The Valle Ancho Volcanic Region: Insights into the Evolution of the Southernmost CVZ.

MPODOZIS, C.; KAY, S.M.; GARDEWEG, M.C. AND B., COIRA

Pucón, Chile

Congreso; General Assembly IAVCEI; 2004

IAVCEI

The Valle Ancho region (VA), located near the southern end of the CVZ in northwestern Argentina (27º30ºS) is a relatively low altitude NW trending structural corridor located between the < 3.5 Ma to Recent Ojos del Salado volcanic chain and the Late Miocene (6-4 Ma) Nevado Pissis volcanic complex. These centers reach > 6800 m a.s.l and include the highest volcanoes on Earth. The oldest rocks of the VA region occur in blocks of Precambrian? to late Paleozoic crystalline basement that are bounded by NW to NNW trending sub-vertical faults. Overlying the basement complex are 26 to 2.3 Ma volcanic rocks that record the processes related with the eastward displacement of the arc magmatic front as the slab shallowed under this region. The oldest Miocene VA volcanic event (9 to 6 Ma) is represented by altered dacitic domes, ignimbrites and minor basaltic flows erupted in the back-arc of the Oligo-Miocene Maricunga arc. Eight to 4 Ma andesitic flows from stratovolcanoes (Dos Hermanos, “older” Pissis) and andesitic and rhyolitic domes and block and ash deposits erupted as the arc front shifted eastward. The arc reached its final position at 4-3.5 Ma when dacites and andesites of the “young” Pissis and Nacimiento de Jagüé centers erupted on the southern margin the VA, before the 3.5 Ma onset of activity in the Ojos del Salado complex to the north. Geochemical changes (e.g., La/Yb & La /Ta) from 26 to 4 Ma can be tied to crustal thickening and forearc subduction erosion during slab shallowing and frontal arc migration.