The Ilgachuz Range and adjacent parts of the Interior Plateau, British Columbia

Abstract

Christensen Creek (93C/11) and Carnlick Creek (93C/14) map areas encompass the Ilgachuz Range, a highly dissected late Tertiary shield volcano that rises 1200 m above the general level of the Interior Plateau of central British Columbia. The drift-covered plateau surface adjacent to the Ilgachuz Range contains isolated exposures of Mesozoic volcanic and sedimentary rocks, early Tertiary felsic volcanic and subvolcanic rocks, and mylonitic orthogneiss of the Eocene Tatla Lake Metamorphic Core Complex. Undeformed late Tertiary volcanic rocks rest unconformably on all of the older strata. They include Chilcotin Group basalt flows and Ilgachuz Group alkaline volcanic and subvolcanic rocks which form the Ilgachuz Range shield volcano. The Ilgachuz Range is a composite volcanic shield comprising a thick lower section of highly alkaline to peralkaline felsic flows, domes, and hypabyssal intrusions, overlain by a relatively thin veneer of alkaline basalt. A small central caldera is filled with pyroclastic deposits, lacustrine beds, and ponded lava. K-Ar dates indicate that the entire volcanic edifice was built between 6 Ma and 4 Ma. The Ilgachuz Range is one of three large shield volcanoes in the central part of the east-trending Anahim Volcanic Belt. Volcanoes along this belt become progressively younger from west to east and are believed to have formed on the American continental plate as it moved westward over a mantle hotspot. The chemistry and stratigraphy of the Ilgachuz Group rocks suggest that they are the product of crustal melting and crystal fractionation, accompanied by underplating with mantle-derived basaltic magma.