Stratigraphy and geochemistry of the Itcha Volcanic Complex, central British Columbia

Abstract

The Itcha Volcanic Complex is the youngest and easternmost felsic shield volcano of the Anahim Volcanic Belt of central British Columbia. The main body of the shield erupted over an area of ~300 km2 forming Itcha Mountain and Mount Downton. Volcanism associated with the Itcha Shield extended 20 km south to the Satah Mountain area, where lavas erupted along a north-northwest – south-southeast fault system and covered an additional area of 250 km2. The Itcha Volcanic Complex is characterized by a bimodal population of volcanic rocks, which are dominated by felsic lavas. There were two stages of volcanism: (i) an early felsic shield-building stage dominated by felsic lavas ranging in composition from phonolite to minor quartz-normative trachytes, which erupted as flows, domes, and pyroclastic deposits to form a low-angled shield; and (ii) a late mafic capping stage, which comprises a thin veneer of hawaiite and more primitive mafic lavas ranging in composition from alkali olivine basalt to basanite. The late mafic capping stage lavas erupted from satellite cinder cones and fissures concentrated on the eastern side of the shield.The hawaiites that dominate the late mafic capping stage cannot have been derived from the more primitive basalts with which they are associated by low-pressure crystal fractionation but may instead have originated from the fractionation of a clinopyroxene-dominated assemblage at high pressures. The presence of mafic xenocrysts in a megacrystic trachyte unit, whose eruption terminated the felsic shield-building stage, and anorthoclase xenocrysts in the most evolved alkali olivine basalts of the mafic capping stage indicate that the mafic and the felsic magmas interacted prior to eruption. An overlap in 87Sr/86Sr ratios and a similarity in the high-field-strength element ratios of the felsic and the mafic lavas suggest that they are genetically related. Elevated ratios of large-ion lithophile elements to high-field-strength elements (e.g., Rb/Zr) in the trachytes, however, indicates that the felsic magmas were not derived by closed-system fractional crystallization from the mafic magmas and may instead suggest the assimilation of a crustal component.