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 coexistingquartz, feldspar and biotite from four lithological units withinthe Aral granite batholith, Altay Mts of China. The data exhibita reversed and an anomalously large positive quartz–feldspar¹⁸O/¹⁶O fractionation. These two ¹⁸O/¹⁶O fractionations, togetherwith a marked decrease in the D values of biotite, are interpretedin terms of a model involving two stages of isotopic exchangewith aqueous fluids. The first stage of ¹⁸O/¹⁶O exchange withan ¹⁸O-rich aqueous fluid occurred during subsolidus cooling.Kinetic effects of the first stage of ¹⁸O/¹⁶O exchange are characterizedby the reversed quartz–feldspar ¹⁸O/¹⁶O fractionationthat was recorded in the megacrystic coarse-grained granitesof Group I and the megacryst-bearing medium- to coarse-grainedgranites of Group II. Robust parameters for the first stageof exchange are obtained by modelling on the hypothesis of initial¹⁸O/¹⁶O heterogeneity in the granite plus subsolidus ¹⁸O/¹⁶Oexchange. The spread in the measured ¹⁸O values of quartz reflectsthe initial heterogeneity of the granite. Calculated isochronsillustrate that an initially heterogeneous system reacting withan externally buffered fluid can generate arrays that mimicisotherms. The second stage of isotopic exchange with ¹⁸O- andD-depleted meteoric water occurred after magma solidification,resulting in the anomalously large positive quartz–feldspar¹⁸O/¹⁶O fractionation and a marked decrease in the D valuesof biotite. These kinetic effects have been recorded in thefiner-grained granites of Groups III and IV. However, the coarser-grainedgranites of Groups I and II have essentially survived the secondstage of ¹⁸O/¹⁶O exchange. KEY WORDS: Altay; granite; initial ¹⁸O/¹⁶O heterogeneity; pseudoisotherm; quartz–feldspar ¹⁸O/¹⁶O reversal