PETROLOGY AND GEOCHEMISTRY OF AN UNUSUAL TRIDYMITE-HERCYNITE XENOLITHIN THOLEIITE FROM SOUTHEASTERN CHINA

A rare variety of Fe-Al-rich xenolith, tridymite-hercynite rock, was e ntrained in the Cenozoic basalt from Niutoushan, southeastern China. T he tridymite-hercynite rock consists of hercynite (40-50%), tridymite (35-45%), ilmenite (approximately 2%), and glass (10-20%). Mineral gra ins are smaller than 0.05 mm with a granular texture. Compared to ''no rmal'' igneous rocks, this tridymite-hercynite rock has higher FeO, Al 2O3, and lower SiO2, CaO, MgO, and alkalis (Na2O + K2O). The origin of this type of xenolith is problematical. Based upon the available mine ralogical and chemical data, three possible origins have been evaluate d: (1) The product of an extreme tholeiitic fractionation under low ox idation conditions; but the major element geochemistry appears to nega te this model. (2) The Fe-rich (Si-poor) portion of silicate-immiscibl e liquids from an extremely fractionated residual tholeiite magma; how ever, the corresponding Si-rich end-member fraction is missing, and th e composition of the tridymite-hercynite rock is different from that o f any Si-poor melt phase of known immiscible silicate liquids. (3) The product of metamorphism, including partial to complete melting, of a sedimentary protolith. An Fe-Al-rich shale (quartz 5%, kaolinite 75%, hematite 20%) has the approximate composition of the tridymite-hercyni te rock. The trace-element abundances of the xenoliths suggest that th e source rock for the Fe-Al-rich sedimentary protolith was an undiffer entiated volcanic rock at the active continental margin.