PETROGENESIS OF DEFORMED INTRUSIVE PODS IN THE QUATSINO FORMATION, OPEN BAY, QUADRA ISLAND, BRITISH COLUMBIA

The boundary between the Insular and Coast geomorphological belts of the North American Cordillera is exposed at Open Bay, on Quadra Island, British Columbia. Here the Karmutsen flood basalts (ca. 225-230 Ma), and overlying Quatsino limestone (upper Carnian) are intruded by diorite and granodiorite that are interpreted as Coast intrusions. At Open Bay the limestone of the Quatsino formation has been subjected to strong ductile deformation that likely formed during a regional tectonic event – possibly the joining of Wrangellia with the Intermontane terrane. The limestone is intruded by several generations of dykes that display various degrees of deformation, and studies of these intrusions should provide temporal and paleogeographic constraints on the accretion of Wrangellia. This poster presents field, petrographic, and geochemical data from strongly deformed intrusions that pre-date deformation of the limestone.

Textures of the deformed intrusions are variable, but most are fine phaneritic and/or plagioclase phyric, consistent with an intrusive origin. Chemistry of the deformed intrusions was compared to published data from both the Karmutsen and Bonanza volcanics. The deformed intrusions were found to be subalkaline basaltic andesite, with consistent enrichment in Al₂O₃ (>15 wt%), Na₂O (>3 wt%), K₂O (>0.5 wt%), TiO₂ (>1 wt%), P₂O₅ (>0.2 wt%), Sr (>500 ppm), Rb (5 – 40 ppm), and Ba (avg = 428 ppm), and consistent depletion in Fe₂O₃ ( <12 wt%), CaO (<10 wt%), MgO (<5 wt%), and Ni (<10 ppm), relative to published values for Karmutsen volcanics. Rare earth element plots show a smooth pattern, moderately enriched in LREEs, that agrees with both E-MORB type Karmutsen volcanics and typical Bonanza volcanics. Ba/Th and Th/Nb ratios are elevated relative to Karmutsen volcanics, and consistent with published geochemical signatures of Bonanza volcanics.

Petrographic, and geochemical data show that these intrusions likely do not represent late-stage Karmutsen volcanism. It is more likely that they are associated with a younger arc-type magmatic event – possibly the Early - Middle Jurassic eruption of the Bonanza arc.