科研之友
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摘要
2.75 to 2.73 Ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the Carajas Province. The Vila Jussara Suite was recognized recently and its geologic, mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the Estrela Complex, Serra do Rabo and Igarape Gelado granites and Planalto Suite. Monzogranites and syenogranites are dominant in most units but in the Igarape Gelado Granite and Vila Jussara Suite granodiorites and tonalites are also relatively abundant. The main mafic phases in all these granitoids are amphibole [potassian hastingsite with subordinate magnesiohastingsite; 0.95 >= Fe/(Fe + Mg) >= 0.47] and biotite [0.88 >= Fe/(Fe + Mg) >= 0.52] with rare occurrence of clinopyroxene relict crystals. Magmatic epidote (% of 'pistacite' component from 26 to 31) occurs only in the Vila Jussara Suite. The rocks of Estrela, Serra do Rabo, and Planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced-ferroan granites [whole rock FeOt/(FeOt + MgO) >= 0.89] that evolved at low fO(2) (FMQ +/- 0.5). The Igarape Gelado Granite needs additional studies but it is formed apparently by reduced-ferroan and oxidized-ferroan granites. The Vila Jussara Suite is also composed of reduced-ferroan granites that are similar to those of the other granite units. However, it has additionally oxidized-ferroan and magnesian granitoids which contain magnetite (+/- ilmenite) as the main Fe-Ti oxide phase and evolved, respectively, at moderate fO(2) (NNO +/- 0.5) and at comparatively higher fO(2) (NNO to NNO + 1). The magmas of these granites were formed at temperatures >= 900 degrees C and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc-alkaline in composition. Plagioclase, orthopyroxene, clinopyroxene and Fe-Ti oxides were the main phases in the melt residues and the liquids were enriched in Ba, Zr, Y, and REE (except for Eu), but not in Sr compared to their sources. The magmas should have relatively high water content (> 4 wt.%). The magmatic evolution was strongly influenced by partial melting processes. The different varieties of the Vila Jussara Suite are not linked by fractional crystallization and derived from distinct magmas. The Neoarchean granites of the Carajas Province were emplaced in a collisional tectonic setting. They are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling. The magmas were generated in the crust at pressures of similar to 800 to 1000 MPa and emplaced at pressures of 200 to 500 MPa. The studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites. The magnesian variety of the Vila Jussara Suite is similar to the pyroxene-free granitoids of the Neoarchean Matok Pluton of the collisional Limpopo Belt. The Neoarchean granites of Carajas are hydrated granites emplaced at comparatively higher crustal levels when compared with the Matok pluton.
- 出版日期2017-4-15
