Lower Paleozoic Volcanic Events in the Northern Famatina System, Argentina: Its Implications for Understanding the Evolution of the Famatinian Arc, as Part of the History of the Central Andes Continental Crust

COIRA, B., KOUKHARSKY, M., CISTERNA, C.E. Y ULBRICH, H.H.

Foz do Iguazu

Congreso; The Meeting of the Americas; 2010

American Geophysical Union

The northern Famatina System exhibits widespread records of Lower Paleozoic sedimentary and volcano-sedimentary sequences as discontinuous outcrops. Fossil content and radiometric ages constrain the evolution of the basins from Early to Mid Ordovician. Petrologic and geochemical studies allowed recognition of three volcanic associations with distinctive characteristics which represent three magmatic events. The oldest has Early Tremadocian age and consists of basaltic, andesitic, dacitic and rhyodacitic lavas interfingered with fine epiclastic facies affected by deformational episodes where low grade regional and contact metamorphic paragenesis were recognized. The Arenigian association is represented by basaltic to rhyolitic lavas, hyaloclastites, syn-eruptive resedimented deposits and volcanogenic sedimentary facies with significative participation of debris flows and turbidite levels of dominant pyroclastic provenance, also with superimposed deformation. The posthumous volcanic association is mainly effusive (mostly hyaloclastic) and consists of rhyolitic, dacitic (as dominat types), andesitic and basaltic lavas. These sequences could be assigned to the Mid Ordovician (Llanvirn) by correlation with volcano-sedimentary associations from northern and central Famatina System, unconformably overlying Arenigian rocks. Geochemical data indicate that Tremadocian and Arenigian associations are subalkaline and peraluminous, but the first one shows tholeitic affinities in the basic members, and in general their rocks have REE patterns similar to E-MORB and at the same time weak subduction characteristics which are compatible with an environment where melts from a depleted mantle (MORB) were produced and enriched during subduction events suggesting supra-subduction zone (SSZ) conditions. The Arenig rocks are calc-alkaline with low to medium K, light LREE enrichment; Nb and Ta negative anomalies related to LREE and Th, and Ba/La=10-22, La/Ta= 28-55 for the siliceous members: transitional characteristics between arc and back-arc environments. Both associations are similar to those observed in the Western Puna region. The last association integrates a series of high silica alkaline rhyolites to basalts with light alkalinity (e.g. Nb/Y=0.43 to 1.14) and relations of Hf, Th, Ta and Nb that point to an E-MORB transitional to within plate signature. These characteristics, joined to the importance of acid effusions accompanied by minor basalts and to a decrease of explosive volcanism, are evidences of a change to extensional conditions in the arc evolution, with a major thinning of the lithosphere, increase of melting processes, and generation of a dominant siliceous volcanism, as the last event of the volcanic arc during the Ordovician. This evolution of the Ordovician arc contributed to the heterogeneity of the Andean continental crust.