Geochemical constraints on the petrogenesis of the Paleozoic granitoids of the Sierra de San Luis, Sierras Pampeanas, Argentina

Abstract

The Paleozoic granitoids of the Sierra de San Luis comprise the Ordovician tonalite suite (OTS; metaluminous to mildly peraluminous calcic tonalite–granodiorites) and granodiorite–granite suite (OGGS; peraluminous calcic to calc-alkaline granodiorite–monzogranites), as well as the Devonian granite suite (DGS; peraluminous alkali-calcic monzogranites) and monzonite–granite suite (DMGS; metaluminous alkali-calcic quartz monzonite–monzogranite ± granodiorite, mildly peraluminous alkalicalcic monzogranites). The OTS has relatively high K 2O, CaO, and Yb N and low Cr, Ni, Ba, Sr, Rb/Sr, Sr/Y, and (La/Yb) N , as well as negative Eu/Eu ∗, high 87Sr/ 86Sr (0.70850–0.71114), and unradiogenic ε Nd(470Ma) (−5.3 to −6.0), which preclude an origin of variably fractionated mantle melts and favour a mafic lower crustal source. The OGGS consists of two granitoids: (1) high-temperature characterized by low Al 2O 3/TiO 2, Rb/Sr, and (La/Yb) N , a smooth negative Eu/Eu ∗, and relatively high CaO and (2) low-temperature with high Al 2O 3/TiO 2 and Rb/Sr, low CaO, (La/Yb) N , and Sr/Y, and negative Eu/Eu ∗. Melting of metagreywackes at pressures below 10 kbar with a variable supply of water could account for the chemistry of the high-T OGGS, whereas dehydration melting of biotite-bearing metasedimentary sources at low pressures is proposed for the low temperature OGGS. Melting of crustal sources relates to a contemporaneous mafic magmatism. Devonian magmatism is characterized by high Ba, Sr, K 2O, Na 2O, Sr/Y, and (La/Yb) N . Sources for the DGS include metasedimentary or metatonalitic protoliths. Biotite dehydration melting triggered by the addition of heat, supplied by mantle-derived magmas, is proposed. High Ba, Sr, LREE, MgO, Cr, Ni, Zr, and V of the monzonites suggest an enriched lithospheric mantle source. Low Yb and Y and high Sr and (La/Yb) N indicate a garnet-rich residual assemblage ( P ⩾ 10 kbar). Melts for the peraluminous rocks may have derived from a metasedimentary or metaigneous source at lower pressures in a process dominated by biotite consumption and plagioclase in the residue. The Ordovician granitoids are synkinematic with compressive deformation related to the early stages of Famatinian convergence. The Devonian magmatism is synkinematic with a system of shear zones that were active during the Achalian cycle.