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Description
Previous studies by Dr. Gary H. Girty and students at San Diego State University have quantified element mobility patterns within metamorphosed mudstones of the post-Cambrian and pre-Upper Devonian Shoo Fly Complex, northern Sierra Nevada, California, as they are traced from outside to inside the aureole of the Middle Jurassic Emigrant Gap composite pluton. Results of these previous studies indicate that mudstone mass, on average, decreased 5.6 +/- 4.7% during development of the contact metamorphic aureole. Loss of mudstone mass was apparently facilitated by the removal of SiO2 under elevated fluid-to-rock ratios, and resulted in an estimated volume strain of -7.1 % +/- 5.0%. These results have lead several investigators to suggest that contact metamorphism may play an important space making role in the emplacement of magmatic bodies at mid-crustal levels. None of this previous work addressed the fate of the removed SiO2 mass. Hence, the purpose of this thesis is to address the question: Was silica mass removed from the aureole, or, was it simply transferred from mudstones to sandstones, both lithologies being prevalent throughout the Shoo Fly Complex? Data presented in this thesis show that during the development of the aureole, sandstone mass as well as volume, on average, increased 13.1 +/- 10.1% and 14.6 +/- 10.2% respectively. The mechanism by which silica mass was introduced to and transferred within the aureole is unclear. However, several observations point to at least two possible mechanisms. First, the fact that all samples analyzed during this and previous studies were absent of any traces of quartz veinlets implies that the transfer of silica mass into sandstone beds must have been within a thin, sub-microscopic, fluid phase around grain boundaries. Such a fluid phase is needed to allow transport of silica over lengths greater than those typical of solid-state diffusion which commonly occurs at the atomic level to scales measured as fractions of individual crystals. Second, the ubiquitous presence of quartz veins and veinlets within mostly sandstones, and, to a lesser extent, mudstones indicates that silica-rich fluids were probably moving through the aureole during its development. Many such veins have strikes that are at a high angle to the contact-parallel foliation and layering within the aureole. Such orientations suggest that the veins may have infiltrated along extensional cracks as layering was flattened and distended against the northwestern contact of the pluton. There is no evidence gathered during my research to support the idea that volume loss driven by removal of elemental mass within wall rocks through elevated fluid-to-rock ratios played a role in making space for the Emigrant Gap pluton.
