Description
The 95 Ma La Posta pluton represents the largest pluton mapped within the Peninsular Ranges batholith, southern California. It intrudes a wide variety of rocks, including the Late Jurassic Cuyamaca Reservoir gneiss. In order to assess element mobility/immobility patterns within the structural aureole of the La Posta pluton, nine samples of the Cuyamaca Reservoir gneiss were collected from outside of the structural aureole and four samples were collected from within the structural aureole. Each sample was analyzed for major elements and was studied previously by C. N. Thomson as part of here Masters thesis at San Diego State University. Because of the constant-sum format of geochemical data, they are not useful m their conventional form for assessmg the chemical changes attending the metamorphic transformation of rocks outside and inside the structural aureole of the La Posta pluton. Hence, major element data were first transformed to a continuous variable format via a log10 transformation utilizing Ti as an immobile normalizing factor. Standard descriptive statistics were then calculated, and the differences m means and associated 95 % confidence limits were determined. The hypothesis, H0 : µ0-uj = 0 was tested utilizing the student-t test. The results of the student-t test indicated that all major elements, with the exception of MgO and K20, are depleted within the structural aureole of the La Posta pluton. Such a conclusion can be explained as the result of (1) metasomatic changes associated with the development of the structural aureole, and/or (2) by chemical changes associated with mineralogical differences m the protolith of the Cuyamaca Reservoir gneiss. However, the gneissosity charateristic of the Cuyamaca Reservoir gneiss is more intensely developed within the structural aureole than in areas that were beyond the influence of the La Posta pluton. Thus, though my study is limited in context, I conclude that the calculated differences in the mean compositions of the Cuyamaca Reservoir gneiss most likely reflect poorly understood mass exchange and/or volume loss processes operating within the aureole during the emplacement of the La Posta pluton.
