The Late Carboniferous to Triassic Volcanic Belt in Northern Chile

Abstract

A prominent feature of the pre-Jurassic Andean basement are outcrops of late Palaeozoic to Triassic, mainly siliceous volcanoplutonic complexes. Published models assume a magmatic trend to have developed from a ``normal'' subduction setting with calc-alkaline magmatism during Carboniferous times towards an ensialic marginal rift during the Permo-Triassic, which gave rise to (calc-)alkaline and peralkaline magmas. In the Precordillera and Cordillera Occidental of northern Chile (20{\textdegree}--25{\textdegree}S), widespread outcrops of late Carboniferous to Triassic volcanosedimentary successions occur (Peine Group). Caldera eruption seems to have been important in this region. The volcanic rocks are of calc-alkaline to slightly alkaline, predominantly siliceous composition (SiO2 range 50--80%). The geochemical features of the volcanic rocks display a clear volcanic arc affiliation. In parts, a diagenesis/anchimetamorphism of the volcanic rocks caused considerable variations of mobile elements (e.g. K, Na, Rb, Sr, Cu, Si). HFS and RE elements display patterns of a non-tensional subduction-controUed setting of the magmatism at a continental margin of moderate thickness. In the Salar de Atacama area, a conspicuous alluvial-limnic volcanosedimentary intercalation occurs. The terrestrial basin which accommodated this sequence formed during the latest Carboniferous-?early Permian on top of the deposits of the pre-existing late Carboniferous composite volcanism. The lower part of the intercalation consists of green limnic and multicoloured alluvial fan deposits and the upper part is made up of red alluvial sedimentary rocks. Throughout the sequence, minor basic volcanic rocks occur. The considerable size of the basin, the limnic-alluvial fan facies association and the accompanying intrabasinal basic volcanism indicate a basin formation controlled by extensional arc tectonics. With regard to the north Chilean Peine Group, this arc-graben represents the only support to the aforementioned model of a tensional regime during the Permo-Triassic.