ZIRCON HF AND O ISOTOPE ANALYSIS OF JURASSIC-EOCENE PLUTONS OF THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA, INDICATES MAGMATIC EVENTS DOMINATED BY MANTLE SOURCES

Magmatism in the southern Coast Mountains batholith (CMB), British Columbia, was active between 180 and 45 Ma and appears episodic, with high magmatic flux events occurring at 161-148 Ma, 114-102 Ma, 85-70 Ma and 61-48 Ma. Similar episodicity is observed in arcs worldwide, yet the mechanism(s) driving such punctuated magmatic behavior is debated. We use Hf and O isotopes in zircon to assess temporal changes in the relative contributions of mantle and crust to melt sources and to evaluate models of crust formation in arc settings. Zircon initial Hf values from the southern CMB range from +5 to +16 but a running mean calculated across all time periods does not vary appreciably from a value of +12. We interpret the overwhelmingly primitive nature of the Hf isotopes to reflect melting dominated by depleted mantle sources, though we cannot rule out contributions from juvenile crust, such as that of the Wrangellia and / or Stikine terranes. Zircon δ¹⁸O values vary between +4.2 and +8.3 ‰ (n=173), with only 15% of analyzed grains having values outside the accepted mantle range (5.3 ± 0.8 ‰ for SIMS data). The fact that southern CMB plutons record uniformly primitive Hf isotopic compositions and mantle-like O isotope compositions, which are both relatively invariant through time and space, suggests that batholithic flare-ups are driven by thermal events in the mantle. We note that those events must be occurring at a scale of ≤ 150 km, as temporal patterns in magmatism are observed to vary significantly along-strike at similar scales. Interestingly, unlike the isotope record, whole rock and zircon geochemical analyses reveal temporal changes in the trace element compositions of magmas, with those being produced at high-flux times having higher whole rock La/Yb and Sr/Y and yielding zircons that scatter to higher U/Th ratios. These trends suggest that high flux events are linked with periods of crustal thickening and deformation / metamorphism in the batholith. We suggest that high-flux magmatic events result from relatively small-scale (<150 km) thermal anomalies in the mantle and that higher heat flow in the arc crustal column during those times weakens the crust, which, in a contractional setting, promotes thickening and metamorphism.