HR: 0800h
AN: V21A-0584    [Abstracts]
TI: Orogenic belts, partial melting and strength of the continental lithosphere: Examples from Fiordland, New Zealand
AU: Price, R
EM: irmtbr@gmail.com
AF: Geologic Services Corporation, 6430 Variel Avenue, Ste 103, Woodland Hills, CA 91367 United States
AU: * Rushmer, T
EM: Tracy.Rushmer@uvm.edu
AF: Department of Geology, Delehanty Hall University of Vermont, Burlington, VT 05405 United States
AB: South Island, New Zealand is composed of two juxtaposed magmatic arcs 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 both high and low Sr/Y ratios 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 but producing only low volumes of partial melt and melt flow through fracture networks to be a possible melt extraction mechanism. Recent experimental studies have investigated several hornblende-bearing metabasaltic and metadioritic source rocks to determine the range of melt compositions that can be derived by partial melting and show that that granitoid compositions are produced from both an alkali basalt dike (SC5; hbl + plg + bt + czo) and amphibole-only dike (PM2, Fe-rich, Si-poor amphibole). SC5 partial melts encompass a broad spectrum of compositions, from granite at low temperatures to tonalite, at higher temperatures, and are controlled by changes in the mineral compositions and assemblages. Partial melts derived from PM2 are only tonalitic. Both bulk compositions produce HREE-depleted compositions but with variable Sr/Y contents. In parallel studies, deformation experiments on the hornblende-bearing lower crustal samples suggest that the mafic lowermost crust in Fiordland was rheologically strong and only weakened transiently during generation of low volume HREE-depleted partial melts. From the experimental deformation results combined with field studies, rheological weakening appears to be transient in this arc. The effects of partial melting on the crustal rheology and thermal structure are short-lived and did not lead to long-term weakening. Rapid extraction along fracture networks may have allowed the mafic lower crust to return to pre-melt generation strength relatively quickly. This relationship contrasts with observations made in significantly weakened orogens, such as the Canadian Cordillera, which are composed of migmatitic paragneiss at mid and lower crustal levels and where much higher melt volumes have been generated during growth. Bulk composition can play an important role in rheologic strength evolution in orogenic belts.
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005