ULTRAHIGH-TEMPERATURE METAMORPHISM OF SAPPHIRINE-BEARING GRANULITE FROM THE 2.0 GA ITABUNA-SALVADOR-CURAÇÁ OROGEN,BAHIA, BRAZIL

 

Leite, C.M.M¹.; Barbosa, J.S.F.²; Nicollet, C³.; Kienast, J.R. ⁴; Fuck, R.A.⁵

 

^1. PETROBRAS, Avenida Antonio Carlos Magalhães, 1113, Pituba, Salvador, Bahia, Brasil. cmml@petrobras.com.br

^2. Universidade Federal da Bahia, Rua Caetano Moura 123, Federação, Salvador, Bahia, Brazil. johildo@cpgg.ufba.br

^3. Université Blaise Pascal 5, rue Kessler, F-63038, Clermont – Ferrand, França. c.nicollet@opgc.univ-bpclermont.fr

^4. PARIS VI, 4, Place Jussieu, Tour 26, 5^eme etage Paris, França. jrk@ccr.jussieu.fr

^5. UNB/IGEO, Campus Univ. Darcy Ribeiro, Brazil, rfuck@unb.br

 

ABSTRACT

 
The studied area is located in the São Francisco Craton, Southern part of Bahia State, Brazil. In this region, the Palaeoproterozoic Itabuna-Salvador-Curaçá Orogen (ISCO) comprises supracrustal rocks associated to tonalite/trondhjemite and monzonite orthogneisses, together with subordinate metamafic rocks of Archean and Palaeoproterozoic ages. All these rocks are strongly deformed and re-equilibrated in granulite conditions. Supracrustal rocks include a sapphirine-bearing granulite associated to tonalitic orthogneiss. These rocks contain quartz, orthopyroxene, sillimanite, garnet, mesoperthite, plagioclase, biotite and cordierite, and the surrounding tonalitic orthogneiss, plagioclase, orthopyroxene and garnet. The main phases define the parageneses of the metamorphic peak (Grt₁+Opx₁+Bt₁+Qtz+Sil₁+Spr₁+Mp) while the microstructures establish the retrograde evolution. The reactions Grt₁± Qtz = Opx₂+Sil₂, Grt₁=Opx₂+Spr₂(± Sil₂); Opx₁+Sil₁+ Qtz=Crd and Grt₁+Qtz=Opx₃+Crd, identified in symplectitic coronae around porphyroblasts, record the retrograde evolution related to orogenic exhumation. Symplectites of Bt₂ +Qtz formed by the reactions Opx₁-₂+Mp+H₂O=Bt₂+Qtz are also retrograde, but were formed during cooling and/or hydration processes. The tonalitic orthogneiss is composed of plagioclase, quartz, orthopyroxene, garnet and opaque minerals. In these rocks, microstructures are interpreted in terms of the reactions Grt₁+Qtz=Opx₂+Pl₂ and Opx_1-2+Mp+H₂O=Bt₁+Qtz. Geothermobarometry data constrain P-T conditions at P = 7 - 11 kbar and T ³ 900 °C in the sapphirine-bearing granulite. One of the main mechanisms that accounts for this high temperature may be related to addition of strongly radioactive material together with the presence of basaltic magmas during the formation of the ISCO. Fractional crystallization of the basalt produced tonalitic intrusions, dated at approximativevely 2.15 - 2.13 Ga, close to the metamorphic peak at 2.07 - 2.08 Ga.

 Keywords: Sapphirine-bearing granulite, tonalitic orthogneiss, Palaeoproterozoic, Itabuna-Salvador-Curaçá Orogen

REGIONAL GEOLOGY AND GEOLOGICAL SETTING

The largest basement exposures of the São Francisco Craton occur in Bahia, where they comprise Archaean to Paleoproterozoic, medium to high-grade metamorphic rocks and remnants of greenstone belts, intruded by Paleoproterozoic granites, syenites and less commonly by basic-ultrabasic bodies. The main litho-geotectonic units recognized within this basement are the Gavião, Serrinha, Jequié and Itabuna-Salvador-Curaçá Blocks. The Gavião Block is composed of gneiss-amphibolite associations, which host an older nucleus formed by trondhjemite – tonalite - granodiorite (TTG) suite (Martin et al. 1991, Santos Pinto, 1996). Within this block Archaean greenstone belts also occur (Marinho, 1991; Cunha et al., 1996; Bastos Leal, 1998; Peucat et al., 2002). The Serrinha Block is composed of banded gneisses, amphibolites and mainly granodioritic orthogneisses (Rios, 2002). Paleoproterozoic greenstone belts are present in this block (Silva, 1996). The Jequié Block (Barbosa, 1990; Barbosa & Sabaté, 2002; 2004) is formed by enderbitic and charnockitic granulites and migmatitic and heterogeneous granulites (Marinho, 1991). In the heterogeneous granulites granulitized supracrustal rocks, including mafic and felsic metavolcanic rocks, quartzites, iron formations and aluminous-magnesian granulites, are present and occasionally cut by anatectic granites. The Itabuna-Salvador-Curaçá Block is mainly formed of tonalitic orthogneisses with basic-ultrabasic enclaves and less abundant supracrustal rocks equilibrated at granulite facies (Barbosa & Sabaté, 2002). These four Archean Blocks collided to form the Itabuna-Salvador-Curaçá Orogen (ISCO; Barbosa & Sabaté 2002, 2004, Leite, 2002).

The studied sapphirine-bearing granulite occurs within a band of supracrustal rocks, tectonically imbricated in tonalitic orthogneiss of the southern part of ISCO. The high-grade supracrustal rocks (quartz-feldspathic granulite, basic granulite, iron-manganese formations, quartzite, calc-silicate rocks, marble and pyroxenite and sapphirine-bearing granulite) are exposed as lenses along a subvertical shear zone. In some of the outcrops there are undeformed leucosome veins, parallel to or cross-cutting the rock layering. They are thought to have been formed during the metamorphic peak, after the end of ductile deformations (Barbosa, 1990). The tonalitic orthogneiss has layering with variable thickness, being composed of quartz, plagioclase, orthopyroxene and opaque minerals. Garnet is dispersed throughout the rocks.        

 PETROGRAPHY, MINERAL CHEMISTRY AND METAMORPHIC REACTIONS

The sapphirine-bearing granulite parageneses formed near the metamorphic peak comprises garnet (Grt1), orthopyroxene (Opx₁), sillimanite (Sil₁), biotite (Bt₁), quartz or sapphirine (Spr₁), plagioclase (Pl₁) and mesoperthite (Mp₁). The accessory minerals are rutile, magnetite / ilmenite, graphite, monazite and zircon. The main mineral phases are in contact with each other. Contacts between quartz and sapphirine have not been observed.

In the sapphirine-bearing granulite, symplectites developed between porphyroblasts provide evidence for the retrograde reactions which occurred in the sapphirine-bearing granulite. Coarse symplectitic intergrowths composed by orthopyroxene (Opx₂) and sillimanite (Sil₂) separate garnet and quartz. These symplectites could be formed according to the divariant FMAS reaction: Grt₁ ± Qtz = Opx₂ + Sil₂ (1). A second generation of sapphirine (Spr₂) is observed in symplectitic intergrowth with orthopyroxene (Opx₂) and sometimes with sillimanite (Sil₂), resorbing large crystals of garnet (Grt₁). This type of symplectite is found only at the rims of orthopyroxene porphyroblasts in contact with garnet. This microstructure suggests the following discontinuous FMAS reaction: Opx₁ + Grt₁ = Opx₂ + Spr₂ (±Sil₂) (2). Cordierite (Crd) moats, sometimes altered to pinnite, occurr around crystals of orthopyroxene. Some of these moats have sillimanite and quartz inclusions. We infer that cordierite was formed by the univariant FMAS reaction: Opx₁ + Sil_l + Qtz = Crd (3). Symplectites of cordierite with blebby orthopyroxene (Opx₃) replace garnet in the presence of quartz. This microstructure could have been produced by the divariant FMAS reaction: Grt₁ + Qtz = Opx₃ + Crd (4). A second generation of biotite (Bt₂) forms intergrowths with quartz, close to orthopyroxene when in contact with mesoperthite, according to the continuous KFMASH reaction: Opx_1-2 + Mp + H₂O = Bt₂ + Qtz (5).

In the sapphirine-bearing granulite garnet, where chemical zonation is subtle, is essentially a solid solution between almandine (44-54 wt.%) and pyrope (48-54 wt.%), while spessartite, grossular and andradite components sum up to 1.5 wt%. Orthopyroxene is present in three different generations: (i) the porphyroblasts (Opx₁) have Al₂O₃ contents close to 11 wt.%, and are usually unzoned, though two of the analysed grains become richer in Al towards one of their borders; (ii) in the symplectites with sapphirine (Spr₂) and sillimanite (Sil₂) (Opx₂) has Al₂O₃ between 8 and 10 wt.%; and (iii) in the symplectites with cordierite, (Opx₃) shows Al₂O₃ between 6 and 7.5 wt.%. Other elements do not show large variations in the three generations. Biotite occurs in two forms: (i) (Bt₁) formed at the metamorphic peak has polygonal contacts and is mostly encountered in the leucosome veins; and (ii) (Bt₂) occurs in symplectites with quartz around orthopyroxene grains, which are in contact with mesoperthite. The compositions of (Bt₁) do not vary even from one sample to another, and are close to the phlogopite end member with X_Mg from 0.78 to 0.86. Sapphirine also occurs in two forms: (i) (Spr₁) included in (Opx₁) and (Grt₁) contains 63-64 wt% Al₂O₃ and (ii) (Spr₂) (ca. 59-61 wt% Al₂O₃) in simplectite intergrowth with (Opx₂) and (Sil₂). Al varies from 4.46 (Spr₁) to 4.20 (Spr₂) cations per formula unit based on 10 oxygens, with the highest values (4.46) correlating with the lowest Si content (12.49). Cordierite is unzoned and has the highest X_Mg of all ferromagnesian phases (0.70 - 0.88). The compositions of sillimanite in long prismatic crystals (Sil₁) and (Sil₂) in symplectites with (Opx₂) are very similar. Its high Fe content is a characteristic of sillimanite in UHT terrains (Sengupta et al., 1990). Plagioclase is essentially oligoclase (Ab > 80%).  K-feldspar in mesoperthite is orthoclase (Or 85 – 92 %).

In the tonalitic orthogneiss, more than 80% of these rocks are made up of antiperthitic plagioclase (Pl₁) and quartz, probably formed in plutonic rocks but now re-equilibrated at granulite facies. A second plagioclase type (Pl₂) is also present in smaller grains. Fe-Mg minerals and opaque grains are concentrated in dark microbands. Orthopyroxene (Opx₁) is most abundant, while the main opaque mineral is ilmenite. Garnet (Grt₁) surrounds globular quartz. Accessory phases are mesoperthite, apatite, monazite and zircon. The mesoperthite occurs as small crystals between antiperthitic plagioclase grains (Pl₁).

     In the tonalitic orthogneiss garnet (Grt₁) reacted with neighbouring quartz to produce symplectites with orthopyroxene (Opx₂) and plagioclase (Pl₂) through the divariant FMAS decompression reaction: Grt₁ + Qtz = Opx₂ + Pl₂ (6). Biotite (Bt₂) and quartz symplectite is observed between orthopyroxene and mesoperthite, according to the continuous KFMASH reaction Opx_1-2 +  Mp + H₂O = Bt₂ + Qtz (5).

In the tonalitic orthogneiss garnet is almandine around 54 wt%, pyrope close to 41 wt%, and 2.30 wt% andradite. Two types of orthopyroxene were found: (i) (Opx₁) is more abundant and forms porphyroblasts, while the (ii) (Opx₂), is rarer, smaller and forms symplectites with plagioclase that are a product of the destruction of garnet (Gt₁) (6). The compositions of the two types are not very different, for exemple, its FeO content (19.38 wt%) is approximately equal.  Mica is phlogopite with X_Mg = 0.78 and it forms symplectites with quartz around orthopyroxene porphyroblasts. Plagioclase occurs as larger (2-3 mm) antiperthites (Pl₁), and less commonly as (Pl₂), a product of reaction (6). The differences between the two types are minimal. (Pl₁) is slightly richer in the anorthite component (An₂₈Ab₇₀) than (Pl₂) (An₂₆Ab₇₁).

THERMOBAROMETRY, MICROSTRUCTURES RELATIONS AND P-T PATH

The sapphirine-bearing granulite paragenesis, Grt₁+Opx₁+Sil₁+Bt₁+Spr₁ is indicative of ultra high-temperature (UHT) metamorphism (>900°C and P= 7 - 11 kbar). The high Al content in orthopyroxene (Opx₁), with Al₂O₃>10 wt%, fits such an argument. According to Hensen (1986) and Bertrand et al. (1991), the assemblage Opx – Sill – Grt - Qz should indicate pressures higher than 11 kbar. In order to constrain the temperature-pressure conditions of metamorphism in the sapphirine-bearing granulite and the tonalitic orthogneiss, using the compositions of the centres of the grains, the thermobarometry softwares by Kohn & Spear (1994), Reche & Martinez (1996), Bermann (1991), were used. For example, the estimated pressure and temperature data calculated with Holland and Powell (1998) - THERMOCALC (6.3 - 11.1 kbar and 900 – 1029 °C) retrieve the metamorphic peak at ultrahigh-temperature conditions. The reactions (2), (3) and (4) that formed symplectitic coronas around the garnet and orthopyroxene porphyroblasts represent a retrograde evolution. Reactions (3) and (4) define shallow curves and may be interpreted either as decompression paths or as the consequences of the influx of a volatile phase into a “dry” assemblage, with or without changing P-T (Newton & Wood, 1979). On the other hand, the symplectites of Bt₂+Qz formed by (5) are also retrograde, related to temperature decrease and/or hydration.

In the tonalitic orthogneiss, the assemblage Pl₁+Qtz+Opx₁+Grt₁ corresponds to the peak-metamorphic paragenesis. The microstructures suggest a retrograde evolution which began with a decompression reaction (6), followed by a temperature decrease and hydration (5). Such microstructures, and their inferred reactions in the FMAS and KFMASH model system, allow the tracing of part of the retrograde PT metamorphic path in a petrogenetic grid constructed for the studied area. This PT path starts with a period of near-isothermal decompression from » 11 kbar to 7 kbar at 900 – 1000 °C, followed by near-isobaric cooling from 900 °C to < 700 °C at < 7 to < 6 kbar.

 The accretion of radioactive material together with the presence of basaltic magma in the roots of the ISCO, could have produced the high tempertures necessary for the formation of the sapphirine-bearing granulites, and also the partial melting which generated the associated leucosome veins. Differentiation of the basalt magma would have lead to the formation of the tonalites (Pinho, 2005). The protoliths of the sapphirine-bearing granulites would have been metapelites, which subjected to extreme temperatures would have partially melted leaving the sapphirine-bearing granulite as the refractory residue. A significant amount of melt must have drained away, although sufficient remained behind in the form of the leucossome veins, which must have formed at the metamorphic peak, after the ductile deformation. The common granulite host rocks, including the tonalitic orthogneiss, may also have experienced UHT metamorphism conditions, but any register of this must have been obliterated during the extensive retrogression that affected the area.

 

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