甘肃寺下河熔结凝灰岩

A CONTRIBUTION TO IGNIMBRITES IN SIXIAHE,GANSU PROVINCE

Shixiahe ignimbrites in Gansu Province were formed as a result of the fracture eruption during Mesozoic time. They are typical welded rocks, the cooling elements of which are still seen clearly, with well developed columnar joints, plastic fragments and fhidal structure. The unit weight of the rocks was found lying between those of lavas and tuffs. Crystallinoclastic plagioclases of both abyssal and volcanic types were recognized. In crystallinoclastic quartz fluid and glass inclusious were identified with the inclusion temperature ranging from 500 to 600℃. In accordance with the petrochemical characteristics these ignimbrites are thought to be rhyotaxitic aluminiumsupersaturated calc-alkali rocks of Pacific type. Studies On the indices σ and τ figured out by A: Rittmann and V. Gottini show that this type of rocks should be assigned to igneous rocks occurring in the orogenic zones and island-arc areas.

Shixiahe ignimbrites in Gausu Province were formed as a result of the fractureeruption during Mesozoic time.They are typical welded rocks,the cooling elementsof which are still seen clearly,with well developed columnar joints,plastic fragmentsand fluidal structure.The unit weight of the rocks was found lying between those oflavas and turfs.Crystallinoclastic plagioclases of both abyssal and volcanic types wererecognized.In erystallinoclastic quartz fluid and glass inclusions were identified withthe inclusion temperature ranging from 500 to 600℃.In accordance with the petro-chemical characteristics these ignimbrites are thought to be rhyotaxitic aluminium-supersaturated ealc-alkali rocks of Pacific type.Studies on the indices σ and τ figuredout by A.Rittmann and V.Gottini show that this type of rocks should be assigned toigneous rocks occurring in the orogenic zones and island-arc areas.