Temporal evolution and spatial variation of early Tertiary volcanism in the Patagonian Andes (40¡ã-42¡ã 30'S)

RAPELA, C.W.; SPALLETTI, L.A.; MERODIO, J. ; ARAG¨®N, E.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 1988 vol. 1 p. 75 - 88

0895-9811

Early to mid-Tertiary igneous activity in the Cordilleran Series (CS) of the Patagonian Andes between 40¡ãS¨C42¡ã30¡äS shows spatial variation and temporal trends that can be correlated with crustal thickness and slab depth. Volcanism in this region is concentrated in two sub-parallel arcs, the Pilcaniyeu Belt to the east and the El Mait¨¦n Belt to the west. Compilation of available K-Ar data and paleogeographic constructions suggests three major periods of volcanic activity. The first is a Paleocene-Eocene (60-42 Ma) event which developed primarily in the Pilcaniyeu belt; the second (Oligocene, 33-23 Ma) event is now exposed primarily along the El Mait¨¦n Belt. Miocene volcanics (16-11 Ma) have also been found in the northern sector of the El Mait¨¦n Belt. After an overall decline in the magnitude of volcanism, the third period of volcanic activity reached its maximum during the Pliocene-Pleistocene as large stratovolcanoes were built along the North Patagonian Cordillera. Temporal variations in the lower Tertiary CS, determined by superposition in selected cross-sections, indicate that the major episodes begin with silicic associations (ignimbritic, plinian, and obsidian rhyolitic facies) and end with intermediate and basic lava flows (stratovolcanoes and monogenetic cones). K2O and total alkalis decrease southward for a given silica content in the Pilcaniyeu and El Mait¨¦n Belts. The calc-alkaline andesites of the northern sector (39¡ã30¡äS¨C41¡ã30¡äS) are similar to many other Central Andes series, but towards the south (Cholila) there are also sequences with mild tholeiitic affinities. Spatial variations in major element composition seem to be related to the southward decrease in thickness of the underlying sialic crust. Variations in subduction geometry during the Cenozoic possible correlate with the compositional recurrence observed in the Cordilleran Series.