Geodynamic interpretations of plate subduction in the northernmost part of the Central Volcanic Zone from the geochemical evolution and quantification of the crustal contamination of the Nevado Solimana volcano, southern Peru

Abstract

The northernmost volcanoes of the Central Volcanic Zone (CVZ, southern Peru) form three isolated composite centres along the western side of the active arc segment. This particular situation is comparable to the Chilean volcanic region at the southernmost end of the CVZ; in both cases, volcanoes lie close to the discontinuity where the Benioff zone becomes less steeply inclined and where crustal thickness decreases. The Solimana volcano (15°25′S), the oldest of the northerly eruptive centres, is built upon a Late Precambrian/Palaeozoic basement intruded by an Upper Cretaceous granite and unconformably overlain by Cenozoic volcanics. After a mid-Miocene andesitic activity along N10° regional faults, the Solimana volcanic history ranged from 4 Ma to Recent and built a composite andesite/dacite centre which consists of two units, preand post-caldera collapses and a youngest intra-caldera basaltic andesite phreatomagmatic activity. Fumaroles with thermal hot springs and precipitates indicate continuing activity of this volcano. North of 16°S, other adjacent volcanoes show the same evolution. Mineral chemistry and geochemical variations of the Solimana series are similar to those obtained from most high-K CVZ lavas and are largely consistent with fractionation processes. A restricted range of compositions of phenocryst assemblages, together with trace element data showing prominent LIL element enrichments relative to HFS element depletions, are typical of subduction-related continental arc magmas. However, this localized volcanism in the vicinity of the Nazca ridge-continent collision shows specific signatures: a weakly positive correlation between 87Sr/ 86Sr and Rb/Sr ratios, a high Ba and Sr content of basaltic andesites and both low ϵ Nd (− 2 to − 4) and high ϵ Sr (+16 to +21) values show differences north of 16°S from those to the south and suggest that distinct processes have controlled this environment. High Ba values for the most primitive lavas may be attributed to a subducted sedimentary component. AFC modelling of the Solimana lavas with a contaminant having a high ϵ Sr value, such as the local upper Palaeozoic gneissic crust ( ϵ Sr = +65), or the lower Precambrian granulitic crust ( ϵ Sr = + 396), requires much less of assimilated material (≦ 5%) than is implied for the Arequipa and Barroso volcanics ( ≧ 20%). For the local Cretaceous granitic intrusion ( ϵ Sr = +19), an AFC model is not compatible with the Solimana data. Contamination of the parental magma by the lower Precambrian crust (e.g. the Charcani gneiss) gives rise to speculation that a Precambrian granulitic component was incorporated from the crust underlying the volcano. This explanation is preferred because the lack of country rocks as crustal xenoliths in this series suggests that a high level of assimilation process was not significantly involved during the differentiated stages of magmatic evolution. The Solimana lavas derived from a parental magma generated in the mantle wedge and contaminated by (1) a subducted sedimentary component and (2) a lower Precambrian crustal component, and were fractionated in the upper crust without significant assimilation. These processes are linked to a thinner crust at the northernmost end of the CVZ.