Petrography and major element chemistry of the volcanic rocks of the Andes, southern Peru

Abstract

More than 200 samples of late Tertiary to Quaternary volcanic rocks have been collected from the northern sector of the central volcanic zone of the Andean belt occupying southern Peru. The most abundant rock type is the pyroxene andesite. Many of the rock samples carry hornblende and/or biotite phenocrysts. Small amounts of shoshonites occur on the back arc side near Puna and Siquani and olivine-augite basanites occur on the western shore of Lake Titicaca. The SiO₂ frequency has, a mode in the 60–65% range which is about 5% higher than the Quaternary volcanic rocks of Japan. The K₂O content shows a distinct tendency to increase away from the front, while the Na₂O content tends to decrease in the same direction. The K, Sr and Ba contents of the late Tertiary to Quaternary volcanic rocks of the northern part of the central zone of the Andean volcanic belt (southern Peru) show regular increase away from the volcanic front. At the same time, a slight northwestward increase along the arc is detected. The Na content regularly decreases away from the front making a strong contrast to the Japanese Quaternary volcanic arcs, which does not show any regular change. The Na content is conspicuously higher in the northwestern frontal zone than the rest.