Petrogenesis of the Late Archean Lac Simard Intrusive Complex, Pontiac Subprovince, Quebec.

Abstract

The Archean Lac Simard Intrusive Complex (LSIC) represents an example of medium-K to shoshonitic and high-K transitional magmatism occurring in the Pontiac sedimentary terrane of the Superior Province. The complex formed from diverse magma batches ranging from weakly quartz saturated diorites to syenites and quartz monzodiorites to quartz monzonites and biotite-clinopyroxene bearing hornblendite magmas. The units of the LSIC are mutually intrusive and typically either display gradational or mingled contacts. All external contacts of the LSIC are variably sheared and intrusive contacts. Granitoids of the LSIC are dominantly monzodioritic Produced through functional crystallization, primarily of amphibole, clinopyroxene, apatite, magnetite, titanite, and biotite, from a melt with shoshonitic affinities emplaced at shallow levels (∼1 kbar.). Relatively oxidising primary conditions are evidenced by the mineral assemblage quartz + magnetite + titanite and a calculated fO2 lies on this buffer at ∼10-13.7 at 800°C. Amphibole and clinopyroxene chemistry provide evidence for polybaric crystallization. Field relationships among units of the LSIC and mafic silicate chemistry suggest that the units of the complex were emplaced over a relatively short time interval from either a zoned magma chamber, or multiple cogenetic chambers at depth. Shoshonitic samples of LSIC granitoids posses high La/YbCN, LREE, LILE/HFSE, Mg#, Cr and Nd indicative of a primary mantle melt with an arc-type trace element signature. Neodymium and Sr isotope compositions of primary mineral separates suggest that the LSIC rocks are similar Nd-Sr- isotopically to Abitibi Subprovince mantle derived rocks. Hornblendites within the LSIC are comagmatic with the granitoids of the LSIC and form two compositional populations. One group of relatively uncontaminated hornblendites appears to have been generated by fractional crystallization whereas a second group of similar parentage records assimilation of up to approximately 40% of a Pontiac sedimentary rock component.