ZIRCON U-PB GEOCHRONOLOGY, TRACER ISOTOPE (LU-HF, δ18O), AND TRACE ELEMENT GEOCHEMISTRY OF THE HOGEM BATHOLITH, QUESNEL TERRANE, NORTH-CENTRAL BRITISH COLUMBIA

The Hogem batholith in Quesnel terrane, British Columbia was emplaced in the Triassic to Early Cretaceous. New zircon U-Pb geochronology indicates this prolonged magmatism (207 to 128 Ma) was punctuated and formed four geochemically distinct intrusive suites. Thane Creek suite (207 to 194 Ma) consists of metaluminous and calc-alkalic diorites with minor hornblendite. Duckling Creek suite (182 to 175 Ma) is comprised of metaluminous and alkalic syenites that intruded pyroxenite. Osilinka suite is not well constrained, but was likely emplaced post 160 Ma and consists of peraluminous and calc-alkalic granites. Mesilinka suite (135 to 128 Ma) is comprised of peraluminous and calcic to calc-alkalic equigranular and K-feldspar porphyritic granites that intruded metaluminous and calcic tonalite. Zircons were analyzed for Hf and O isotopes and trace elements to assess temporal changes in relative contributions of mantle and crustal sources and magma conditions. Zircon initial Hf isotope compositions range from +6.4 to +12.1ɛ in Thane Creek (mean=10.0ɛ, n=94), Duckling Creek (mean=9.1ɛ, n=74), and Osilinka (mean=9.2ɛ, n=24) suites. Mesilinka suite has a more evolved Hf range (+4.5 to +9.8ɛ). This range is due to more unradiogenic compositions in the porphyritic (mean=6.3ɛ, n=42) and equigranular (mean=7.5ɛ, n=48) granites, compared to tonalite (mean=8.8ɛ, n=9). Lower Hf values in Mesilinka granites correspond with higher δ¹⁸O values. Zircon δ¹⁸O results vary from +5.0 to +7.6 ‰ for Thane Creek, Duckling Creek, and Osilinka suites and Mesilinka tonalite, while Mesilinka granites range from +4.7 to +12.2 ‰. Ti in zircon temperature estimates range from ~600 to 900°C and do not correlate with age, which may reflect magma pulsations. Zircon U-Pb, Hf, O, and TE results indicate punctuated, prolonged magmatism and suggest a change in tectonic processes. Triassic to Middle Jurassic magmatism reflects a mostly depleted mantle source. After 160 Ma, zircons record increased hydrosphere interaction and less radiogenic Hf, indicating crustal contamination. However, Hf isotope results do not indicate North American basement was involved in Hogem magmatism. These changes in zircon chemistry may represent the transition from subduction within an island arc setting to post-accretionary crustal thickening in Quesnel terrane.