δ18O values of the Heavitree Quartzite, central Australia, vary systematically as a function of deformation and recrystallization in a duplex composed of 5 superposed thrust sheets which formed under greenschist-grade conditions. Undeformed quartzite of the basal thrust sheet 1 and moderately strained and partially recrystallized quartzite of thrust sheet 2 range over 4%. in δ18O values and have an average value of ~12.8%., Entirely recrystallized quartzite of overlying thrust sheet 3 retains a similar average value of ~12.3%. but shows a narrow range of ~1.5%., suggesting isotopic homogenization. Recrystallized quartzites of sheet 5 have a lower δ18O value than the other thrust sheets. The homogenized δ18O values of sheet 3 relative to sheets 1 and 2 could not have resulted from mechanical mixing, isotopic exchange during dissolution-reprecipitation, or exchange by diffusion between the quartzite and a fluid in microfractures or along grain boundaries. The homogenized δ18O values of sheet 3 and lowered values of sheet 5 are interpreted to have been produced by isotopic exchange during recrystallization by grain-boundary migration. This is probably an important mechanism of isotopic exchange for rocks undergoing dynamic recrystallization.
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Acknowledgements-We thank Jim Dunlap, Greg Hirth and Shun Karat0 for many useful discussions on the processes of quartzite deformation. Mark Brandon, Jim Dunlap, Emi Ito and Jan Tullis made many constructive comments on the paper. We thank Steven Wojtal for the editorial handling of this paper. Support from National Science Foundation grants EAR-8720755 (CT) and EAR-9005583 (CT and RTG) are gratefully acknowledged.