MESOZOIC GRANITOID MAGMATISM IN SOUTHEAST BRITISH-COLUMBIA - IMPLICATIONS FOR THE ORIGIN OF GRANITOID BELTS IN THE NORTH-AMERICAN CORDILLERA

The trace element and isotopic compositions of Mesozoic granitoids in southeast British Columbia constrain the timing and mechanisms for granitoid production in the western North American Cordillera. The granitoids were emplaced in Middle Jurassic and mid-Cretaceous pulses into Paleozoic-Mesozoic accreted terranes, Paleozoic metasediments, and Proterozoic metasediments of the Purcell-Windermere Group. Middle Jurassic compositions range from quartz diorite to tonalite to granite, with high Ba and low Rb and Nb abundances similar to those in granitoids found within Mesozoic arc complexes of western North America. Isotopic compositions range for (epsilon)Sr(T) from -27 to +33 and (epsilon)Nd(T) from 0 to -7 with initial Pb-206/Pb-204, Pb-208/Pb-204, and Pb-207/Pb-204 lying in an array between upper and lower crust reservoirs. These compositions can be produced by addition of 5-25% of Precambrian crust to basalt. The Mid-Jurassic granitoids formed as part of an arc magmatic complex during subduction of ocean crust and as accretion of outboard terranes occurred west of the North American continent. In contrast, mid-Cretaceous batholiths are composed of weakly peraluminous hornblende-biotite granitoids and strongly peraluminous two-mica granites. These granitoids have lower (epsilon)Nd(T), and epsilon(Sr(T), Pb-206/Pb-204, Pb-208/Pb-204, and Pb-207/Pb-204 ratios extending to more radiogenic compositions than the Middle Jurassic granitoids and have complete overlap with isotopic compositions of southeast British Columbia Precambrian crust. Their trace element and isotopic compositions are consistent with a model in which crustal anatexis of Precambrian basement gneisses produced the hornblende-biotite granitoids and anatexis of Proterozoic metapelites produced the two-mica granites as a melt zone migrated up through the crust. Collision and accretion of exotic terranes resulted in a maximum in crustal thickening in the Late Jurassic to Early Cretaceous (approximately 150-130 Ma) in the Canadian Cordillera, 20-40 m.y. prior to mid-Cretaceous (approximately 110 Ma) granitoid magmatism. The timing of these events are consistent with anatexis being a response to crustal thickening.