Crustal and Enriched Mantle Derived Melts in the Devonian Magmatism of the Sierra de San Luis, Argentina.

LÓPEZ DE LUCHI, M. G., ; SIEGESMUND, S.; STEENKEN, A.; WEMEMR, K.

Baltimore

Otro; AGU-CGU Joint Assembly 2004; 2004

American Geophysical Union

Devonian granitoids of the Sierra de San Luis consist of voluminous batholiths that were broadly synchronous with the collision between the Chilenia Terrane and southwestern Gondwana and the development/reactivation of NE-trending ductile shear zones. Batholiths were emplaced at around 3.5 kbars and are made up by high K calc-alkaline porphyric granodiorites and equigranular monzogranites and are associated with high Mg-K syenites and monzonites, equivalents to vaugnerites and durbachites that appears as stocks, syn-plutonic dykes and enclaves in the porphyric facies and shows textural and chemical evidences of mingling and mixing processes. Granitoids are contemporaneous to ultrapotassic lamprophyre dykes (minettes, that intruded the country rocks next to the contacts and preferentially emplaced in an E-W fracture system. The less evolved high Mg-K series rocks present strong enrichment in K, Rb, Ba, Th, Zr anf Hf, high Mg, Ni, Cr, V and relatively low Ti and Nb contents which indicate a basic primitive magma enriched source. Sucessives stages of mixing and mingling with the evolving calc-alkaline melt are indicated by field and petrographic data in synplutonic dykes and enclaves: identical feldspar megacrysts, desaggregation of enclaves that appear as mafic rafts in the porphyroid facies, quartz ocelli with reaction rims, the general predominance of amphibole and euhedral sphene versus biotite in areas where the synplutonic dykes are intruded, transitional contact between hybridized and ¨normal porphyroid facies, etc. These processes took place mainly prior to emplacement and continue during the entire emplacement as shown by the parallelism between internal and external fabrics. The mantle origin of the less evolved hybrid granitoids is indicated by their isotopic composition initial 87Sr/86Sr 0.70380 and epsilon Nd of ?1.4. Therefore high K, Ba and Sr and high field strength element contents of these rocks must be considered as a primary feature. The more evolved granitoids with initial 87Sr/86