HR: 0800h
AN: V51C-0723    [Abstracts]
TI: Contrasting Sources Of Granites In The Fosdick Migmatite Dome, West Antarctica
AU: * Saito, S (
EM: ssaito@geol.umd.edu
AF: Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Brown, M
EM: mbrown@geol.umd.edu
AF: Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Korhonen, F
EM: korhonen@geol.umd.edu
AF: Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, United States
AU: Siddoway, C S
EM: csiddoway@ColoradoCollege.edu
AF: Department of Geology, Colorado College, Colorado Springs, CO 80903, United States
AB: The Fosdick Mountains in Marie Byrd Land, West Antarctica comprise a migmatite dome 80x15km composed of paragneiss, orthogneiss and associated granites. The dome represents exposed middle-crust that experienced a polymetamorphic evolution associated with the active margin of Gondwana during the Devonian-Carboniferous and the Cretaceous. Research to date has focused on the metamorphic and structural evolution of the dome, with little attention to the origin and petrogenesis of the granites and migmatite leucosomes. Here, we report new geochemical and Sr-Nd isotope data for granites and migmatite leucosomes from the dome that bear on the source and petrogenesis of the parent melts (see also Saito et al., 2007, isaes.confex.com, 2.PS-38). We have obtained whole-rock major and trace element, and Sr-Nd isotope data for a suite of samples comprising paragneiss and orthogneiss, leucosome from paragneiss, and various types of granite from within the migmatite dome, and for possible protolith lithologies from outside the dome. Possible protoliths are Devonian biotite- and hornblende- bearing Ford Granodiorite and pre-Upper Ordovician Swanson Formation metapelites. Geochemical and isotopic compositions of the paragneiss and orthogneiss are comparable to those of the Swanson Formation metapelites and the Ford Granodiorite, respectively, consistent with previous interpretations regarding the protoliths of the gneisses (Siddoway et al., 2004, GSA SP380). Granites and leucosomes within the dome are silicic (71-78 wt % SiO₂) and slightly peraluminous (1.02-1.18 alumina saturation index), and are granite sensu stricto in terms of normative mineralogy. The major element composition of granites and leucosomes is broadly similar to those of high-pressure experimental melts produced from granodiorite sources (Skjerlie and Johnston, 1993, J. Pet.34; Patin~{o}-Douce, 1997, Geol.25) and sedimentary sources (Montel and Vielzeuf, 1997, CMP128; Koester et al., 2002, J. Pet.43). Our data suggest that the granites and leucosomes likely formed by partial melting of the Ford Granodiorite or a sedimentary protolith represented by the Swanson Formation. In terms of trace element compositions, the granites in the dome may be grouped into high-Sr and low-Sr type. Our initial Sr-Nd isotope data suggest that sources of the high-Sr and low-Sr granites are the Ford Granodiorite and the Swanson Formation, respectively. The difference in Sr contents of two granite types may be attributed to different hydrate-breakdown melting reactions in the contrasting sources. Mica-breakdown melting will not contribute a significant amount of Sr to the melt, so we infer that the low-Sr granite likely formed by a mica-breakdown melting reaction in the metasedimentary source. In contrast, hornblende-breakdown melting or hornblende-biotite-breakdown melting of a granodiorite source may contribute Sr to the melt, and we infer the high-Sr granite likely formed by this process. Continuing research will investigate whether these attributes may be used for correlation with contemporaneous magmatism recorded in contiguous parts of the East Gondwana margin, particularly New Zealand (e.g. Tulloch and Kimbrough, 2003, GSA SP374) or Thurston Island (e.g. Bradshaw et al., 1997, Ter. Ant. Pub.).
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting