U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA

Understanding the evolution of the Coast Mountains batholith (CMB) in British Columbia provides insight into processes of batholith formation and terrane accretion along the cordilleran margin. This study uses U-Pb geochronology and zircon hafnium (Hf) isotope analyses to investigate the magmatic history and petrogenesis of Late Jurassic to Eocene plutons of the southern CMB.

Whereas U-Pb ages in northern and central areas of the CMB ages show a younging eastward trend, the southern CMB displays a more complicated pattern. Late Jurassic plutons commonly occur inboard (east) of Early Cretaceous plutons. We document a number of plutons with ages ranging from 140 – 120 Ma, an age population that is uncommon elsewhere along the Cordilleran margin.

Hafnium isotope signatures in plutons of all ages are juvenile, with ɛHf values ranging from +15 to +5. The highest ɛHf, which approach those in MORB, are found in plutons crystallizing at 150-120 and 80-60 Ma. The overall positive values suggest that plutons of the southern CMB were generated mainly from melting of Paleozoic arc-type terranes (e.g., Wrangellia, Alexander, and/or Stikine), with little or no melt contribution from older continental crust. ɛHf excursions may have resulted from basinal extension at ~150-120 Ma and a phase of extension between ~85 and ~60 Ma. However, at this time, we do not have structural evidence to verify an extensional regime in the southern CMB between 85 – 60 Ma.