Determination of an arc-related signature in Late Miocene volcanism over the San Rafael Block,Southern Central Andres (34º30"-37º) Argentina: The Payenia shallow subduction zone

LITVAK, VANESA D.; ANDRÉS FOLGUERA; VICTOR A. RAMOS

Niza, Francia

Simposio; 7º International Symposium on Andean Geodynamics; 2008

Institut de Recherche pour le Développement (IRD) y la Université de Nice - Sophia Antipolis

A series of isolated volcanic centers can be located over the San Rafael block 300 km away of the Present arc front. They correspond to highly eroded stratrovolcanoes of Late Miocene age. Two groups of volcanic events with particular spatial distributions and age can be defined: an older sequence (15-10 Ma) and a younger one (8-3.5 Ma) located in an eastern position relative to the first group. Petrographical features characterize typical calk-alkaline volcanic andesites. Geochemical classifications correlate with the petrographic one. The San Rafael andesitic to dacitic stratovolcanoes represent Late Miocene volcanic arc activity. Evidence for their arc-like pattern comes from their petrographical characteristics and from their major and trace element chemical signatures. The recognition of these volcanic sequences, and their affinity with an arc-like signature, allow understanding real dimensions of arc-expansion at that time over the San Rafael block. The Chachahuén Volcanic Complex is located further with an age range from 7.3 to 4.9 Ma. Andesitic volcanism of Chachahuén shows similar chemical behavior than other San Rafael Block volcanics: Main differences are seen in the increase of Sm/Yb ratios in the first volcanic stage of San Rafael Block and Chachahuén Volcanic Complex towards the youngest San Rafael Block volcanics, that reflect an increase in pressure conditions at the melt generation site. The absence of plagioclase fractionation in younger San Rafael volcanics also evidences a relatively deeper site of magma generation. Higher pressure conditions are consistent with tectonic shortening and crustal thickening as a result of subducted slab flattening during the development of the Payenia shallow subduction zone.