Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt, Bahia, Brazil (original) (raw)
Related papers
2017
A área de pesquisa se situa no Cráton do São Francisco, no sul da Bahia, Brasil. Nessa região, no segmento crustal Bloco Itabuna-Salvador-Curaçá (BISC), ocorrem rochas supracrustais associadas a gnaisses tonalíticos/trondhjemíticos a monzoníticos e rochas máficas subordinadas, de idades arqueana e paleoproterozoica, todos fortemente deformados e reequilibrados na fácies granulito em 2,07 a 2,08 Ga. As rochas supracrustais incluem granulitos com safirina constituídos por quartzo, ortopiroxênio, silimanita, granada, mesopertita, plagioclásio, biotita e cordierita. Com base nos arranjos microestruturais entre os minerais constituintes dos granulitos com safirina, a paragênese de pico metamórfico é Grt1 + Opx1 + Bt1 + Qtz + Sil1 + Spr1 + Mp. Microestruturas de reações entre minerais, como coroas de simplectitos ao redor de porfiroblastos, permitiram identificar as seguintes paragêneses retrógradas, interpretadas como formadas durante a ascensão tectônica dessas rochas: Grt1 ± Qtz = Opx2...
Journal of Petrology, 2002
GO 74823-430, BRAZIL 4 COMPANHIA DE PESQUISA DE RECURSOS MINERAIS (CPRM), SGAN 603 CONJ. 'J', PARTE 'A', 1°, ANDAR BRASÍLIA, DF 70830 030, BRAZIL RECEIVED SEPTEMBER 10, 2001; REVISED TYPESCRIPT ACCEPTED MARCH 8, 2002
Journal of Metamorphic Geology, 2003
The Mollendo-Camana Block (MCB) is a 50 • 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U-Pb and U-Th-Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K-feldspar (Kfs), orthopyroxene (X Mg Opx ¼ 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X Mg Grt ¼ 0.60), sapphirine (X Mg Spr ¼ 0.87), cordierite (X Mg Crd ¼ 0.92) and biotite (X Mg Bt ¼ 0.83). The ubiquitous peak mineral assemblage is Opx-Sil-Kfs-Qtz-(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al 2 O 3) and the local coexistence of sapphirine-quartz, attest to regional UHT metamorphism (> 900°C) at pressures in excess of 1.0 GPa. Fluid-absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo-Cocachacra areas). In this area, a first stage of decompression from 1.1-1.2 to 0.8-0.9 GPa at temperatures in excess of 950°C, is marked by the breakdown of Sil-Opx to Spr-Opx-Crd assemblages according to several bivariant FMAS reactions. High-T decompression is also shown by Mg-rich garnet being replaced by Crd-Spr-and Crd-Opxbearing symplectites, and reacting with quartz to produce low-Al-Opx-Sil symplectites in quartz-rich migmatites. Neither osumilite nor spinel-quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt-Qtz-(± pl) symplectites in quartz-rich migmatites (melt + Opx + Sil ¼ Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid-absent melting, compatible with a second stage of decompression below 900°C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large-scale penetration of hot asthenospheric mantle at the base of an over-thickened crust.
Journal of the Geological Society, 2009
Nappes in the southern sector of the Southern Brasília Belt record suturing of the Paranapanema Block and Socorro-Guaxupé Arc with a subducted passive margin on the western side of the São Francisco Craton. We report secondary ion mass spectrometry U-Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a 206 Pb/ 238 U age of 678 AE 29 Ma, which we interpret as most likely to date close-to-peak-P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields 206 Pb/ 238 U ages of 648 AE 12 and 647 AE 11 Ma, and Ti-in-zircon crystallization temperatures from c. 860 8C down to c. 785 8C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high-T retrograde P-T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb-Sr multi-mineral-whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c. 590 Ma at temperatures of c. 750 8C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a 206 Pb/ 238 U age of 622 AE 28 Ma and Ti-in-zircon crystallization temperatures from c. 970 8C down to c. 820 8C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred.
Gondwana Research, 2006
The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperature metamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provoked generalized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean to Paleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatite and leuconorite within the high-T shear zones that make up the eastern continuation of the huge E-W Patos shear belt. These data were also constrained by using the Sm-Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The results suggest an age of about 578 to 574Ma for the peak of the widespread high-T metamorphism. This event is best recorded by Sm-Nd garnet-whole rock ages. The U-Th-Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553 ± 10 Ma. When compared to the Sm-Nd garnet-whole rock ages, the U-Th-Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peak of the high-T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, the U-Th-Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete reopening and resetting of the U-Th-Pb isotopic system produced by the action of low-T Ca-rich fluid.
Metamorphic PT evolution of granulites in the central Ribeira Fold Belt, SE Brazil
Geosciences …, 2011
were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 ± 50°C; P = 8 ± 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T ≈ 500°C and P ≈ 5 kbar coupled with a H2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. Hightemperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/ retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/ greenschist facies, promoting the replacement of the plagioclasequartz-garnet-hypersthene peak assemblage by quartz-biotite-K-feldspar symplectites.
2017
Manuscript ID: 20160097. Received in: 08/12/2016. Approved in: 03/27/2017. ABSTRACT: The Porangatu Granulite Complex is exposed in the central part of the Neoproterozoic Tocantins province in central Brazil, along the boundary between the Brasília Belt to the east and the Araguaia Belt to the west. This is part of the transcontinental Transbrasiliano-Kandi shear system. The complex includes garnet-rich enderbite and charnockite, high-grade gneisses as well as lenses of garnet-bearing mafic granulite or amphibolites, and in situ anatectic charnockite, elongated in the NNE-SSW direction along the Talismã Shear Zone (TSZ). These rocks represent suites of ortho-derived rocks of calc-alkaline affinity and small contributions of tholeiitic basalts and aluminous paragneisses. The structural framework records thrust components probably related to the early stages of an oblique collision during the evolution of Neoproterozoic Brasiliano orogens, and can be understood as involving a collision...
Rem: Revista Escola de Minas, 2016
The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulites as well as lower grade gneisses and mylonites. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometric calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulites based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 ºC and pressures between of 5.0 and 9.9 kbar and a retrometamorphic path characterized by near-isobaric cooling. Part of the granulites was affected by anatexis. The melting of felsic granulites resulted in the generation of pegmatites and two aluminous lithotypes. These are: i) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an anatectic melt, and ii) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronology of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronologically related to the lower grade Mantiqueira Complex.