PLUTONIC AND STRUCTURAL FRAMEWORK, SOUTHERN COAST MOUNTAINS BATHOLITH, BUTE INLET TO MT WADDINGTON: PRELIMINARY RESULTS

New field and geochronologic studies across the southern Coast Mountains batholith build on pioneering regional mapping by Roddick, Tipper, Woodsworth, and Monger. This previous work established the foundation for our investigation into the influence of crustal deformation on episodic magmatism in large continental arcs. Plutonic rocks dominate the batholith in our study area from Bute Inlet to Mt Waddington, and most are similar to two previously dated Late Jurassic plutons on Bute Inlet and elsewhere in the CMB. A few small bodies may be Late Cretaceous, but the widespread post-100 Ma magmatism found in much of the batholith appears to be absent on Bute Inlet. The Late Jurassic (?) plutons intrude an amphibolite-facies schist with quartzofeldspathic sedimentary and mafic volcanic(?) protoliths. The NE-edge of the schist body is composed of markedly lower grade and less deformed mafic metavolcanic rocks with relic clastic and pillowed textures and a distinctive rhyolitic body. These lower grade rocks resemble the Hauterivian Ottarasko volcanics found in the eastern Waddington thrust belt NE of Bute Inlet. Four large ductile shear zones are present on Bute Inlet. Two of these, in the SW part of the inlet, were previously unrecognized and show sinistral-oblique sense of shear in the field. Regionally, these shear zones may join shear zones mapped by Monger near Vancouver. In the northern part of the inlet, two previously recognized NW-striking shear zones are steeply dipping reverse faults and may belong to the Late Cretaceous eastern Waddington thrust belt. Minor NE-striking high-angle faults disrupt the structures and plutons.

Regional mapping, geochronologic and paleomagnetic studies suggest that plutonic rocks in Bute Inlet are part of the eastern of two Late Jurassic-Early Cretaceous arcs present elsewhere in the Coast Mountains batholith. The anomalously western position of this eastern arc may be explained major displacement (>100 km) along the sinistral shear zones during regionally developed Late Jurassic-Early Cretaceous sinistral faulting. The Late Cretaceous eastern Waddington thrust belt and coeval magmatism truncated and obscured the older fault system. Plutonism appears to have ceased by ~ 60 Ma.