CRUSTAL MELTING IN THE CORDILLERAN INTERIOR - THE MIDCRETACEOUS WHITE CREEK 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 the batholith, which are in turn crosscut by two-mica granite. This range in rock type is similar to the range displayed by Mesozoic granitoid suites found in the Cordilleran interior of western North America. The lithological zones in the White Creek batholith correlate with distinct jumps in major element, trace element, and isotopic compositions, and indicate that several pulses of magma were emplaced within the White Creek magma chamber. The hornblende- and biotite-bearing granitoids are metaluminous to weakly peraluminous, have strong light rare earth element (LREE) enrichment, and small negative Eu anomalies. These granitoids have initial epsilon(Sr), ranging from +32 to +84 (Sr-87/Sr-86T 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 ranging from 18.3 to 18.7, 15.58 to 15.65, and 38.3 to 39.0, respectively. The two-mica granites and associated aplites are strongly peraluminous, and show only moderate LREE enrichment and strong negative Eu anomalies. 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, and more radiogenic initial Pb isotope ratios than the hornblende- and biotite-bearing granitoids. Oxygen, Sr, Pb, and Nd isotopes, REE modelling, and phase equilibrium constraints are consistent with crustal anatexis of Precambrian basement gneisses and Proterozoic metapelites exposed in southeast British Columbia, the product being the hornblende-biotite granitoids and two-mica granites, respectively. The sequence of intrusion in the White Creek batholith constrains the melting sequence. A zone of anatexis proceeded upwards through the crust, first melting basement gneisses then melting overlying metapelites. A model for basaltic magmatic underplating as a primary cause of anatexis of the crust during the mid-Cretaceous magmatic episode is difficult to reconcile with the absence of early Cretaceous basalt in the southern Canadian Cordillera. A much more likely petrogenetic model is that crustal anatexis was probably a response to crustal thickening in association with terrane accretion and collision along the western margin of the North American continent.