Department of Earth Sciences

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Browsing Department of Earth Sciences by browse.metadata.advisor "De Alkmin, Fernando Flecha"

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    Repeated partial melting events in the polymetamorphic Carlos Chagas batholith : implications for the tectono-metamorphic evolution of the Araçuaí orogen (southeastern Brazil)
    (Stellenbosch : Stellenbosch University, 2017-03) De Melo, Marilane Gonzaga; De Carvalho Lana, Cristiano; Stevens, Gary; De Alkmin, Fernando Flecha; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Araçuaí orogen (AO), located in the southeastern Brazil, and its African counterpart, the West Congo belt, is part of the Pan-African-Brasiliano orogenic system developed between the Congo and São Francisco paleocontinents. From the earliest (ca. 630 Ma) pre-collisional plutons to the latest (ca. 480 Ma) post-collisional intrusions, the Araçuaí orogen records an outstanding succession of granite production events in space and time. This study investigates the petrogenesis of the Carlos Chagas batholith (CCB), a very large body (~ 14,000 km2) composed of S-type granites ascribed to the collisional plutonism (G2 supersuite) in the back-arc region of the AO, to the east of the Rio Doce magmatic arc. CCB extends in a N-S direction between latitudes 17°S and 19°30’S in the states of Espírito Santo, Minas Gerais and Bahia. The CCB includes a dominant granite richer in garnet than in biotite, in which three mineral assemblages can be identified: 1) Qz + Pl + Kfs + Bt + Grt + Ilm ± Rt; 2) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil; and 3) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil + Spl. Rocks which contain mineral assemblage 2 and 3 all contain two generations of garnet. Textural evidence for the presence of former melt, recognized in all studied CCB samples, includes: silicate melt inclusions in poikiloblastic garnet, pseudomorphed thin films of melt surrounding both generations of garnet, pseudomorphed melt pools adjacent to garnet and biotite, and plagioclase and quartz with cuspate-lobate shapes occurring among matrix grains. Both generations of garnet crystals (Grt1 and Grt2) are unzoned in terms of major element concentration, contain small rounded inclusions of Ti-rich biotite and, in addition, the Grt2 crystals also contain inclusions of remnant sillimanite needles. Microstructural evidence, in combination with mineral chemistry, indicates that the garnet crystals grew during two distinct metamorphic-anatectic events, as the peritectic products of fluid-absent melting reactions which consumed biotite, quartz and plagioclase, in the case of Grt1, and which consumed biotite, quartz, plagioclase and sillimanite in the case of Grt2. P-T pseudosections calculated via Theriak-Domino, in combination with U-Pb monazite and zircon dating, provide new constraints on the thermal evolution of the back-arc region of the Araçuaí orogen. Data from assemblage 1 suggests P-T conditions for the first granulite-facies metamorphic event (M1) at 790-820 ºC and 9.5- 10.5 kbar, while the assemblage 2 records P-T conditions for a second granulite-facies metamorphism (M2) of around 770 ºC and 6.6 kbar. Ti-in-zircon thermometry is consistent with the phase equilibrium modelling, indicating that the two metamorphic events in the batholith reached granulite facies conditions. A wide range of monazite and zircon ages (> 825 Ma to ca. 490 Ma) have been found in CCB rocks, recording a complex history of crustal recycling and inheritance, magmatic crystallization and anateksis during the evolution of the AO. 582 Ma magmatic zircons are marked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825-600 Ma), indicating that the chemical signature of these crystals has likely been inherited from the source. The U-Pb ages and initial 176Hf/177Hf ratios from anatectic and/or metamorphic zircon domains are consistent with a two-stage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. The oldest metamorphic episode (ca. 570-550 Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of new anatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grew contemporaneously with peritectic Grt1 through biotite-absent fluid partial melting. U-Pb ages obtained in monazite and zircon (ca. 569-552 Ma) included in Grt1, further support the interpretation that the Grt1 crystals grew during the first anatectic episode. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535-500 Ma) is only recorded in part from the CCB rocks (assemblage 2 and 3). In these rocks, the growth of new anatectic zircon and/or overgrowth is marked by high initial 176Hf/177Hf values and also by generation of Grt2, as indicated by petrographic observations and REE patterns. On the other hand, some rocks have zircon grains formed by solidstate recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition those to inherited/magmatic core domains. The M1 event likely corresponds to the striking crustal thickening and widespread anateksis in the CCB. The M2 event can be associated to asthenosphere upwelling during extensional thinning and gravitational collapse of the orogen, this produced anateksis in parts from the CCB that had been re-fertilized by retrogression along shear zones following the first granulite facies event.