Geochemical constraints on the petrogenesis of the Kamiskotia gabbroic complex and related basalts, Western Abitibi Subprovince, Ontario, Canada

Abstract

The Kamiskotia gabbroic complex (KGC) is a large (170 km 2), late Archean synvolcanic intrusion located in the western Abitibi Subprovince, Canada. It is divided into four stratigraphic units: a Lower Zone (LZ), comprised of adcumulus and mesocumulus peridotites, troctolites and gabbro-norites (Mg number = 73–86); a Middle Zone (MZ) comprised of mesocumulus gabbro-norites (Mg numbemr = 60–74); mesocumulus and orthocumulus gabbro-norites and ferroan gabbro-norites of the Upper Zone (UZ, Mg number=37–64) which are overtain and partly along strike with granophyric rocks of intermediate to felsic composition. The cumulates are characterized by flat REE patterns with La N/Yb N = 0.4–2.6, and range from 0.6–2× chondrites for olivine-bearing LZ adcumulates to 10–25× chondrites for upper UZ meso-to orthocumulates. Europium anomalies are strongly positive for the LZ and and diminish upsection to the upper UZ which has slight negative Eu anomalies. Chilled rocks are found in mixed magma and contact chill outcrops, and have a Mg number of 54–58, La N/Yb N = 0.8–1.2 and slight negative Eu anomalies. Phase relationships and mass balance calculations using the REE indicate that LZ cumulates may be derived from the average chill composition. Similarly upper UZ cumulates may be derived from evolved basalts which overlie the KGC. Assimilation-fractional crystallization modeling indicates that the evolved basalts may be derived from the average chill composition by 70–80% fractionation at low pressures; or lower degrees of fractionation accompanied by recharge of the averagr chill composition into the system. The basalt compositions do not reflect significant assimilation of nearby felsic rocks or average Archean crust during fractionation. Primitive KGC liquids are estimated by adding an average olivine-bearing adcumulate composition to the KGC chill composition in equal proportions to obtain a liquid with an Mg number of 70. The REE content of such a liquid is consistent with a derivation by 34% partial melting of a chemically primitive mantle. Neodymium isotope ratios for KGC cumulates, mineral separates and overlying volcanic rocks plot on a 2710 Ma isochron with ϵ Nd( t) = 2.5 ± 0.8, in close agreement with U-Pb zircon ages for the KGC and Kamiskotia felsic volcanic rocks at 2707 ± 2 Ma and 2702 ± 2 Ma, respectively. The initial value is identical to the isotopically depleted, MORB-like Abitibi mantle signature at 2.7 Ga, and provides supporting evidence for fractionation-dominated supracrustal processes, with no detectable assimilation of an isotopically enriched crustal component. Kamiskotia mafic magmatism is similar to more recent, rift-related systems represented by the Skaergaard Intrusion, East Greenland, and differentiated tholeiitic volcanic fields in Eastern Iceland and along the Galapagos Spreading Center.