THE CHILCOTIN BASALTS, BRITISH-COLUMBIA (CANADA) - GEOCHEMISTRY, PETROGENESIS AND TECTONIC SIGNIFICANCE

The Chilcotin basalts are a contiguous suite of lavas which occurs in the central and southern interior region of British Columbia. The ages of the lavas, which are spread thinly over an area of 25,000 km(2), r ange from 24 Ma to Recent. The basalts, which show only a small variat ion in composition, are predominantly transitional in character (olivi ne tholeiites and mildly silica undersaturated alkali basalts). Most o f the basalts, many of which contain spinel peridotite xenoliths, rese mble oceanic island basalts and were derived from a garnet peridotite source, probably the asthenospheric mantle. The subordinate quartz-nor mative tholeiites are characterized by a distinct LILE enrichment rela tive to high-field-strength elements. The source of these rocks, which compositionally resemble continental flood basalts, was probably subc ontinental lithospheric mantle which was enriched by earlier subductio n processes. Upwelling of asthenospheric mantle, a source of the major ity of the basalts, probably triggered melting in the subcontinental l ithosphere.