Crystallography of Exsolution Lamellae in Garnet: A New Tool for Interpreting Metamorphic Environments of UHP Rocks

Abstract:

Discerning the prograde history of rocks metamorphosed to high grades at convergent margins is challenging, especially because metamorphic fabrics may be overprinted by retrogression. Exsolution lamellae in metamorphic garnet are a record of the extreme pressures and temperatures reached by their host rocks, and have been used to deduce peak conditions from precursor garnet chemistry. Crystallographic studies have made great progress documenting and explaining the crystallographic orientation relationships (COR) between lamellae and host garnet, which signify an exsolution origin, but other uses for these data have not been developed.

We reviewed the available COR data for pyroxene and rutile, the two lamellae minerals for which COR have been measured from the most localities. From each locality, we identified COR which satisfy criteria for low-energy lattice matching between host and lamellae lattices. Through comparisons with results from the metallurgical literature, we find that, as metals do, garnets with lower-strain histories preserve higher fractions of lamellae with these favorable matches. For example, slowly upwelled mantle peridotites preserve far more lamellae with low-energy matches than deeply subducted ultrahigh-pressure (UHP) rocks which did not efficiently anneal prograde strain. We conclude that these COR distributions preserve information about prograde conditions and discuss the ways in which COR distributions may be developed as petrogenetic indicators of pre-exsolution metamorphic environment.