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

LOPEZ DE LUCHI, M. G.; SIEGESMUND, S.; WEMMER,K.; STEENKEN, A.; NAUMANN, R.

Stellenbosch

Simposio; Sixth Hutton Symposium on the Origin of Granites and related rocks; 2007

Universidad de Stellenbosch, South Africa

The Paleozoic granitoids of the Sierra de San Luis comprise the Ordovician Tonalite Suite (OTS: metaluminous to mildly peraluminous calcic tonalite-granodiorites), the Granodiorite-Granite suite (OGGS: peraluminous calcic to calc-alkaline granodiorite-monzogranites), the Devonian Granite suite (DGS: peraluminous alkali-calcic monzogranites), and the Devonian Monzonite-Granite Suite (DMGS: metaluminous alkali-calcic quartz monzonite-monzogranite ¡Àgranodiorite and mildly peraluminous alkali- calcic monzogranites. The OTS has relatively high K2O, CaO, YbN, low Cr, Ni, Ba, Sr, Rb/Sr, Sr/Y, (La/Yb)N, negative Eu/Eu*, high 87Sr/86Sr (0.70850-0.71114), and unradiogenic ¦ÅNd(470 Ma (-5.3 to -6.0) which preclude an origin from variably fractionated mantle melts and favour a mafic lower crustal source. The OGGS consists of two different granitoids: The high-temperature OGGS characterized by low Al2O3/TiO2, Rb/Sr, (La/Yb)N, a smooth negative Eu/Eu*and relatively high CaO and the low-temperature OGGS with high Al2O3/TiO2, Rb/Sr, low CaO, (La/Yb)N, Sr/Y and a 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 is related to the coetaneous mafic magmatism. Devonian magmatism is characterized by high Ba, Sr, K2O, Na2O, Sr/Y and (La/Yb)N. Sources for the DGS involved metasedimentary or metatonalitic protolith. 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 point to an enriched lithospheric mantle source. Low Yb, Y, high Sr, and (La/Yb)N indicate garnet-rich residual assemblage, (P¡Ý10 kbar). Melts for the peraluminous rocks may have derived from a metasedimentary or metaigneous source at lower pressure in a process dominated by biotite consumption and plagioclase in the residue- The Ordovician granitoids are syn-kinematic to compressive deformations related to the early stages of the Famatinian convergence. The Devonian magmatism is syn-kinematic to a deformation event that is expressed as shear zones controlled by the Achalian cycle.