HR: 0800h
AN: T11C-0394 INVITED     [Abstracts]
TI: Geochemical Characteristics of Intra-oceanic Plateaus and Recognition of Ancient Plateau Remnants on the Continents
AU: * Mahoney, J J
EM: jmahoney@hawaii.edu
AF: SOEST, University of Hawaii, Honolulu, HI 96822 United States
AB: Early geochemical work on Cretaceous oceanic plateaus suggested that tholeiitic plateau basalts might have a distinctive isotopic and trace element signature that could be diagnostic for identifying fragments of ancient plateau crust now incorporated in the continents [e.g. Mahoney, 1987]. Subsequent work has demonstrated that basalts of intra-oceanic plateaus formed far from continental lithosphere indeed have general similarities in Nd-Sr-Pb-Hf isotope ratios and alteration-resistant incompatible elements [e.g. Neal et al., 1997; Kerr et al., 1997; Fitton et al., 2004; Hoernle et al., 2004]. These characteristics differ from those of most N-MORB and island arc basalts. For example, most basalts of the ca. 120 Ma Ontong Java Plateau have relatively flat primitive-mantle-normalized element patterns from Nb to Lu and a small range of age-corrected isotope ratios (e.g., epsilon_Nd = +5.4 to +6.5) distinct from those of present-day and pre-120 Ma Pacific N-MORB. Similar signatures are found in several accreted mafic terranes interpreted to be Mesozoic plateau fragments [e.g. Lassiter et al., 1995; Kerr et al. 2002], and in Precambrian examples that may have a plateau origin [e.g. Abouchami et al., 1990; Puchtel et al., 1998; Polat et al., 1998]. However, many E- (or T-) MORB have broadly similar isotopic and incompatible-element characteristics, and recent sampling has revealed that such MORB are common, even dominant, along some segments of the ocean-ridge system far from hotspots. Data for the central Southeast Indian Ridge, for example, overlap with those for the Ontong Java Plateau. The E-MORB and plateau basalts differ when major-element indicators of partial melting are combined with isotope and trace element data. However, major-element compositions can be modified significantly by alteration and even relatively low-grade metamorphism. Disregarding possible long-term temporal changes in plateau and/or ocean-ridge composition, these effects make it difficult to prove a plateau origin on geochemical grounds alone, particularly for smaller and/or poorly preserved terranes. Other evidence (on thickness and extent of lava flows, total thickness of basalt lava section, etc.) is required in addition to geochemical data.
DE: 1021 Composition of the oceanic crust
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3038 Oceanic plateaus and microcontinents
SC: Tectonophysics [T]
MN: Fall Meeting 2005