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Norite and charnockites from the Venda Nova Pluton,SE Brazil:Intensive parameters and some petrogenetic constraints

Geoscience Frontiers 2012年第6期3卷 789-800页

作者： Julio Cezar Mendes Cristina M.P.De Campos Departamento de Geologia IGEOUniversidade Federal do Rio de Janeiro21910-949Cidade UniversitariaRio de JaneiroBrazil Department of Earth and Environmental Sciences GeoCenterLudwig Maximillians UniversitatTheresienstrasse 41/III80333 MunichGermany

The Venda Nova Pluton （VNP） is a zoned ring structure emplaced in the southern portion of the Neoproterozoic araquai belt, in Espirito Santo, Brazil. It is a slightly westward tilted cylinder-like intrusion, with an almost circular horizontal section. In the center of this structure, an off-centered gabbro-noritic core, surrounded by syeno-monzonitic rocks, intrudes an outer ring of charnockites and norite. These envelop the syeno-monzonitic and gabbro-noritic center, as a narrow discontinuous belt. While, in the core intrusion, mingling and mixing processes are widespread and well documented in the literature, in the outer ring, the norite and charnockite layers show predominantly homogeneous and isotropic internal structures. Nevertheless, smaller interaction zones between charnockites and norite denote a comparatively more restricted mingling process. The norite is a fine-grained rock with hypidio- morphic granular to intergranular texture. The charnockites are medium-grained and made up of：（a） orthopyroxene-tonalite, （h） orthopyroxene-quartz-diorite, and （c） orthopyroxene-granodiorite with hy- pidiomorphic granular to porphyritic textures. In all lithotypes both ortho- and clinopyroxene are replaced by hornblende and biotite. Two contrasting compositional sequences have been recognized, based on whole rock geochemistry：（1） a basic, with tholeiitic affinities （norite） and, （2） an intermediate, medium-K calc-alkalinc, comprising the charnockites. Estimated crystallization temperatures, which have been calculated from micro-probe analysis of pyroxenes, range from 915± 25 ℃ to 960 ± 50 ℃. Re-equilibration temperature （ihnenite-magnetite calibration） is around 600 ~ 50 ℃. This indicates oxygen fugacities four order of magnitude below the FMQ-buffer and a reduced environment. Coeval pressure conditions estimated from the Al-content in hornblende range from 5.5 ± 0.6 kbar. Data obtained for the norite point toward an evolution from the partial melting of an anhydrous

关键词： Chamockites Petrology Intensive parametersVenda Nova Pluton araqua~ belt Southeast Brazil

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