Geochemical characteristics of the Late Mesozoic Andean volcanics.

Abstract

The authors evaluate new trace-element, Nd and Sr isotope analyses for two late Mesozoic volcanic series located in S Peru: the Jurassic Chocolate and the late Cretaceous Toquepala group. For the former formation, major- and trace-element analyses indicate a calc-alkaline composition for the whole volcanic suite; the new trace-element data enable these volcanics to be subdivided into four groups or cycles. The data base and unique differentiation of each of these suggests that Chocolate basaltic andesites and andesites are related by a process of fractional crystallization. The preferred age adopted for Chocolate volcanics is 185 m.y., giving initial 87Sr/86Sr approx 0.7038, however, several sub-groups yield ages ranging 140-205 m.y. suggesting that some volcanics may be younger or that alteration and metamorphism strongly affected some samples. Nd isotopes clearly indicate a depleted source for the entire suite (epsilon Ndi = +2.7 to '+7.9'). The Toquepala and Mid-Cretaceous rocks show great variability in trace-element abundance and abundance ratios, but the dacites and rhyolites possess an overall highly incompatible trace-element content. Petrogenesis of the Toquepala volcanics remains unclear, but Nd isotopes indicate a depleted source (epsilon Ndi = +3.0 to +6.5). -J.M.H.