PETROLOGY OF RECENT LAVA FLOWS, VOLCANO-MOUNTAIN, YUKON-TERRITORY, CANADA

Volcano Mountain, Yukon Territory, a Quaternary edifice located near t he junction of the Pelly and Yukon Rivers, erupted two Recent nephelin ite lava flows between 3000 and 7300 years BP and a third Recent lava flow that is probably younger. The porphyritic nephelinites have olivi ne phenocrysts, Fo(91)-Fo(79), and Ti-rich diopside microphenocrysts. The groundmass consists of olivine, Fo(83)-Fo(80), Ti-rich diopside, u lvospinel-magnetite solid solutions, nepheline, leucite, patches of re sidual glass and traces of calcite. The nephelinites also carry spinel -lherzolite xenoliths and xenocrystic olivine, Fo(87)-Fo(90). The xeno crysts have lower CaO contents than the phenocrysts. Samples from the two older flows have similar Thompson space representations whereas th e youngest flow has a distinctly different representation. The Thompso n space representations of the Volcano Mountain nephelinites are diffe rent from the representations of nephelinites from other centers in th e Cordillera. Thompson space representations and Pearce element-ratio diagrams suggest the two older flows are comagmatic whereas the younge st flow is from a separate magma batch. Thermodynamic modeling suggest s the magmas that supplied the lava flows originated at pressures near 2.5 GPa, left behind a residue containing olivine, and rose along an isenthalpic path after separating from the source rocks.