CRUSTAL MELTING IN THE CORDILLERAN INTERIOR - THE MIDCRETACEOUS WHITECREEK BATHOLITH IN THE SOUTHERN CANADIAN CORDILLERA

The mid-Cretaceous White Creek batholith in southeast British Columbia is a zoned pluton ranging from quartz monzodiorite on the margin, to hornblende- and biotite-bearing granodiorite towards the interior of t he batholith, which are in turn crosscut by two-mica granite. This ran ge in rock type is similar to the range displayed by Mesozoic granitoi d suites found in the Cordilleran interior of western North America. T he lithological zones in the White Creek batholith correlate with dist inct jumps in major element, trace element, and isotopic compositions, and indicate that several pulses of magma were emplaced within the Wh ite Creek magma chamber. The hornblende- and biotite-bearing granitoid s are metaluminous to weakly peraluminous, have strong light rare eart h element (LREE) enrichment, and small negative Eu anomalies. These gr anitoids have initial epsilon(Sr), ranging from +32 to +84 (Sr-87/Sr-8 6T from 0.7069 to 0.7106), initial epsilon(Nd) ranging from -5 to -10, and initial Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 Pb rangin g from 18.3 to 18.7, 15.58 to 15.65, and 38.3 to 39.0, respectively. T he two-mica granites and associated aplites are strongly peraluminous, and show only moderate LREE enrichment and strong negative Eu anomali es. These granites have epsilon(Sr), ranging from +174 to +436 (Sr-87/ Sr-86, from 0.7169 to 0.7354), epsilon(Nd) ranging from -12 to -16, an d more radiogenic initial Pb isotope ratios than the hornblende- and b iotite-bearing granitoids. Oxygen, Sr, Pb, and Nd isotopes, REE modell ing, and phase equilibrium constraints are consistent with crustal ana texis of Precambrian basement gneisses and Proterozoic metapelites exp osed in southeast British Columbia, the product being the hornblende-b iotite granitoids and two-mica granites, respectively. The sequence of intrusion in the White Creek batholith constrains the melting sequenc e. A zone of anatexis proceeded upwards through the crust, first melti ng basement gneisses then melting overlying metapelites. A model for b asaltic magmatic underplating as a primary cause of anatexis of the cr ust during the mid-Cretaceous magmatic episode is difficult to reconci le with the absence of early Cretaceous basalt in the southern Canadia n Cordillera. A much more likely petrogenetic model is that crustal an atexis was probably a response to crustal thickening in association wi th terrane accretion and collision along the western margin of the Nor th American continent.