Fault controlled Carboniferous A-type magmatism in the proto-Andean forelandud(Sierras Pampeanas, Argentina): Geochemical constraints and petrogenesis

摘要

The intrusion of granitoids into the Eastern Sierras Pampeanas in the Early Carboniferous took place after audlong period of mainly compressional deformation that included the Famatinian (Ordovician) and Achalianud(Devonian) orogenies. These granitoids occur as small scattered plutons emplaced in a dominant extensionaludsetting, within older metamorphic and igneous rocks, and many of them are arranged along a reactivatedudlarge shear zone. A set of 46 samples from different granitic rocks: Huaco granitic complex, San Blas pluton,udand the La Chinchilla stock from the Sierra de Velasco, Zapata granitic complex from Sierra de Zapata, and theudLos Árboles pluton from Sierra de Fiambalá, display high and restricted SiO2 contents between 69.2 andud76.4 wt.%. On both FeO/(FeO+MgO) vs. SiO2 and [(Na2O+K2O)−CaO] vs. SiO2 plots the samples plot in theudferroan and alkaline-calcic to calco-alkaline fields (FeO/(FeO+MgO)=0.88–1.0%;[(Na2O+K2O)−CaO]=ud6.3–8.3%), thus showing an A-type granitoid signature. The high concentrations for the High Field StrengthudElements (HSFE), such as Y, Nb, Ga, Ta, U, Th, etc. and flat REE patterns showing significant negative Euudanomalies are also typical features of A-type granites. Our petrogenetic model supports progressiveudfractional crystallization with dominant fractionation of feldspar and a source mineral assemblage enrichedudin plagioclase. Biotites have distinctive compositions with high FeO/MgO ratios (7.8–61.5), F (360–ud5610 ppm), and Cl (120–1050 ppm). The FeO/MgO ratios together with the F and Cl content of igneousudbiotites seem to reflect the nature of their parental host magmas and may be useful in identifying A-typeudgranitoids. The isotopic data (Rb–Sr and Sm–Nd) confirm that the A-type granites represent variableudmixtures of asthenospheric mantle and continental crust and different mixtures lead to different subtypes ofudA-type granite (illustrating the lack of consensus about A-type magma origin). We conclude that prominentudshear zones play an important role in providing suitable conduits for ascending asthenospheric material andudheat influx in the crust, a hypothesis that is in accord with other recent work on A-type granites.