The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil

Journal of South American Earth Sciences, Vol. 8, No. 1, pp. 55-63, 1995 Copyright © 1995 Elsevier Science Ltd& Earth Sciences & Resources Institute Printed in Great Britain. All rights reaurved

Pergamon 0895-9811(94)00041-7

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The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil L.V.S. NARDI and J.C. FRANTZ Centro de Estudos em Petrologia e Geoqufmica, CPGq, Instituto de Geociancias/UFRGS Av. Bento Gongalves 9500, 91540.000, Porto Alegre, RS, Brazil

(Received July 1993; Revisions Accepted March 1994)

Abstract--The Cordilheira Intrusive Suite is made up of peraluminoustwo-micaleucogranitesrelated to the Brasiliano Cycle. Located in the east region of the Sul-rio-grandenseShield, in southernmostBrazil, the granitic bodies were emplaced mainly inside mylonitic zones belongingto a transcurrentsystem with NE orientation.The structuralrelationshipof this intrusion with shear zones indicates a syn- to late-kinematicemplacement.The CordilheiraIntrusiveSuite rocks are mainly syenogranitesand monzogranites,with inequigranulartexture, sometimesmodifiedby mylonitic deformationaleffects. Mineralogicallythey are composed of alkali feldspar, plagioclase,quartz, muscoviteand biotite,with minor amountsof garuet, tourmaline,apatite, zircon, sphene and fluorite.The analyzed samples show a SiO2 range of 71.55 to 77.68%, and a reduction in the K20/Na20 ratio with increasing differentiation.Trace-elementcontents, includingREE, are similar to that of granites produced by partial melting of quartz-feldspathic rocks. Field relationships and geochemicaldata are suggestive of partial crustal melting along transcurrent shear zones developed during the late orogenic stages of the BrasilianoCycle. The evidenceconsidered in this paper shows the remarkable similarityof the CordilheiraIntrusiveSuite to Hercyniangranitoidsof Portugal and France. R e s u m e n - - A Suite Intrusiva Cordilheira6 constitufdapor leucogranitosperaluminososcom duas micas relacionados ao Ciclo

Brasiliano. Situados na regi~o leste do Escudo Sul-rio-grandense,os corpos granfticostSm seu posicionamentocontroladopor zonas milonfticasde um sistematranscorrentede dire~goNE. A relag~o estruturaldas intrus6escom as rochas milonfticasindica condi~6essin a tardi-cinemfiticasde posicionamento.Os corpos da S.I.C. s~o dominantementesienograniticosa monzogranfticos, com textura inequigranularafetada pelos efeitos da deforma~o milonftica.A mineralogia6 coustituidapor K-feldspato, plagioclgsio, qualXzo,muscovitae biotita, corn granada, turmalina,apatita, zircfio,esfeno e fluorita.As amostras analisadasapresentam uma variag~o em SiO2 situada entre 71,55 a 77,68%, com uma raz~o K20/Na20 decrescentecoma diferencia~o. Os valores detectados para elementostra~os na Suite IntrusivaCordilheira,bem como de ETR, sgo compatfveiscorn os de granitosproduzidos pot fus~o crustalde rochas quartzo-feldspfiticas.Os dados existentessugerema geragao de magmasgranfticosde fus~o crustal ao longo de zonas de cisalhamentotranscorrentesdesenvolvidasao final do Ciclo Brasiliano.As evid~nciaspermitemidentificar uma forte semelhan~aentre as rochas da Suite IntrusivaCordilheirae os granitosHercinianosde Portugale da Fran~a. INTRODUCTION

This paper presents field, petrographic and chemical data concerning peraluminous granites of Late Proterozoic age from southernmost Brazil, comparing them with similar Hercynian granites.

PERALUMINOUS GRANITIC magmas are generally associated with calc-alkaline suites and genetically related to fractional crystallization from basic to intermediate primary liquids or to crustal melting processes related to collisional tectonic events. Pitcher (1983) associated S-type granites with c o n t i n e n t a l c o l l i s i o n zones or encratonic ductile shear belts where the crust is thick enough for geothermal gradients to promote melting. Petrogenetic processes involved in the generation of these magmas have recently been discussed by Miller (1985), WinNer (1983), and Harris et al. (1986), among others.

Two-mica leucocratic granites, in the Sul-rio-grandense Shield (Fig. 1), were described originally by Picada (1965) under the designation of Cordilheira Granite and Associated Migmatites. Frantz and Remus (1986), Fragoso-C6sar et al. (1986) and F e r n a n d e s et al. (1990) related these granites to a Brasiliano Orogenesis and suggested that they were produced by crustal melting processes. Issler (1984) compared two-mica granites from south Brazil, including the Cordilheira granites, with those of collisional belts.

W h e n the c r u s t a l m e l t i n g p r o c e s s i n v o l v e s m o s t l y metasedimentary sequences the granites so produced are referred to as S-type (White and Chappell, 1977). Nevertheless, p e r a l u m i n o u s melts can also be generated from quartz-feldspathic orthogneissic sequences (Halliday et al., 1981) or even from more basic compositions.

Cordilheira granitic intrusions are located mainly along the contact between two larger units (Fig. 1): the Porongos Metamorphic Suite (Jost and Bitencourt, 1980) and a calca l k a l i n e granitic a s s o c i a t i o n ( F i g u e i r e d o et al., 1990; Frantz and Nardi, 1992).

Address all correspondence and reprint requests to Lauro V.S. Nardi, Centro de Estudos em Petrologia e Geoqufmica, Instituto de Geoci~ncias/UFRGS, Av. Bento Gon~alves 9500, 91540.000, Porto Alegre, RS; Brazil 55 SAES 8 / 1 ~

56

L.V.S. NARDI and J.C. FRANTZ

This magmatism is associated with, and partially affected by, transcurrent shear zones named the Aqot6ia and Vigia-Roque systems by Jost et al. (1984). Tin and tungsten mineralization, greisens and tourmalinites, besides some beryl-bearing pegmatites, emphasize the importance of peraluminous magmatism in southernmost Brazil. These granitic intrusions are included in this paper under the designation Cordilheira Intrusive Suite. Analytical data were produced in the Geochemical Laboratories of the Universidade Federal do Rio Grande do Sul and G E O L A B , while m i c r o p r o b e data w e r e obtained in the Universit6 Pads-Sud/Orsay and Universit6 Pierre et Made Curie.

F I E L D RELATIONSHIPS The most prominent structural feature in the eastern part of the Sul-rio-grandense Shield is represented by transcurrent shear zones, marked by mylonitic rocks where the strain is maximum. This deformational event affects the Porongos Metamorphic Suite and most of the calc-alkaline granitoids.

The shear zones are mostly oriented NE-SW (Fig. 1) and are marked mainly by mineral stretching, mica orientation and quartz recrystallization leading to the development of horizontal and sub-horizontal lineation and a dominant sub-vertical foliation. Emplacement of peraluminous granitic intrusions is controlled mainly by these transcurrent shear zones. These instrusions have lenticular shapes and are elongated in a northeasterly direction, following the general trend of the shear zones. Deformation effects are reflected in the elongated shape of the intrusions, in a mylonitic foliation with oriented muscovite and biotite, a NE sub-horizontal lineation defined by quartz and feldspar stretching, and a locally developed irregular banding. The structural relationship of intrusions and transcurrent shear zones, as well as the parallelism of magmatic mica orientation and the mylonitic foliation in the granitic bodies, suggest a syn- to late-kinematic emplacement. Fernandes et al. (1990) found the same structural relationship for similar granitic rocks, occurring to the north of this area. Besides the peraluminous magmatism, typical calcalkaline metaluminous granitoids were also intruded at this same stage, displaying similar deformational features.

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The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil Both granitic types are intrusive in less evolved calc-alkaline granitoids, affected by a pre-transcurrence, low-angle deformation with W-NW vergence (Fernandes et al., 1990; Frantz and Nardi, 1992). The tectono-stratigraphic subdivision adopted in this work has been previously suggested by Frantz and Remus (1986) and Frantz and Nardi (1992). According to these authors, granitic rocks with low-angle deformation may be considered syn-orogenic while those emplaced during the development of transcurrent shear zones are late-orogenic. The two-mica granites described in this paper are included in the late-orogenic group. They generally occur as elongated bodies concordant with the main shear zones and show widespread effects of hydrothermal activity, such as muscovitization, tourmalinization, epidotization and iron-oxide formation. Two main contact relationships are observed: granitic bodies intruded inside the mylonitic zones have diffuse limits with host lithologies, while those outside the highly deformed zones have sharp limits and may project veins crosscutting the host rocks. Xenoliths of biotite-muscovite gneisses, with granodioritic composition, occur in some of these intrusions, and may represent the protoliths of the peraluminous melts. PETROGRAPHY

The Cordilheira Intrusive Suite is composed mainly of monzogranites and syenogranites, with minor proportions of granodiorites. Textures vary between inequigranular and porphyrytic, with grain sizes of about 1 to 6 mm, although some K-feldspar megacrysts may reach up to 15 mm. With increasing deformation, textures range from hypidiomorphic and isotropic to granoblastic with polygonized intergranular contacts, with intermediate types characterized by foliated and banded fabrics. Mylonitic deformational effects are generally superimposed on magmatic mineral orientation and banding. They are registered as undulatory extinctions and subgrain development in quartz and feldspar, bending and kinking in magmatic micas, bending of twin planes in plagioclase, development of quartz ribbons and mortar textures and, less frequently, the appearance of pressure shadows associated with ptagioclase and alkali feldspar porphyroclasts. The Cordilheira Intrusive Suite rocks are made up of plagioclase, alkali-feldspar, quartz, muscovite, biotite, and minor amounts of garnet, tourmaline, apatite, zircon, sphene, and fluorite. Chlorite, epidote, iron oxides, secondary muscovite and calcite are produced by hydrotherreal activity and retrogressive metamorphism associated with mylonitization. Plagioclase is generally euhedral to subhedral, except where recrystallization effects have caused grain size reduction along its borders. Some grains are zoned, with core compositions about An30_34, reaching up to An7_11 at the rims. The most albitic rims may be related to metamorphic recrystallization, while more calcic ones probably reflect magmatic re-equilibration. Albite, Albite-Carlsbad, and Pericline twinning are widespread. Fine muscovite

57

lamellae develop mainly along cleavage and twin planes of plagioclase, probably resulting from hydrothermal activity. Although some muscovite and biotite may occur sporadically as inclusions in plagioclase grains, they generally present sharp contacts, suggesting simultaneous magmatic crystallization. Two different generations of alkali feldspar are present: a perthitic feldspar that occurs as subhedral megacrysts, including micas, plagioclase and garnet; and another one that is anhedral to subhedral, shows tartan twinning and displays reactive contact with plagioclase and perthitic feldspar. While the former represents a magmatic phase, the second, with higher triclinicity, probably crystallized under post-magmatic or metamorphic conditions. Perthites in megacrystic alkali feldspar are patchy or veined. Myrmekites are common and their origin may be related to mylonitic recrystallization as proposed by Simpson (1985). Exsolution textures, such as quartz blebs, are common in plagioclases and perthitic alkali feldspar, mainly in those samples where the deformational effects are more evident. Anhedral quartz with undulatory extinction is present in the less deformed rocks. With increasing deformation, subgrains and ribbon textures are developed. Quartz is mainly late magmatic since it includes most of the other phases, and is also present in granophyric textures related to late-magmatic stages. Primary muscovite occurs as large euhedral to subhedral lamellae, locally bent as a result of deformational processes or even with some recrystallization and kink band formation. Modal percentages are as high as 10%. Secondary muscovite is probably produced by dynamic metamorphism and hydrothermal activity. The chemical composition of primary muscovites is represented in Table 1, and shows low values of the celadonitic component. This composition is consistent with a magmatic crystallization at relatively low temperature, high H20 fugacity and pressure above 3 kb, as referred by Clemens and Wall (1988). Biotite is less abundant than muscovite, with maximum modal contents of about 4%. It is euhedral to subhedral, and sometimes shows deformed tamellae and kink band formation. It may be locally transformed to muscovite with iron oxide segregation along cleavage planes, probably reflecting hydrothermal alteration. Intergrowths of muscovite and biotite, partially engulfed by plagioclase or perthitic feldspar, are frequent and suggestive of a magmatic origin for these minerals. Biotites of the Cordilheira Intrusive Suite rocks (Table 1) are compositionally similar to those of the peraluminous Hercynian granites from central and northern Portugal (Neiva et al., 1987; Neiva and Gomes, 1991) as far as major elements are concerned. According to A1 versus Mg content parameters (Nachit et al., 1985), the studied biotites (Fig. 2) are similar to those of biotite-muscovite granites associated with the South Armorican Shear Zone. Ti content is lower than that reported as normal for S-type granites (Clemens and Wall, 1988). Garnet is associated with the most differentiated quartzenriched muscovite granites, without biotite. It is euhedral

58

L.V.S. NARDI and J.C. FRANTZ

Table 1. Representative analyses and chemical formulate of micas, garnet and tournaline from the Cordilheira Instrusive Suite grantoids. % SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 I(20 Total Si+4 Ti+4 A1+3 Fe+2 Mn+2 Mg+2 Ca+2 Na+1 K +1 Total

Musc. 47.74 0.04 34.59 2.90 -0.57 0.02 0.34 10.48 96.68 6.86 -5.86 0.35 -0.12 0.02 0.09 1.92

Biot. 35.43 2.98 19.35 23.19 .56 5.11 -0.07 9.49 96.18 5.94 0.38 3.83 3.25 0.08 1.28 --2.03

Garnet 36.42 .17 20.79 29.35 11.21 .70 1.73 0.01 -100.38 5.94 0.02 3.99 4.01 1.55 0.17 0.30 ---

15.21

16.78

16.04

24

24

24

0 -2

Tour, 36.94 0.04 35.80 11.83

0.08 2.95 0.19 1.62 0.06 89.51

5

3.5

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to subhedral and may be included by plagioclase and Kfeldspar. Microprobe analyses show their composition varying in the range: Alm (77-56%), Spe (42-17%), Pyr (4-1.5%), Gros (2,5-1.5%). These compositions are similar to the ones described as typical of magmatic garnets crystallized from peraluminous melts (Miller and Stoddard, 1981; du Bray, 1988; Searle and Fryer, 1986). They are probably a product of reaction between early crystallized marie phases and the magmatic liquids, as suggested by Miller and Stoddard (1981). Early magmatic or restite garnets, with a lower MnO content, generally show evidence of back reaction with the melt (Clemens and Wall, 1988), which has not been observed in the Cordilheira Intrusive Suite samples. Most of the analyzed grains show a reverse zoning with Mn e n r i c h m e n t towards the border, as described by several authors in magmatic garnets (Allan and Clarke, 1981). Tourmaline is generally euhedral and occurs in the most differentiated rocks. It can be observed as poikilitic inclusions in quartz and K-feldspar grains. Based on chemical composition, it was classified as a schorl (Table 1). According to Manning and Pichavant (1988) this is the normal composition of tourmaline in peraluminous granites which also contains magmatic muscovite, besides pera l u m i n o u s m i n e r a l s such as c o r d i e r i t e , g a r n e t and aluminum silicates. Zircon, apatite, and sphene are euhedral to subhedral, and the first was observed as inclusions in biotite. Fluorite is present as discrete anhedral grains or as a constituent of microveins and is in both cases a product of the late stages of crystallization. LITHOCHEMISTRY The twenty-eight studied samples of the Cordilheira Intrusive Suite were analyzed for major and trace elements. They present SiO 2 contents ranging from 71.55 to 77.68%. Representative analyses are listed on Table 2. Their agpaitic index has values between 0.70 and 0.90. The K 2 0 / N a 2 0 ratio decreases from 2.00 to 0.60 with increasing differentiation (Fig. 3a), while the normative corundum has no correlation with differentiation and shows values of 0.19 to 4.42%, below values considered typical for partial melting products of metapelites (Miller, 1985). The molar ratio A/CNK (Fig. 3b) also illustrates the peraluminous character of these granites and the lack of correlation between this ratio and differentiation. As far as major elements are concerned, the Cordilheira Intrusive Suite rocks are similar in composition to Hercynian granites from northern Portugal (Neiva and Gomes, 1991), and southern Brittany (Strong and Hanmer, 1981), or even to some peraluminous leucogranites from the Higher Himalayas (Searle and Fryer, 1986). Normative (CIPW) albite, quartz, orthoclase and anorthite were considered in relation to the Qz-Ab-Or-An-H20 system (Winkler, 1983). Although normative orthoclase is overestimated because of the mica effect in the CIPW norm, the studied samples plot close to minimum melts at 5 kb water pressure (Fig. 4).

The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil Their composition resembles that of some Damara orogenic granites produced, according to Winkler (1983), by anatexis of metasedimentary rocks. Harris et al. (1986) characterized the syn-collisional peraluminous intrusions based upon trace-element composition. Taking into account the REE and other measured trace-elements and the high Rb/Zr ratios, the Cordilheira Intrusive Suite granites can be included in this group. Figure 5 illustrates patterns normalized against hypothetical ocean ridge granite, compared to the peraluminous syntectonic granites referred by Harris et al. (1986). The trace-element contents of the Cordilheira Intrusive Suite rocks (Table 2) are comparable to those of granites produced by crustal partial melting. This process is also consistent with the observed moderate depletion of compatible elements during differentiation. The relatively low Ba and Sr contents, as well as the significant negative Eu-anomaly in REE chondrite-nor-

realized patterns, suggest the presence of alkali-feldspar or plagioclase among the major residual phases. Four samples (Fig. 6) show prominent Ba, Sr, Pb and Zr enrichment, plotting out of major geochemical trends. Textural evidence and the high partition coefficients of these elements in alkali feldspars (Henderson, 1982; Nash and Crecraft, 1985) suggest local cumulative processes to explain these cases. REE content in the Cordilheira Intrusive Suite granites is relatively lower than in acid rocks of calc-alkaline series (Fig. 7), and LREE are more compatible than the HREE during differentiation processes. Negative-Eu anomalies are well developed in most of the studied samples. Exceptional HREE enrichment in very differentiated garnetbearing terms is probably related to local accumulations of this mineral as suggested by Sultan et al. (1988) and Nardi and Bitencourt (1989) for similar situations. The general features of chondrite normalised REE patterns, including

Table 2. Representative analytical data of granitoids. Sample

C004

C008

C017

SiO2 TiO2 A1203 Fe203* MnO MgO CaO Na20 K20 P205 Total LOI Rb Cs Ba Sr Ga Li Nb Zr Y La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu

74.15 0.10 13.45 2.03 0.01 0.12 0.37 3.24 5.54 0.07 99.08 0.99 322 <5 390 129 35 25 18 63 <5 6.60 13.65 7.72 1.68 0.20 1.11 0.71 0.14 0.42 0.39 0.06

72.71 0.17 13.51 2.17 0.02 0.18 0.50 3.50 5.78 0.14 98.68 0.95 316 <5 323 126 28 22 16 57 <5 7.18 15.81 9.52 2.18 0.26 1.51 0.95 0.16 0.41 0.31 0.06

75.11 75.63 0.15 0.04 14.64 14.31 1.03 1.04 0.01 0.05 0.08 0.03 0.70 0.60 3.37 3.77 4.27 4.61 0.11 0.05 9 9 . 4 7 100.13 1.02 0.46 288 408 8 -51 44 111 38 30 -18 17 <10 -<30 <30 22 -4.37 4.12 11.35 9.72 6.45 4.49 1.65 1.05 0.22 0.12 1.44 0.96 1.16 1.09 0.24 0.26 0.60 0.88 0.49 1.20 0.90 0.18

C019

59

c020

C023

C024

73.37 0.22 14.21 1.34 0.01 0.23 0.91 3.10 5.16 0.20 98.75 0.71 326 <5 300 113 34 37 11 <30 10 15.56 38,81 19.97 4.16 0,37 2.56 1,30 0.22 0.50 0.36 0.07

75.97 76.54 75.58 0.04 0.05 0.08 14.00 13.61 14.80 0.97 1.04 0.99 0.02 0.02 0.01 0.02 0.06 0.08 0.55 0.54 1.13 4.04 3.50 4,18 4.01 3.96 3.14 0.02 0.07 0.03 9 9 . 6 4 9 9 . 3 9 100.02 0.39 0.70 0.61 359 401 219 12 19 <5 33 119 43 30 63 46 15 26 26 16 29 27 <10 20 <10 <30 <30 41 <5 12 70 2.58 6.45 9.56 4.76 10.66 19.56 2.07 6.79 9.02 0.44 1.55 1.70 0.08 0.33 0.47 0.53 1.61 2.00 0.71 1.71 2.55 0.15 0.33 0.46 0.40 0.87 1,39 0.48 0.96 1.51 0.07 0.11 0.20

C025

C026 75.66 0.07 14.85 1.04 0.01 0.09 0.72 3,60 3.82 0.07 99,93 0,82 329 14 43 46 30 62 17 <30 <5 10.08 17.42 8.36 1.87 0.67 1.75 1.55 0.28 0.75 0.89 0.18

60

L.V.S. NARDI and J.C. FRANTZ

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their concave shape frequently present in the HREE segment, are consistent with liquids produced by partial melting of quartz-feldspathic rocks containing amphibole among the residual phases. REE patterns of the Cordilheira Intrusive Suite rocks are similar to those referred to by Neiva and Gomes (1991) in syntectonic muscovitebiotite Hercynian granites from northern Portugal, or those of the Manaslu leucogranite, described by Vidal et al. (1982).

PETROGENETIC CONSIDERATIONS

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Field relationships of, and petrographic and chemical data from the Cordilheira Intrusive Suite granitoids show similarities with other peraluminous granitic rocks typically occuring in deeply dissected parts of orogenic terranes, where they are associated with calc-alkaline metaluminous granites (Strong and Hanmer, 1981). As observed previously by Frantz and Remus (1986) and Fernandes et al. (1990), they are syntectonic with transcurrent movements related to the late orogenic stages of the Brasiliano Orogenesis. Typical calc-alkaline metaluminous granitic magmas were concomitantly emplaced in this same region (Fernandes et al., 1990; Gomes, 1990; Frantz and Nardi, 1992) testifying to the final stages of a subduction-related magmatism. The high geothermal gradient has probably played an important role in allowing the production of granitic melt by frictional heating along the shear zones, where the fluid activity was high enough. Similar anatectic mechanisms have been proposed by Nicolas et al. (1977), Strong and Hanmer (1981) and Winkler (1983), among others. On the other hand, the peraluminous granitic magmatism could represent fluid-absent melting of the lower crust, induced by basaltic magmatism, as suggested by Vielzeuf and Holloway (1988) for Hercynian granites. Evidence of basic magmatism has up

The Cordilheira Intrusive Suite: Late Proterozoic peraluminous granitoids from southern Brazil

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to now been restricted to enclaves produced by magma mingling in calc-alkaline granites of the same age and also by syn-transcurrence mafic dikes. Halliday et al. (1981) admitted the following mechanisms capable of producing peraluminous granitic melts: 1) Di-normative rocks submitted to partial melting with hornblende in the residue (Cawthorn et al., 1976), 2) fracdonation of amphibole (Cawthorn et aI., 1976), 3) melting of peraluminous compositions (White and Chappell, 1977), 4) assimilation of aluminous sediments, and vaporphase transfer of alkalis from metaluminous granitic liquids (Luthet al., 1964). The behavior of alumina during the Cordilheira Intrusive Suite differentiation showing no evidence of enrichment in the less evolved terms, the relatively low normative corundum values, and the trace-element trends, particularly the REE patterns, are more consistent with the first mechanism. The field relationships, including the presence of quartz-feldspathic gneisses as xenoliths, suggest as well that granitic gneisses could represent a suitable source for these melts, even though a more complete model should be supported by isotopic data. Turpin et al. (1990) proposed a similar genetic hypothesis for Hercynian peraluminous granites, particularly the Saint Sylvestre granite, based upon isotopic data. A metasedimentary source (FragosoCesar et al., 1986), as reported for S-type granites, cannot be ruled out. Magmatic muscovite composition indicates that these intrusions crystallized at pressures above 3 kb and high H20 fugacity. According to Vielzeuf and Holloway (1988), orthopyroxene-cordierite would be expected from similar liquids crystallized under pressures lower than 3 kb. The plot of the normative composition of the Cordilheira Intrusive Suite samples in the Ab-An-Or-Qz-H20 system (Winkler, 1983) is also suggestive of crystallization under pressures of about 4-5 kb. Dynamic metamorphism associated with shear zones was probably developed under

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Fig. 6. Samples with prominent Ba, Sr, Pb and Zr enrichment projecting out of the general geochemical trends,

Fig. 7. REE patterns of the Cordilheira Intrusive Suite rocks normalised against the chondritic values of Haskin et al. (1968).

62

L.V.S. NARDI and J.C. FRANTZ

greenschist facies conditions, as shown by the mineralogical and textural transformations in the Cordilheira Intrusive Suite, and reported by Fernandes et al. (1992) to the north o f this area. This evidence is consistent with a mesozonal emplacement o f the Cordilheira intrusions. The major processes causing differentiation within each granitic body are p r o b a b l y partial melting and local mineral segregation. W h i l e the former process is suggested by the patterns o f the compatible element (Hanson, 1978), the second involves mainly K-feldspar and garnet. Differences in the g e o c h e m i c a l features observed in the several intrus i o n s are related either to different protoliths or to variations on the amount o f extracted melt. L a c k o f analytical data concerning volatile elements prevents a better comprehension of their role in the differentiation of these magmas. A s o b s e r v e d in o t h e r p e r a l u m i n o u s granites, such as those related to the South A r m o r i c a n Shear Zone (Strong and Hanmer, 1981) or the Lac-du-Bonnet batholith (Goad and Cerny, 1981) the Cordilheira Intrusive Suite shows tin and tungsten d e p o s i t s f o r m e d by late to p o s t - m a g m a t i c hydrothermal activity.

Du Bray, E.A., 1988. Garnet compositions and their use as indicators of peraluminous granitoid petrogenesis- southeastern Arabian Shield. Contributions to Mineralogy and Petrology 100, 205-212. Fernandes, L.A.D., Tommasi,A., and Porcher, C.C., 1990. Esbo~o estrutural de parte do Batolito Pelotas-Regi~ode Quitrfia-Capivarita.Acta Geologica Leopoldensia XIII, 117-138. Fernandes, L.A.D., Tommasi, A., and Porcher, C.C., 1992. Deformation patterns in the southern Brazilian branch of the Dom Feliciano Belt: a reappraisal. Journal of South American Earth Science 5, 77-96. Fragoso-Crsar, A.R.S., Figueiredo, M.C.H., Soliani Jr. E., and Faccini, U.F., 1986. O Batolito Pelotas no Escudo do Rio Grande do Sul. Anais, XXXIV Congresso Brasileiro de Geologia, Goiania 3, 13221342. Frantz, J.C. and Nardi, L.V.S., 1992. Litoqutmica e evolu~o de granit6ides cfilcio-alcalinosda regi~o leste do Escudo Sul-rio-grandense. Pesquisas 19, 13-25. Frantz, J.C. and Remus, M.V.D., 1986. Geologia da regi~o de Cangu~tiTorrinhas, RS- caractedza~ao petrogr~ifica,estrutural e estratigrtlfica. Anais, XXXIV Congresso Brasileiro de Geologia, Goiania 2, 931948. Goad, B.E. and Cerny, P., 1981. Peraluminous pegmatitic granites and their pegmatite aureoles in the Winnipeg River District, Southeastern Manitoba. Canadian Mineralogist 19, 177-194. Gomes, M.E.B., 1990. Petrologia do Granito Arroio Moinho (Cangucu, RS) geoqufmica e deforma~o. Unpublished MSc thesis, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS Brazil, 209 p. -

CONCLUSIONS The Cordilheira Intrusive Suite is c o m p o s e d of peralum i n o u s l e u c o c r a t i c granites, c o n t a i n i n g t o u r m a l i n e and s p e s s a r t i n e - r i c h g a r n e t in the m o s t e v o l v e d t e r m s . Its e m p l a c e m e n t is c o n t r o l l e d by l a t e - B r a s i l i a n o O r o g e n y transcurrent shear zones, which also deform and promote dynamic metamorphism along mylonitic zones. Field relationships, m i n e r a l o g i c and c h e m i c a l data are consistent with an o r i g i n b y p a r t i a l m e l t i n g o f a m p h i b o l e - b e a r i n g granitic gneisses, although a metapelitic source cannot be ruled out in identifying thes,~ rocks as true S-type granites. Geochemical and petrographic evidence is consistent with a differentiation controlled by partial melting, associated with local m i n e r a l s e g r e g a t i o n processes. The e v i d e n c e p r e s e n t e d in this p a p e r suggests a r e m a r k a b l e similarity between the Cordilheira Intrusive Suite rocks and the Hercynian granitic rocks o f Portugal and France. Acknowledgements--This research has been supported by the Brazilian

PADCT/FINEPprogram. The authors are particularly indebted to professors R. Dall'Agnol, M.F. Bitencourt and L.A. Hartmann for comments and review.

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