PARTIAL MELTING OF A HORNBLENDE-BIOTITE-CLINOZOISITE BEARING METADIORITE: APPLICATIONS TO THE DEEP CRUST, FIORDLAND, NEW ZEALAND

Piston-cylinder experiments have been conducted to test 1) melt composition 2) volume change of the melting reaction and 3) the water activity of the melt produced during partial melting of a metadiorite. A natural un-melted sample of metadiorite was collected from the Pembroke Valley, Milford Sound, 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 plag + qtz with tschermakitic hbl, czo and bt 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. Our field observations are consistent with melt segregation and transport having been aided by fracture propagation. The fracture networks are inferred to have been induced by cracking associated with the positive volume change during fluid-absent melting. Migration of melt through surrounding gabbroic gneiss resulted in dehydration of the host rock, requiring a partial melt with a low water activity. Experiments have been performed at 14 kbar, between 800-900°C under fluid-absent and fluid-present conditions (2 wt% added H₂O, for comparison) at fO₂=Ni-NiO+2. Melt is observed in the hydrous experiments under all conditions; the fluid-absent solidus is at 850°C. Under fluid-absent conditions, preliminary results suggest that biotite reacts out initially followed by reaction between hornblende and clinozoisite with garnet and melt as the primary reaction products. Reacting hornblende becomes more magnesium rich and new garnet (Gro_14-20Alm_45-48Pyr_31-33Sp_2-4) increases in Gro and decreases in Alm with increasing temperature. Melt compositions are granodioritic but become trondhjemitic with temperature, as the dominant reaction shifts to fluid-absent melting of hornblende +/- clinozoisite.