Large- and Fine-Scale Geochemical Variations Along the Andean Arc of Northern Chile (17.5°– 22°S)

Abstract

Geochemical data from 37 volcanic centres from the active volcanic front in the Central Andes between 17.5° and 22°S of northern Chile provide constraints on crustal contributions to arc magma genesis in that region. Crustal thickness, distance from the trench, height above the seismically active subduction zone, and sediment supply to the trench are all constant along this segment of the arc. The only significant variable along the current arc segment is in mean crustal age (Palaeozoic in the south to Proterozoic in the north). In addition, the crustal thickness has varied through time from around 40 km in the Lower Miocene to about 70 km today. Variations along the N-S chain of the volcanic front include: (1) regional segmentation into zones of abundant and voluminous volcanic complexes that evolve from rare basaltic andesites to rhyodacites and rare rhyolites; (2) northern centres with higher incompatible trace element contents than southern ones; (3) constant isotopic composition with differentiation for most centres, with the exception of V. Ollague where AFC trends are observed; (4) Sr- and, in particular, Pb — isotopic compositions which differ north and south of the Alto de Pica area at about 19.5°S, and reflect interaction of magmas with domains of different crustal type and mean crustal age; and (5) interruption of the volcanic chain north and south of the Alto de Pica area where recent volcanism is absent over 180 km in a N-S direction, except for a single rhyodacite mega-dome (Co. Porquesa). The Alto de Pica sector is also unusual in that it lacks the tectonic horst of the Western Cordillera and may correlate with a major transverse crustal domain boundary. There is also a striking correlation between the variation in isotopie composition of erupted magmas through time and the crustal uplift/crustal thickening history of this region. Together these are interpreted as clear evidence that isotopie (and thus bulk chemical) compositions are strongly affected by magma-crust interaction rather than by changes in the mantle source. Crust-magma interaction is viewed as a MASH-type process (melting, assimilation, storage, homogenization) in lower crustal melting regions. Homogenization of deeply-derived contaminated magmas produced typical “MASH-baseline values”, i.e. chemical and isotopie characteristics that are constant throughout the entire compositional range of an individual volcanic structure. However, for a number of unusual basalts and basaltic andesites in small and rare isolated monogenetic centres, MASH homogenization is not indicated and is in conflict with chemical characteristics.

In order to account for variation of magma composition in time and space, we envisage a transitional MASH zone which has responded to crustal thickening since the Upper Miocene, involving breakdown of mineral phases such as plagioclase and amphibole, and/or variable degrees of partial melting of the crust. In contrast, small-volume basaltic andesites show regional geochemical domain boundaries that are distinct from the much larger stratovolcanoes (andesites to rhyodacites). Their trace element and isotopie signature is regarded as being derived from the deepest crust, with the northern crustal domain extending further south at depth rather than nearer the surface.