GEOCHEMISTRY AND AGE OF TIMISKAMING ALKALI VOLCANICS AND THE OTTO SYENITE STOCK, ABITIBI, ONTARIO

Tephrites and trachytes of the Timiskaming volcanics from the Kirkland Lake area (Ontario) and syenites and a granite from the nearby Otto Stock are characterized by extreme enrichment of incompatible elements coupled with relative depletion of Nb, Ti, and to a lesser extent Zr and Y. The volcanic rocks have a whole-rock Sm-Nd isochron age of 2740 +/- 117 Ma (2-sigma error), and minerals separated from the Otto Stock, a Sm-Nd age of 2544 +/- 50 Ma. Conventional and ion probe U-Pb analyses of zircons from the Otto Stock yielded an upper intercept age of 2700 +/- 19 Ma, whereas the more concordant ion probe analyses had a mean 207-Pb/206-Pb age of 2671 +/- 8 Ma (2-sigma). The latter is interpreted as the age of emplacement of both the volcanics and the pluton, and the Sm-Nd mineral isochron age is thought to reflect a period of later disturbance, probably during regional metamorphism. A high initial epsilon(Nd) of 2.5 +/- 1.5 for Kirkland Lake volcanics indicates long-term isotopic depletion of their source. This value is the same as that for volcanic rocks throughout the Abitibi belt and indicates that any chemically enriched material in the source cannot have been much older than the volcanics themselves. An environment remote from older continents is inferred.