INENCO   05446
INSTITUTO DE INVESTIGACIONES EN ENERGIA NO CONVENCIONAL
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Petrogenesis leucogranites - trondjhemites of Molinos, Salta. NW Argentina: constrains from a geochemical modeling of migmatitic leucosomes
Autor/es:
SOLA ALFONSO; RAUL BECCHIO
Lugar:
Foz de Iguazu
Reunión:
Simposio; The Meeting of the Americas; 2010
Institución organizadora:
American Geophysical Union
Resumen:
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.