Evolution of perthite composition and microstructure during progressive metamorphism of hypersolvus granite,Rhode Island,USA

The Scituate Granite in central Rhode Island, USA contains very coarse alkali feldspar mesoperthite and has been subjected to prograde metamorphism subsequent to original igneous cooling. The modal abundance and grain size of relics of alkali feldspar mesoperthite decrease systematically as metamorphic grade increases. The composition and microstructures of coexisting phases in the relict perthite grains also vary systematically with increasing metamorphic grade. Microstructures coarsen and become more complex, and compositions record increasing metamorphic temperatures.We suggest that the microstructures and compositions have been produced by exsolution, either during post-igneous cooling or early metamorphism, followed by partial homogenization during prograde metamorphism. The principal control on the evolution of the microstructures and compositions was probably the maximum temperature achieved during prograde metamorphism, with the abundance and size of perthite relics determined by recrystallization during deformation.