Brianna Force

Date of Award

2011

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Daniel R. Lux

Second Committee Member

Scott E. Johnson

Third Committee Member

Christopher C. Gerbi

Abstract

Schlieren preserve evidence of the physical processes and crystallization events that occur within a magma chamber and if they are properly interpreted can provide significant insights into these dynamic systems. In this study schlieren of the Mount Waldo granite are characterized and a mechanism for schlieren structure formation is proposed. More than thirty schlieren structures, with a variety of forms including ellipsoids/rings, planar and trough structures, blobs/diapirs, and tube-like structures were observed in the 371 million year old Mount Waldo granitic complex, located near Bucksport, Maine. The schlieren typically have one sharp and one gradational contact with the host granite. Mineral abundance varies with position; ferromagnesian minerals are concentrated at the sharp contact and their abundance decreases in the direction perpendicular to that contact. Grain size shows systematic variations away from the sharp contact as well, with smaller grains concentrated near the sharp contact. Potassium feldspars and rare plagioclase megacrysts are aligned parallel to the schlieren margins or may be imbricated immediately below the base of planar structures. Textures associated with solid state deformation are lacking and suggests that these structures are of magmatic origin. Multiple mechanisms for schlieren formation, including melt extraction, disaggregation of xenoliths and/or enclave, gravitational settling, magmatic stoping, and magmatic flow, were tested. Field observations suggest that the geometry of Mt. Waldo schlieren are not consistent with formation by melt extraction. The compositional similarity of biotite grains in the host granite, schlieren, and in enclaves suggests the disaggregation of xenoliths as a mechanism to form schlieren is improbable, therefore indicating that the biotite within the schlieren originate from the granite and not from a foreign source. All contacts between enclaves and the host granite are sharp, so there is no evidence to suggest enclave disaggregation was an important mechanism. Calculated crystal settling velocities predict mineral grading opposite that which is observed in Mount Waldo schlieren. Furthermore, very slow rate of settling, casts doubt on the gravitational settling as a viable mechanism. A few schlieren structures have geometries that are consistent with formation via magmatic stoping, but there are not enough such structures to support stoping as a dominant formation process. Electron backscatter diffraction (EBSD) was used to test the remaining hypothesis that crystal sorting during magmatic flow produced the schlieren structures. The results demonstrate that biotite crystals have crystallographic preferred orientations that are similar to the simplified theoretical orientations predicted by this mechanism. The basal planes (001) are oriented parallel to the schlieren plane near the base of the schlieren, whereas in the upper regions of the schlieren they are oriented close to perpendicular to the schlieren plane. In combination, field, microstructural, and EBDS data support crystal segregation during magmatic flow as the dominant formation mechanism of the Mount Waldo granite schlieren structures.

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