Paper No. 2
Presentation Time: 8:20 AM
MELT GENERATION AND EXTRACTION IN THE LOWERMOST CRUST: AN EXPERIMENTAL TEST OF A NATURAL SETTING
South Island, New Zealand is composed of a magmatic arc of early Mesozoic age. The Fiordland portion of the belt exposes high-pressure intermediate to mafic granulites (14-16 kbar), and layered mafic to intermediate intrusions that formed in the lower to middle crust during the early Cretaceous. The magmatic units are sodic and have high Sr/Y ratios ("adakitic") and have been suggested to originate from partial melting of lowermost mafic crust. Field evidence collected from dioritic to gabbroic granulite units shows partial melting has occurred and melt flow through fracture networks to be a possible melt extraction mechanism. We are testing this hypothesis and determining melt geochemistry by performing fluid-absent partial melting experiments on lowermost crustal samples collected from the base of this arc. We report results from metadiorite experiments under both static and dynamic conditions and from metabasaltic dikes that have been interpreted to represent underplate material emplaced during arc generation. Experimental results from the hbl+czo+bio bearing metadiorite show that partial melt volumes remain low (<2 vol% at 800oC to 13 vol.% at 975oC) and that melting is associated with high volume change (dilatational strain), producing melt fractures. These results, plus results from the deformation experiments, suggest that the mafic lowermost crust in Fiordland was rheologically strong and represents a different mechanical response to far-field stresses than other orogens which undergo extensive partial melting in the mid crust and can behave rheologically weak for extended periods of time. Results from experiments performed at 14 kbar and 825oC to 975oC on metabasalt show that hbl + pl +/- czo reacting to form melt + gnt is the dominant reaction. Granitic to granodioritic melt compositions are observed as a function of increasing temperature. LA-ICPMS results show glass to be HREE depleted, LREE enriched compositions, with high Sr/Y ratios. Direct partial melting of basaltic underplate, in addition to that of dioritic lowermost crust, may provide a source for the observed middle to upper crustal magmas. These results may also bear on current magmatism observed in the oceanic trench offshore of southern Fiordland. Solander Island is composed of young (<2 Ma) andesitic volcanics bearing adakite signatures.