GEOCHEMICAL CHARACTERISTICS OF BENGGE SYENITES IN THE ZHONGDIAN AREA,YUNNAN PROVINCE AND ITS GEOLOGICAL SIGNIFICANCE

Abstract

Bengge syenites has been always regarded as the product of alkaline magma activity controlled by the strike-slip structures caused by collision between Yindia and Euroasia. The Bengge syenites belongs to shoshonite series, and the rocks are very enriched in K2O (5.88%-9.08%), rare earth elements (REE: particularly light REE) and large ion lithophile elements, but are relatively low in high field strength elements, with the element character like arc magmas. The geochemical data suggest that the primitive magma of the syenites partially melted under high pressure, and most likely formed via fractional crystallization. Zircon U Pb dating of isotope dilution method indicates that the Bengge intrusions have Late Triassic(201.4Ma) crystallization ages, respectively. Our data suggest that the syenites formed under extensional geological setting behind continent arc. Combined with previous regional structural, and sedimentary data, we suggest that emplacement of the Bengge extrusion took place at the transitional period from extrusion to extension.