Repetitive cyclical volcanism in the Late Archean Larder Lake Group near Kirkland Lake, Ontario: implications of geochemistry on magma genesis

Abstract

The Larder Lake Group is the basal stratigraphic unit of the Upper Supergroup, the younger magmatic cycle of the Abitibi Greenstone Belt of the Superior Province. Three major suites of extrusive and hypabyssal rocks are recognized: (a) a high-magnesian suite of komatiite to Mg-basalt; (b) a tholeiitic series with Mg-rich and Fe-rich basaltic varieties, and (c) a low-magnesian series of basalt to rhyodacite. Fourteen such subcycles were recognized in the Boston Iron Range south of Kirkland Lake, and are grouped in four major stratigraphic intervals. The entire Larder Lake Group magma series evolved from a peridotitic mantle source through various stages of partial melting and fractional crystallization. LREE-enriched tholeiites are interpreted as the initial products of low degrees of melting of a chondritic mantle. Both magnesian suites show LREE-depleted patterns and identical Zr/Y and Zr/Ti ratios, suggesting that their parental magmas were generated by partial melting of the residual mantle from which the tholeiitic melts had been previously derived. The low-magnesian suite, which is genetically related to a komatiitic magma series, includes compositions as silicic as rhyodacite.