High-Alumina Hypersthene in a Basalt Flow from Mt. Sundook, Siberian Traps, Russia

Electron probe microanalysis (EPMA) and preliminary x- ray diffraction studies on a basalt flow from Mt.Sundook have revealed the presence of rare high-alumina hypersthene in the groundmass. The basalt occurs at the base of a 800 m flow sequence (Ivakinsky and Syverminsky suite) that unconformably overlies a Permo-Carboniferous sedimentary sequences. It is postulated that besides normal processes of fractionation, assimilation-fractional Crystallization (AFC) processes involving the parental magma and crustal components (pelitic sedinientary rocks) were responsible for the formation of hypersthene in the basalt. The presence of a complex feldspar and pyroxene assemblage may also be an additional evidence for the operation of such an AFC process.