HR: 1340h
AN: V33E-1500    [Abstracts]
TI: Trace Element and Isotopic (Re-Os, O) Systematics of Roberts Victor Eclogites: Evidence for 3 Ga Subduction-Incorporation of Archean Oceanic Lithosphere into the South African Kaapvaal Craton Keel
AU: * Shirey, S B
EM: shirey@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd, NW, Washington, DC 20015 United States
AU: Schmitz, M D
EM: markschmitz@boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725-1535 United States
AU: Wiechert, U
EM: wiechert@erdw.ethz.ch
AF: Institute of Isotope Geology, ETH-Zentrum, Zurich, 8092 Switzerland
AB: Eclogite xenoliths from the 125 Ma old, Group II, Roberts Victor kimberlite have long been of interest in studies of Kaapvaal craton evolution because of their diversity, abundance, availability in large size, occurrence with peridotite and their sometimes high carbon/diamond content. From among the coesite, corundum, kyanite, Ca-, Mg-, and Fe- rich eclogites available, we have chosen to work on those that can be classified as Group I, Group II or diamondiferous with the goal to better understand their petrogenesis, the evolution of the Kaapvaal craton keel, the role of eclogites in diamond formation and the behavior of the Re-Os system. Group I vs Group II eclogites can be distinguished by texture (isolated gt in a cpx matrix vs subhedral, interlocking gt-cpx) and mineral chemistry (higher Na$_{2}$O in gt and K$_{2}$O in cpx for GI vs lower for GII). Such differences have been thought to result from higher vs lower pressures of equilibration. Recent laser fluorination oxygen isotopes ($\delta^{18}$O) on gt (GI = 5.8 to 6.9; GII = 2.1 to 5.1), ion-probe trace elements (e.g. Ce chondrite normalized) on gt (G1 = 0.2 to 0.5; GII = 0.002 to 0.07) and cpx (G1 = 7 to 20; GII = 0.2 to 2) and whole-rock Re-Os (G1 Re = 0.19 to 3.41 ppb; GII Re = 0.006 to 0.38 ppb) highlight even more distinct differences between Groups I and II. These differences are likely a pre-metamorphic signature of their original protoliths and not just due to pressure differences or magmagenesis during emplacement into the lithosphere. Using the stratigraphic variation of O isotopic composition and trace element content of ophiolites and drill core from DSDP/ODP holes 735B and 504B as a guide, Group I eclogites might represent the volcanic rocks of Layer 2 of Archean oceanic crust whereas Group II might represent the cumulate, intrusive rocks of Layer 3. This idea is supported by the presence, only in Group II eclogites, of positive Eu anomalies in reconstructed ion-probe whole rock rare earth element patterns. The Re-Os sytematics of the oceanic lithosphere is poorly known, especially in the Archean, but Roberts Victor eclogite Re-Os and trace element abundances and major element compositions suggest a basaltic komatiitic protolith as might typify slightly hotter ocean ridges in the Archean. A U-Pb age of 3.061$\pm$0.006 Ga on zircon grains separated from a Group I Roberts Victor eclogite and a same-age but scattered whole-rock Re-Os isotope array containing the diamondiferous and some Group I eclogites (including the zircon-bearing eclogite), firmly date the eclogite protoliths as Meso-Archean. The direct covariation of Re content (and hence $^{187}$Re/$^{188}$Os) with O isotopic composition allows the low-T alteration process occurring on the seafloor to be firmly dated at that time. For Kaapvaal craton evolution, this age is interesting because it predates by about 100 Ma the terrane collision that sutured the Kimberley block to the Eastern Kaapvaal along the Colesburg magnetic lineament, the stabilization of a thickened lithospheric mantle keel, and the generation of a widely distributed suite of eclogitic diamonds. Incorporation of these eclogites in the lithosphere is further evidence for Meso-Archean plate tectonics and the role of subduction in cratonization and diamond genesis.
DE: 8125 Evolution of the Earth
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1025 Composition of the mantle
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting