NEW ZIRCON U-PB-HF RESULTS FROM A TRANSECT ACROSS THE CRETACEOUS TO EOCENE NORTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA

Pluton geochronology and isotope geochemistry are important for understanding the tempo and petrogenesis of crustal growth in continental arc settings. We present new zircon U-Pb-Hf results from a transect in the northern Coast Mountains batholith (CMB), collected in order to evaluate models of batholith development and for along-strike comparison with well-studied transects to the south. We obtained U-Pb ages from 26 samples and Lu-Hf isotope data from 15 of those via laser-ablation ICP-MS techniques. The weighted mean ages of dated plutons range from 197 to 53 Ma, with apparent high-flux events occurring during Late Cretaceous (93 – 90 Ma) and Paleocene-Eocene time (59 – 53 Ma). We observe a pronounced magmatic gap at ca. 80 – 65 Ma. These U-Pb ages follow an along-strike trend in the CMB toward more voluminous Paleocene-Eocene magmatism in the northern part of the batholith. The Late Cretaceous plutons outcrop in the western (outboard) part of our transect, whereas the younger intrusions are found in the axial and eastern parts. Many of the Late Cretaceous zircons (30%) have elevated U/Th ratios (grains as high as 300), indicating widespread growth or recrystallization of metamorphic zircon at that time. Initial epsilon Hf values of individual zircons range from -4 to +11, with uniform and high values (+5 to +11) recorded in the western, Late Cretaceous plutons. Paleocene-Eocene plutons have highly variable Hf signatures, with those located in the axial parts of the transect pulling down to negative values (-5 to +5). Some of the most evolved plutons locally intrude older Cretaceous granitoids that have distinctly higher Hf isotopes. These isotopic patterns indicate a significant change in magma sources and the incorporation of older crustal components through time in the highly-deformed axial belt. Relative to the rest of the CMB to the south, zircon Hf isotope values are lower in our transect, and reveal increasing northward contributions of Precambrian (?) crustal components in the CMB.