Publication: Fault controlled Carboniferous A-type magmatism in the proto-Andean foreland
(Sierras Pampeanas, Argentina): Geochemical constraints and petrogenesis
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2010
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Elsevier Science B.V., Amsterdam
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Abstract
The intrusion of granitoids into the Eastern Sierras Pampeanas in the Early Carboniferous took place after a
long period of mainly compressional deformation that included the Famatinian (Ordovician) and Achalian
(Devonian) orogenies. These granitoids occur as small scattered plutons emplaced in a dominant extensional
setting, within older metamorphic and igneous rocks, and many of them are arranged along a reactivated
large shear zone. A set of 46 samples from different granitic rocks: Huaco granitic complex, San Blas pluton,
and the La Chinchilla stock from the Sierra de Velasco, Zapata granitic complex from Sierra de Zapata, and the
Los Árboles pluton from Sierra de Fiambalá, display high and restricted SiO2 contents between 69.2 and
76.4 wt.%. On both FeO/(FeO+MgO) vs. SiO2 and [(Na2O+K2O)−CaO] vs. SiO2 plots the samples plot in the
ferroan and alkaline-calcic to calco-alkaline fields (FeO/(FeO+MgO)=0.88–1.0%;[(Na2O+K2O)−CaO]=
6.3–8.3%), thus showing an A-type granitoid signature. The high concentrations for the High Field Strength
Elements (HSFE), such as Y, Nb, Ga, Ta, U, Th, etc. and flat REE patterns showing significant negative Eu
anomalies are also typical features of A-type granites. Our petrogenetic model supports progressive
fractional crystallization with dominant fractionation of feldspar and a source mineral assemblage enriched
in plagioclase. Biotites have distinctive compositions with high FeO/MgO ratios (7.8–61.5), F (360–
5610 ppm), and Cl (120–1050 ppm). The FeO/MgO ratios together with the F and Cl content of igneous
biotites seem to reflect the nature of their parental host magmas and may be useful in identifying A-type
granitoids. The isotopic data (Rb–Sr and Sm–Nd) confirm that the A-type granites represent variable
mixtures of asthenospheric mantle and continental crust and different mixtures lead to different subtypes of
A-type granite (illustrating the lack of consensus about A-type magma origin). We conclude that prominent
shear zones play an important role in providing suitable conduits for ascending asthenospheric material and
heat influx in the crust, a hypothesis that is in accord with other recent work on A-type granites.