Description
Plutons are derived from melting of the crust deep within the earth. The resulting magmas then rise through the lithosphere until they reach an equilibrium position within the mid to shallow levels of the crust. If large volumes of wall rock are not incorporated into the rising magma, then what arc the mechanisms involved to allow large volumes of magma to be emplaced within the crust? Recent studies within the Peninsular Ranges batholith, California involving the emplacement of the 94 ± 2 Ma La Posta pluton, indicate that one possible space-making mechanism may be significant elemental mass loss and resulting volume change. The ~105 Ma Cibbets Flat pluton lies adjacent to the western edge of the La Posta pluton and contains a well-developed dynamothermal aureole. In order to determine if similar mass loss occurred within the Cibbets Flat pluton as a result of the emplacement of the La Posta pluton, I mapped the location of the aureole within the Cibbets Flat pluton and completed a statistical and chemical study of samples collected from outside and inside the aureole. Though no statistical change in mass occurred in 42 of the 43 elements analyzed during this study, the one element that does exhibit a statistically significant change in mass is silica (~3%). Although quartz veins are abundant within the dynamothermal aureole, the change in mass is not a result of the redistribution of silica by metamorphic or magmatic fluids, but rather reflects original heterogeneities in magmatic chemistry. If this is true, then the Cibbets Flat pluton exhibits reverse zoning, displaying an increase in the amount of silica within the aureole relative to samples from outside the aureole. Rocks of the Cibbets Flat pluton outside the aureole contain on average 22% quartz while those inside the aureole contain on average 32% quartz. The results of the work presented within this thesis suggest that the development of textures indicative of plastic strain within the dynamothermal aureole of the Cibbets Flat pluton may be dependent upon some critical amount of silica Little is known about how quartz abundances control the development of plastic strains in dynamothermal aureoles that have formed in older rocks. These results indicate that there is much to discover about space-making mechanisms during pluton emplacement.
