GEOCHEMICAL EFFECTS OF HYDROUS PHASE STABILITY ON METADIORITE PARTIAL MELTS: APPLICATIONS TO TONALITE PRODUCTION IN CONVERGENT REGIMES

Piston-cylinder experiments have been conducted to test the geochemical effects of hydrous phase stability on partial melt compositions during melting. A natural un-melted sample of metadiorite was collected from the Pembroke Valley, Fiordland, New Zealand. The Pembroke suite forms part of a well-exposed, regionally extensive high-pressure (P=12-14 kbar) granulite belt representative of the lower crust of an Early Cretaceous convergent orogen. XRF data show that the metadiorite is calc-alkaline in composition with a mineral assemblage consisting of plagioclase + quartz with tschermakitic hornblende, clinozoisite and biotite as the hydrous-bearing phases. The metadiorite of the Pembroke granulite experienced partial melting simultaneously with fracturing at granulite facies conditions T> 750°C and P ˜14 kbar during the Early Cretaceous. The metadiorite preserves macro- and microscopic textural evidence of partial melting (Clark et. al., 2000, JMG v18. pg. 359-374).

Experiments have been performed at 14 kbar, between 800-950°C under fluid-absent and fluid-present conditions (2 wt% added H2O) at f_O2=Ni-NiO+2. The fluid-absent solidus occurs between 800-825°C. Under fluid-absent conditions, biotite reacts out initially followed by reaction between hornblende and clinozoisite, with garnet, clinopyroxene and melt as the primary reaction products. Melt compositions are tonalitic but become granodioritic with temperature and in-coming garnet stability, as the dominant reaction shifts to fluid-absent melting of hornblende +/- clinozoisite.

Under fluid-present conditions, textural evidence suggests the presence of H₂O stabilizes hornblende and clinozoisite, requiring higher temperatures to shift the dominant reaction from that of biotite to hornblende. Clinozoisite remains stable in all the hydrous experiments, though exsolution of the iron component resulting in end-member clinozoisite and iron oxides does occur. This hydrous phase stability results in melt compositions that are tonalitic, becoming become less anorthitic with temperature. These experiments provide supporting evidence that tonalitic compositions can be produced at relatively low temperatures (T=825°C) by partial melting of mafic crustal assemblages under fluid-present conditions.