SEISMIC REFRACTION EVIDENCE FOR A LARGE VOLUME OF MELT RESIDUAL BENEATH THE COAST MOUNTAINS BATHOLITH COMPLEX NEAR BELLA COOLA, BRITISH COLUMBIA

In 2009, the interdisciplinary Batholiths project acquired a seismic refraction and wide-angle reflection survey across the Coast Plutonic Complex and the Stikine terrane of British Columbia at the latitude of Bella Coola. Seismic travel times were inverted to produce 2-D compressional-wave and shear-wave velocity models of the crust and upper-most mantle. Seismic velocities indicative of a felsic upper crust and a felsic to intermediate middle crust are observed beneath both the arc batholith complex and the accreted Stikine island-arc terrane. The P and S wave data observe a ~10-km thick high-velocity layer (Vp ≥ 7.0 km/s, Vs ≥ 3.8 km/s, Vp/Vs ≥ 1.81) in the lower crust beneath the late Cretaceous to Eocene arc complex east of the Coast Shear Zone. In contrast, much lower velocity indicative of felsic-intermediate rocks extends to the Moho under the Jurassic to Cretaceous arc west of the Coast Shear zone. The lower crust under the Stikine terrane has modestly high velocities consistent with amphibolite or other hydrated mafic rocks. The Moho under the mountains of the eastern arc is ~3 km deeper than that under Stikinia. The high velocity zone in the lower crust under the younger, eastern portion of the Coast Plutonic Complex is interpreted to have a bulk composition of mafic garnet granulite. This garnet granulite and a large volume of granodiorite-dominated melt were created by arc dehydration melting of an amphibolitic or hydrated gabbroic protolith in the lower crust. Unlike most arcs, a substantial portion of the garnet granulite melt residual was not delaminated during subduction or arc collapse, but remains within the crust.