Geology of the Coquihalla Volcanic Complex, southwestern British Columbia

Abstract

The Coquihalla Volcanic Complex consists of calc-alkaline acid to intermediate extrusive and intrusive rocks that have an areal extent of roughly 30 km2 near Hope, British Columbia. The oldest and most voluminous members of the complex are rhyolitic pyroclastic rocks that have an overall thickness of approximately 1600 m. Later igneous activity produced numerous andesite to dacite domes, dykes, and sills. A late stage diorite to quartz diorite stock forms the core of Coquihalla Mountain.Pyroclastic rocks rest unconformably on the Jurassic to Cretaceous Eagle pluton and Lower Cretaceous Pasayten Group rocks. Monolithologic avalanche breccias were deposited against a fault scarp of uplifted Pasayten Group rocks in the southwestern portion of the map area. In the southeastern part of the area, monolithologic granitic avalanche breccias formed in response to lilting and uplift of the underlying Eagle pluton as the basin subsided.Three K–Ar dates average 21.4 ± 0.7 Ma, and are concordant with a Rb–Sr isochron (22.3 ± 4 Ma with initial 87Sr/86Sr = 0.70370 ± 0.00008) based on seven whole-rock samples which span the entire compositional range of the suite. These results indicate that the Coquihalla Volcanic Complex is coeval with calc-alkaline centres in the Pemberton Volcanic Belt (PVB).The north-northwest trend of the PVB is parallel to, but roughly 75 km east, of the Pleistocene to Recent Garibaldi Volcanic Belt (GVB). Both volcanic belts appear to have formed as a result of subduction of the Juan de Fuca plate. Genetic models must explain the easterly displacement of the PVB, the development of larger volumes of rhyolitic compositions in this belt as compared to the GVB, and the decrease in age of igneous rocks from south to north within the PVB.