ORIGIN AND SIGNIFICANCE OF SCHLIEREN IN GRANITE ON VINALHAVEN ISLAND, MAINE

The Silurian Vinalhaven pluton (Penobscot Bay, Maine) is composed of three units: coarse-grained granite, fine-grained granite of identical composition, and gabbro. The goal of this study is to determine the origin and significance of schlieren in the Vinalhaven granites. Schlieren are usually vertical, and they commonly occur in two settings: 1) in sheets and pipes of granite associated with map-scale gabbro units, and 2) associated with large enclaves in otherwise relatively homogeneous coarse-grained granite. Although there is no general model for schlieren formation in granites, there are several theories: deformation or deterioration of mafic enclaves, crystal settling, shear sorting against a rigid wall (of a more crystalline and viscous magma), and steep chemical gradients at flow margins resulting in preferential crystallization of mafic minerals. From preliminary analysis of mineral sizes, locations, and modes, there are more mafic minerals in schlieren than in the adjacent granite, and there appears to be a weak, sub-vertical alignment of plagioclase feldspar in the schlieren. In the adjacent granite there is significantly less mafic mineral content, and there is also a sub-vertical alignment of the feldspars. The alignment is in the same orientation for grains in both the schlieren band and the adjacent granite; whatever aligned these feldspars in the schlieren had the same effect on the grains in the granite. This observation is consistent with a schlieren formation model of strain imposed on the crystal mush or shear sorting of grains during flow of the crystal mush. From field relations and preliminary computer analysis we conclude that the schlieren did not form by crystal settling because schlieren are often found in semi-circular arrangements, and it would be highly unlikely that this arrangement would develop as a result of settling. Additionally, crystal settling implies a horizontal layering (as in sedimentary processes), but the schlieren on Vinalhaven are sub-vertical. Instead, schlieren on Vinalhaven may be a result of deformation or deterioration of mafic enclaves, shear sorting, or steep chemical gradients at flow margins that record vertical movement.