Development of continental magmatic systems : insights from amphibole chemistry of the Altiplano Puna Volcanic Complex, Central Andes

Abstract

The pressure history of a continental magmatic system can be deciphered by analyzing the composition of amphiboles in the eruptive products where the pressure of equilibration correlates with the depth of the magmatic system. This can reveal vertical evolution of the magma as amphibole composition varies significantly with temperature and pressure. The Altiplano Puna Volcanic Complex (APVC) is a long-lived and large continental magmatic system that has produced episodic ignimbrite eruptions during the last 10 m.y. The amphiboles from ignimbrites of the various stages during the 10 Ma history have been analyzed, classified and the pressure and temperature calculated using thermodynamic calculation. The APVC amphiboles are calcic amphiboles and are magnesiohornblende, tschermakite, magnesiohastingsite, and edenite based on the Leake et al. (1997) classification and based on the Deer et al. (1992) scheme. The amphiboles are also calcic, namely, hornblende, tschermakite, pargasite, and edenite. They are broadly similar to amphiboles from other calc-alkaline dacitic systems through space and time. The calculated P-T conditions range from 0.2 to 2.5 kbar and 765°C to 871°C. The P-T conditions are generally similar throughout the 10 Ma time frame of the APVC, although higher minimum and maximum pressures are recorded in the most voluminous 4 Ma pulse. The APVC magmas are representative of calc-alkaline dacitic magmas associated with subduction and therefore it is a useful model for how large calc-alkaline dacitic systems might evolve. The lack of an obvious trend in P and T with time during the 10 million years history of the APVC, suggests that evolution of dacitic magmas prior to eruption is limited to a narrow depth range in the crust, which is probably primarily controlled by the density of the magmas.