CHARACTERISTICS OF DIFFUSION ZONING IN GARNET AND ITS IMPLICATIONS IN GARNET-BIOTITE GEOTHERMOMETER FROM PELITIC GRANULITE OF THE JINGSHAN GROUP IN NORTH JIAODONG, CHINA

The detailed EPMA analysis indicates that the diffusion zoning is ubiquitous in garnets from Al-rich rocks of the Jingshan group that have experienced granulite facies metamorphism in northern Jiaodong, China. However, the intensity and pattern of the garnet zoning are rather different. The rim of garnet has the lowest Mg content and shows a well-developed diffusion zoning when contacting biotite, a slightly higher Mg content and less-developed diffusion zoning by contacting to cordierite, and nearly same Mg content as that in the core of garnet and the poor-developed diffusion zoning when adjacent to felsic minerals. The characteristic of Mg zoning of garnet indicates that the Fe-Mg exchange between garnet and biotite or other mafic minerals results from the transfer of interface contact. The grain-boundary fluid seems taking less important role on the process of Fe-Mg exchange. However, Fe and Mg also could be transferred through grain-boundary fluid owing to increasing H2O activity in the rocks provided that the presence of large volume of biotite and low garnet content (VGrt/VBt°‹0.25). As a result, the weak diffusion zoning could be formed in the garnet rim even though they contact felsic minerals. Therefore, it is necessary to analysis the mineral contents of rocks before studying on the effect of grain-boundary fluid. The Mg component profiles of garnets display that intense diffusion distance of Mg ion is no more than 200~300 um from edge to core of the grain, the longest distance is less than 700 um. The cores of coarse garnet grains over 1500 um in size have nearly same chemical composition. Therefore, it can be concluded that the metamorphic peak composition of the garnet could be persevered in the core of coarse grains over 1500 um in size. Accordingly, the core of large biotite far away from garnet and among felsic minerals might also have kept peak composition. Thus, a reliable peak temperature could be get by garnet-biotite geothermometer. The peak temperatures of the Jingshan group yielded from this approach range from 720 to 770oC, which is confirmed by the peak assemblage.