ND-ISOTOPE AND SR-ISOTOPE SYSTEMATICS FOR THE KAMISKOTIA MONTCALM AREA - IMPLICATIONS FOR THE FORMATION OF LATE ARCHEAN CRUST IN THE WESTERN ABITIBI SUBPROVINCE, CANADA

Geochemistry and Nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic, calc-alkalic, and lamprophyric suites in the Kamiskotia-Montcalm area. The Kamiskotia gabbroic complex (KGC) and cogenetic, bimodal volcanic rocks have epsilon-Nd(t) = +2.2 to +2.6, consistent with a direct derivation from a long-term, light rare-earth element (LREE)-depleted mantle. The Montcalm gabbroic complex has decreasing epsilon-Nd(t) upsection from +2.8 to 1.0, consistent with contamination by long-term, LREE-enriched (with respect to the long-term, LREE-depleted Abitibi mantle) crust during fractionation. Two calc-alkalic lamprophyre samples, characterized by large-ion lithophile element (LILE) and LREE enrichment and high MgO, Ni, and Cr contents, have epsilon-Nd(t) of +2.5 and +2.8, indicating a derivation from a depleted mantle source that had undergone recent trace-element enrichment. A different lamprophyre suite is extremely LILE and LREE enriched and has an epsilon-Nd(t) of +1.0, indicating a derivation from a slightly different source that had earlier LREE enrichment. Granitoid rocks internal and external to greenstone belt rocks have epsilon-Nd(t) = +2.5 to +3.8 and +0.6 to -0.4, respectively. The lower values provided additional evidence for the existence of LREE-enriched crust in this area. Considering these data along with other radiogenic isotope studies, a petrogenetic and tectonic model is suggested for the crustal development of the southern Abitibi Subprovince. From > 2740 to 2698 Ma-the major period of volcanic activity-komatiitic and tholeiitic suites and one lamprophyre suite were derived from a uniformly LREE-depleted mantle reservoir with epsilon-Nd(t) = +2 to +3. Calc-alkalic granitoids were emplaced generally after 2700 Ma. Their long-term, LREE-depleted and LREE-enriched Nd isotopic signatures are similar to signatures in continental-arc settings (e.g., the Coastal Batholith of Peru). Form 2690 to 2670 Ma, when transpressional tectonism prevailed, mantle-derived magmatism was represented by long-term, LREE-enriched (epsilon-Nd(t) = +1 to +2) lamprophyric and alkalic volcanic suites. The Kamiskotia suite has a seven-point, whole-rock - mineral isochron Sm-Nd age of 2710 +/- 30 Ma, identical to U-Pb zircon ages for the suite, indicating closed-system behavior. An Rb-Sr mineral - whole-rock isochron age from one KGC sample is 2450 +/- 30 Ma, identical to U-Pb ages for the Hearst-Matachewan dike swarm, a prominent feature in the KGC area. Regression of whole-rock and mineral-isotope data for one granitoid sample with a U-Pb zircon age of 2696 +/- 1.5 Ma gives identical ages of 2530 +/- 30 Ma in the Sm-Nd and Rb-Sr systems. The latter data add to an increasing body of evidence for cryptic, late thermal events after granitoid-greenstone belt development in the southern Abitibi Subprovince.