U-PB GEOCHRONOLOGY AND ZIRCON HF AND O ISOTOPE ANALYSIS OF PLUTONS OF THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA: INSIGHT INTO EPISODIC MAGMATISM IN CONTINENTAL ARCS

Magmatism in many continental arcs is characterized by episodes of high magmatic flux (flare-ups), alternating with periods of relatively low magmatic activity (lulls). The processes that control the tempo of magmatism, however, remain unclear. In this study, new U-Pb ages, together with zircon Hf and O isotope analyses of plutons of the southern Coast Mountains batholith (CMB), British Columbia, are used to constrain changing magma sources through time and space and evaluate potential mechanisms controlling magmatic tempo in continental arcs.

Seventeen new U-Pb ages and Hf isotope signatures were compiled with previously determined zircon U-Pb-Hf data from 34 plutons to create a 150 km transect across the batholith. The compiled dataset reveals apparent flare-ups ca. 160-140 Ma, 120-90 Ma, and 80-70 Ma. Zircon U/Th ratios are generally <10, but scatter to higher values (up to 80) in plutons emplaced during the 160-140 Ma and 80-70 Ma events, recording metamorphic zircon growth at those times. Hf isotope signatures (εHf(t)) from all samples are juvenile, with pluton averages ranging from +7.7 to +14.4. εHf(t) scatters to lower values during the 160-140 Ma and 120-90 Ma apparent flare-ups. The 80-70 Ma apparent flare-up has not been documented elsewhere in the batholith and does not appear to coincide with a Hf isotope “pull-down”, suggesting it may have had a different triggering mechanism. Zircon O isotope analysis yielded nearly mantle-like signatures (δ¹⁸O sample averages range from +4.4 to +6.5‰). Results from Hf-O binary mixture modeling of depleted mantle and potentially underthrusted crustal sources (e.g. Wrangellia and Alexander terranes) indicate limited involvement (<<20%) of crustal material. The model results, together with the homogeneity and mantle-like nature of δ¹⁸O and the overall primitive εHf signatures, suggests that intermediate plutons of the southern CMB were likely generated from a mantle source, with little input from supra-crustal sources. Variability in the εHf(t) data are attributed to: 1) limited incorporation of young (Paleozoic) crustal material, and/or 2) an isotopically enriched and heterogeneous mantle source. These findings suggest that input of crustal material via cyclic retroarc underthrusting is not a likely driving mechanism for high flux events in the southern CMB.