Gemmological and Geochemical Characteristic of Zircon Megacryst Related to Cenozoic Alkaline Basalt from Eastern China

Gemmological/mineralogical characteristics and geochemical characteristics of zircon megacrysts related to Cenozoic alkaline basalt from eastern China are systematically collected and reanalyzed. These zircon megacrysts those common contain CO₂-rich gas-liquid and mineral inclusions(zircon, apatite, etc) with flat ribbon are generally reddish brown, glassy-oily lustre, weak fluorescence and most of them are 3~10 mm. The tetragonal dipyramid and prism constitute the most developed crystal forms. The UV-Vis-NIR and the results of heating treatment show that the mechanism of coloration should be closely related to lattice defect caused by radioactive elements. Valence elements and roman spectral features show that these zircon megacrysts are high-type zircon. However, there is no differences of gemmological characteristic of these zircons from different deposits. According to Ti-in-zircon geothermometer after correcting the crystallization pressure and the activity of TiO₂ and SiO₂, the crystallization temperatures are above 700℃. Combination of formation conditions of clinopyroxene megacrysts and basaltic xenoliths, the zircon megacrysts should crystallize in upper mantle. Ce positive anomaly and hematite inclusions indicate that zircon megacrysts crystalize in the oxidizing environment. Hf-O isotope composition show that the crystallization of zircon megacrysts should be related to the mantle environment. Combined with the zircon geochemical data and the tectonic background, the zircon megacrysts should be resulted from retrogression of lithospheric thinning and crystallize from SiO₂-rich and CO₂-rich melt condition. The crystallization of zircon megacrysts should be considered as a symbol of the lithosphere tending to be stable, and the subduction of western Pacific plate is the mainly geodynamic of lithosphere thinning, carbon cycling and silica rich melt existing in lithosphere mantle of eastern China.