METAMORPHISM DURING HIGH-FLUX EVENTS IN THE SOUTHERN COAST MOUNTAINS BATHOLITH

A large and growing number of U-Pb zircon ages linked to geologic maps indicate that most of the Coast Mountains batholith (CMB) was emplaced during high-flux events (HFE) concurrent with metamorphism. The central segment of the CMB has pluton compositions requiring a crustal contribution to HFE magmas and records widespread partial melting of crustal material during exhumation from c. 10 to 4 kbar at 650-850°C. We evaluate the role of partial melting in magma production in the southern CMB by comparing metamorphic P-T-t paths to the timing of HFE between latitudes 50 and 52°N, British Columbia.

Garnet Sm-Nd ages, thermobarometry, and pressure-temperature paths indicate that the timing and peak conditions of metamorphism varied between screens and pendants of metamorphic rocks. In four locations garnet growth occurred at 99-90, 80, 72, 72-65, and 64 Ma at peak conditions of c. 3 kbar-575°C, c. 8 kbar-620°C, c. 4 kbar-650°C, c. 6 kbar-640°C, and c. 6 kbar-675°C. The simplest interpretation for all of the P-T-t paths is a single contractional event from 99 to 64 Ma. HFE is documented at 161-148 Ma, 114-102 Ma, 85-70 Ma, and 61-48 Ma. Only the third of these HFE overlaps with metamorphism, suggesting crustal melting is not significantly affecting the tempo of arc magmatism here. Additionally, the lack of widespread partial melting precludes any correlation between metamorphism and HFE for any time interval at the currently exposed crustal level in the southern CMB. This result differs from observations of synchronous partial melting and HFE in the central CMB.

Taken together, the data indicate shallower crustal exposures, less melting, and a lack of temporal correlation between HFE and metamorphism compared to the northern CMB. Thus the processes controlling HFE vary along the length of the batholith and melting of sediments does not drive the magmatic tempo.