HIGH-ALUMINA HYPERSTHENE IN A BASALT FLOW FROM MT SUNDOOK, SIBERIAN TRAPS, RUSSIA

Electron probe miciroanalysis (EPMA) and preliminary x-ray diffraction studies on a basalt flow from Mt.Sundook have revealed the presence o f rare high-alumina hypersthene in the groundmass. The basalt occurs a t the base of a 800 m flow sequence (Ivakinsky and Syverminsky suite) that unconformably overlies a Permo-Carboniferous sedimentary sequence s. It is postulated that besides normal processes of fractionation, as similation-fractional Crystallization (AFC) processes involving the pa rental magma and Crustal components (pelitic sedimentary rocks) were r esponsible for the formation of hypersthene in the basalt. The presenc e of a complex feldspar and pyroxene assemblage may also be an additio nal evidence for the operation of such an AFC process.