Andesitic to Dacitic Volcanism in the Miocene Back-Arc (30°30'S-69°30'W) over the Pampean Flat Slab, Southern Central Andes, Argentina

POMA, STELLA; LITVAK, VANESA D.

Foz de Iguazú, Brasil

Congreso; The Meeting of the Americas; 2010

American Geophysical Union

During late Oligocene, an Andean type arc was developed and a concomitant magmatic activity was continuous along western margin of Southern Central Andes. Volcanic front gradually migrated towards the east due to the shallowing of the subducted slab. By the same time, volcanic activity was developed in a back-arc position cropping out in the eastern margin of Cordillera de Colangüil, along the Valle del Iglesia of San Juan. Two main lithological facies are exposed: pyroclastic deposits and subvolcanic bodies associated to lava flows. First ones correspond to pyroclastic flows with subordinate fall and surge-type deposits that covered Carboniferous and Triassic basement rocks; green and pale grey porphyritic andesites and dacites are representatives of the second facies. Volcanic rocks are enriched in large ion lithophile elements relatively to the high field strength typical of subduction related magmas. Ba/Ta and La/Ta are also consistent with a volcanic arc signature. Available ages, of Middle Miocene age, associated with its petrographic and chemical characteristic allow correlating this magmatism with the andesitic volcanism of similar age in the volcanic front, but developed, in these case, in a back-arc position.