STABLE ISOTOPE EVIDENCE FOR A COMPLEX FLUID EVOLUTION OF THE NORTHWESTERN BRITISH COLUMBIA COAST RANGES RELATED TO TERRANE ACCRETION

Stable isotope geochemistry reveals a complex fluid evolution for the Western Metamorphic Belt (WMB), Coast Mountain Batholith (CMB), Central Gneiss Complex (CGC) and Coast Shear Zone (CSZ). These fluids are a product of a complex tectonic history related to terrane accretion that includes oblique convergence, metamorphism, magmatism, and orogenic collapse. From W-to-E, these fluid systems are as follows. High-pressure greenschist-to-amphibolite facies metasedimentary rocks of the WMB record variable mineral δD (–61 to –111‰) and δ¹⁸O (e.g., quartz +9.6 to +13.4‰) values with multiple minerals in apparent isotopic equilibrium (T ~ 450-550°C) suggest a low W/R system dominated by metamorphic fluids. Low δ¹³C values (< ­–10‰) and δ¹⁸O values ~ 12‰ recorded in WMB calcite from metagraywacke suggest a protolith rich in organics. Variable and non-equilibrium δD (–62 to –143‰) and δ¹⁸O (e.g., biotite +2.3 to +6.2‰) values from diorites of the Quottoon pluton affected by the ductile CSZ suggest a complex evolution that involved both magmatic and meteoric-hydrothermal fluids in this dextral shear zone; these results differ from those 300 km along strike to the north that documented only metamorphic fluids in the CSZ (Goldfarb et al., 1988). Our data and those of Magaritz and Taylor (1976) from granulite facies metasediments of the CGC and plutons of the western CMB reveal homogeneous δD values (–62 to –78‰) and a restricted range of δ¹⁸O values (e.g., quartz +8.5 to +11.5‰) with all minerals in equilibrium at T > 570°C indicate a system dominated by magmatic fluids. Calculated whole-rock δ¹⁸O values (~ +7‰) for the Quottoon pluton and CMB intrusive rocks suggest a mantle origin for these magmas. Reinterpretation of very low δD (< –150‰) and quartz-feldspar δ¹⁸O pairs that display extreme disequilibrium (feldspar δ¹⁸O values as low as –5‰) from the Ponder pluton, eastern CMB, and Hazelton Group point reveals that the major meteoric-hydrothermal system that affected these rocks was related to Eocene detachment faulting along the Shames Lake fault system, a regional-scale extensional structure. A steady decrease in mineral δD values from a 350 km-long traverse across the Coast Ranges orogeny record a progressive lowering of meteoric water δD values due to the rain out effect.