GEOCHEMICAL CHARACTERIZATION OF MIDCRETACEOUS GRANITOIDS OF THE KOOTENAY ARC IN THE SOUTHERN CANADIAN CORDILLERA

Within southeast British Columbia, mid-Cretaceous granitoid batholiths are exposed in the Omineca Belt, a north-south-trending metamorphic a nd plutonic orogenic belt' The Bugaboo, Horsediief Creek, and Fry Cree k batholiths are post-kinematic with respect to regional metamorphism that affected their host rocks, and are composed of hornblende and bio tite granodiorites and granites in all three batholiths, and two-mica granites in Fry Creek. The biotite granites are weakly peraluminous, h ave initial epsilon(Sr) ranging from +36 to +56 and initial epsilon(Nd ) ranging from -4.8 to -7.5, and overlap the range of Nd-Sr isotopic c ompositions for Precambrian basement gneisses and Proterozoic metasedi ments found in southeast British Columbia. The initial Pb-206/Pb-204 v ersus Pb-208/Pb-204 and Pb-207/Pb-204 for these granitoids lie in band s between model lower and upper crustal Pb isotopic compositions above the Northern Hemisphere Reference Line for oceanic basalts. These gra nitoids have high Nb and Rb and low Ba abundances relative to granites found in volcanic arcs. The two-mica granites are strongly peralumino us and have epsilon(Sr) ranging from +170 to +470, epsilon(Nd) ranging from -10 to -21, and more radiogenic initial Pb-isotope ratios than t he biotite granites. The two-mica granites have trace element composit ions similar to those of granites found in within-plate and collisiona l tectonic settings. We favor a model for crustal anatexis of Precambr ian basement gneisses and Proterozoic metapelites to produce the mid-C retaceous biotite granites and two-mica granites, respectively. Crusta l anatexis was likely in response to crustal thickening that occurred during the Mesozoic in southeast British Columbia as exotic terranes c ollided with and were accreted to the western edge of the North Americ an continent.