MAGMA MIXING IN EARLY CRETACEOUS COMPOSITE LAVA FLOWS IN XINCHANG, ZHEJIANG
浙江新昌早白垩世复合岩流中的岩浆混合作用

Early Cretaceous composite lava flows from Bamao of Xinchang. Zhejiang , consist of olivine tholeiite, high-K basaltic andesite, high-K andesite, high-K dacite and rhyolite, which are assigned to high-K cafe-alkaline series. The high-K andesite enclaves with rounded and complicated shape and quenched rim have been found in the rhyolite in the composite lava flows. There also are rich-Ca plagioclases with sieve texture in the rhyolite. The high-K andesite and the enclaves with same compositions contain elastic quartz and K-feldspar grains derived from the rhyolitic magma. The foreign quartz grains in the high--K andesites have pyroxene rim. Above petrographic features suggest that magma mixing is an important process which causes magmatic diversity of the composite lava flows. Moreover, isotopic studies Indicate that olivine tholeiite, high-K andesite and rhyolite in the composite lava flows have similar ENd (t) values (their Nd (t) are -4. 58, - 5. 56 and -5. 44 respectively) and REE patterns. It implies that materials exchange among these relative magmas occurred. On SiO2 vs. major oxides, compatible element vs compatible element (e. g. Ni vs. Co) and incompatible element vs. incompatible element (e. g. Nb vs. Th) plots, composition data of volcanic rocks in the composite lava flows exhibit linear relations. On Co/Th vs. Nb/Ni plot occurs hyperbolic relation. On companion plot of Co/Th vs. Ni/Th, the data plot as a straight line. These features support the magma mixing.petrogenesis for the high-K andesite in the area. On the basis of the space distributions of every rock types in Bamao area of Xinchang and their petrographic and geochemical characteristics, we have reason to believe the following magma mixing model to be reliable. While both the upper rhyolitic and the lower basaltic magmas were stored in a common magma reservoir, the chemical mixing between them occurred near their boundary layer for long-term residence prior to eruption. This chemical mixing resulted in the formation of the high-K andesitic magma. When the upper rhyoliic and the underlying high-K andesitic magmas were simultaneously withdrawn from the chamber,the physical mixing between them occurred in the conduit.