The Neoproterozoic Dorsal de Canguçú strike-slip shear zone: its nature and role in the tectonic evolution of southern Brazil

Journal of African Earth Sciences, Vol. 29, No. 1, 0 1999

Pll:SO899-5382(99)00076-7

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reserved. 089%5362/99

PP.3-24.

Elsevier

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Printed in Great $- see front

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The Neoproterozoic Dorsal de Canguqii strike-slip shear zone: its nature and role in the tectonic evolution of southern Brazil LUIS ALBERT0

D’AVILA

FERNANDES’-*

and EDINEI

KOESTER*

‘Departamento de Geologia, lnstituto de Geociencias, Universidade Federal do Rio Grande do Sul & Pesquisador do CNPq, Caixa Postal 15065, 91501/970, Porto Alegre, RS-Brazil %urso de P&s-GraduacBo em Geocibncias, Universidade Federal do Rio Grande do Sul, Brazil

ABSTRACT-The Dorsal de Cangucu Shear Zone IDCSZ) is part of a strike-slip fault system showing trends parallel to the Neoproterozoic Dom Feliciano Belt in southern Brazil. As an attempt to assess the role played by this fault system in the tectonic evolution of the continental crust in southern Brazil, a re-evaluation of the main structural, magmatic and geochronological characteristics of the best known shear zone of this system was conducted. Magmatism syntectonic to the strike-slip shear zone is represented by mantle-derived granodioritic magmas emplaced into transtensional segments. These were followed by crustal melts, represented by successively younger peraluminous granites. The porphyritic granodiorites have a mixed origin involving a parental dioritic magma that suffered fractional crystallisation and assimilation of crustal rocks. They present a well-developed subvertical magmatic fabric with northeast to north-south trending foliation and low plunging lineations defined by dimensional orientation of K-feldspar megacrysts. Partial melting of the country rocks is the most likely petrogenetic process for the origin of the peraluminous granites. Microstructures produced by solid-state deformation under lower amphiboliteto greenschist-facies metamorphic conditions exhibit ubiquitous kinematic indicators of sinistral displacement. The contribution of the transcurrent fault zones to crustal growth was limited to the emplacement of relatively small volumes of mantle-derived dioritic magma during their early stages of development. Large-scale tectonic control of the overall strain field responsible for the nucleation and sinistral displacement of these faults is likely to be a far-field effect of convergence between the Kalahari and Zaire Cratons during the final stages of Neoproterozoic amalgamation of West Gondwana. Q 1999 Elsevier Science Limited. All rights reserved. RESUME-La zone de cisaillement de la dorsale de Cangucu (ZCDC) fait partie d’un systeme de failles en decrochement d’orientation NE, parallele a la ceinture neoproterozoi’que de Dom Feliciano dans le sud du Bresil. Nous proposons ici une reevaluation de la structure principale, des caracteristiques magmatiques et des donnees geochronologiques de la zone de cisaillement avec comme but la comprehension du role joue par ce systeme de faille dans I’evolution tectonique de la croute continentale. Le magmatisme syn-tectonique de la zone de cisaillement en decrochement est represente par des magmas granodioritiques d’origine mantellique mis en place dans des segments transtensifs. Ils sont suivis par des liquides crustaux represent& par des leucogranites hyperalumineux de plus en plus jeunes. La petrologic des granodiorites porphyroydes indiquent une origine par melange d’un magma parental dioritique ayant subi une cristallisation fractionnee avec un materiel crustal. La modelisation geochimique des leucogranites indique une fusion partielle des roches du socle. Les granodiorites presentent une petrofabrique magmatique subverticale tres developpee avec une foliation orientee NE-NS et une lineation de faible pendage definie par I’orientation des megacristaux de feldspath

*Corresponding [email protected]

author (L.A.D. Fernandes)

Journal

of AfricanEarth

Sciences 3

L.A.D. FERNANDES and E. KOESTER potassique. Les microstructures produites par la deformation B If&at solide a temperature decroissante (metamorphisme du facies schistes verts) presentent les indices cinematiques d’un deplacement senestre. La contribution de la zone de failles transcurrentes a la croissance crustale se limite aux faibles volumes de magma dioritique mantellique lors des stades precoces du developpement de ces failles. Le controle tectonique a grande dchelle de I’ensemble du champ de contrainte responsable de la nucleation et du deplacement senestre de ces failles semble resulter de I’effet Bloigne de la convergence entre les cratons du Kalahari et du Congo lors des stades terminaux du collage neoproterozoi’que de I’ouest du Gondwana. o 1999

Elsevier Science Limited. All rights reserved. (Received l/7/98:

revised version received 2013199: accepted 2713199)

INTRODUCTION The early work on shear zones focussed on their structural evolution and general aspects of magma transport along these structures (e.g. Ramsay and Graham, 1970; Nicolas et a/., 1977; Spera, 1980; Shaw, 1980). This was followed by a profusion of interest on mechanisms of magma emplacement. It became necessary to establish empirical criteria to distinguish between magmatic and solid-state structures and this was the emphasis during the 1980s and early 1990s (e.g. Vernon, 1983; Vernon et al., 1983; Castro, 1987; Paterson et a/., 1989; Hutton et al., 1990). The recognition of the genetic links between fault zones and magmatism directed the application of petrogenetic studies of syntectonic granites. These studies were directed to the investigation of the role played by fault zones as sources of magmas and as channelways for melt migration, from anatectic migmatites to shallow crustal levels (e.g. Mackenzie, 1985; Clemens and Vielzeuf, 1987; Holtz and Barbey, 1991; Hutton and Reavy, 1992). Most studies on shear zones were developed in strike-slip systems. Not only are these the most abundant in nature (Woodcock, 19861, but, as compared to extensional or contractional structures, strike-slip shear zones represent tectonic environments where the effects of deep-seated geological processes are most favourably exposed. These structures often reveal deep crustal levels and are common in Precambrian erogenic belts. An example of such a large-scale strike-slip fault with syntectonic magmatism is the Dorsal de Cangucti Shear Zone that crops out in southern Brazil and Uruguay (Fig. 1). This shear zone has been variously interpreted as a suture between two lithospheric plates, produced by oblique collision between the Kalahari and Rio de La Plata Cratons (e.g. Fragoso-Cesar et al., 1986; Fragoso-Cesar, 1991) or between the latter and the Dom Feliciano Craton (Issler, 1982). More recently it has been interpreted as an intracontinental transcrustal strike-slip shear zone (Fernandes eta/., 19931, responsible for late-collisional strain softening at the

4 Journal of African Earth Sciences

continental-scale (Tommasi et al., 1994) and as a terrane boundary (Soares and Rostirolla, 1997; Fragoso-Cesar and Machado, 1997). In an attempt to discuss the nature of the Dorsal de Cangucu transcurrent fault system and its role in the tectonic evolution of the continental crust in southern Brazil, an overview of the structural, magmatic and geochronological characteristics of this shear zone is presented. After a short account of the tectonic setting of this shear zone, the main conclusions of a preliminary study on the sources and petrological processes responsible for the origin of the syntectonic granitoids are presented. This is followed by a brief description of the main structures displayed by these granites and a short section about mechanisms of emplacement and their relations with the recognised stages of shear zone development. Short comments on the nature, timing and history of these shear zones and speculations about a possible origin for these faults are the subject of the last part of this paper.

TECTONIC SETTING OF THE DORSAL DE CANGUCU SHEAR ZONE The Dorsal de Canguclj Shear Zone cross-cuts the central gravity and magnetic domains of the Dom Feliciano Belt that is correlated with the Kaoko Belt in Namibia (Fig. 1). This erogenic collage was formed by convergence between the Kalahari and Rio de La Plata Cratons during the Neoproterozoic (Porada, 1979). Closure of the Adamastor Ocean and of a back-arc basin gave rise to a continental margin and an intra-oceanic magmatic arc with intervening fragments of reworked continental crust (Fernandes et al., 1992a; Leite et al., in prep.). Lithotectonic units of the Dom Feliciano Belt (DFB) and their collisional east-west trending fabrics developed under high T conditions and were overprinted by a lower T (amphibolite- to greenschist-facie4 deformation with tectonic transport parallel to the belt’s trend. This deformation was accommodated along

The Dorsal de Cangu@ strike-slip shear zone: the tectonic evolution of southern Brazil

Fig.

2a

Y-

_.

.- -

Movement direction

:m

collisional 3

i

’

I-

a 7

/ /

i a m m

transcurrent

Phanerozoic

sequences

Post-tectonic

granites

Eo-paleozoic sequences Syntranscurrent granitoids Western magmatic arc assemblage Back-arc basin assemblage Eastern magmatic arc assemblage Transamazonian

basement

Figure 1. la) Tectonic setting of the Dom Feliciano Belt and Dorsal de Cangucci Shear Zone (DCSZJ. The cratonic areas are represented in grey, while the intervening deformation belts and major shear zones are shown as white areas and black lines, respectively. The Dom Feliciano Belt (DFJ is located between the Rio de La Plata lRPJ and Kalahari IKJ Cratons. The Kaoko Belt (KOJ represents its physical continuity in Namibia. The Damara Belt IDA), belonging to the same geodynamic system, is situated between the Zaire (ZJ and Kalahari IKJ Cratons. lb) Outline of the main tectonic units of the Dom Feliciano Belt in the Urugua yan-Southern Brazilian Shield showing the position of the DCSZ, represented in the map by their synkinematic granites (shades of greyl. Open arrows represent the stretching lineations of the collisional deformation, with arrow heads pointing the top-to-west movement along (originally) flat-lying shear zones. The tectonic transport directions of this deformation are at high angles to the length of the belt. Black arrows represent stretching lineations belonging to the younger belt-parallel tectonic transport along both (i) the strike-slip shear zones in the eastern region, where rocks of the magmatic arc and basement are reworked; and (ii) the flat-lying shear zones in the cent&part of the DFB, where reworking of Palasoproterozoic basement is predominant. In the latter, upthrust sense towards the northeast is indicated by arrow heads. To the south, in Uruguay, the possible physical continuity of the Dorsal de Cangucci Shear Zone (DCSZJ is named the Zona de Cisalhamento de Sierra Ballena (ZCSBJ. To the northeast, despite the large cover sequence of the Parana Basin, the DCSZ can be correlated to the Zona de Cisalhamento Major Gercino IZCMGJ.

Journal of African Earth Sciences 5

-3O”OO’s

-3OO30’ s

I- Eastern domain II- Central domain

a

Figure 3

Figure 2. Gravimetric and aeromagnetic models of the central and eastern parts of the Dom Feliciano Belt (DFBJ showing (al the geophysical domains: eastern (I), central (II) and western (Ill). The position of the main tectonic discontinuities between the geophysical domains and the younger Dorsal de Canguru Shear Zone (DCTSZ’J is marked in the diagram. An enlargement shows gravimetric contours forpart of this area with northeast-southwest elongation of the gravity lows corresponding to the peraluminous granites cropping out along the shear zone. Diagram (b) presents magnetic data for the region studied. Note the boundary between the eastern (I) and central (II) magnetic domains separated by the Port0 Alegre Suture (PDASJ. This structure is a magnetic and gravimetric discontinuity, possibly related to the late stages of collision between the eastern geophysical domain (I), strongly reworked by magmatism and deformation, and the more preserved basement rocks of the central domain (II). The Port0 Alegre Suture (PDATSZJ is older than the transcurrent shear zone system, being cut by the Transcurrent Shear Zone of Port0 Alegre (ZCPDAJ, marked by the outcrop of the Lomba do SabSo porphyritic granodiorite (cf. Menegat et al., 1998). This relationship is more evident in the eastern domain (I) where the east-west magnetic anomalies, which track the attitude of the main fabric of the deformed rocks of the magmatic arc, are truncated by the northeast trending discontinuities representing the transcurrent shear zones and their reactivations. In the central domain (IIJ, the Dorsal de Cango@ Shear Zone (DCTSZ) converges towards the Port0 Alegre Suture along its southwestern segment, suggesting that the older discontinuity has behaved as a zone of pre-existing weakness during the development of the transcurrent shear zones (modified after Fernandes et al., 1995b. figs 1 and 21.

The Dorsal de Canguqli

strike-slip

shear zone: the tectonic

mid-crustal flat-lying shear zones in regions where PalEoproterozoic basement rocks crop out. Deformation with similar metamorphic grade and compatible kinematics was developed along the transcurrent shear zones in the eastern segment of the DFB, where they overprint collision-related talc-alkaline plutonic rock and their higher T fabrics. The compatibility of flat-lying and transcurrent deformations in terms of kinematics, metamorphic conditions and relative ages and the lack of any direct evidence about their mutual relationship, led to speculations about their possible cogenetic nature (Percher and Fernandes, 1990; Percher, 1992; Fernandes et al., 1993). According to this model, the transcurrent shear zones may be interpreted as ‘lateral ramps’ of northeast-southwest moving blocks of crustal material or as roots of a large-scale ‘flowerstructure’. The mantle signature of the oldest syntranscurrent granites and the geophysical data (Fig. 2) would favour the last interpretation. However, a cogenetic origin cannot be directly demonstrated since there is no continuity between the flat-lying and transcurrent shear zones. It is obliterated by low T phyllonites and the intrusion of post-transcurrent alkaline granites. Nonetheless, it is evident that the northeast trending shear zones of the Dorsal de Cangucu system cut across tectonic units of the erogenic collage as well as their boundaries. This can be clearly observed towards the south, where the shear zone merges into a geophysical discontinuity known as the Port0 Alegre Suture Zone (Fig. 2bI. This latter structure was interpreted as a suture between the central and eastern geophysical domains of the Dom Feliciano Belt (Fernandes et al., 1995b). The Dorsal de Canguclj Shear Zone is the best known fault of the transcurrent system in the central and eastern part of the Uruguayan-Southern Brazilian Shield (Picada, 1969, 1971; Fernandes et al., 1988, 1990, 1993; Tommasi etal., 1994; Costa etal., 19941. The main petrological, structural and geochronological characteristics of this shear zone will be presented below and are used as the empirical basis for the discussion on tectonics that follows.

GEOLOGY OF THE DORSAL DE CANGUClj SHEAR ZONE The Dorsal de Canguclj Shear Zone (DCSZ) runs for several hundred kilometres, reaching a maximum thickness of 20 km, as recognised from the mapping of its syntectonic granites. It also shows variable strikes, from a northeast trending structure along its northern segment in Brazil, to a north-south trending fault in its southernmost outcrop area in Uruguay (cf. Fig. 1).

evolution

of southern

Brazil

Although high strains are commonly assumed for large-scale shear zones, it was not possible to obtain an estimate about the displacement along the DCSZ. This was precluded by heterogeneity of deformation, absence of adequate strain markers and a complex history of reactivation. Estimations based on the relationship between length and thickness are within a few tens of kilometres, despite difficulties in determining the entire length of the Dorsal de Cangucu Shear Zone. The presence of the same rock units on both sides of this shear zone, north of where it merges into the Port0 Alegre Suture (Fig. 2b) is suggestive of small displacements (Fernandes et al., 19931. Tectonites formed during relatively high T deformation within this structure are restricted to the older granitoids, being associated with the development of decametre-thick amphibolite-facies shear zones. Solid-state deformation of the alkaline granites are predominantly cataclastic or typical of the brittleplastic transition. The rare microstructures characteristic of ductile behaviour observed in these rocks are restricted to the fault-bounded margins of the intrusions and therefore ascribed to advective heat (Frantz and Fernandes, 1994). Rb-Sr (whole rock) and K-Ar ages recently obtained show good agreement with the field-based stratigraphy, despite the error intrinsic to the methods adopted (Koester et al., 1997). Generation and emplacement of granites with different ages and compositions were controlled by the Dorsal de Cangucli Shear Zone and its several episodes of reactivation. The relative ages of these rocks were established on the basis of intrusive relationships, presence of xenoliths of older units within successively younger granites and the relations between intrusive features and tectonic fabrics. The composition of granites genetically or spatially related to the Dorsal de Cangucu Shear Zone present a distinct petrological pattern, from less to more differentiated magmas. The oldest and less differentiated porphyritic granodiorites with a mantle signature were the first magmas to be emplaced in the transtensional segments of these faults. They were followed by the emplacement of crustal-derived peraluminous (‘two-mica’) leucogranites. The latter represent the largest volume of syntranscurrent paraautochthonous magmas, so they can be used as a diagnostic tool for the recognition of similar shear zones elsewhere (cf. Menegat et al., 1998). The peraluminous leucogranites are represented by two major intrusions recognised on the basis of intrusive relationships, diverse deformation history, geochemical characteristics and radiometric dating. The genetic relationship between the peraluminous granites and the shear zones was first proposed by

Journal of African Earth Sciences 7

L.A.D. FERNANDES and E. KOESTER 52’05’

I

N

I

N

I”

52”45’ 30030 I~_

+ +I m

Phanerozoic sediments tectonic Dom Feliciano granitoids

+ +

+ + + + + + + + + +

+

Cataclasites

KI I

LJ

Phyllonites and qz-mylonites Isotropic peraluminous granite Banded peraluminous granite

+

Porphyritic granodiorite

a +

m

A

-

+

Capivarita anorthosite Calc-alkaline granitoids

4 + +

B.

+

Hugh-grade supracrustal rocks

+ +

+

+

+

+

+

0

Encruzilhada do Sul

-3

Tectonic Lineation

A

Tectonic Foliation

+ -

> +++u+++++++ + + + + + + + + + + + + + + + + + + + + + + +

&

Magmatic Foliation

+

Magmatic Lineation

Figure 3. Simplified geological map of the northern segment of the DCSZ, Encruzilhada do Sul region, showing the distribution of the syntranscurrent granitoids. Note the small proportion of the host rocks in relation to the younger granite intrusions. Foliations and lineations represented in the stereonets are collision-related structures and therefore older than the transcurrent deformation.

Picada (1969), who referred to these rocks as ‘estratoide’ (layered) granites due to their northeast elongated shapes and the well-developed banding (of

8 Journal of African Earth Sciences

old peraluminous intrusion). However, in his paper, the origin of granites, such as the porphyritic granodiorite, was ascribed to metasomatic processes

the

t

I

All ages in Ma. ?? : KIAr ages in biotite; * ?? : K/Ar ages

Collisionrelated granitoids

jyntranscurren granitoids

Post-to latetranscurrent granitoids

I-

in

muscovites;

et al.

et al.

da Silva et al.

Chemale

Koester

?? : U/Pb Shrimp ages; Fib/% are whole rock ages.

????

Table 1. Radiometric ages of granitic rocks related to the Dorsal de CanguClj Shear Zone

(1997a)

(1995)

(I 997)

Thrusting with east-west tectonic transport

cataclasis

L.A.D. FERNANDES and E. KOESTER

Figure 4. Deformation features of the porph yritic granodiorite and the old two-mica leucogranite. (al Shear zone (dark band of the southwest half of picture) developed during the high temperature solid-state deformation episode is marked by strong grain-size reduction and stretching of kinzigite xenoliths (dark) in the porphyritic granodiorite. 16) Slightly deformed quartz fills a fracture in plagioclase IPI and shows optical continuity with interstitial quartz grains. This feature is interpreted as having developed during the submagmatic deformation stage. Enlargement is x 102. (cl Mylonitic foliation with fine-grained matrix due to low T dynamic recrystallisation of feldspar and quartz ribbons. Arrow indicates preserved m yrmekites that are typical of the previous stage of solid-state deformation under higher temperature conditions. Enlargement is x 128. fdl Leucogranite showing mullions and stretching lineation of quartz and feldspars marking the northeast-southwest direction of tectonic transport. fel K-feldspar IKI with interpenetrating boundaries and domains showing different optical orientation. Myrmekites developed along high-strain faces. Enlargement is x 102. If) Ultramylonite showing complete recrystallisation of feldspar-rich bands IFI and a few quartz IQ) crystalloclasts. Enlargement is x 102.

10 Journal of African Earth Sciences

The Dorsal de Cangucli

strike-slip

Table 2. Major and trace element porphyritic

shear zone: the tectonic

representative

analyses

old two-mica

granodiorite

evolution

of southern

for the syntranscurrent

leucogranite

Brazil

granitoids

young two-mica

leucogranite

1

2

3

4

5

6

7

8

9

SiO2

65.58

67.30

68.05

71.54

72.13

72.59

71.29

72.09

72.72

TiOz

0.64

0.59

0.63

0.07

0.09

0.03

0.13

0.07

n.d.

16.12

15.92

15.91

16.44

3.88 0.04 1.80 2.50

4.00 0.04 1.90 2.68

0.95

1 .I4

0.77

1.20

0.99

0.70

0.01

0.01

MgG CaO

4.17 0.06 1.97 2.82

0.67 0.81

0.71 0.77

n.d. 0.59 0.66

0.02 0.78 1.03

0.01 0.67 0.60

0.01 0.54 0.42

NazO

3.18

3.81

3.43

3.75

3.63

4.78

3.89

3.65

4.39

KzC

3.49

3.41

3.09

4.70

4.74

3.88

3.94

4.36

4.80

A1203 Fe0 * MnO

p205

total v Fib Ba Sr Zr La Ce Nd Sm ELI Gd DY Ho Er Yb LU

0.23

0.19

98.26 40 190 503 273

99.44 34 192 463 266 178 34.43 80.12 33.60 6.44

184 30.17 70.98 31.46 6.04 1 .Ol 4.50 3.52 0.66 1.54

1.25 0.19

1.01

4.67 3.41 0.62 1.39 1.05 0.17

15.98

15.82

16.35

16.28

15.59

0.08

0.16

0.09

0.24

0.15

0.09

99.94

99.02

99.36

37 171 434 255 191 42.95 92.78 38.85 7.51 1.05 5.31 3.64 0.64 1.27 0.85 0.17

n.d. 216 530 243 100 13.73 24.26 14.87 3.78 1.44 4.26 4.31 0.83 2.03

0.4 276 337 191 87 10.17 23.12 9.98 2.52 0.74 1.61 1.74 0.38 0.91 0.74 0.1 1

99.21 n.d. 231 467 272 117 5.65 12.21 3.92 0.99 0.29 1.10 1.65 0.34 1 .Ol 1.08 0.17

98.87 n.d. 302 218 145 92 15.04 27.78 19.40 4.70 1.12 4.35 4.27 0.82 2.03 1.62 0.22

98.87 n.d. 362 180 102 87 9.83 21.03 10.26 2.36 0.52 2.10 2.06 0.39 0.96 0.88 0.13

99.26 n.d. 354 106 45 66 4.50 8.26 3.19 0.61 0.18 0.80 1.29 0.26 0.75 0.98 0.14

0.21

1.38

0.20

1 to 3: porphyritic granodiorite; 4 to 6: old peraluminous leucogranite; 7 to 9: young peraluminous leucogranite. Analyses and 8 are averages of 28, 18 and 14 samples, respectively. Oxides are in % and trace elements in ppm. n.d.: not detected.

based on the presence of K-feldspar megacrysts. The occurrence of two peraluminous granite intrusions of similar composition but different ages was not recognised then. The last magmatic episode related to the brittle reactivation of the large-scale shear zones and the Pot-to Alegre Suture in this region is represented by alkaline, post-transcurrent (ca 545 Ma), sub-alkaline granite intrusions. These granites present typical features of epizonal intrusions, showing geneticallyrelated volcanic and pyroclastic rocks belonging to the cupola, miarolitic cavities and boundaries marked by both narrow lobate chilled margins as well as wide and straight cataclastic zones (Fig. 3). These rocks show no evidence of the decametre-thick ultramylonites and phyllonites, observed in the latetranscurrent, ca 580 Ma monzogranites (cf. ‘Encruzilhada do Sul Granitic Suite’; Table 1).

2, 5

A summary of the available Rb/Sr (whole rock) radiometric ages of rocks genetically or spatially related to the Dorsal de Cangucli Shear Zone along its northern segment is presented in Table 1. These data will be the basis for discussion about the nature and history of the evolution of this fault in the final section of this paper.

PETROLOGICAL CHARACTERISTICS OF SYNTRANSCURRENT GRANITOIDS The oldest of all intrusions along the shear zone, the porphyritic granodiorite, shows compositions ranging from granodiorite to monzogranites and, very locally, syenogranite. The main textural feature is a subvertical planar magmatic fabric registered by Kfeldspar megacrysts. This fabric was overprinted by solid-state deformation under decreasing temperature

whole

process

rock

Mineral

assemblages

Rge - KlAr

without

(Ma)

xenoliths

to 1.2 2.0%

6.93

20.87

+ crust)

(f

sum to 51%

59B-cll”

672&21

paragneiss)

and orthogneiss

magma

(mantle

0.716 AFC basaltic

mixed

Ba, Rb and Sr highly

moderate 0.55

moderate

high V and Zr,

<

0.9

64 to 71%

restricted

regular

high talc-alkaline

a percentage

Eu/Eu*

Eu anomalies

Lur

LaJLu,

fractionation

(Ma) 1’muscovita; ” biotite)

&ge - Rb/Sr

Protolith

Source

Wagmatic

“SrS’Sr

+ KsO + CaO)

diagrams

corindom

elements

REE

rrace

Vormative

nol Als03/fNa20

NO2

variation

Geochemistry

3xide

rocks

microgranular

2.90%

0.70%

xenoliths

granodiorite

and

to 4.4%

> 1 .l

high

xenoliths

Rb

melting

Host

1.60%

0.90%

xenoliths

to4.5%

> 1.2

melting

+ 22

+41’ &31”

624 578

617*48

orthogneiss

crust

crustal

0.740

incipient

5.05

9.86

( k paragneiss) 628

high

Ba and Sr slightly

moderate

orthogneiss

586+10’

and xenolith

low V and Zr, high Rb

2.1

70 to 75%

- relatively

irregular

peraluminous

granodiorite

rocks

Al72Pv1r3S~10

restricted

lorphyritic

3.3% 18.5%

( k paragneiss)

crust

crustal

0.732

0.93

incipient

4.50

11.24

slightly

moderate

low V and Zr, high Ba and Sr

2.0

70 to 74%

- relatively

irregular

peraluminous restricted

lorphyritic

rocks

and host

mafic

3.1% 18.4%

Cnclaves

1 .f30%

1.20%

17.4%

3.5%

zircon apatite

zircon apatite

tourmaline

cassiterite

fluorite

tourmaline

fluorite

Host

(2 22% 1 I+ 28%) (+ 5%) ( f 10% 1

garnet

muscovite

biotite

K-feldspar

(-+ 35%)

cassiterite

Al75Pv15S~10

Fe0

TiO2

A1203

quartz oligoclase

sphene

Garnet composition

:omposition

ttlurcovite

:omposition

granite syenogranite

to monzogranite

two-mica

young two-mica sotropic

monazite

garnet

( + 9%)

(+ 35% 1 ( + 27% 1 (+ 23%) (i 6% )

tourmaline

muscovite

biotite

K-feldspar

oligoclase

quartz

zircon

3iotite

granite syenogranite

to monzogranite

two-mica

apatite

IInclaves

t-C30% ) (+ 30% ) (+- 25%) (+ 15% )

muscovite

biotite

K-feldspar

andesine

quartz

banded

old two-mica

granitoids

Bssemblage

TiOz

monzogranite

granodiorite

of the syntranscurrent

to granodiorite

porphyritic

porphyritic

characteristics

Hineral

types

Jetrographic

Table 3. Main petrological

The Dorsal de Cangu@ strike-slip shear zone: the tectonic evolution of southern Brazil

Field of normal cabalkakne

rocks

0

-1 0

_ I

65

60

70

75

SlO*

1

I

I

I

I

I

I

I

I

I

I

I

La Ce Nd SmEu Gd Dy Ho Er Yb Lu Figure 5. Petrological diagrams for the porph yritic granodiorite. (a) Plot of log CaO/alkalis (molar) versus SiO,. ibl Bulk chondrite-normalised REE diagram.

conditions (Fig. 4a, b, c). Essential magmatic minerals include quartz, K-feldspar, andesine and biotite, with muscovite and tourmaline. K-feldpar appears as large, subidiomorphic grains, with inclusions of biotite, quartz and plagioclase. Development of tartan twining (microcline) and string perthites are common features. Plagioclase crystals are subidiomorphic with an average composition of Ans3. Acessory minerals include apatite, zircon, monazite, allanite and sphene. Secondary minerals include chlorite, epidote and opaque minerals. Enclaves are common and consist mainly of mafic dioritic to quartz dioritic microgranular autoliths (< 1 m in diameter), and subordinate xenoliths and roof-pendants of the host rocks, varying in size between a few centimetres and several kilometres. The porphyritic granodiorites show a SiO, content ranging from 64 to 71%, with an Aluminium Saturation Index (mol A&O&a0 + Na,O + K,O) between 0.9 and 1.2, which means that they can be characterised as metaluminous to slightly peraluminous (Tables 2 and 3). Samples of these rocks present a

good linear correlation in Harker diagrams, except for K,O, Na,O, Rb, Ba and Sr. The high-K talc-alkaline affinity of these rocks is evident in the log CaO/alkalis diagram (Fig. 5a) and the FeO/(FeO + MgO) ratio (< 0.760). Other characteristics include low Ba, Sr, V and Zr contents and strongly fractioned REE patterns (Fig. 5b), with La,/ Lu, = 20.87 and Lu, = 6.93 on average, as well as moderate Eu/Eu* anomalies (0.55 on average). This granitoid shows an l-type signature considering its mineralogy, type of enclaves and chemistry, whereas the initial isotope ratios fe7Sr/*‘Sr = 0.7161, and low Zr (average of 178 ppm) and V (34 ppm) contents suggest a mixed origin, involving both crustal and mantle sources. Petrogenetic modelling using the linear equation method (Hofmann and Feigenson, 1983; Vieira Jr, 1990) was applied to the syntranscurrent granitoids in an attempt to identify the magmatic processes and recognise the sources of magma responsible for the origin of these rocks (Koester, 1995). Several rock types of this region, in addition to data from the literature, were tested as the original material (e.g. granulites, kinzigites and orthogneisses) assimilated by a more primitive mantle-derived magma to produce these granites. The best-fit results were obtained using a basaltic composition for the parental magma, which, for modelling purposes, was assumed to be represented by the mafic microgranular dioritic enclaves (Koester, 1995). The results of this study indicate that assimilation of the talc-alkaline gneisses (which are the most common host rock) by a primitive basaltic magma, and fractional crystallisation of the resulting material, is the most likely process responsible for the origin of the porphyritic granodiorites. The two peraluminous leucogranites present similar petrographical and mineral compositions, ranging from syenogranite to monzogranite (Table 3) with only a few granodiorite portions (Fig. 4d, e, f). Essential minerals of these rocks include quartz, K-feldspar and plagioclase, with muscovite, biotite and tourmaline as accessory minerals. K-feldspar appears as subidiomorphic crystals, with inclusions of biotite, muscovite and plagioclase. Plagioclases are subidiomorphic with oligoclase (An,, on average) predominant in the old (banded) leucogranite, and zoned albite in the young (isotropic) leucogranite (An,, in the core and An,_,, along the rims). Accessory minerals include apatite, zircon, fluorite, ilmenite, sphene and cassiterite. Textures indicate that muscovite, biotite and garnet are primary minerals, which seems to be confirmed by the chemistry of both peraluminous granites. Enclaves are rare in these rocks and consist of xenoliths and roof-pendants of

Journal

of African

Earth

Sciences

13

L.A.D. FERNANDES and E. KOESTER

Peraluminous

Metaluminous

2.0 z h

0.4 0.5

2.0

1.5

A/CNK 300

I

I

I

I

I

I

I

I

I

I

I

1

I

I

I

I

I

I

I

I

I

I

I

La CeNdSmEu

Gd Dy Ho Er Yb Lu

Figure 6. Petrological diagrams for the peraluminous leucogranites. la) Shand’s reference diagram. 0: old peraluminous leucogranite; V: young leucogranite. (b) Bulk chondrite-normalised REE diagram.

the older high-grade host rocks and of the porphyritic granodiorites. Microgranular mafic enclaves were not observed in these rocks.

14 Journal of African Earth Sciences

Both old and young two-mica leucogranites show compositions ranging from 70 to 75% SiO,. Using Harker’s and others differentiation indices, the rocks

The Dorsal de Canguqti strike-slip shear zone: the tectonic evolution of southern Brazil do not present a clear linear correlation. They are strongly peraluminous (1.1 < AKNK c 1.81, as indicated by Shand’s classification diagram (Fig. 6a). The old leucogranite shows higher contents of Sr and Ba, and lower Rb than the young leucogranite. Both leucogranites show weakly fractionated REE patterns (Fig. 6b) with the old leucogranite having La,/Lu, = 11.24 and Lu, = 4.58, while the young leucogranite has ratios of La,/Lu,= 9.86 and Lu, = 5.05. The leucogranites are sligthly anomalous with respect to Eu, and Eu/Eu* ratios with values of 0.93 and 0.7 1, for the old and young, respectively. Both rocks have signatures resembling ‘S’-type granites, as suggested by their mineralogy (aluminous minerals but no cordierite), chemical composition (high SiO,), normative corundum ( > 2%) and initial high *7Sr/86Sr isotope ratios (0.732 for the old and 0.740 for the young leucogranite). Accordingly, these rocks have been traditionally interpreted as ‘S’-type granites formed by partial melts of the high-grade supracrustal gneisses (Fragoso-Cesar et al., 1986; Figueiredo et al., 1990; Philipp et al., 1993). They were also thought to mark a collisional suture between the magmatic arc and the marginal basin (Fragoso-Cesar et al., 1986). However, these interpretations are not supported by either petrological data or field studies. In fact, these rocks do not present typical characteristics of ‘S-type’ granites. Field relations, enclave typology and petrogenetic modelling indicate that they are in situ partial melts of the main host rock, represented by talc-alkaline quartzofeldspathic gneisses (CL Table 3). This explanation is in agreement with results of independent work along the southern segment of the DCSZ (Nardi and Frantz, 19951, being also consistent with most petrological studies, where two-mica granites without cordierite are interpreted as partial melts of quartzofeldspathic gneisses (cf. Barbarin 1996 for a review). It is still supported by the time sequence of generation of these rocks, according to which large volumes of talc-alkaline erogenic gneisses of the magmatic arc are younger than the high-grade supracrustal rocks, but older than the peraluminous granites. The period of time between the partial melting event that affected the gneisses and the emplacement of the peraluminous granites must be considerable, since there is a general agreement that large volumes of talc-alkaline magmas, such as those observed along the eastern segment ot the Dom Feliciano Belt (Tommasi et al., 19921, are produced by subduction of oceanic lithosphere. The small volume of paragneisses in this region, and the fact that their outcrop area does not coincide with the one occupied by the peraluminous granites, is an additional indication that these

rocks were not formed by the same partial melting event registered in the main fabric of the high-grade supracrustals of this region (see later).

STRUCTURAL EVOLUTION OF SYNTRANSCURRENT GRANITOIDS The porphyritic and peraluminous granitoids were first recognised as syntranscurrent magmas due to the following characteristics: (1 I northeast elongated shapes; (2) northeast trending magmatic and solidstate deformation fabrics with subvertical foliation and gently-plunging lineations marked by K-feldspar megacrysts, features that are (3) parallel to the structures of the nearby country rocks and to those in mylonites developed during solid-state deformation of the granites themselves (i.e. deformation outlasted emplacement and cooling). In addition, the porphyritic granodiorites contain xenoliths with large aspect ratios and long axes of microdiorite enclaves orientated parallel to the trend of the magmatic foliation and lineation. The latter is marked by the dimensional orientation of the (010) faces and the long axes of euhedral K-feldspar megacrysts, that, together with biotites and amphibole crystals, are deflected around xenoliths of the host rocks. These and other structures belonging to several scales and stages of deformation of these rocks (magmatic, transitional and solid-state), that are considered as diagnostic of their syntectonic nature (e.g. Paterson etal., 1989; Schofield and D’Lemos, 19981, are documented in more detail by Fernandes et al. (1990, 1993) and Tommasi et al. (19941. Solid-state deformation of the porphyritic granodiorite gave rise to low-strain lenses where magmatic and transitional structures are better preserved. These lenses are confined between decametre-thick high-strain zones showing strong grain-size reduction (Fig. 4a). The old (banded) peraluminous granite shows strongly heterogeneous deformation with an eastnortheast trending composite fabric and conspicuous development of several generations of foliations, stretching lineations and folds (Fig. 4dI. Coaxial refolding of folds, foliations, lineations and kinematic indicators that originated during previous increments of deformation, posed many problems for the determination of the sense of displacement for the shear zone during the kinematic analysis of these rocks (Fernandes et al., 1990, 1993). While the first set of folds affecting the banding are typically noncylindrical, the last set of these structures are gently plunging, normal buckle folds with small amplitude/ wavelength ratios, remarkably cylindrical geometries and with north-northeast trends. Although less

Journal of African Earth Sciences 15

magmatic deformation

solid-state deformation

-St

(early)

n=49

cl 00

+0

00

St (early)

Lt (early)

QQ + St (late)

Figure 7. (a) Diagram showing the ‘tension gash ‘/‘fault-bend-fold’geometry of the magma chambers of the porphyritic granodiorites proposed to account for the pattern of magmatic flow structures (foliation-Sm and lineation-Lm). Diagram lb) shows the variations of attitude of the tectonic planar structures presented mainly by theperaluminous leucogranites. Note the more east-northeast trending higher temperature and ductility tectonic banding (St) of the first stage of deformation (average attitude within brackets) as compared to the more north-northeast trends of the lower temperature foliation of the later deformationalstage. Structures formed during each of these two stages of deformation are interpreted as tracking the shearing planes of the finite strain elipsoids (represented by the ellipses) and register deformation under increasing strain rates or decreasing temperatures. ‘Wand ‘Lt’ stand for tectonic foliation and lineation, respectively.

The Dorsal de Canguqci strike-slip shear zone: the tectonic evolution of southern Brazil conspicuous, structures similar to the latter are developed in the mylonites and phyllonites cropping out mainly along the rims of the coarse-grained peraluminous granite. The younger age of this intrusion is indicated by its less deformed state, with the absence of the well-developed banding and other high T structures observed in the old peraluminous granite. This interpretation seems to be confirmed by the presence in the latter of thin and slightly deformed apophysis of the former. This, and the similar distribution of both granites within the shear zone, so that they should have suffered the same deformational episodes if they were contemporaneous, are the best evidence of their differing ages. Structures of several scales and typical of decreasing temperature conditions ranging from amphiboliteto greenschist-facies can be observed in these rocks (Fig. 4b-f).

EMPLACEMENT

OF THE SYNTRANSCURRENT GRANITES

The porphyritic granodiorites crop out as three isolated bodies and small roof-pendants and xenoliths within the peraluminous and younger granites (cf. Fig. 3). Strong reworking during late-stage and low temperature deformational events, that formed thick sequences of phyllonites and cataclasites, left few tectonites along the original contacts with the orogenie talc-alkaline host rocks intact. In the rare high T transcurrent-related mylonites, interpretation of kinematic indicators are confused due to the complex geometry caused by folding. In this context, the distribution pattern of the magmatic foliation in the oldest porphyritic granitoids is the only source of evidence of the sense of displacement along the shear zone. It was also used to consider the mechanical behaviour of the continental crust during the early stages of nucleation of these shear zones, as discussed below. Anti-clockwise rotation of country rock banding, that could be interpreted as evidence of left-lateral movement, could easily be produced by deformation during the phyllonite-producing episode, with abundant kinematic indicators of sinistral shear, or even by reactivation of these faults during the Phanerozoic (cf. Fernandes et al., 1995c). An attempt to constrain the original geometry of the magma chambers using the pattern of magmatic structures presented by the porphyritic granitoids was made. The magmatic foliations and lineations show predominantly northeast-southwest trends, with some dispersion towards the north-south and east-west. Based on this pattern of flow and assuming the need for space to emplace the considerably large volume of granodioritic to monzogranitic (vis-

cous) magmas, a model involving emplacement of the primitive magma in originally north-south trending extensional fractures (‘tension gashes’) was proposed by Fernandes et al. (1993). These structures are interpreted as having formed initially on a right-stepping ‘en echelon’ pattern or along ‘S’-shaped fault-bend-folds, which were subsequently interconnected during progressive deformation. Although partially delimited by younger rocks, the southernmost outcrop area of this rock might have preserved the original geometry of the spaces generated (Fig. 7). Detailed studies of the magmatic fabric at both outcrop and regional scales seem to confirm this general model (Koester eta/., 1995) and the magnetic anisotropy in the younger peraluminous granite is presently being studied. A variation of the interpretation presented is the intrusion of magma along originally north-south trending segments of releasing bends of the Dorsal de Cangucu Shear Zone. This would also explain the ‘S’-shape shown by this structure at large scales. A structure with this geometry and a left-lateral sense of displacement would be capable of providing adequate space for the emplacement of the porphyritic granitoids. The ‘mantle signature’ of these rocks, and the interpretation of a basaltic magma as the parental magma of the porphyritic granitoids, means that the initial structural discontinuities have probably reached the upper mantle. Although there is no direct evidence allowing an estimation of the length and width of these fractures, the younging ages of the syntranscurrent granitoids, from northeast to southwest, shows evidence for the speculation that the large area occupied by the granites of the present study corresponds to the southern segment of a single tension gash or an ‘S/-shaped bend of the fault. According to this model, emplacement of progressively younger magmas would be expected towards the southwest, something apparently confirmed by the available geochronological data (cf* Table 1). The corroboration of the extensional fracture model would indicate loss of cohesion of the continental crust during the nucleation stages of this transcurrent fault. Despite the fact that such a behaviour is typical of brittle-plastic deformation conditions along shear zones, it can also be produced under the influence of other factors. These include the occurrence of partial melting under low fluid pressures, producing a local increase of volume leading to high pore fluid pressures which, in turn, favours melt-enhanced embrittlement (Brown et al., 1995; Rushmer, 1995). In this case, effective confining pressures would not be high enough to inhibit fracturing (Rutter, 1997).

Journal of African Earth Sciences 17

L.A.D. FERNANDES and E. KOESTER

?? Phyllonites,

quartz-mylonites and cataclasites

!ZI Peraluminous

? ?Potphyriiic ?? Host

leucogranites

granodiorite

25 km

Rocks

w Mantelic magma Mantle

Figure 8. Diagrammatic representation reached by peraluminous leucogranites reproduce the mapped outcrop area.

of the model of emplacement are constrained by gravimetric

Alternatively, high strain rates may promote fracturing (e.g. Dell’Angello and Tullis, 1987). The mode of failure of the highly deformed and heterogeneous crust represented by the old calcalkaline erogenic granitoids and high-grade supracrustals was possibly controlled by one or a combination of the factors mentioned above. Experimental deformation has demonstrated that necessary stresses for brittle failure are very low in a fluid-rich

18 Journal of African Earth Sciences

proposed for the syntranscurrent granitoids. Depths data and the top of the last diagram is an attempt to

environment. The well-known weakness of rocks to resist effective tensile forces (Etheridge, 1983) might have contributed to the loss of cohesion of the host rocks, despite indications that relatively high temperatures were prevalent (cf. Tommasi et a/. , 1994). Opening of the extensional structures and magma emplacement must have been simultaneous due to the impossible existence of open voids at mid-crustal

The Dorsal de Cangucti strike-slip shear zone: the tectonic evolution of southern Brazil levels necessary for the emplacement of such large volumes of magmas (Paterson and Tobisch, 1992; Paterson and Fowler, 1993). After initial emplacement, magma overpressure and the effects of a thermal contrast between mantle-derived magmas and the host rocks could have contributed to the evolution and nucleation of these extensional fractures, even at deep crustal levels (cf Clemens and Mawer, 1992). Emplacement of the first magmas with the introduction of large quantities of advective heat were likely to promote large-scale strain softening (cf. Tommasi eta/., 19941, but progressive deformation along the shear zone might have obliterated evidence for the original geometry of these structures and of their rheological conditions of development. However, the striking similarity of the geometry and mechanism of emplacement deduced for the porphyritic granitoids from the pattern of magmatic flow, with the nature and geometry of structures produced by experimental deformation of the partially molten Westerly Granite (cf. Rutter and Neumann, 1995; Fig. 81, means that the above suggested model for the emplacement of the syntranscurrent porphyritic granitoids deserves further investigation. Experimental deformation tests performed under temperature conditions estimated for these rocks by studies of geothermobarometry can provide information on the strength of the host rocks and the stress values necessary to form these structures. Such results can provide useful constraints on the conditions of natural deformation in this and similar tectonic situations. A hypothetical sequence of events responsible for the emplacement of the syntranscurrent granites is presented in Fig. 8. It is also based on subsurface data presented by Costa et al. (19941, where the modelling of gravity data of the DCSZ suggest the presence of a wedge-shaped low gravimetric anomaly ( < -19 mgal) with roots reaching depths of more than 25 km. This shape was attributed to the large volume of leucogranites representing in situ or little mobilised partial melts. The older and higher T banding of the peraluminous leucogranites, where kinematic indicators are abundant, is attributed to deformation under middle crustal (amphibolite- to upper greenschist-facies) temperature conditions. During this deformation episode, as a function of the higher ductility of these quartz/ biotite/muscovite-rich rocks, the planes of no finite strain would tend to show east-northeast trends. It is known that under conditions of progressive deformation and decreasing temperature, during which the largest phyllonite sequence was formed, as a function of the lowering of ductility, the values of the dihedral angle (28) would tend to decrease

(Kligfield et a/., 1984). This could provide a simple explanation for the preferential north-northeast trends of the low T mylonites and the left-lateral vorticity presented by the kinematic indicators in these rocks. They would have been formed along shearing surfaces originated under an overall strain field presenting a general north-south orientation for the main compressive direction (Fig. 7). The model set up to explain the intrusion and deformation of the porphyritic granitoids is also capable of accommodating the evidence for progressive deformation of the peraluminous granites under decreasing temperature conditions and within the same overall tectonic regime. This applies, provided that an anti-clockwise reorientation of less than 30° of the main compression direction had occurred between the early and later stages of evolution of the shear zones. The main stress direction would have to be reorientated from a north-northeast to a more north-south direction, something that could have been produced by tectonic episodes leading to the final assemblage of Neoproterozoic West Gondwana (see below).

DISCUSSION Field and petrological evidence suggest that the syntectonic granitoids were formed by partial melting of rocks older than the nucleation and development of the Dorsal de Cangucu Shear Zone. While assimilation of the country rocks by a parental basaltic magma and fractional crystallisation of the resulting material is capable of explaining the petrological characteristics of the porphyritic granodiorite, the model that best explains the origin of the peraluminous granites is melting of the high-grade orthogneisses and their parametamorphic xenoliths. The petrological, structural and geophysical characteristics of the Dorsal de Cangucti Shear Zone indicate that they are post-collisional faults which made a minor contribution to crustal growth. It was restricted to the emplacement of a small volume of mantle-derived dioritic magma presumably during their initial stages of development. The transcrustal nature of the shear zones is indicated by both (1) the presence of dioritic magma from mantle sources; and (21 the results of gravity studies showing that despite all the erosion suffered in this region, the roots of the peraluminous granites can be traced to a depth of 25 km (Costa et al., 1994). On the other hand, the intracontinental nature, relative ages and metamorphic and deformational characteristics of these shear zones can be used to rule out the possibility that there are translithospheric faults, be it a suture zone, a terrane boundary fault or a transcurrent

Journal of African Earth Sciences 19

L.A.D. FERNANDES and E. KOESTER shear zone produced by oblique collision, as suggested in the literature. In this context, the event of partial melting responsible for the origin of the peraluminous granites is envisaged as a younger and more localised episode, restricted to the vicinity of the transcurrent shear zones. The invariably older ages presented by the porphyritic granodiorites, in relation to the peraluminous granites and their close spatial distribution along the entire length of the shear zone, suggest a genetic relationship between these rocks. Advective heat transfer due to emplacement of the porphyritic granodiorites, triggering partial melting of the host rocks, is a likely mechanism (Tommasi et a/., 1994). If this is the case, the time-span of magma emplacement and partial melting along the shear zone should be substantially short (within a few million years). This interpretation is currently being tested with the use of high resolution radiometric dating methods. The attitude and ages of the shear zones, in relation to the orientation kinematics and ages of the collisional granitoids and their structures, suggest that there is no straightforward genetic relationship between collision and strike-slip tectonic processes, as previously suggested (e.g. Fernandes et a/., 1992a; Tommasi et al., 1994). Showing upper amphibolite parageneses and syntectonic partial melts, the polyphase metamorphism exhibited by the host rocks (kinzigites and talc-alkaline gneisses) does not belong to the same thermal episode responsible for the genesis of the peraluminous granitoids, as suggested by interpretations based on simplistic analogies between the DCSZ and the Main Central Thrust (e.g. Fragoso-Cesar et a/., 1986). Minerals of the high-grade metamorphic assemblage are marking the older collisional fabrics and typical examples include centimetre-long prismatic sillimanites as mineral lineations, as well as partial melts along pressure shadows of large garnet porphyroblasts. These rocks and fabrics are clearly older than the nucleation of the shear zones, based on evidence from field relations and available radiometric dating. In addition, stretching lineations ascribed to latecollisional episodes show predominantly east-west trends in this area (Fernandes et al., 1992a). If the shear zones were formed during the late stages of this deformation, they should display a right-lateral sense of movement, for which there is no evidence. The ages of collision-related host rocks show a Ma by large time span - between ca 800-609 both Rb/Sr and U/Pb methods (Table 1). Considering the polymetamorphic nature and composite sources of these rocks, these ages, regardless of their accuracy, should be interpreted with caution, particularly where no structural analysis and metamor-

20 Journal of African Earth Sciences

phic petrology data are available. Values of 631 + 13 Ma WPb-SHRIMP) obtained for the calc-alkaline tonalitic gneisses, intrusive in the supracrustals along the northern segment of outcrop of the DCSZ and northwest of Pot-to Alegre Suture Zone, are interpreted as the ‘Brasiliano resetting’ of older crust (da Silva et al., 1997a). However, some of these rocks show evidence for several episodes of regional high-grade metamorphism and at least three episodes of partial melting. The oldest one, represented by quartzofeldspathic lenses parallel to the banding and pressure shadows of garnets in kinzigites, has certainly happened before the emplacement of the talc-alkaline granitoids, since these features are only observed in xenoliths of kinzigites within those highly deformed igneous rocks. The other two melt-producing episodes are registered by the presence of the (autochtonous) peraluminous granites emplaced along the shear zone. The similarity in age obtained by U/Pb dating of the gneisses with the Rb-Sr results for the peraluminous granites (particularly the old intrusion) suggests that this ‘Brasiliano resetting’ might be more precisely the partial melting event responsible for the origin of the old peraluminous granites emplaced along the transcurrent shear zones. In the southern segment of the DCSZ and east of the Pot-to Alegre Suture (cf Fig. 2b), U/Pb and Pb/ Pb ages of around 610 Ma were obtained for the metamorphism and deformation of ‘collision-related gneisses’ of the Pinheiro Machado Complex Khemale et al., 1995a, b; Babinsky et a/., 1997). This unit was also interpreted as a magmatic arc, but younger than the orthogneisses that host the transcurrent shear zones, based on marked differences in the preservation of primary magmatic features and lower metamorphic conditions. This applies, in spite of the fact that both complexes show similar kinematic patterns (Fernandes eta/., 1992b, 1995b). However, what is relevant to the present discussion is the fact that they are both older than the transcurrent shear zones and that the samples for U/Pb radiometric dating were taken from the vicinity of the DCSZ (Chemale et a/., 1995b; Babinsky et a/., 1996; da Silva et a/., 1997b). The similarity of these results with the Rb/Sr ages determined for the emplacement of each of the syntranscurrent peraluminous granites, suggests the possibility that these ages are not only related to major episodes of metamorphism (and zircon growth) in these two regions, but are also reflecting the ages of the episodes of metamorphism and localised partial melting associated with the development of the shear zones. This would provide a much simpler explanation for the ‘reworking of the older crust’ in

The Dorsal de Canguqti strike-slip shear zone: the tectonic evolution of southern Brazil the eastern part of the Sul-rio-grandense Shield, than having to propose major collisional episodes as late as 610 Ma in the history of this belt. It also seems to be a good reason to question the interpretation that collisional and strike-slip deformations are genetically related, and that the presence of magmas within the shear zone would induce strain softening at a continental-scale during collision (e.g. Tommasi et al., 1994). Until the results of high-resolution radiometric dating of the protoliths and the tectonites developed during both collisional and strike-slip episodes are obtained, suggestions about their cogenetic nature are only speculation. Large-scale tectonic processes that have possibly controlled the origin and evolution of the strike-slip shear zones include the post-erogenic extensional collapse of the Dom Feliciano Belt and the simultaneous tectonic episodes related to the amalgamation of Neoproterozoic West Gondwana. Closure of the Khomas Sea that originated the Damara Belt post-dates the Rio de la Plata-Kalahari collision. This episode could account for the belt-parallel deformation along the transcurrent shear zones, such as the Dorsal de CangucQ (Tommasi, 1991). A very similar kinematic pattern to the one reported here for the transcurrent and flat-lying shear zones was recognised in the Kaoko Belt of Namibia - the physical continuity of the Dom Feliciano Belt (Dtirr and Dingeldey, 19961. In this orogen, the belt-parallel deformation seem to be younger than the convergence between the Kalahari and Zaire Cratons, belonging to the interval between 550-530 Ma (Haack and Martin, 1983; Dtirr and Dingeldey, 1996). Correlation of this deformation with the ages available for southern Brazil are difficult. Even if the ca 670 Ma (Rb/SrI age of the oldest syntranscurrent granodiorite emplaced along releasing bends of the shear zone, or the U/Pb ages of 595 + 1 Ma for the emplacement of the late-transcurrent granites such as the Arroio Moinho Granite (Chemale et al., 1995a, b; Babinsky et al., 19971, are assumed to be geologically significant and representing the nucleation stages of these structures, a difference of 50-l 00 Ma between the absolute ages obtained for the belt-parallel deformation in both continents still persists.

CONCLUSIONS The history of large-scale Precambrian shear zones, such as the Dorsal de Cangucu, can provide valuable insights into the nature and characteristics of fundamental processes responsible for the evolution of the continental crust in southern Brazil during the Neoproterozoic.

Out of the several interpretations proposed for the Dorsal de Cangucu Shear Zone, the interpretation of an intracontinental fault, seems to be the most consistent with the characteristics and history presented by this structure. The question whether the continental evolution of this region during the Neoproterozoic was mainly by the reworking of old crust or an amalgamation of a magmatic arc along an active continental margin remains a subject of controversy (cf. Chemale et al., 1995a, b; Fernandes et al., 1995a). What can not be denied is that the contribution of the DCSZ to crustal growth was extremely modest in terms of volume. It seems that this fault was more important for having favoured the development of episodes of partial melting of the host rocks, which are registered by two generations of peraluminous magmas. Petrological characteristics of these granites suggests they were produced by partial melting of the host (or similar) rocks that are represented by high-grade talc-alkaline gneisses and their supracrustal enclaves, The occurrence of mafic magmatism, although restricted in volume and only occurring during the initial stages of the evolution of this shear zone, could be an indication that this fault might have reached the mantle, being characterised in this case, as a transcrustal fault. The cohesionless behaviour of a fluid-rich crust within a transtensional environment proposed for the nucleation of this fault is, no doubt, highly speculative. However, it would be in agreement with the intracontinental nature of this structure being also consistent with the occurrence of mafic magma restricted to the initial stages of development of the fault. The sequential emplacement of magmas and development of structures characteristic of magmatic flow and solid-state deformation has made it possible to recognise the occurrence of several distinct episodes of displacement and granite intrusion along this shear zone. The reworking of structures typical of higher T deformation under retrogressive metamorphic conditions, with consistent evidence for sinistral displacement, was accompanied by the intrusion of progressively more differentiated and younger granites. The data obtained so far suggest a complex history of evolution spreading over a much longer period than most cases reported in the literature. The difficulties of establishing reliable correlation even between different segments of a single shear zone below Phanerozoic sediments, such as is the case of the northern and southern extremes of the presently discussed structure (cf. Fig. 1 I, is a good measure of the problems which need to be overcome in order to achieve any successful correlations across

Journal of African Earth Sciences 2 1

L.A.D. FERNANDES and E. KOESTER the Southern Atlantic. In the virtual absence of data obtained by the combined application of several geological techniques to unveil the nature and history of these shear zones in both continents, any such attempts (e.g. Rogers et a/., 1995) should be regarded as a working hypothesis to be tested.

ACKNOWLEDGEMENTS Comments by 0. T. Tobisch and A. McCaig helped to improve the manuscript. The Conselho National de Pesquisas (CNPqI is thanked for a post-doctorate scholarship (Proc.n” 200.858/82) during which part of this paper was outlined. CAPES - British Council Project (Proc. 081/98) and PRONEX (Proc.76.97. 1006) are thanked for support during several stages of the work.

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