INENCO   05446
Unidad Ejecutora - UE
congresos y reuniones científicas
Petrogenesis leucogranites - trondjhemites of Molinos, Salta. NW Argentina: constrains from a geochemical modeling of migmatitic leucosomes
Foz de Iguazu
Simposio; The Meeting of the Americas; 2010
Institución organizadora:
American Geophysical Union
In Molinos range, Eastern Cordillera, NW Argentina crop out basement units of low to high metamorphic grade (Puncoviscana metaturbidites and La Paya Formations) intruded by granitoids and numerous pegmatitic and aplitic dykes. The granitoids belong to Cachi Formation with ages ranging between 460-480 Ma which overlap with the age of migmatization. In this area, can be observed the relationship between different components of an anatectic system developed in upper amphibolite facies conditions (HTLP) allowing the direct observations on the origin of granites by fusion of paragneisses. Field observations and provenance studies suggest that ‘fertile’ metasediments of the upper Neoproterozoic – Early Paleozoic Puncoviscana supracrustal sequence were the source of the granitoids. The granites are intimately linked with migmatitic rocks and characterized by the presence of accessory minerals such as garnet, cordierite, sillimanite and tourmaline. All transitions were found in the field between granite veins in migmatite complexes to thick sheets and plutons. Two compositional groups were distinguished within migmatites leucosomes and granite facies. The first one is syeno-granitic and the second one displays a trondjhemitic trend. Geochemical modeling of migmatites leucosomes is used to understand the nature and behavior of granitic magmas in other locations along the Calchaquí valley. Through mass balance linear regression was possible to determine that both compositions can be obtained from the fusion (~16-25%) of the same metasedimentary protolith. The key parameter controlling the chemistry of melts would be H2O activity during anatexis, which produce changes in the stoichiometry of melt producing reactions. The mineralogy of modeled residuum matches with that of cordierite-biotite-bearing residual migmatites that crop out in the study area. In addition, the isotopic data is consistent with all these rocks being genetically related, with the granitoids having been generated by anatexis of its high-grade host rocks and not as a result of a mixed source or mantle component. Trondjhemites have particular interest since they have controversial origin. Previous work suggested depleted mantle rocks and lower crust rocks as possible sources with no conclusive results or related with a coeval TTG (trondjhemite-tonalite-granodiorite) magmatism extending along the Pampean Ranges. 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 at conditions equivalent to that expected for the S-type leucogranites and migmatites (4-6 Kbar/700-750ºC). The geochemical modeling reveals that is possible to generate Na-rich melts with trondjhemitic tendency 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.