HR: 16:00h
AN: V22H-01    [PDF]
TI: The Thetford Mines Ophiolite Complex: Focus on the Petrology, Mineralogy and Geochemistry (REE, PGE) of a Supra-Subduction Mantle Section
AU: * Page, P
EM: ppage@nrcan.gc.ca
AF: INRS Eau-Terre-Environnement, 880 chemin Ste-Foy bureau 840, C.P.7500, Ste-Foy, Qc G1V 4C7 Canada
AU: Bedard, J H
EM: jbedard@nrcan.gc.ca
AF: Geological Survey of Canada, Centre Geoscientifique de Quebec, 880 chemin Ste-Foy bureau 840, C.P.7500, Ste-Foy, Qc G1V 4C7 Canada
AU: Tremblay, A
EM: tremblay.a@uqam.ca
AF: Departement des Sciences de la Terre et de l'Atmosphere, 201 President-Kennedy, local PK-6151, Montreal, Qc H3C 3P8 Canada
AU: Schroetter, J
EM: jschroet@nrcan.gc.ca
AF: INRS Eau-Terre-Environnement, 880 chemin Ste-Foy bureau 840, C.P.7500, Ste-Foy, Qc G1V 4C7 Canada
AB: The Ordovician Thetford Mines Ophiolitic Complex of southern Quebec preserves a complete ophiolite sequence, with a 5 km thick mantle section, a 1.5 - 2.5 km thick ultramafic and mafic cumulate crust, subdivided into Dunitic, Pyroxenitic and Gabbroic Zones; a partly-eroded sheeted dyke complex, subvolcanic breccias, tholeiitic (subordinate) and boninitic (dominant) lavas and associated dacitic tuffs, followed by a fore-arc flysch deposit. Mantle rocks are mostly harzburgite tectonites with diffuse compositional layering. Most rocks exhibit porphyroclastic to weakly deformed, nearly equigranular coarse textures. Dunite forms irregular masses and dykes ($<$ 0.1 - 5 m). The larger dunitic conduits may contain massive chromitite pods and nodules at their centers, while smaller dunites may contain chromite stringers. Occasional pyroxenite dykes are also encountered. Chromian spinels in harzburgites show a wide range in Cr2O3 and Al2O3 content, with Cr\# varying from 0.28 to 0.71. Dunite spinels have Cr\# from 0.41 to 0.86. Olivine is magnesian (Fo $=$ 90.4 to 92.0 in harzburgite, and from 90.5 to 94.9 in dunite), NiO-rich (0.38 - 0.49 wt%) and MnO-poor ($<$ 0.15 wt%). Orthopyroxene is magnesian (En $=$ 90.3 - 93.0), Ca-poor (Wo from 0.86 to 2.41, av.: 1.51) and has low to moderate Al2O3 (1.10 - 3.18 wt%). Diopsidic clinopyroxene represents 1-7 modal %, has En49.1 - 51.3, Wo47.0 - 50.2, and is slightly poorer in Al2O3 than opx, with 1.08 - 2.61 wt%. All phases have low TiO2 (spinel: 0-0.054 wt%, opx: 0.001-0.025 wt% and cpx: 0.007-0.046 wt%). The whole rock chemistry of the TMOC mantle peridotites have low to moderate CaO and Al2O3 contents (0.01 - 1.52 and 0.07 - 1.38 wt% respectively). They have U-shaped REE chondrite-normalized patterns, and are depleted in most incompatible elements, e.g. HREE from 0.05 to 0.5 x chondrite. Primitive mantle-normalized PGE patterns for harzburgite show negative to slightly positive slopes with Pd$/$Ir ranging from 0.21 to 3.1 and Pt$/$Ir ranging from 0.20 to 8.60. Dunites show the same tendencies (negative to slightly positive slopes) but with lower concentrations of Pd and Pt, with Pd$/$Ir ranging from 0.01 to 0.60 and Pt$/$Ir from 0.007 to 2.26. The refractory mineralogy, low whole-rock CaO, Al2O3 and HREE imply that the mantle peridotites are slightly to highly melt-depleted residues. However, the TMOC peridotites show enrichment in LREE and some of them show preferential enrichment of Pt-Pd over Ir-Os. This enrichment, which cannot be explained by partial melting, could be attributed to metasomatism of previously-depleted peridotites in the mantle wedge by a subducted slab-derived fluid or melt.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting