INENCO   05446
INSTITUTO DE INVESTIGACIONES EN ENERGIA NO CONVENCIONAL
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
PETROGENESIS OF LEUCOGRANITES AND TRONDJHEMITES OF MOLINOS, SALTA, NW ARGENTINA: CONSTRAINTS FROM GEOCHEMICAL MODELING OF MIGMATITIC LEUCOSOMES
Autor/es:
SOLA, A.; BECCHIO, R.A.
Lugar:
Foz do Iguaçu
Reunión:
Simposio; The Meeting of the Americas; 2010
Institución organizadora:
AGU
Resumen:
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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.
Key words: Leucogranites, Trondjhemites, Anatexis, Eastern Cordillera, NW Argentina