a core of metaigneous rocks surrounded by a mantle of gneiss, 

which he thought was derived from ultrametamorphism of sedimentary rocks. In three other areas of

the Quadrilatero, Herz (1970) was able to show contacts over limited distances between other foliated 

granitoids and metasedimentary gneiss.

Radiometric age determinations on granitoid 

rocks and gneiss from the Quadrilatero were published by Herz and others (1961), Aldrich and 

others (1964), Pinson and others (1967), and Herz 

(1970) and have been summarized by Herz (1970). 

These data for the Quadrilatero yield a wide scatter
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS B5

EXPLANATION

Itacolomi Series undivided

Piracicaba Group 

DISCONFORMITY

Itabira and Caraca Groups, undivided 

DISCONFORMITY

Nova Lima Group 

Clastic and metavolcanic rocks

Ultramafic and mafic rocks 

TECTONIC CONTACT

Gneiss and foliated granitoid rocks 

Contact

Thrust fault 

Sawteeth on upper plate

Fault

Syncline 

FIGURE 2.—Continued.

of ages ranging from 2,790 to 420 m.y. In general, 

K/Ar ages are younger than the corresponding 

Rb/Sr ages from the same sample, frequently by 

more than a billion years. Age determinations on

biotites show a particularly large range and a large 

number of dates between 975 and 420 m.y. that 

probably are the result of Ar loss. Table 1 gives a 

summary of age data from the Bagao and Bonfim 

Complexes. All of the multiply determined ages from 

the same samples are discordant for these complexes. 

K/Ar ages on biotite from foliated granodiorite in 

the core of the Bac.ao Complex (2,440 and 2,510 

m.y.), Rb/Sr ages on feldspar from gneiss from 

the Bonfim Complex (2,300 m.y.), Rb/Sr ages from 

muscovite from amphibolite in the Bagao paragneiss 

(2,675 m.y.), and Rb/Sr ages on muscovite from the 

metasedimentary rocks immediately north of the 

Bagao Complex (2,790 m.y.) give a more restricted 

range of older ages for the district, despite the wide 

diversity of rock types. Contrary to the conclusion 

of Herz (1970, p. B9), we conclude that as these 

ages were determined from minerals derived by 

metamorphic processes, the ages must be partly or 

completely reset 'by metamorphism and do not reflect the time of implacement of plutonic sequences 

such as the granodiorite in the core of the Bagao 

Complex. Until detailed work has been carried out 

using U/Pb methods on zircons from these granitoid 

rocks, we feel that the ages of these rocks are uncertain ; however, the rocks most probably are older 

than 2.5 b.y. and were affected by metamorphism 

at about that time, as well as by later thermal events 

(in agreement with Herz, 1970) at about 1,350, 

1,000, and 500 m.y.

Prior to the work of the D.N.P.M.—U.S.G.S. 

team, most workers interpreted the gneiss and foliated granitoid rocks of the area to be basement to 

the Nova Lima Group in the Rio das Velhas Series, 

although some workers in the 1950's interpreted 

some of the gneiss to be the result of granitization. 

Dorr (1969) and Herz (1970) differed with the 

earlier interpretations and considered all granitoid 

rocks to be intrusive into the metasedimentary rocks 

of the area. Herz divided these rocks into an earlier 

pre-Minas suite of granodiorite and quartzo-feldspathic gneiss and a later post-Minas suite of granite 

and gneiss. He concluded that the gneiss may have 

originated by ultrametamorphism of sedimentary 

rocks and also that some of the granodiorite and 

granite may have crystallized from anatectic melts. 

The granodiorite and granite were considered to be 

younger than the Rio das Velhas Series, and much 

of the gneiss was thought to be granitized Nova 

Lima schist. The interpretation of Dorr and Herz 

on the intrusive nature of the granitoid rocks has 

been disputed in several more recent publications. 

For example, de Almeida and others (1973), in a
B6 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

TABLE 1.—Summary of isotopic age dates for the Bagao and 

Bonfim Complexes, Quadrilatero Ferrifero. Minas Gerais

Age (m.y.)

Sample Eock Mineral Rb/Sr K/Ar

Ba^ao Complex

Ha-4 ___ Granitoid from 

core of complex Biotite 

do

Feldspar 

Ha-2 _ Gneiss Biotite

Ha-3 do _ _ do

_ _do

DTM-1 _ do do

Feldspar 

Ha-28 __ Granitoid rock 

(palingenetic?) Biotite 

do

Feldspar 

Ha-29 _ do Biotite

DTM-2 __ Amphibolite in 

gneiss Muscovite 

DTM-3 __ Mica schist from 

Nova Lima 

Group _ _ do ____

———

1,080

495 

1,420

1,640 

2,675 

2,790

2,440 

2,510

1~340 

760 

720

1,080 

1,050

~870 

975

1,700

Bonfim Complex

Ha-21 _ _ Granitoid rock Biotite 

Ha-24 do do

Ha-33 do do

DTM-5 do Biotite

Feldspar 

DTM-6 _ _ do _ Biotite

Feldspar 

DTM-7 _ do Biotite

Feldspar 

Ha-13 __ Gneiss Biotite 

Ha-23 _ do do

Ha-26 do do

DTM-4 _ do do

Feldspar

— —

510 

975 

1,320 

1,985 

630 

1,050

530 

2,300

514 

460 

493 

420

"§50

640 

470 

550 

600 

790 

600

summary of the Precambrian geology of Brazil, continued to regard these rocks as a basement for 

the overlying metasedimentary strata. Similarly, 

Fleischer and Routhier (1973), in a study of gold deposits in the southeastern part of the Quadrilatero 

Ferriferro, considered the gneiss and granitoid rocks 

as basement to the metasedimentary sequence.

Our interpretation of the gneiss and granitoid 

rocks in the Quadrilatero differs markedly from that 

of Dorr (1969) and Herz (1970). On the basis of 

investigations of these rocks in the Belo Horizonte 

area as well as in the Bonfim and Bacao Complexes, 

we have concluded that the gneiss and granitoid 

rocks represent a part of the ancient Sao Francisco 

craton and have an age in excess of 2.5 b.y. The 

Rio das Velhas Series, a graywacke-shale (flysch) 

sequence containing exotic blocks of disrupted 

ophiolite, was thrust over that craton and metamorphosed for about 2.5 m.y. After a period of erosion, 

the Minas and Itacolomi Series were deposited on 

the eroded remnants of the Rio das Velhas rocks and 

the exposed craton. At an unknown time (but perhaps at about 1,350 m.y.), a major erogenic event

was characterized by the diapiric protrusion of the 

granitoid and gneissic cratonic rocks upward into 

the overlying sedimentary rocks, resulting in the 

formation of a series of mantled gneiss domes. 

Minor palingenetic magmas probably were generated at this time and intruded the mantling rocks 

in the form of small sills and apophyses; high heat 

flow associated with the doming also may have resulted in some contact-metamorphic effects. The 

evidence for this interpretation of the history of 

the granitoid rock and gneiss in this area is given 

in the following paragraphs.

Large-scale, crosscutting intrusive relationships 

are not characteristic of the contacts between granitoid rocks and the metasedimentary rocks of the 

Quadrilatero. A careful examination of the 1:25,000- 

scale maps made by the D.N.P.M.-U.S.G.S. team 

shows that the metasedimentary rocks are often exposed in thin sinuous ribbons around the boundaries 

of the gneiss and granitoid rocks. For example, the 

rocks of the Minas Series may be traced continuously around the Bonfim dome along the Serra do Moeda 

and westward to Brumadinho. (See fig. 2.) Nowhere along this trace of nearly 100 km are any 

typical intrusive contacts shown on the detailed 

maps (Simmons, 1968a; Pomerene, 1964; Wallace, 

1965; Guild, 1957). A similar relationship is also 

apparent in the Rio Piracicaba area in the eastern 

part of the Quadilatero (Reeves, 1966). In the area 

•of the Bacao dome, the Minas Series rocks form 

thin continuous ribbons around the south and western flanks of the dome; however, palingenetic magmas have intruded Minas rocks at one locality on 

the south flank of the dome, and a larger apophysis 

of magma invaded the Nova Lima rocks on the north 

side of the dome (Dorr, 1969, pi. 6).

Relationships of granitoid rocks and gneiss with 

metasedimentary rock appear to be faulted rather 

than intrusive or unconformable, as evidence of extensive shearing and cataclastic textures is found 

along contacts. The contact zones between Nova Lima 

rocks (including an unnamed staurolite schist unit) 

and granitoid terrane are zones of profound shearing where rocks have been almost completely pulverized into mylonite.

Mylonitized granitoid rocks and gneiss have a 

cataclastic fabric along the northwest front of the 

Quadrilatero from Igarape to Ibirite and Belo 

Horizonte, and eastward to the Piedade syncline 

(fig. 2). The granitic rocks within the Bonfim dome 

near Brumadinho and south along the west side of 

the Serra do Moeda have a similar fabric as do those 

in contact with metasedimentary rock near Caete
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS B7

B

FIGURE 3.—Photomicrographs of cataclastic granitoid rocks from the Quadrilatero Ferrifero. Plane polarized light. Width 

of view is 12 mm. A, Granite beneath the Piracicaba Group near Igarape. B, Granite beneath Moeda Quartzite, Serra 

do Moeda (X on fig. 2). C, Granodiorite beneath chlorite phyllonite after staurolite schist on highway from Barreiro 

to the Industrial City of Brazil (CC on fig. 2).
B8 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

and within the Bac,ao Complex. Typical cataclastic 

fabrics are shown in figure 3. Other evidence of 

shearing within the granitoid terrane consists of 

zones of "quartzite" near contacts with pre-Minas 

rocks (q on fig. 2). This "quartzite" was interpreted 

by Pomerene (1964) to be recrystallized chert of 

the Sabara Formation and to have survived granitization because of its bulk composition. A careful 

study of this material "shows that it is ultramylonite 

derived by crushing and chemical alteration of the 

granitoid rocks and that its outcrop pattern parallels 

the cataclastic fabric in the granitoid terrane. Similar "quartzite" was described by Simmons (1968a) 

and may have had a similar origin, although we did 

not investigate these rocks in the field.

The pre-Minas rocks are also sheared where they 

are in contact with the granitoid rocks, the shear 

fabric being parallel to the contact. This relation is 

particularly well exposed along the highway between Barreiro de Cima and the Industrial City of 

Brazil, where a wide zone of chlorite phyllonite 

after staurolite schist is in contact with granodiorite; the phyllonite is retrograded to the greenschist 

metamorphic fades and has two cataclastic foliations (CC in fig. 2). The rocks at this locality are 

so severely deformed that it is virtually impossible 

to locate the contact between the schist and granitoid rock. Another excellent exposure showing highly sheared pre-Minas rocks is found on the road to 

Piedade do Paraopeda on the west flank of the Serra 

do Moeda (just east of PP in fig. 2). At this place, 

severely sheared ultramafic rock and Nova Lima 

phyllonite are in contact with ultramylonite, mylonite, and granitic augen gneiss. At this locality, the 

cataclastic fabric also parallels the contact.

A comparison of tectonite fabrics of the g-anitoid 

rocks and gneiss and the sedimentary rocks shows 

that the granitoid rocks and gneiss have a complex 

tectonite fabric coupled with metamorphic differentiation into a layered sequence. The gneisses are 

in the amphibolite facies of the regional metamorphism and were derived from sedimentary or volcanic 

rock or from metamorphically differentiated intrusive rock. The gneisses have undergone severe strain 

as shown by floating folds and multiply folded folds; 

thus, these rocks are complexly polydeformed. An 

example of fold interference that is very common in 

these rocks is shown in figure 4. We found no examples of a fabric of this kind within the metasedimentary-metavolcanic terrane of the Quadrilatero. A staurolite schist unit (see below) is polydeformed, but it shows no evidence of metamorphic 

differentiation or beginning anatexis. Rocks of the

Nova Lima Group have a good penetrative cleavage 

(schistosity) and in most places a strain-slip fabric; 

however, the Nova Lima rocks never have the extent 

or style of deformation of the granitoid terrane. 

Younger rocks in the Quadrilatero are often greatly 

deformed and are involved in overturned to recumbent folds, but, except for the staunolite schist, we 

have not observed more than one fabric element in 

these rocks; in many exposures, even a penetrative 

axial surface cleavage is lacking. An excellent example of this is in the overturned fold along the 

Serra do Curral south of Belo Horizonte, where a 

distinct tectonic break is evident between the rocks 

of the granitoid and gneiss terrane and those rocks 

above it.

Comparison of major rock types and chemical 

compositions of granitoid rocks and gneiss and those 

of the sedimentary rocks shows a great dissimilarity 

between the two groups of rocks. In table 2, average 

analyses of granitoid rocks and gneiss taken from 

Herz (1970) are compared with averages for graywackes, carbonate-rich phyllites, and pelites taken 

from Herz (1978). Although there is some chemical 

similarity between the analyses of the gneiss and the 

graywacke, the amount of graywacke in the Nova 

Lima Group is very minor. Most of the Nova Lima 

Group in the Rio das Velhas Series is composed of 

rocks that have lower SiO2 content and either higher 

Al aO3 (pelitic rock) or CaO content (calcareous phyllite) than does the quartz-feldspar-rich gneiss. Conversion of the pelites and calcareous phyllites to a 

quartz-feldspar gneiss would involve very extensive 

metasomatism, including migration of aluminum, an 

element usually considered to be relatively immobile

TABLE 2.—Comparison of the chemistry of gneiss with graywacke, pelite, and calcareous phyllite of the Nova Lima 

Group

Gneiss

Nova Lima 

graywacke 2

Nova Lima 

pelite 3

Nova Lima

calcareous *

phyllite

S,02

ALOs

Fe2O3

FeO

MgO

CaO

Na20

K20 _

H2O

MnO 

CO2

74.4

13.2

.65

1.2

.40

.90

2.4

5.0

.•:o

.20 

.05 

.04

.27

66.6

16.2

1.1

2.8

1.6

3.3

4.4

1.8

.54

.19

.06

1.5

.22

58.4 

18.8 

3.8 

5.7 

3.7 

.36 

.60 

3.6 

4.1 

.53 

.10 

.09 

<.05

54.4

13.2

1.8

5.8

7.3

3.9

2.0

1.8

4.0

.45

.07

.05

6.2

Total _ 99 100 100 100

- * Analyses from samples Ha-21, Ha-23, J-82, Ha-11, Ha-19, Ha-7, 

Ha-36, Ha-38 (Herz, 1970).

2 Analysis from sample JG 241-55 (Herz, 1978).

3 Analyses from samples 2-842, A-4953 (Herz, 1978).

4 Analyses from samples C-5, C-8, 2148, 2554 (Herz, 1978).
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS B9

FIGURE 4.—Polyphase folds in the granitoid terrane along the north front of the Quadrilatero Ferrifero. A and B, Two 

sets of recumbent folds in gneiss beneath staurolite schist along Rio das Velhas north of Sabara. C, Type III foldinterference patterns (between arrows) in gneiss beneath staurolite schist near Igarape. Z, M, and S refere to rotation sense of second-generation folds.

during metasomatic exchange. The greenschist metamorphic facies of the Nova Lima Group and the 

amphibolite facies of the gneiss (now retrograded 

to greenschist facies) do not suggest that temperatures were attained during regional metamorphism 

that would result in the ultrametamorphism of the 

Nova Lima rocks and conversion of a large part of 

it to a monotonous sequence of quartz-feldspar
BIO TECTONIC STUDIES IN THE BRAZILIAN SHIELD

AI 2 03

Staurolite

EXPLANATION

Mineral assemblages containing in addition 

to quartz, plagioclase, and muscovite:

1. Chlorite

2. Chlorite and biotite

3. Chlorite and biotite and garnet

4. Chlorite and garnet and staurolite

5. Chlorite and staurolite

Garnet

MgO

Biotite 

FIGURE 5.—Al2Os-Mgo-FeO-K2O projection with tielines for mineral assemblages reported for the staurolite schist.

gneiss in the areas of the Bonfim and Bagao domes, 

as suggested by Simmons (1968a) and Herz (1970). 

The presence of staurolite schist along many contacts with granitoid rocks has been interpreted as 

a contact-metamorphic aureole produced by the intrusion of the granitoid rock (Dorr, 1969). A compilation of metamorphic facies and zones by Herz 

(1978, pi. 1) has shown that staurolite schist in contact with granitoid rock extends all along the Serra 

do Curral, partly around the granitoid core of the 

Bac.ao dome, and over a large area in the extreme 

southern part of the Quadrilatero; however, the 

staurolite rocks are absent in the northwest part of

the Quadrilatero. We have reexamined the staurolite schists in the Rio das Velhas section first described by Gair (1962) and along the south flank of 

the Bagao dome and have concluded that these 

higher grade schists are pre-Nova Lima in age and 

metamorphism and either rest unconformably on 

the granitoid terrane or form a thrust sheet emplaced over the granitoid rocks and gneiss before 

the thrusting of Nova Lima rocks.

The staurolite schists described by Gair (1962) 

are in the Rio das Velhas area north of Sabara 

(just north of S in fig. 2) ; Gair placed these rocks in 

the Sabara Formation within the Minas Series. This
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS Bll

FIGURE 6.—Schists from the Quadrilatero Ferrifero. 

A, Staurolite-garnet schist from valley of the Rio das 

Velhas north of Sabara. Flat foliation is secondgeneration schistosity that is axial surface cleavage 

to folds in first schistosity. Hinges can be seen to 

right of pen. B, Proximal graywacke of Sabara Formation from new railroad cuts south of Ibirite. Beds 

top to the left (north) as shown by grading (dark 

part of beds are coarse, light parts are fine).

formation was mapped in two parts, one part altered 

by contact metamorphism and another part affected 

only by low-grade regional metamorphism. We have 

found that the low-grade part of the sequence is 

both pelitic and semipelitic and contains abundant 

interbedded graywacke and intervals 50 to 100 m 

thick of very proximal graywacke. Graded beds, Cinterval cross lamination, and abundant sole marks 

are beautifully developed. These Sabara rocks are in 

the regional greenschist facies and have only one 

penetrative cleavage, a cleavage often absent in the 

thicker graywackes. Bedding in these rocks can be 

seen in most exposures. The staurolite-bearing part 

of the sequence is almost entirely pelitic. The basic 

mineral assemblage is quartz-muscovite-plagioclase 

and biotite-chlorite-garnet or chlorite-garnet-staurolite. The most common assemblage, however, 

is quartz-muscovite-plagioclase-chlorite. The presence of coarse-grained conspicuous biotite, garnet, 

and staurolite is dependent on more extreme compositional variations in the rock sequence. Quite 

coarse-grained staurolite was found by Gair (1962) 

and by us near the granitoid terrane; staurolite was 

also found by us as fine-grained crystals in thin sections of rocks from areas of supposed lower grade,

\\ \\Vvl \\ \\v\i

where rocks are dominantly quartz-muscovite-plagioclase-chlorite. The suggested prograde zonation 

crosses no tielines of a Thompson-type Al2 03-MgOFeO-K2 0 projection (fig. 5), and the appearance of 

coarse-grained staurolite is the function of sediment 

composition rather than a thermal gradient.

The staurolite-bearing part of the Sabara section 

is so severely deformed that sedimentary structures 

other than sparse disrupted beds cannot be seen. The 

staurolite-bearing rocks have been deformed at least 

three times (fig. 6). Staurolite and garnet porphyroblasts, both megascopic and microscopic, grow in 

the second schistosity (the flat foliation in fig. 6), 

which is axial surface to folds in the first schistosity. 

This texture is not typical of those formed by contact metamorphism.

The contact between these two rocks units is 

abrupt, knife-edged, and well exposed in new highway cuts. We conclude, therefore, that the staurolite-bearing rocks are a different sedimentary sequence that has a faulted contact with the low-grade 

rocks and that cannot be a part of a thermal aureole.

The staurolite schist mapped as Sabara Formation can be traced along the Serra do Curral both 

east and west of the valley of the Rio das Velhas.
B12 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

It is especially well exposed along1 the highway between Barreiro de Cima and the Industrial City of 

Brazil (east of CC in fig. 2), and in the Ibirite 

area. The rock at these places is exactly the same as 

that north of Sabara in both lithology and structural fabric; the staurolite forms porphyroblasts in 

the second schistosity.

Staurolite-garnet schist identical in lithology and 

tectonic style was mapped as Nova Lima Group in 

exposures on the southeast border of the Bagao 

Complex (RC on fig. 2). In this area, the staurolite 

.schist unit tectonically overlies sheared felsic gneiss 

and is unconformably overlain by Moeda Quartzite 

(fig. 7). The tectonic fabric in the staurolite schist 

is truncated by the unconformity and is folded with 

the bedding of the Moeda. There is no petrographic 

evidence of retrogressive metamorphism. Strain-slip 

cleavage is axial surface to these folds in the staurolite .schist and parallels the cleavage in the Moeda. 

The staurolite schist in this area is severely deformed, and the lower part is a tectonic melange 

containing blocks of gneiss and pegmatite as well 

as disrupted blocks of the staurolite unit.

Along the east contact of the Bagao Complex, the 

staurolite schist is cut out by a thrust fault that 

brings greenschist facies Nova Lima rocks in contact with granitoid rocks. Farther north along this 

contact, the staurolite schist emerges from beneath 

the Nova Lima thrust sheet and can be traced for

FIGURE 7.—Unconformable contact between staurolite 

garnet schist to left and Moeda Quartzite on southern flank of Bagao dome. Both first and second 

schistosities in the staurolite unit are truncated by 

the unconformity.

about another 7 km, where it again is covered (Herz, 

1978, pi. 1). The mapping by the D.N.P.M.-U.S.G.S. 

team clearly shows that both greenschist and amphibolite facies rocks are in contact with granitoid 

rocks of the Bagao Complex, and we conclude that 

the staurolite schist does not result from contact 

metamorphism.

We propose that all the staurolite schist rocks are 

the same unit, whether mapped as Nova Lima Group 

or Sabara Formation. The distribution of this unit 

suggests strongly that it lies in tectonic lenses directly above the granitoid craton. Along the Serra 

do Curral, it is beneath a major thrust that brings 

rocks of the Piracicaba Group in contact with older 

rocks all along this mountain front. Around the 

Bagao and Bonfim domes, it lies beneath thrusted 

Nova Lima Group or unconformably beneath rocks 

of the Minas Series.

Stratigraphically, the staurolite schist could be 

considered as the basal part of the Nova Lima Group 

of the Rio das Velhas Series on the basis of the 

original definition of that series as comprising all 

pre-Minas rocks. However, we believe that these 

rocks should be considered as a different sequence 

because of their different metamorphic and tectonic 

aspect. Whether the staurolite schist is parautochthonous or whether it is a different thrust sheet 

beneath the Nova Lima allochthon cannot be conclusively proved. If this unit is parautochthonous, it 

probably unconformably overlies the granitoid rocks, 

a view held by Brazilian geologists currently working in the Quadrilatero (Pedro Gervasio Ferrari, 

oral commun., 1975). If this is true, the lenses of 

staurolite schist might be tectonic slices that have 

been overridden and partly dragged along by the 

advancing Nova Lima thrust sheet. If the staurolite 

schist is a separate allochthon, then it arrived before the Nova Lima sheet.

In summary, our interpretation of the granitoid 

rocks and gneiss in the Quadrilatero is based on (1) 

a scarcity of well-defined, crosscutting relationships 

between metasedimentary rocks and granitoid rocks, 

(2) evidence for faulting between metasedimentary 

rocks and granitoid rocks, (3) contrasting tectonite 

fabrics between metasedimentary rocks, granitoid 

rocks, and gneiss, (4) contrasting chemistry between gneiss and the Nova Lima Group, and (5) a 

reinterpretation of the staurolite schist unit, which 

would rule out evidence for widespread contact 

metamorphism in the area. However, many workers 

of the D.N.P.M.-U.S.G.S. team have given evidence 

for small-scale transgressive contacts between 

granitoid rocks and metasedimentary rocks (for
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS B13

example, Moore, 1969; Wallace, 1965; Maxwell, 

1972) and evidence for thermal metamorphism by 

the presence of small migmatite zones and the formation of feldspar- and tourmaline-rich zones in 

metasedimentary rocks at their contacts with granitoid rocks and gneiss (for example, Moore, 1969; 

Simmons, 1968b).

We suggest that all these data can be made compatible by interpreting the later history of these 

rocks in terms of the mantled-gneiss-dome concept 

first advocated by Eskola (1949) and later modified 

by Thompson and others (1968) and by Fletcher 

(1972). We also hasten to point out that both Simmons (1968a) and Herz (1970) have used the term 

"gneiss dome" in describing the Bonfim dome. They 

interpreted the dome as having been formed by the 

intrusion of granodiorite accompanied by extensive 

migmatization and ultrametamorphism of Nova 

Lima rocks to produce a mantle of gneiss. We contend, however, that the domes were produced during 

a later orogeny in which masses of granite and 

gneiss rose as diapirs and plastically accommodated 

themselves to the domal form structures in the 

heavier overlying (mantling) rocks. The formation 

of the mantled gneiss domes probably was accompanied by local palingenetic generation of magma; 

a higher than normal heat flow may have resulted in 

some contact metamorphism; however, the process 

was dominantly a plastic rather than a magmatic 

deformation. This interpretation is essentially the 

same as that advocated for the origin of gneiss 

domes in New England by Thompson and others 

(1968).

Our evidence for the presence of late-formed 

gneiss domes in the Quadrilatero is based in part 

on a reinterpretation of the maps and reports made 

by the D.N.P.M.—U.S.G.S. workers and compiled 

by Dorr (1969) and shown as our figure 2 and also 

by a study of the minor fold structures associated 

with a beautifully exposed contact of the Bagao 

Complex with metasedimentary rocks in a railroad 

cut south of Santo Antonio dos Tabuoes (RC on fig. 

2). In this exposure, staurolite schist is tectonically 

above sheared granitic gneiss and is unconformably overlain by Moeda Quartzite. Planar structures 

(schistosities in the schist, bedding, and cleavage in 

the Moeda) dip to the southeast off the gneiss dome. 

Small folds in schistosity in the staurolite schist and 

bedding in the Moeda cascade down the flank of 

the dome, forming "Christmas tree" folds. The 

"Christmas tree" folds occur in both the old polydeformed rock and the Moeda Quartzite; clearly, 

no decollement is present between the two rock units.

The folds here resulted from a rotational couple 

caused by the drag on cover by the rising core of 

the dome. Attenuation of the cover by basement rise 

is shown by the severe tectonic thinning of the 

Moeda Quartzite along the south flank of the Bagao 

dome (fig. 2).

These structural relations indicate that the Moeda 

and the rest of the Minas Series were in place on 

the staurolite schist and granitic basement here before the doming event. The doming, therefore, could 

not have been the result of pre-Minas intrusion, as 

visualized by Herz (1970). Neither could the doming be the result of post-Minas intrusion, as intrusive contacts are lacking and Minas Series pelitic 

rocks are at chlorite grade. Therefore, the granitoid rocks of the Bagao Complex must have been 

deformed into a domelike structure along with the 

overlying metasedimentary and metavolcanic rocks. 

The doming is post-Minas and probably post-Itacolomi. The doming may have been coincident with 

a higher heat flow, which may be reflected in the 

wide scatter of age determinations shown in table 1 

and reported by Herz (1970).

Other gneiss domes are probably present within 

the Quadrilatero (fig. 2). The Bonfim terrane is 

probably a mantled dome and was so called by Simmons (1968a). Relations on the west flank of this 

structure are somewhat more complicated in places, 

as along the road to Moeda on the Serra do Moeda 

(X on fig. 2), where Moeda Quartzite has been 

thrust onto sheared basement during post-Minas deformation. The roughly circular outcrop area of 

granitoid rocks east of Caete (Ca on fig. 2) is perhaps another gneiss dome. The uplifted area west of 

the Serra do Caraga is also possibly a mantled dome. 

If this last structure is a dome, it is still "roofed," 

but it is flanked by rocks of the Minas Series in a 

domelike pattern and has a dimensional similarity to 

the Bagao and Bonfim domes.

The structural pattern of the Quadrilatero has 

been interpreted as resulting from the folding and 

faulting of a thick sequence of metasedimentary and 

metavolcanic rocks in a regional terrane dominated 

by giant buttresses of pre-Minas granitoid rocks. 

Our interpretation, however, is that the structural 

pattern results from the emplacement of broad 

gneiss domes and the attendant pinching of cover 

into tight polyoriented synforms.

THE RIO DAS VELHAS SERIES

A sequence of rocks older than the Minas Series 

and lying below an angular unconformity was first 

described by Dorr and others (1957) as the Rio das
B14 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

Velhas Series. This series was correlated with an 

unnamed "greenschist" series described by Guild 

(1957) and believed by him to be contemporaneous 

and in fault contact with the Minas rocks in the 

Congonhas district in the southwest part of the 

Quadrilatero Ferrifero. Dorr and others (1957) 

considered that the base of the Rio das Velhas Series 

was not exposed and that all contacts with granitoid 

rocks and gneiss were intrusive. Descriptions of the 

Rio das Velhas Series are found in Guild (1957), 

Gair (1962), Johnson (1962), Dorr and Barbosa 

(1963), Wallace (1965), Reeves (1966), Simmons 

(1968a), Moore (1969), Dorr (1969), and Maxwell 

(1972). Exceptionally complete lithologic descriptions are in Gair (1962), and a summary of stratigraphic interpretations was made by Dorr (1969). 

Dorr divided the series into two major groups—the 

widespread Nova Lima Group and the more restricted Maquine Group.

The Nova Lima Group consists dominantly of 

chlorite, sericite, quartz-sericite, and, to a lesser extent, graphite-sericite phyllite. Chlorite-carbonate 

greenschist, probably of volcanic origin, is also 

abundant. Gair (1962) has estimated that the minimum thickness is at least 4,000 m in the northcentral part of the area shown in figure 2 in the Rio 

Acima quadrangle. Graywacke is an additional but 

minor component of the group. Iron formation of 

the carbonate facies is also present as small discontinuous lenses throughout the group, having an average thickness of 5 to 10 m. In some areas, the iron 

formation grades into bedded chert (Dorr 1969). 

Because of extensive weathering, which selectively 

destroys volcanic rocks and graywackes, no worker 

in the area has been able to estimate with any degree of confidence the ratio of volcanic, volcaniclastic, 

and graywacke rocks to the more dominant phyllite 

units. Extensive weathering, low economic interest, 

and lack of key beds have all contributed to a very 

poor understanding of the structure and thickness 

of the Nova Lima Group.

Ultramafic and related mafic rocks are widespread 

in the Nova Lima Group and are not present in any 

other group of rocks within the entire Quadrilatero 

Ferrifero (Dorr, 1969). Figure 2 shows the general 

distribution of the larger ultramafic bodies within 

the area. All the ultramafic rocks are completely 

serpentinized to antigorite, talc, magnetite, and 

minor carbonate and are present as small, highly 

sheared tectonic blocks that crop out over areas of 

as much as 8 km2 . The ultramafic rocks are intimately associated with metagabbro (for example, in the 

Rio Acima quadrangle (Gair, 1962) and the Itabira

district (Dorr and Barbosa, 1963)) and with greenstone (or greenschist) sequence of metavolcanic 

rocks (for example, the Congonhas area (Guild, 

1957) and the Serra do Caraca (Maxwell, 1972)). 

Outcrops of ultramafic rocks are characteristically 

(although not invariably, see fig. 2) in a section of 

the Nova Lima Group just at or near the sheared 

contact with granitoid rocks. At places, however, 

there are intervening tectonic lenses of staurolite 

schist. The association of lenses or tectonic blocks 

of ultramafic rocks with fragments of metagabbro 

and extensive metavolcanic rocks, all in a part of a 

thick flysch sequence, suggests to us that fragments 

of a disrupted ophiolitic sequence are present in 

the Nova Lima Group. In the following paragraphs, 

the principal occurrences of the ultramafic and associated rocks in the Quadrilatero Ferrifero are 

described.

The Maquine Group is exposed in a syncline east 

of the Rio das Velhas and is believed by Gair (1962) 

and by Dorr (1969) to overlie the Nova Lima Group 

with an angular unconformity; however, many of 

the contacts appear to be faulted (Moore, 1969). 

The group contains sericitic quartzite, quartzose 

phyllite, and conglomerate beds. Total thickness 

ranges from a composite total of about 2,000 m down 

to a featheredge along erosional contacts. Volcaniclastic rocks and greenschist are absent, as are ironformation and carbonate rocks.

Dorr (1969) has considered the Nova Lima Group 

to be typical of a graywacke shale (flysch) suite, as 

described by Pettijohn (1957), on the basis of its 

thickness, predominantly argillaceous nature, and 

general scarcity of carbonate rocks and quartzite. 

The Maquine Group is considered to be a typical 

molasse suite on the basis of the presence of basal 

conglomerates and an increase upwards in the section of quartz and subgraywacke, finally progressing 

to clean quartzite and grits. The Rio das Velhas 

Series rocks "seem to form a single evolutionary 

stratigraphic entity reflecting a progression from 

eugeosynclinal to paralic sedimentation" (Dorr, 

1969, p. 27).

Serpentinite and talc schist underlie an area of 

about 8 km2 in the Rio Acima and Macacos quadrangles (u on fig. 2) ; a smaller outcrop of metagabbro covers an area of about 4 km2 . The field relations of these rocks are shown in figure 8. Serpentinite is massive, dark to pale green, and contains no 

relict minerals. The mineral assemblage is antigorite and minor talc, magnetite, and carbonate. Many 

rocks are nearly monomineralic antigorite. Weathering textures suggest that the rocks originally were
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS B15

EXPLANATION

Iron formation

Talc schist

Metagabbro

m

Serpentinite

Contact

\ 55

Strike and dip of foliation

Overturned beds

JG-11-55

•

Sample analyses on table 3

FIGURE 8.—Geologic map showing distribution of ultramafic and mafic rocks in the Rio Acima and Macacos quadrangles.

Modified from Gair (1962) and Pomerene (1964).
B16 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

TABLE 3.—Chemical analyses and normative minerals from gabbro and ultramafic rocks, Quadrildtero Ferrifero

[All samples from Rio Acima and Macacos quadrangles. Samples JG-11-55 and JG-10-55 from Herz (1970, p. B19); other samples, this report. Chemical analyses are rapid-rock analyses using method of Shapiro and Brannock (1956). Cr and Ni by atomic-absorption spectroscopy]

Sample

Si02 _ _ _____ ___ .

A1203 ______________ __ _____

Fe203 _

FeO _ _ _ _ _ __ __.

MnO _ _

MgO _____

CaO _ _

Na_0 __ _ _ ___ _ .

K,>0 _

Ti02 __ _ __ _ _ _.

P2 05 — __ ___________ __ .

H20 _________________ _ __.

C02 _ _ _ __ ____ _.

Total

Cr _ _ _ ___ _ ___ _ _.

Ni _ _ _ _ _ ___ __ _ .

Gabbro

JG-11-55

Chemical analyses

. _ 50.6

. __ _ 15.5

1 Q

. _ 8.8

OQ

7 0

. _ __ 9.3

2.8

48

.__ __ .41

.08

2.8

._ _ _ <.05

.- __ 101

._ _ 10

._ 30

MG-4

(weight percent;

54.8 

14.3 

1.1 

8.0 

.12 

6.9 

7.5 

4.4 

.31 

.43 

.10 

1.82 

.03

100

<20 

220

Normative minerals (mol.

Q _

Or ____ _ __ _ __ .

Ab _ _____________________

An

C _ _ _ __ ____ ______ .

Di ______

Hy ________________________

01 _______ __ ____ .

Ar.

11 _____ _ _ _.

Mt

Hm __________ _

2.6

23.4

26.3

. __ 16.0

23.9

5.4

.2

.7

1.5

1.7 

36.5 

16.9

16.2 

26.4 

.3

.4 

.7 

.9

Pyroxenite

JG-10-55

Cr and Ni in ppm)

51.8 

3.4 

2.2 

5.0 

.16 

28.1 

1.2 

.04 

.02 

.10 

.04 

7.6 

.07

100

1,000 

300

percent)

3.8 

.1 

.3

4.8

.7

88.5

.1 

.1 

1.6

MG-8

41.9 

1.1

1.9 

5.5 

.18 

36.5 

.02 

.02 

.10 

.05 

.05 

11.26 

.03

99

6,800 

1,600

.5 

.1

.6

47.7 

49.5 

.1 

.1 

1.4

Peridotite

MG-9

43.6 

1.7

3.2 

1.3 

.04 

40.7 

.00 

.01 

.00 

.05 

.04 

8.88 

.02

100

8,000 

1,800

.6

.9

47.3 

49.0

.1 

2.0 

.1

either dunite or orthopyroxenite. "Bastite textures" 

suggesting serpentinized orthopyroxene are present 

in some rocks and make up the greater part of those 

samples. Chemical analyses of three ultramafic rocks 

(table 3) confirm these observations.

Sample JG-10-55 is an orthopyroxenite, normative orthopyroxene making up nearly 80 percent of 

the sample. The chromium content of 1,000 ppm is 

typical of ultramafic mantle-derived rocks, and the 

somewhat low nickel content of 300 ppm is in accord 

with a low modal abundance of olivine in the original rocks.

The metagabbro consists of massive medium- to 

coarse-grained rocks, which are completely fractured and are penetrated by numerous small veins 

of altered plagioclase. These rocks consist of hornblende and plagioclase altered to actinolite, albite, 

clinozoisite, zoisite, sphene, quartz, and carbonate. 

The grain size of the gabbro is erratic and changes 

abruptly within even a single outcrop. Chemical analyses of two samples of metagabbros given in table 

3 are similar to low-potassium tholeiite basalt containing minor olivine and considerable orthopyroxene in the norm.

Structural relationships between serpentinite and 

metagabbro are not clear because of very poor outcrops and extensive shearing in the southern part 

of the area, which has produced talc schists within 

both serpentinite and metagabbro. In addition, 

structural relationships with the surrounding Nova 

Lima rocks are not clear. However, the parallel contacts of gabbro with structurally overlying Nova 

Lima metavolcanic rocks, which contain interlayered 

carbonate iron formation, on the east side of the 

complex shown in figure 8, suggest the possibility 

that a stratigraphic sequence from serpentinite to 

gabbro to metavolcanic rock (containing intercalated carbonate iron formation) may be present 

within this area. No granitoid rocks are exposed 

near the ultramafic-mafic complex. Along the southwest margin, the complex lies unconformably beneath quartz-rich rocks of the Minas Series.

Occurrence of ultramafic rocks altered to serpentinite steatite, and talc schists have been described 

and mapped by Guild (1957) in the Congonhas district, an area famous in Brazil for the production of 

ornamental stone, statuary, and pottery from steatite. All ultramafic rocks are completely altered to
PRECAMBRIAN PLATE TECTONICS, BRAZILIAN SHIELD, MINAS GERAIS BIT

antigorite and talc and minor carbonate and magnetite. No gabbro is associated with the ultramafic 

rocks, but the ultramafic rocks are overlain by 

greenschists correlated with the metavolcanic rocks 

of the Nova Lima Group. The greenschists include 

quartz-muscovite plagioclase-biotite schists retrograded to chlorite and carbonate assemblages and 

more mafic rocks containing quartz-zoisite-hornblende; both rocks probably were derived from volcanic tuff or reworked pyroclastic rocks.

The ultramafic-greenschist assemblage is underlain by granodiorite, which is massive near the center of the exposed area. It is more sheared and foliated as the contacts with the ultramafic rocks and 

greenschists are approached. Guild (1957) considered that the greenschists were intruded by an 

ultramafic magma, forming a thick sill. The sill 

served to localize a later postorogenic intrusion of 

granodiorite that contained fragments of the older 

rocks as xenoliths within the granodiorite.

Most additional occurrences of ultramafic "sills" 

in the Quadrilatero Ferrif ero are at or near the contact of the Nova Lima Group and granitic gneiss 

and are exposed on the flanks of the geiss domes. 

These occurrences include outcrops on the easter^i 

flank of the Bonfim dome on the road to Piedade do 

Parapeba, on the eastern flank of the Bagao dome, 

and on the western flank of the Caete dome (fig. 2).

Ultramafic rocks at all the above localities crop 

out in long relatively thin bodies virtually conformable to the foliation of the rocks of the Nova Lima 

Group. These bodies are either at the contact of the 

Nova Lima with the granitoid rocks or are within a 

few hundred meters of that contact. The ultramafic 

rocks here are also composed of antigorite-talc-magnetite and minor carbonate. At several localities, the 

rocks are nearly monomineralic talc. On the Piedade 

do Parapeba road at the contact with the gneisses of 

the Bonfim dome, the contact of the ultramafic rocks 

with the overlying phyllite of the Nova Lima Group 

is marked by a zone of "blackwall" alteration. 

Here, the ultramafic rocks have the assemblage antigorite-magnetite and are separated from the phyllite 

by an inner rind of talc-carbonate and an outer rind 

of chlorite, similar to metasomatic alterations of 

serpentinite described by Chidester (1962) in Vermont. Thin dikes or sills of foliated mafic rock composed of coarse-grained hornblende, epidote, chlorite, 

tourmaline, and quartz are common in the area 

along the contact with the phyllite. Extreme weathering and rugged terrain make it difficult to determine the extent of ultramafic rocks in this area; 

however, the contact with the mylonitic granitoid 

rock of the Bonfim dome is easily traced.

At Santa Barbara, about 60 km east of Belo 

Horizonte, a "greenstone sequence" including antigorite, antigorite-talc, and antigorite-tremolite assemblages forms a long sill-like mass between the 

Nova Lima Group and the granitoid basement (Maxwell, 1972; Simmons, 1968b). Southward toward the 

Serra do Caraga, the structural relationships become more compilcated, and the compilation map of 

Dorr (1969) in this area provides no unique structural interpretation. The "greenstone sequence" of 

Maxwell (1972) extends in a broad arc to the east 

and southeast on the eastern flank of the Serra do 

Caraga.

In the Itabira district, about 80 km northeast of 

Belo Horizonte, rocks of the Nova Lima Group crop 

out between the granitoid terrane and the iron formations of the Minas Series. The structure of this 

famous iron-producing region is that of a tight syncline, which has undergone at least two periods of 

deformation (Dorr and Barbosa, 1963). Here, the 

lower contacts of the Nova Lima Group are also 

marked by extensive but irregular masses of antigorite and antigorite-talc serpentinites as well as 

by small bodies of amphibolite, which Dorr and 

Barbosa (1963) believe to have been derived from 

gabbro. Chromitite containing about 45 volume percent chromite was collected from this district (Dorr 

and Barbosa, 1963, p. 35) ; however, in all other 

localities, chromite is present only in very minor 

amounts.

Most workers in the Quadrilatero Ferrifero have 

believed that the base of the Rio das Velhas Series 

is not exposed and that the sill-like masses of ultramafic rocks represent intrusions during an early 

period of igneous activity in the area (Dorr, 1969). 

We, however, depart from the interpretation on the 

basis of the following observations: (1) Intrusion 

of sills of highly refractory rocks such as orthopyroxenite, dunite, and chromitite is unlikely to have 

taken place except at exceedingly high temperatures. 

No contact metamorphism has been observed around 

any of these bodies. (2) The ultramafic rocks generally are found at or near the lowest exposed section of a rock sequence containing flysch-type metasedimentary rocks as well as metavolcanic rocks. 

Such an association is suggestive of a disrupted 

ophiolite at the base of a deep marine sequence of 

rocks. (3) The ultramafic rocks are always at or 

near the contact between the Nova Lima Group and 

granitoid rocks, except possibly in the Rio Acima 

area. The contact is always a zone of profound 

shearing and cataclasis, suggesting a major thrust 

zone. From the above three relations, it seems more 

reasonable to us to interpret the Rio das Velhas
B18 TECTONIC STUDIES IN THE BRAZILIAN SHIELD

Series, including- broken ultramafic rocks of uppermantle derivation, as representing oceanic lithosphere that has been thrust over an ancient granitic 

cratonie massif.

CONCLUSION

In this paper, we interpret the older metasedimentary and metavolcanic rocks of the Quadrilatero 

Ferrifero as being in thrust contact with a granitoid 

terrane, which may be a part of the Sao Francisco 

craton. Ultramafic and related rocks previously 

thought to be intrusive into the Nova Lima Group 

are considered to be dismembered parts of an ophiolite sequence mixed within flyschoid rocks of that 

group. These rocks were thrust, with the rocks of 

the Rio das Velhas Series, onto the granitoid craton. 

After deposition of the Minas Series, and probably 

the Itacolomi Series, the granitoid craton was deformed into gneiss domes, the process being attended 

by severe folding of the metasedimentary-metavolcanic cover.

Our most important observation in this study is 

that the Quadrilatero Ferrifero contains remnants 

of an allochthon containing ophiolitic material obducted onto a cratonie margin during Archean time. 

Rubidium-strontium age determinations on muscovite from the Nova Lima Group on the flank of the 

Bagao gneiss dome gave an age of 2,790 m.y., and 

muscovite from within the dome gave an age of 

2,675 m.y. (Herz, 1970). These dates are substantially the same, are obviously from metamorphic. 

minerals, and therefore must represent a time of 

metamorphism for both rocks. Thus, the Nova Lima 

must have been emplaced on the granitic rocks of 

the Bagao dome prior to 2.6 or 2.7 b.y. These dates 

also give a minimum age for the deposition of the 

Rio das Velhas Series.

The considerable age of the 'Nova Lima rocks and 

their relationship to a possible plate-tectonic event 

prior to 2.6 b.y. suggests that at least some rigidity of lithospheric rocks could have been obtained by 

that time. At least in this region, the Permabile 

Phase of Burke and Dewey (1973) would have 

ended at some earlier time. The dominant mode of 

tectonics envisaged is the obduetion of a large 

oceanic section onto a craton; therefore, this region 

is not an example of intercratonic block tectonics 

but is more analogous to Phanerozoic tectonics, 

which may involve the large-scale transport of 

oceanic lithosphere onto continental margins (Coleman, 1977).

The direction of thrusting of the obducted plate

can only be speculative, but the plate may have come 

from the southeast. This conclusion is based on recent sedimentological studies of the Moeda Formation (D. A. Lindsay, written commun., 1975), which 

found that the ocean was to the south and east during early Minas time. We have no proof, however, 

that the older Rio das Velhas Series ocean was in 

that direction.

Almost certainly, the Rio das Velhas Series ocean 

is completely closed; any intraeratonic suture is 

cryptic and may be within the vast terrane of granitoid rocks and gneisses between the Quadrilatero 

and the Atlantic Ocean. A likely place to look for 

remanents of autochthonous Rio das Velhas rocks, 

in our opinion, would be in the area of the contact of 

the multiply deformed granitoid terrane characteristic of the area east of the Quadrilatero amd the 

homogenized granulite terrane farther east that 

passes from Sao Paulo through southern Minas 

Gerais to Espirito Santo.

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