Common Crustal Source for Contrasting Peraluminous Facies in the Early Paleozoic Capillitas Batholith, NW Argentina

Abstract

The Ordovician Capillitas batholith, part of the northern Pampean ranges, NW Argentina, exhibits two peraluminous granitic facies in its eastern portion: (a) coarse- to medium-grained, porphyritic mafic facies with biotite, cordierite and aluminosilicates, carrying sillimanite-, cordierite-, and andalusite-bearing migmatitic enclaves and schlieren and (b) enclave free, mica poor, coarse-grained, porphyritic felsic facies, with andalusite and sillimanite. Banded hornfels aureoles contain cordierite poikiloblasts, biotite, plagioclase and quartz. The low-P mineral assemblage in these granites, enclaves and restites, suggests partial fusion of a supracrustal protolith. The two facies plot as two separate groups in geochemical variation diagrams, suggesting that they evolved from different magma batches derived from the same source, rather than from in situ fractional crystallization. The composition of felsic facies granites corresponds to pelite and metagraywacke-derived melts, whereas cordierite-bearing mafic granites follow a trend indicating mixing of pelite-derived melts and corresponding restites. The mafic-facies granites approach more the continental crust composition than the felsic-facies ones, which display more pronounced Ba and Sr negative anomalies. The average (La/Yb) N ∼11 rules out a high pressure garnet-rich source and the low normalized Sr contents, in both granitic facies, suggest a recycled metasedimentary protolith. Absence of mafic intrusives that could have assimilated pelitic schists, allow us to infer that melting took place at rather low T and under high water activity. Heat to trigger partial fusion could have been radiogenically generated and stored in the upper crust during deformation and thickening of the continental crust, with further release during decompression. The Capillitas batholith, emplaced close to an I, S-type granite boundary line in this region appears to be an Argentinean analogue of the Cooma Series supersuite in the Lachlan Fold Belt, emplaced close to the eastern Australian I, S-type granite boundary line.