Trace element distribution in the cainozoic lavas of Nevado Coropuna and Andagua Valley, Central Andes of Southern Peru
Major and minor elements have been determined on 26 samples of andesitie to rhyolitic lavas from Nevado Coropuna and Andagua valley in Southern Peru. Nevado Coropuna dating back since late Miocene is the highest stratovolcano of Peru. It is located at 150 km NW of Arequipa and at 110 km E of the Pacific coast. Andagua valley is situated at about 30 km E of Coropuna. The magmatic activity there, as shown by the presence of several cones, is more recent than that of Corpouna and is related to the tectonic graben characterizing this valley. The geological position of the valley is very important because it is near the transverse line separating the zone of rather flat subduction of the Nazca plate from another one dipping more steeply to the SE. The lavas from Andagua show higher Ti, P, Sr and alkali contents than those from Coropuna, and several display some alkaline tendency with Na affinity. No shoshonitic rocks have been found in the area. According to their geochemistry, Corpuna and Andagua andesites do not seem to have been originated by a single process. In particular, the distribution of Ni, Cr, Ti, Zr, Y, P, Nb, and Sr would exclude either a direct origin from pyrolitic materials, or a progressive crustal contamination as the most important factors for their origin. Calculations of mineral/melt equilibria for Coropana andesites suggests crystallization processes at depth less than 35 km and H2O-understurated conditons at the time of the phenocrysts precipitation, indicating a possible high undersaturation at depth of the source zone. As lar as the rhyolites are concerned, their geochemical characteristics do not preclude a crustal origin. A statistical study of the chemical zonation of the Plio-Quaternary lavas of southern Peru has shown an increase of Ti and P contents eastward of the Chile-Peru trench.