Multiple sources for basaltic arc rocks from the southern volcanic zone of the Andes (34°–41°S): Trace element and isotopic evidence for contributions from subducted oceanic crust, mantle, and continental crust

Abstract

Potential sources for arc magmas include subducted midocean ridge basalt and sediment, subarc mantle peridotite, and overlying crust. Geochemical data for arc rocks in general, including those for the southern volcanic zone (SVZ) of the Andes (34°–41°S in Chile), cannot be explained by one‐stage partial melting of subducted oceanic crust or overlying mantle peridotite. Sr and Nd isotope systematics in arc rocks are similar to those in oceanic island basalts (OIB's). However, plots of 207Pb/204Pb versus 208Pb/204Pb show clearly that there is an input from subducted sediment in most arcs. In arc rocks, radiogenic isotope ratios are decoupled from abundance anomalies of Th, U, Pb, alkali metals, Sr, and Ba. This decoupling is expected if the trace element abundance anomalies are caused by a process such as partial melting of or volatile release from the subducted oceanic crust. In basalts from different volcanoes in the SVZ, anomalous trace element abundance ratios, such as Ba/Nb, are coupled with incompatible element abundances. This feature is consistent with mixing of a component derived from the subducted oceanic crust (i.e., having high Ba/Nb, Cs/Rb, and 207Pb204Pb) with enriched asthenosphere or lithosphere. Although crustal contamination may have had minor effects in all SVZ volcanoes, basaltic lavas from volcanoes 34°–38°S, which are sited on thicker crust, exhibit more evidence for continental crustal contamination, specifically, lower 143Nd/144Nd and higher Al2O3/CaO, than lavas from volcanoes 38°–41°S, which are sited on thinner crust.