科研之友
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摘要
A-type granitoids and rhyolites are here considered as those that fulfill at least one of the following four criteria: (i) genetic association with magmatic rocks of sodic or ultrapotassic alkaline affinity. These in general include syenitic rocks, peralkaline rocks, and trachybasalts, basaltic trachyandesites or their plutonic equivalents; (ii) (Na(2)O + K(2)O) and FeO(T)/(FeO(T) + MgO) whole-rock values respectively higher than 9 wt% and 0.9; (iii) peralkaline compositions; (iv) (10(4)*Ga)/Al > 2.6, and Ce + Y + Nb + Zr > 340 ppm, plotting in the within-plate field in the (Nb + Y) versus Rb diagram. A-type granitic rocks are widespread in the Neoproterozoic post-collisional settings in southern Brazil. They are positioned either within large shear-zones or in thick volcanosedimentary sequences formed in strike-slip basins. They predominate in the late to final stages of post-collisional magmatism, and follow medium-to high-K subalkaline and shoshonitic associations. The intensity of transcurrent tectonics has an important role in promoting crustal contamination of magmas intruded where shear zones are active. The main effects of crustal contamination observed are increased magmatic oxidation, depletion of HFS elements, and reduction of agpaitic indices. The post-collisional rhyolitic or granitic A-type magmas from southern Brazil are coeval with mafic magmas and, in some cases, with trachytes or syenites of the same chemical affinity, i. e., most of them belong to the sodic silica-saturated alkaline series and to the ultrapotassic silica-saturated series. Trace-element and isotope data suggest that the plutonic and volcanic A-type rocks from southern Brazil have a variable contribution of crustal components, and in some cases none at all, and have a very strong contribution of OIB-EM1 sources. On the basis of such evidence, we propose that the post-collisional A-type magmas in southern Brazil are derived from basic or intermediate mantle-derived magmas, mostly by fractional crystallization processes, in some cases with contamination by crustal melts, particularly where transcurrent tectonics were active.
- 出版日期2009-12
