Coexisting quartz, feldspar and biotite vary widely in their δ18O values and display a remarkable 18O/16O disequilibrium relation; especially, a quartz-feldspar reversal (△ 18OQUartz_feldspar< 0) exsists in the Aral granite pluton of the Altay Mountains, northern Xinjiang. The 18O / 16O exchange reaction definitely occurred between granite and water. Initial δ18O values of the granite and exotic fluid are evaluated by the mass balance consideration. The conventional method of discrimination between various magma derivations simply with δ18O values of either whole rock or separate minerals is misleading and unreliable. Experiments carried out by the authors show that the 18O / 16O exchange reaction is not accompanied by what geologists describe as petrological and mineralogical alteration effect. This decoupling relation implies that exchange reaction occurs at a relatively high temperature during subsolidus-postmagmatic cooling of magmas. The exchange mechanism is mainly diffusion-controlled. It is demonstrated through quantitative modelling that the hydrothermal system associated with the Aral pluton is long-lived (0.8-6 Ma), with a relatively high fluid flow rate (3 ×10-14 mol. s-1) and high W/ R ratio (0.79-6.14). This means that an intense flow and convection may exist at the midcrustal level of erogenic magmatic arcs.
Liu Wei,Research Center of Mineral Resources Exploration, Chinese Academy of Sciences, Chinese Academy of Sciences, BeijingHe Baichu, Changsha Institute of Geotectonics, Chinese Academy of Sciences, Changsha, Hunanand Chen ZhenshengYichang Institute of Geology and Mineral Resources, Chinese Academy of GeologicalSciences, Yichang, Hubei
