Two Disequilibrium Quartz–Feldspar 18O/16O Fractionations within the Aral Granite Batholith, Altay Mountains of China: Evidence for Occurrence of Two Stages of O and H Isotopic Exchange of a Heterogeneous Granite System with Aqueous Fluids

18O/16O and D/H isotope compositions are reported for coexisting quartz, feldspar and biotite from four lithological units within the Aral granite batholith, Altay Mts of China. The data exhibit a reversed and an anomalously large positive quartz–feldspar 18O/16O fractionation. These two 18O/16O fractionations, together with a marked decrease in the δD values of biotite, are interpreted in terms of a model involving two stages of isotopic exchange with aqueous fluids. The first stage of 18O/16O exchange with an 18O-rich aqueous fluid occurred during subsolidus cooling. Kinetic effects of the first stage of 18O/16O exchange are characterized by the reversed quartz–feldspar 18O/16O fractionation that was recorded in the megacrystic coarse-grained granites of Group I and the megacryst-bearing medium- to coarse-grained granites of Group II. Robust parameters for the first stage of exchange are obtained by modelling on the hypothesis of initial 18O/16O heterogeneity in the granite plus subsolidus 18O/16O exchange. The spread in the measured δ18O values of quartz reflects the initial heterogeneity of the granite. Calculated isochrons illustrate that an initially heterogeneous system reacting with an externally buffered fluid can generate arrays that mimic isotherms. The second stage of isotopic exchange with 18O- and D-depleted meteoric water occurred after magma solidification, resulting in the anomalously large positive quartz–feldspar 18O/16O fractionation and a marked decrease in the δD values of biotite. These kinetic effects have been recorded in the finer-grained granites of Groups III and IV. However, the coarser-grained granites of Groups I and II have essentially survived the second stage of 18O/16O exchange.