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Costa F.G.,CPRM Geological Survey of Brazil | Oliveira E.P.,University of Campinas | McNaughton N.J.,Curtin University Australia
Journal of South American Earth Sciences | Year: 2011

Several granitic plutons have intruded the Palaeoproterozoic Rio Itapicuru greenstone belt, São Francisco craton, Brazil, in the time interval 2163-2080 Ma, but their tectonic significance is poorly understood. The Fazenda Gavião granodiorite (FGG) is one of a set of plutons emplaced along the western boundary of the greenstone belt with Archaean migmatite-gneiss basement. The pluton is mostly composed of hornblende granodiorite, occasionally crosscut by syn-plutonic mafic dykes. The FGG is metaluminous, medium- to high-K calc-alkaline with relatively constant silica abundances (SiO2 ~ 63-66 wt%), high Sr (900-800 ppm) and high Ba (1000-1500 ppm). The associated mafic dykes are ultrapotassic, with high abundances of Ba, Sr, MgO, Ni, Cr, and light rare earth elements, suggesting derivation from partial melts of an enriched mantle source. The FGG originated probably by fractional crystallization from a primitive K-rich mafic magma that interacted with crustal melts. Its zircon U-Pb SHRIMP age of 2106 ± 6 Ma indicates that the FGG is younger than the early (2163-2127 Ma) tonalite-trondhjemite-granodiorite (TTG) and calc-alkaline arc plutons of the greenstone belt, and is closely related in time and space with potassic to ultrapotassic plutons (ca. 2110-2105 Ma). The negative εNd(t) of FGG and coeval K-rich plutons of the Rio Itapicuru greenstone belt contrasts markedly with the positive εNd(t) of the older arc plutons, indicating a major change of isotope signatures in granites of the Rio Itapicuru greenstone belt with time. This isotope shift may be related to magma contamination with older continental material and/or derivation of the parental potassic magma from enriched lithospheric mantle sources. We suggest that the K-rich plutons were emplaced during or shortly after Palaeoproterozoic arc-continent collision. © 2011 Elsevier Ltd.

Faleiros F.M.,CPRM Geological Survey of Brazil | Faleiros F.M.,University of Sao Paulo | Campanha G.A.D.C.,University of Sao Paulo | Martins L.,University of Sao Paulo | And 2 more authors.
Precambrian Research | Year: 2011

The Curitiba Terrane represents a major segment of the southern Ribeira Belt (SE Brazil), which was derived from the collision between the São Francisco, Congo, Paranapanema and Luís Alves Cratons during the Neoproterozoic (Brasiliano/Pan-African Orogeny). The tectonic setting and the metamorphic records of two major juxtaposed units from the Curitiba Terrane, a Neoproterozoic shallow continental-shelf metasedimentary assemblage (Turvo-Cajati Formation) and an Archaean to Paleoproterozoic TTG-type orthogneiss assemblage (Atuba Complex), were investigated. Migmatitic paragneisses from the Turvo-Cajati Formation underwent a deep collision metamorphism. Conventional geothermobarometry and petrological modelling in the system NCKFMASHTi indicate peak metamorphic conditions between 670 and 810°C at 9.5-12kbar. Metamorphic paths calculated from zoned garnet and plagioclase using the Gibbs method of differential thermodynamics indicate distinct evolution for two major groups of migmatites from the Turvo-Cajati Formation: (i) kyanite migmatites evolved from low-temperature eclogite to high-pressure granulite facies conditions following near isobaric heating; (ii) sillimanite migmatites underwent near isothermal decompression and apparently evolved from high-temperature eclogite facies conditions. Chemical dating of monazite indicates that the peak metamorphism of the Turvo-Cajati Formation occurred at 589±12Ma, followed by a greenschist facies metamorphic overprint at 579±8Ma related with late transcurrent shear zones. 40Ar-39Ar biotite ages indicate that the Turvo-Cajati Formation cooled below 250-300°C at 555±4Ma. P-T data and petrological evidence of rocks from the Atuba Complex suggest a retrograde metamorphic path with cooling from 730 to 630-650°C at 6-7kbar. Available K-Ar and 40Ar-39Ar data indicate that the Atuba Complex had cooled to below 300-500°C between ca. 590 and 580Ma. Geochronological data indicate that the main metamorphism of the Turvo-Cajati Formation and the Atuba Complex are coeval, but very contrasting metamorphic signatures reflect formation in different parts of a collisional suture. The integration of structural and petrological data indicates that the structural pattern of the Curitiba Terrane is related to Ediacaran westward directioned nappes during the late- to postmetamorphic period. This is concomitant with a main, crustal-scale, strike-slip regime, dominant throughout the Ribeira Belt. The nappe stack was later deformed by cylindrical folds with E-W trending sub-horizontal axes parallel to the synthrusting stretching lineation and was dismembered and dispersed by late sinistral strike-slip shear zones. The late tectonic assembly of the Ribeira Belt was controlled by significant postcollision terrane dispersion along major strike-slip shear zones. © 2011 Elsevier B.V.

Sawakuchi A.O.,University of Sao Paulo | Dewitt R.,Oklahoma State University | Faleiros F.M.,CPRM Geological Survey of Brazil
Radiation Measurements | Year: 2011

The thermoluminescence (TL) characteristics of quartz are highly dependent of its thermal history. Based on the enhancement of quartz luminescence occurred after heating, some authors proposed to use quartz TL to recover thermal events that affected quartz crystals. However, little is know about the influence of the temperature of quartz crystallization on its TL characteristics. In the present study, we evaluate the TL sensitivity and dose-response curves of hydrothermal and metamorphic quartz with crystallization temperatures from 209 ± 15 to 633 ± 27 °C determined through fluid inclusion and mineral chemistry analysis. The studied crystals present a cooling thermal history, which allow the acquiring of their natural TL without influence of heating after crystallization. The TL curves of the studied samples present two main components formed by different peaks overlapped around 110 °C and 200-400 °C. The TL sensitivity in the 200-400 °C region increases linearly with the temperature of quartz crystallization. No relationship was observed between temperatures of quartz crystallization and saturation doses (<100 Gy). The elevated TL sensitivity of the high temperature quartz is attributed to the control exerted by the temperature of crystallization on the substitution of Si4+ by ions such as Al3+ and Ti 4+, which produce defects responsible for luminescence phenomena. The linear relationship observed between TL in the 200-400 °C region and crystallization temperature has potential use as a quartz geothermometer. The relative abundance of quartz in the earth crust and the easiness to measure TL are advantageous in relation to geothermometry methods based on chemistry of other minerals. © 2010 Elsevier Ltd. All rights reserved.

The Neoproterozoic Gurupi Belt in northern Brazil developed at the southwestern margin of the Palaeoproterozoic São Luís-West Africa Craton. Orogenic gold deposits of this belt are hosted in Palaeoproterozoic (2160-2147 Ma) metavolcano-sedimentary and calc-alkaline granitoid rocks formed in arc and/or back-arc settings during a protracted Rhyacian orogeny (2240-2080 Ma). These host rock assemblages were tectonically and isotopically reworked during the Neoproterozoic and represent the reworked margin of the craton, that is, the external domain of the Neoproterozoic (Brasiliano-Pan African) orogen. The location of the gold deposits is controlled by the Tentugal shear zone, which represents the tectonic boundary between craton and the Gurupi Belt, and its subsidiary structures. Gold occurs in veins and in association with pyrite, and subordinately arsenopyrite and chalcopyrite, in strongly altered and variable deformed host rocks. Geological characteristics, petrographic, fluid inclusion, and isotopic evidence indicate near-neutral, reduced aqueous-carbonic metamorphic fluids, with local contributions from host rocks at the deposit site. Ore deposition occurred at about 300-370 °C and up to 3 kbars in response to fluid immiscibility and fluid-rock reactions (sulphidation, desulphidation, carbonatization, CO2 removal) and local fluid mixing and oxidation. © The Geological Society of London 2014.

Klein E.L.,CPRM Geological Survey of Brazil | Fuzikawa K.,Brazilian National Nuclear Energy Commission
Ore Geology Reviews | Year: 2010

The Carará gold deposit, located in the Ipitinga Auriferous District, south-eastern portion of the Guiana Shield, northern Brazil, is a typical orogenic, greenstone-hosted, auriferous quartz vein. Mineralisation was post-metamorphic and syn-tectonic in relation to the host Palaeoproterozoic (ca. 2.03 Ga) shear zone developed close to the tectonic boundary between a Palaeoproterozoic continental arc and an Archaean block. The deposit style is very simple, consisting of a quartz vein and its hydrothermal envelope, which is composed of muscovite and tourmaline; sulphides are rare. Muscovite and tourmaline, in addition to gold, fill small fractures in the quartz vein. The fluid inclusion assemblage trapped in high- and low-grade portions of the Au-quartz vein is rather enigmatic, consisting of one-phase CO2 inclusions with no visible water at room or sub-zero temperatures, although small amounts of water have been detected by micro-Raman analysis. In this aspect Carará differs from the other gold showings in the same district, which are characterized by abundant aqueous-carbonic fluid inclusions. The carbonic fluid is composed predominantly of CO2 in addition to < 2 mol.% N2 and traces of CH4 and C2H6. The carbonic fluid show very variable densities, which is interpreted to result from post-entrapment re-equilibration. Inclusions in the high-grade quartz are the densest (0.89 to 1.07 g/cm3) and with less effects of re-equilibration. These inclusions approximate the physico-chemical characteristics of the parental fluid that started to be trapped at least around the amphibolite facies metamorphic conditions and then followed a retrograde path. Most of the inclusions appear to have been trapped and/or re-equilibrated at 350 to 475 °C and 1.8 to 3.6 kbar, which implies a 7 to 12 km depth of vein formation and gold mineralisation. Both phase separation of a carbonic-aqueous fluid (XCO2 > 0.8) and the existence of an originally CO2-dominated fluid could account for the observed fluid inclusion properties and the absence of H2O-bearing inclusions in the mineralised vein. The fluid inclusion characteristics, combined with published geological and isotopic information, indicate a deep-seated source for CO2 that could be mantle, magmatic or metamorphic in origin. We suggest that the likely sources are fluid produced by the 2.07 Ga-old charnockites that occur in the region and/or the coeval high grade metamorphism that is widespread in the Guiana Shield. © 2009 Elsevier B.V. All rights reserved.

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