Eocene Volcanic Complex from Central British Columbia: The Role of Fractional Crystallization during the Magmatic Evolution

Abstract

Abstract The Challis-Kamloops belt of south-central British Columbia is a regionally extensive (>65,000 km2) magmatic province that erupted within the North American Cordillera during the Eocene (55-45 Ma). The inland volcanic belt runs parallel to the coast, and the rocks were emplaced mainly within extensional basins indicating volcanism was attributed to rift-related decompressional melting. The rocks include both calc-alkaline and tholeiitic mafic and intermediate types (i.e., low-Fe, medium-Fe, and high-Fe suites). Voluminous volcanic units (Buck Creek, Goosly Lake, Swans Lake) of the Buck Creek volcanic complex (~3,000 km2 in area) within the Nechako plateau erupted within 1-2 million years and show significant internal chemical variability. All rock types have similar Sr-Nd isotopic (87Sr/86Sri=0.70435-0.70487; εNdt=+2.6-+4.0) ratios indicating they originated from the same sub-Cordilleran mantle source. Petrological modeling using the most primitive rocks of the Buck Creek, Goosly Lake, and Swans Lake magmatic pulses demonstrates that the chemical variability observed in each system can be explained by hydrous fractional crystallization in the upper crust (≤0.1 GPa) under moderately oxidizing to oxidizing conditions (ΔFMQ 0 to +0.7). The primary difference between the low-Fe to medium-Fe (calc-alkaline) Buck Creek suite model and the high-Fe to medium-Fe (tholeiitic) Swans Lake suite model is water content as the Swans Lake model has lower (H2O=0.75 wt.%) starting water than the Buck Creek and also the Goosly Lake models (H2O=1.25-2.00 wt.%). Moreover, the intermediate to silicic rocks of the complexes are compositionally similar to rocks associated with “slab failure” suggesting that rifting and mantle melting were related to asthenospheric upwelling through a slab tear. The implications are that the chemical variability of the rock suites are primarily related to fractional crystallization and that the mantle source is heterogeneous with respect to water content which is likely due to heterogeneities in the processes related to pre-Eocene subduction.