Coexisting calcalkaline and high‐niobium basalts from Turrialba Volcano, Costa Rica: Implications for residual titanates in arc magma sources

Abstract

Turrialba volcano, the southeasternmost volcano in the Central American arc, is constructed of medium to high‐K calcalkaline basalts, andesites, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high‐Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc‐tholeiitic basalts. The association of calcalkaline and high‐Nb basalts is rare in arc front volcanoes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high‐Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab‐derived fluid was present during calcalkaline basalt genesis but not during high‐Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid‐ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end‐members for both the calcalkaline and the high‐Nb basalts. The calcalkaline basalts represent mixtures of the high‐degree melts and oxidized small‐degree melts generated by amphibole breakdown in mantle overlying the subducting slab. This small‐degree melt has high incompatible element concentrations and is saturated in rutile. Arc‐related lamprophyric rocks have compositions that are appropriate for these small‐degree melts. High‐Nb basalts are mixtures of the high‐degree melts and more reduced small‐degree melts that are undersaturated in rutile. These reduced melts may migrate around or through the subducting slab into the wedge to become involved in arc magma genesis.