THE CHILCOTIN GROUP: ENIGMATIC NEOGENE BASALTIC VOLCANISM IN INTERMONTANE BRITISH COLUMBIA

The Chilcotin Group (CG) is the largest of several contemporaneous and adjacent Neogene basaltic provinces across the plateaus of central British Columbia. It forms an extensive (~17,500 km²) veneer of basalt lavas and associated valley-filling hyaloclastite-pillow successions, interspersed with remnants of small shields and dolerite plugs. The CG is comprised of multiple localised basaltic fields erupted on to a peneplain spasmodically between about 25 Ma and 100,000 years. Due to its longevity it should serve as a excellent gauge of tectonic and geomorphic changes in the Canadian Cordillera through the Neogene; for example, derangement of the Fraser River and uplift of the Coast and Cariboo Mountains. The CG occurs in the retro-arc region (sensu latto) to the Garibaldi and preceding Pemberton volcanic arcs of the northern Cascades, however it does not convincingly appear to be a 'typical' back-arc regime. Furthermore, it appears to have been formed as British Columbia passed over at least one slab window.

There is no tectono-magmatic synthesis for the CG, largely due to a paucity of accurate dates and good geochemical analyses with temporal-spatial context. It is enigmatic in several ways: (1) Why has basaltic volcanism been so long-lasting? (2) Why is the magmatic volume flux so low? (3) Is there a temporal-spatial or temporal-spatial-geochemical control? (4) Why is there no evidence of extension or translation (i.e. faults) in the CG? This presentation will introduce some of our recent work in the CG in an attempt to: (1) address some of these questions; (2) highlight it for further and more extensive study; and (3) to look for analogues in similar tectonic environments.