INSTITUTO DE INVESTIGACIONES EN ENERGIA NO CONVENCIONAL
Unidad Ejecutora - UE
congresos y reuniones científicas
MIGMATITES LEUCOSOMES AS MAGMA ANALOGUES FOR LEUCOGRANITES AND TRONDHJEMITES OF THE FAMATINIAN BELT: A CASE STUDY FROM MOLINOS RANGE, SALTA, NW ARGENTINA
SOLA ALFONSO; BECCHIO, R.
Congreso; XVIII Congreso Geológico Argentino; 2011
This work is focused on the partial melting process affecting rocks that constitutes part of the Famatinian Belt, in the NW Argentina. The occurrence of migmatites and anatectic rocks has been widely described along the Famatinian belt, however, the study of partial melting remains qualitative and the degree of partial melting has not been estimated quantitatively. We discuss the importance of migmatites in terms of possible magma feeders for leucogranites widespread in the Famatinian belt. The aim of our work is to describe the association between lower grade Puncoviscana metasediments, schists, migmatites and leucogranites from a point of view of petrogenesis providing new field, petrographic, geochemical, geochronological (U-Pb). In Molinos range, Eastern Cordillera, NW Argentina (Fig.1) can be observed the relationship between different components of an anatectic system allowing the direct observations on the origin of ca. ~470 Ma S-type granites by anatexis of pelitic schists. Anatexis may have been triggered by decompression during back-arc extensional tectonism during the Famatinian Orogenic Cycle. Field observations suggest that ?fertile? metasediments of the upper Neoproterozoic ? Early Paleozoic Puncoviscana sequence were the source of the leucogranites. The granitoids are intimately linked with migmatitic (Fig.1) rocks and characterized by the presence of accessory minerals such as garnet, cordierite, sillimanite and tourmaline [see 4]. Two compositional groups were distinguished within migmatites leucosomes and leucogranites from Molinos igneous/metamorphic complex (Fig.2). The first one is syenogranitic (K-rich, high K/Na and Rb/Sr ratios) and the second one displays trondhjemitic character (Na-rich, low K/Na and Rb/Sr ratios). Estimation of the degree of partial melting in the pelitic migmatite zones is crucial for understanding how much melt is produced, segregated and extracted from the region to produce leucogranite and trondhjemite bodies like those that formed the Cachi Formation, in other locations along the Calchaquí valley. Partial melting of metapelites under conditions of increasing temperature results from the incongruent melting firstly of muscovite and secondly of biotite. The breakdown of muscovite may occur under fluid present or fluid absent conditions which in turns exert strong control on the chemistry of resulting melts  and . Mass balance calculations are used to estimate the degree of partial melting in migmatites. As a result, degrees of partial melting F = 0.12 and 0.15 were calculated for K-poor trondhjemitic leucosomes whereas F~0.20 is required to form the syenogranitic leucosomes. Trondhjemites have particular significance since they have controversial origin. Previous work suggested depleted mantle rocks and lower crust rocks as possible sources with no conclusive results or related them with a coeval TTG (trondhjemite-tonalite-granodiorite) magmatism extending along the Pampean Ranges e.g. . The conditions required to form trondjhemitic melts from depleted grabbroid source (5-10% batch melting) are estimated at 10-12 Kbar/900ºC . An alternative to these proposals is to explain its genesis by H2O-fluxed melting of metasediments at conditions equivalent to that expected for the S-type leucogranites and migmatites (4-6 Kbar/650-750ºC). As in migmatites leucosomes, the lack of convection and the high viscosity of these relatively ?cold? magmas may inhibit homogenization even within pluton-scale. The geochemical modeling reveals that is possible to generate Na-rich melts with trondhjemitic character resembling the magmas that originated some plutons of the Cachi Formation from melting of K-rich Puncoviscana sediments without any participation of mantle or lower crust component. Trondhjemitic and syenogranitic compositions then can represent different melt fractions of the same metapelitic source and its chemical diversity is understood as the consequence of H2O activity during muscovite breakdown.