HR: 08:15h
AN: V31F-02    [Abstracts]
TI: Roberts Victor Eclogites: The MacGregor Legacy of Archean Oceanic Lithosphere Subduction and its Role in Craton Formation
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
AU: Carlson, R W
EM: carlson@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd, NW, Washington, DC 20015 United States
AB: Eclogite xenoliths from the 125 Ma old, Group II, Roberts Victor kimberlite have long been of interest (MacGregor et al, 1968) because of their diversity, abundance, large size, occurrence with peridotite and their high carbon/diamond content. Coesite, corundum, kyanite, Ca-, Mg-, and Fe- rich eclogites are available but those classified as Group I, Group II (as defined by MacGregor, 1970) or diamondiferous were selected with the goal to better understand eclogite petrogenesis, Kaapvaal cratonic keel evolution, diamond formation, and eclogite metasomatism. Recent laser fluorination oxygen isotope data (δ¹⁸O) on gt (GI = 5.8 to 6.9; GII = 2.1 to 5.1) match earlier data (Garlick et al, 1971; MacGregor and Manton, 1986), while ion-probe trace element contents of gt (e.g. chondrite normalized Ce 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 must be a pre-metamorphic signature of their original protoliths and not just due to pressure differences or partial melting during emplacement. Using ophiolites and composites of oceanic crust as a guide (e.g. MacGregor and Manton, 1986), Group I eclogites could represent the volcanic rocks of Layer 2 of Archean oceanic crust whereas Group II might represent the cumulate, intrusive rocks of Layer 3. Group II eclogites have positive Eu anomalies and lower REE and Re contents which support this idea. The Re-Os systematics 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±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 ¹⁸⁷Re/¹⁸⁸Os) with oxygen isotopic composition allows the low-T alteration process occurring on the seafloor to be 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 (e.g. MacGregor and Manton, 1986) in cratonization and diamond genesis. MacGregor,ID, Carter,JL, Davis, BTC (1968) GSA Spec. Paper 115, 136-137. MacGregor, ID and Carter, JL (1970) PEPI 3, 391-197. Garlick, GD, MacGregor, ID, Vogel, DE (1971) Science 172, 1025-1027. MacGregor, ID and Manton, WI (1986) JGR 91, 14,063-14,079.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 8103 Continental cratons
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005