HR: 1330h
AN: V13B-03    [Abstracts]
TI: Nd and Hf Isotopic Compositions of Pan-African High-Pressure Mafic Granulites
AU: * Attoh, K
EM: ka17@cornell.edu
AF: Kodjopa Attoh, Dept of Earth and Atmos Sciences Cornell University, Ithaca, NY 14853-1504 United States
AU: Schmitz, M D
EM: markschmitz@boisestate.edu
AF: Mark D. Schmitz, Dept of Geosciences Boise State University, Boise, ID 83725 United States
AB: High-pressure (HP) granulites and eclogites which define the Pan-African Dahomeyide suture zone in West Africa have major and trace element compositions which indicate geochemical imprints of their magmatic protoliths. Samples representing suites characterized by island arc tholeiitic (IAT) and MORB affinities have been analyzed for Nd and Hf isotopic compositions. The MORB-like sample has relatively high radiogenic Nd isotopic composition (143Nd/144Nd= .513167; epsilon Nd= 8.17) and Hf isotopic composition of (176Hf/177Hf=.283496; epsilon Hf=16.14) that yielded TDM= 0.77 Ga. Two samples with IAT-affinities gave lower radiogenic Hf isotopic composition (176Hf/177Hf ~ .282877, .282985; epsilon Hf= 9.43 and10.79) and lower epsilon Nd (5.3 and 6.6), that plot in the OIB field of the mantle Hf-Nd array and yield older TDM~0.96 Ga. The data suggest that magmatic protoliths of the HP granulites may have erupted ~200-300 my prior to suturing involving collisional orogeny and thus provide, for the first time, some constraint on the timing of the pre-Gondwana rifting of the WAC. The data also provide the opportunity to test models of the decoupling of Hf-Nd isotopic fractionation by garnet; these models predict that garnet production during partial melting could result in anomalous fractionation of crustal rocks out of the mantle Hf-Nd array.This is apparently not the case for the samples analyzed although the HP granulites which recrystallized ca 0.60 Ga (at 800 C, P >15 kbars) with as high as 20 percent modal garnet, require only ~450 my to evolve out of the juvenile crustal array (if partial melting is > 20%). Thus a plausible explanation for the preservation of the mantle array in the Pan-African HP mafic granulites array is that the degree of partial melting was significantly less than 10% and that this partial melt was largely retained as pervasive veins.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly