The Generation of Arc Andesites and Dacites in the Lower Crust of a Cordilleran Arc, Fiordland, New Zealand

Abstract We present microbeam major- and trace-element data from 14 monzodiorites collected the Malaspina Pluton (Fiordland, New Zealand) with goal of evaluating processes involved in production andesites lower arc crust. focus on relict igneous assemblages consisting plagioclase amphibole lesser amounts clinopyroxene, orthopyroxene, biotite quartz. These are heterogeneously preserved crust within sheeted intrusions that display hypersolidus fabrics defined by alignment unstrained amphibole. Trace-element amphiboles these rocks reveal two distinct groups: one relatively enriched high field strength element concentrations depleted. The group has Zr values range ∼25–110 ppm, Nb ∼5–32 Th up to 2·4 ppm. depleted group, contrast, shows <35 ppm <0·25 is generally below level detection. Amphibole crystallization temperatures calculated major elements ∼960 830 °C for all samples pluton; however, we do not observe significant differences between (∼960–840 °C) groups (∼940–830 °C). Bulk-rock Sr Nd isotopes also remarkably homogeneous show no apparent difference (εNdi = 0·1 –0·1; 87Sr/86Sri 0·70420–0·70413) 0·3 –0·4; 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate melts were highly fractionated (molar Mg# <50), andesitic dacitic composition, much more evolved than bulk continental or primitive basalts predicted have formed hydrous melting mantle-wedge peridotite beneath an arc. suggest originated a common, isotopically source Pluton, reflect varying contributions subducted sediment-derived fluid, respectively. Our demonstrate dacites dominant intruded crust, their mirror middle upper bulk-continental estimates. Continental crust-like geochemical signatures acquired region interaction recycled sediment rather assimilation and/or remelting pre-existing