HR: 10:50h
AN: V41D-03    [PDF]
TI: Generation and Evolution of Lowermost Crust of an Arc: Examples From Fiordland, New Zealand
AU: * Rushmer, T
EM: Tracy.Rushmer@uvm.edu
AF: Department of Geology, University of Vermont, Burlington, VT 05405 United States
AB: Questions regarding the composition, structure and rheology of the lowermost crust continue to arise due to the acknowledgement that heterogeneity, both spatially and temporally, is the rule rather than the exception in most studied examples. To better understand the origin of lowermost crust in a magmatic arc setting, we are using Fiordland, New Zealand as a natural laboratory. Rock exposures in southwest NZ once formed the ancient Gondwana margin and now represent exhumed crustal levels of 50 km depth within the magmatic arc. In the Fiordland block, we find that the lowermost crust is heterogeneous, particularly in terms of heat and rheologic behaviour. Studies point to several elements which compose the lowermost arc crust and suggest that its heterogeneity resulted from the relative contributions of three components: (1) lower crust generated by the injection of mafic, mantle-derived partial melts and the addition of cooler mafic material through collision and arc building; (2) modification of lower crustal composition by tectonically burying supercrustal material; (3) compositional modification of the lowermost crust and arc crust by in-situ melting and remobilization of crustal-derived partial melts. Integration of field studies with targeted partial melting experiments has helped elucidate the relative contribution of this latter process and has established possible partial melt compositions derived from potential source material of magmas now observed in the arc. 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 also help evaluate whether or not there are different extraction mechanisms for partial melt that lead to different temperature and heat distribution in the lowermost crust. Experimental deformation studies suggest that the mafic lowermost crust in Fiordland remained rheologically strong during melting. LA-ICPMS results show glasses from both compositions to be HREE depleted, LREE enriched, 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 Sr/Y enriched magmas observed in middle to upper Fiordland crust.
DE: 1020 Composition of the crust
DE: 3630 Experimental mineralogy and petrology
DE: 8102 Continental contractional orogenic belts
DE: 8159 Rheology--crust and lithosphere
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting