Evaluating the Metamorphic History of the Coast Mountains Batholith in the Stikine River Area: Alaska and British Columbia

The Coast Mountains batholith stretches more than 1,700 km along the western edge of North America and preserves a record of terrane amalgamation and magmatic arc processes. I present garnet Sm-Nd ages and pressure, temperature, time (P-T-t) paths for rocks along a transect crossing the Coast Mountains batholith near the Stikine River and Wrangell, AK. These sample locations straddle the Coast Shear Zone (CSZ): a fundamental, arc-parallel crustal break west of the batholith. The three new garnet Sm-Nd ages for metamorphism are 86.5 ± 3.6 (n=5) and 95.3 ± 4.8 (n=6) Ma in the Western Metamorphic Belt (WMB), and 57.0 ± 0.7 (n=6) Ma in the Eastern Metamorphic Belt (EMB). These ages, coupled with associated P-T-t paths provide new constraints on the plutonic and metamorphic history across the Coast Mountains. The WMB, marked by kyanite and staurolite zone mineral assemblages, was metamorphosed at 7.5 ± 1 kbar and 600-650°C between 86 and 95 Ma. Metamorphic conditions in the EMB are constrained by sillimanite replacing andalusite, and the P-T-t path indicates garnet growth at pressures from 4 to 6 kbar and temperatures from 550 to 650°C at ~57 Ma. These results indicate that garnet growth and cooling below garnet growth temperatures was ~30 million years earlier in the WMB than in the EMB. The metamorphic pressure estimates require > 10 km more exhumation in the WMB, which must have occurred significantly earlier in this area than further east. The new metamorphic data can be explained by exhumation due to focused uplift west of the CSZ and/or west-side-up tilting of the batholith ~80 Ma. This was followed by lesser amounts of exhumation in the central and eastern parts of the batholith.