STEEP SCHLIEREN AND ASSOCIATED ENCLAVES IN THE VINALHAVEN GRANITE, MAINE: POSSIBLE INDICATORS FOR GRANITE RHEOLOGY

Abstract

Schlieren that form above the solidus are potentially diagnostic of the rheology of crystallizing granitic magmas and could provide insights into magma chamber structure. Here we focus on steep schlieren associated with comagmatic enclaves in the Vinalhaven granite. Steep schlieren extend 1–3 m upward from the sides of enclaves and appear to have steep cylindrical shapes in 3D. Analyses of schlieren widths and the sizes of associated enclaves suggest that granitic crystal mush in which they occur had a plastic rheology probably characterized by a yield strength. The enclaves, now enclosed in coarse-grained granite, must have existed at higher levels in a crystal-poor part of a magma chamber and settled downward until reaching material with a yield strength exceeding the buoyancy of the enclaves. In addition to constraining the local rheology of the granite, their relative positions may indicate vertical rheological variations and possibly the chamber floor.