HERCYNITE + QUARTZ -BEARING GRANULITES OF BREJÕES DOME AREA, JEQUIÉ BLOCK BAHIA, BRAZIL: INFLUENCE OF CHARNOCKITE INTRUSION ON GRANULITE FACIES METAMORPHISM

Barbosa, J.S.F.¹; Leite, C.M.M.²; Nicollet, C.³; Kienast, J.R.⁴; Fuck, R.A.⁵

 

¹Universidade Federal da Bahia, Rua Caetano Moura 123, Federação, Salvador, Bahia, Brazil, johildo@cpgg.ufba.br

²PETROBRAS, Avenida Antônio Carlos Magalhães,1113, Pituba, Salvador, Bahia, Brasil, cmml@petrobras.com.br

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

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

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

 

ABSTRACT 

In the present study, we describe and discuss the geology of aluminous-magnesian granulites and associated garnet-bearing charnockitic granulite from the Brejões Dome area, located in the Jequié Block, basement of the São francisco Craton in Bahia, Brazil. Investigation of  metamorphic mineral assemblages allows to evaluate P-T conditions of formation of these rocks and, therefore, to bring about constraints to better understand the geological evolution of the area. We conclude that the rocks from the Brejões Dome area were formed under granulite facies conditions of low to intermediate pressure (5-8 kbar). Temperatures determined in samples collected away from the dome are in the order of 850°C, similar to the temperature determined elsewhere in the southern part of the Itabuna-Salvador-Curaçá Orogen. However, samples from close to the Brejões Dome record higher temperatures of about 900-1000°C. It is suggested that the intrusion of the charnockitic diapir of Brejões is responsible for the local rise in temperature in relation to peak temperature of the regional granulite metamorphism.

 

Keywords: Jequié Block, Paleoproterozoic metamorphism, spinel+quartz, high-temperature granulites

 

 

REGIONAL GEOLOGY AND GEOLOGICAL SETTING

The Archean Gavião, Serrinha, Jequié and Itabuna-Salvador-Curaçá blocks are part of the basement framework of the São Francisco Craton in Bahia, Brazil. The Archean blocks were amalgamated during ca. 2.08 Ga Paleoproterozoic collision, which gave birth to the Itabuna-Salvador-Curaça Orogen (ISCO; Barbosa and Sabaté, 2002, 2004).

The granulite rocks, located in the roots of the ISCO, underline one of the largest high-grade metamorphic provinces in the world. Many geological studies, including petrochemical and mineralogical studies, were undertaken in the area to clarify geological relations, composition of metamorphic products, pressure-temperature conditions of metamorphism, as well as ages of protholiths and of high-grade metamorphism (Wilson, 1987; Barbosa and Fonteilles, 1989; Barbosa, 1990, 1996; Marinho et al., 1992; Fornari & Barbosa, 1992; Figueirêdo and Barbosa, 1993; Iyer et al., 1995; Leite, 2002; Barbosa & Sabaté, 2002, 2004; Kosin et al., 2003; Barbosa et al., 2004).

In the Jequié Block, where the Brejões Dome is located, enderbitic-charnockitic granulites were dated at 2.7-2.8 Ga (Alibert and Barbosa, 1992). They intruded a heterogeneous association of different kinds of high-grade rocks of igneous descent, that are labelled as heterogeneous granulites (HG) in this paper. These heterogeneous granulites are associated with granulite facies supracrustal rocks (SP).

ISCO rocks were deformed under three continuous ductile deformation phases, contemporaneous with the Paleoproterozoic high-grade metamorphism which affected the region ca. 2.1-2.0 Ga (Wilson, 1987; Ledru et al., 1994). The first deformation phase (F1) is represented by large top to the west recumbent folds, with sub-horizontal axial planes and N-S fold axes. The coaxial second phase (F2) comprises open folds in the western part of Jequié Block and tight folds in the eastern part, both with near vertical axial planes. The third phase (F3) is represented by vertical transposition zones, which are sub-parallel to the upright axial planes of the tight F2 folds. High-grade metamorphic conditions of Jequié Block were obtained through petrological investigation of rare garnet-bearing mafic granulites.

The Brejões Dome area comprises the following rock units: (i) heterogeneous granulites (HG) are exposed in the central part of the study area, surrounding the Brejões Dome, comprising an association of charnockitic rocks, with mafic and felsic intercalations of varied composition, equilibrated under granulite facies conditions; (ii) granulitic supracrustals (SP), including quartzite, iron formation, quartz-feldspar layers, Al-Mg granulites, graphite-rich rocks, and mafic granulites, appear as enclaves or intercalated within heterogenous granulites; and (iii) intrusive charnockite bodies (CH6)  that  constitute the cores of the Brejões Dome and several other smaller unnamed domes dated at ca. 2.0 Ga (Barbosa et al., 2004).

This work deals with garnet-bearing charnockitic granulite, belonging to the heterogenous granulites (HG) unit, and Al-Mg granulites, from the granulitic supracrustal layers (SP) collected in the country rocks surrounding the Brejões Dome charnockite intrusion (CH6). Previously obtained thermobarometric results (5-7 kbar, 850-870 °C; Barbosa, 1990; Barbosa and Fonteilles, 1991), will be compared with the results determined on Al-Mg granulites of the supracrustal rock layers and garnet-bearing charnockitic granulite from the heterogeneous granulites.

 

MINERALOGY, METAMORPHIC REACTIONS, MINERAL CHEMISTRY AND THERMOBAROMETRY

The garnet-bearing charnockitic granulite is constituted of mesoperthite (Mp1, 40-50%), quartz (30%), antiperthitic plagioclase (Pl1, 10%), orthopyroxene (Opx1, 10%), garnet (Grt1, 5-10%), and biotite (Bi1, 2-5%). Ilmenite, pyrite, apatite, zircon, and monazite are accessory phases. Myrmekite along contacts between plagioclase, mesoperthite, and quartz is also found in these plutonic rocks re-equilibrated under granulite facies metamorphic conditions. In general, equilibrium contacts are observed between main mineral phases. However, reaction of garnet and quartz, producing orthopyroxene (Opx2) plus plagioclase (Pl2) symplectites, is observed sometimes. Also, fine biotite (Bi2) plus quartz symplectite may substitute for orthopyroxene, garnet, and opaque minerals. These features are due to retrograde metamorphism, according to metamorphic reactions Grt1+Qtz=Opx2+Pl2 and Opx+Fk+H₂0=Bi2+Qtz, respectively.

Composition of antiperthitic plagioclase (Pl1) and mesoperthite (Mp1) in the garnet-bearing charnockitic granulite, measured between exsolution lamellae, is andesine and orthoclase, respectively. Orthopyroxene (Opx1) with hypersthene-ferrosilite-type composition appears in 2-5 mm porphyroblasts oriented whithin the rock foliation. More or less well formed garnet grains in equilibrium with mesoperthite, plagioclase, and orthopyroxene contain biotite (Bi1) and opaque inclusions. Garnet (Grt1) grains, product of granulite facies metamorphism of a plutonic protolith, are unzoned and rich in almandine (74-75%) and spessartite (2,5-3%) end members, whereas pyrope concentration is around 12-16%. Biotite (Bi1) is rich in Fe.

Textural relations in the aluminous-magnesian granulite show that the peak metamorphic  paragenesis is composed of quartz (35-40%), plagioclase (Pl1, 30-40%), cordierite (Cd1, 10-15%), garnet (Grt1, 8-10%), sillimanite (Sil1, 5%), and biotite (Bi1, 5%), with ilmenite (1-2%), magnetite/pyrite (1-2%), spinel (1-3%), rutile (1-2%), graphite (1%), monazite, and zircon as accessory minerals. Orthopyroxene (Opx1) is observed in addition to the above peak metamorphic paragenesis. Rare interstitial mesoperthite (Mp1) crystals are also present in these rocks. In several thin sections of Al-Mg granulite, particularly in samples collected at the contact of Brejões Dome charnockite, hercynite type spinel crystals are in direct contact with quartz (Sp/Qtz). In many thin sections of Al-Mg granulites, equilibrium metamorphic peak mineral paragenesis was partially replaced by secondary mineral assemblages. Greenish spots, oriented parallel to banding or foliation, found in several outcrops, are composed of cordierite (Cd1) plus spinel (Sp). These minerals were formed by prograde reaction Gt+Sill±Qz=Cd1+Sp. Orthopyroxene plus K-feldspar coronae around biotite (Bi1) and magnetite were produced by dehydratation metamorphic reactions Bi1+Qz=Opx2+Fk+ H₂O and Opac+Qz+K⁺=Opx2+Fk+H₂O. On the other hand, retrograde phenomena in the Al-Mg granulite rocks are also indicated by (i) the presence of cordierite (Cd2), formed through the decompression reaction Sp+Qz (+H₂0)=Cd2; (ii) the presence of orthopyroxene (Opx3) plus cordierite (Cd2), produced through decompression reaction Gt+Qz (+H₂0)=Opx3+Cd2; and (iii) the presence of Bi+Qz symplectites formed due to cooling and/or hydratation reactions Opx+Fk+H₂0=Bi+Qz, Gt+Fk+H₂O=Bi+Qz, and Op+Fk+H₂O=Bi+Qz.

In the aluminous-magnesian granulite, electron microprobe analyses have determined that plagioclase (Pl1) is close to andesine. On the other hand, there are slight variations in chemical composition of the two cordierite generations (Cd1, Cd2) from different samples. For example, cordierite Cd1 has variable X_Mg ratios between 0.77 and 0,80, whereas cordierite Cd2 displays lower X_Mg values, around 0,74-0,75. Garnet (Grt1) displays short-ranged chemical variations from one sample to another, and is relatively rich in the almandine component (about 60%); pyrope contents are around 30-35% and the other end members, like grossular and spessartite, are low (<5%). Microprobe profiles throughout two garnet porphyroblasts show slight zoning of Fe and Mg contents. Two generations of biotite (Bi1, Bi2) have been found. The peak metamorphic biotite (Bi1), sometimes surrounded by an orthopyroxene corona (Opx2), is richer in Mg than secondary retrograde biotite (Bi2), in symplectitic intergrowth. The three generations of orthopyroxene (Opx1, Opx2, Opx3) observed in several samples differ in chemistry: (i) Opx1 is richer in Al and poorer in silica than Opx2 and Opx3, and (ii) Opx3 has higher contents of Fe, Mn, and Cr than Opx1 and Opx2. The two types of spinel, one associated with cordierite (Sp/Cd), the other in direct contact with quartz (Sp/Qz), show important differences in composition. Both spinel generations are hercynite-type solid solutions, with X_Mg ratios varying between 0.19 and 0.34. Cr₂O₃ content varies between 0.15 and 2.5% and ZnO content between 0.8 and 2.0–2.7% in spinel associated with cordierite (Sp/Cd) and spinel in contact with quartz (Sp/Qz), respectively. Potassic feldspar (Mp1) is essentially orthoclase. Ilmenite has TiO₂ contents of ca. 54% and Fe0 contents of ca. 46%. Magnetite, often associated with spinel, has X_Fe ratio (Fe⁺³/Fe⁺³ + Fe⁺²) of 0,66.

Temperature and pressure conditions of the studied rocks were estimated using the geothermobarometric softwares of Reche and Martinez (1996) and  Holland & Powell (1998). These geothermobarometers were used on rocks sampled close to the Brejões Dome and on samples collected away from this structure. These samples clearly display the prograde and retrograde parageneses, as well as the main metamorphic reactions. T and P values represent an average obtained from different pairs and groups of mineral phases. The majority of calculated P values are between 5 and 8 kbar in all analyzed samples, irrespective of the method that was used. Pressure estimates by the method of Holland & Powell (1998) are closer to 8 Kbar, either for the Al-Mg granulite sampled close to the Brejões Dome, or for the garnet-bearing charnockitic granulite collected away from the dome.

T values obtained with Gt-Cd and Gt-Opx thermometers are in the range of 700-830 °C. Calculated temperatures for the secondary paragenesis of Al-Mg granulites samples are sligthly lower. The highest temperature values (900- 1150°C) were determined with Gt-Ilm and Cd-Sp thermometers for the samples of hercynite+quartz-bearing Al-Mg granulite near the border of the Brejões charnockite intrusion. On the other hand, the method of Holland & Powel (1998) gives high temperatures (900-1000 °C) for these same samples, and lower temperatures (850-870 °C) for the garnet-bearing charnockitic granulite sample collected farther away from the Brejões Dome. It is reminded that the hercynite plus quartz assemblage is a very high temperature assemblage, exceeding 800°C (Waters, 1991; Tobi et al., 1995).

In summary, we conclude that the rocks from the Brejões Dome area were formed under granulite facies conditions of low to intermediate pressure (5-8 kbar). Temperatures determined in samples collected away from the dome are in the order of 850°C, similar to the temperature determined elsewhere in the southern Bahia high-grade region (Barbosa, 1986; 1990). However, samples from close to the dome record higher temperatures of about 900-1000 °C, suggesting that there was an additional heat source available from the charnockite intrusion, aside from the regional granulite facies metamorphism.

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