Petrogenesis of the Devonian-Early Carboniferous Granitoids of the Eastern Sierras Pampeanas

LÓPEZ DE LUCHI, M. G.

Foz de Iguacu

Congreso; Meeting of the Americas; 2010

American Geophysical Union

During the Achalian orogeny extensive Devonian-Early Carboniferous granitoids intrude the basement of Central Argentina and form a belt almost parallel to the trend of the Ordovician Famatinian orogen from the Sierra de San Luis (SL) in the south, the Sierra de Córdoba (SC) in the central part to the Sierra de Velasco (SV) in the north. In SL voluminous Devonian batholiths are made up by a hybrid monzonite-granite suite with metaluminous alkali-calcic (393-385 Ma) quartz monzonite- granodiorite±monzogranite and peraluminous alkali-calcic monzogranites (López de Luchi et al. 2007). Monzonites are characterized by high K2O, Na2O, Sr/Y, Ba, Sr, LREE, MgO, Cr, Ni, Zr, V and (La/Yb)N. which suggest an enriched lithospheric mantle source. Two groups of peraluminous monzogranites are present: one with low Rb/Sr, Yb and Y and high Sr and (La/Yb)N which could derive from biotite dehydration melting of a greywacke/metaigneous source with plagioclase consumed and garnet stable and another with high Rb/Sr, (ca.3) Ga/Al and low Sr/Y which could derive from melting of a mica rich source at lower pressure since plagioclase does not contribute to the melt. Rapela et al. (2008) indicated that the Achala batholith, in SC is dominated by aluminous A-type calc-alkalic to alkali-calcic (379 ± 4 Ma) monzogranites with enrichments in F, Ga, Nb, high Ga/Al ratios and low K/Rb ratios which represent variable proportions of juvenile mantle components and crustal melts formed by dehydration melting of biotite-bearing sources. In SV the Early Carboniferous (ca 350 Ma) magmatism which is dominantly granitic (SiO2>70%) would have a main crustal source, consisting of peraluminous metagranitoids, and a mantle-derived source (Grosse et al. 2008). An important episode of crustal growth by juvenile magma input is indicated by TDM (one stage) between 1.0-1.2 Ga of these granitoids which is considerably younger than the Eastern Sierras Pampeanas background of 1.6-1.8 Ga. Partial melting of crustal sources was probably triggered by multiple, mafic intrusions, distributed over a protracted period that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Hybrid petrographic features and the abundance of monzonitic rocks in the south suggest a protracted interaction between mantle and crustal melts whereas to the north the younger dominant lithologies are crustal derived and the interaction is apparently more limited. On the other hand it seems that in a particular area like in San Luis, melting of the crust sources proceeded along a decompression path or alternatively affected different segments of the crust.