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Description
A ~1400 m by ~300 m (map view) inclusion of the Julian Schist occurs within the structural aureole of the La Posta pluton, Peninsular Ranges batholith, southern California and Baja California. Field observations and stereonet analysis of schistosity orientations from the main outcrop belt and inclusion of the Julian Schist suggest that emplacement of the La Posta pluton dextrally rotated (map view) the long axis of the inclusion ~44° eastward into parallelism with the margin of the pluton. Metamorphic parageneses of pelites from the inclusion of Julian Schist suggest that prograde conditions within the La Posta pluton structural aureole were somewhere between ~2.0 to 2.7 kb and ~635 to 675 °C; retrograde temperatures probably fell below ~510 °C. Using an average geobaric gradient of .33 kb/km, metamorphism in the structural aureole of the pluton probably occurred at depths of ~6.0 to 8.0 km. A geochemical and statistical based mass balance and volume strain analysis was performed on pelites from the inclusion and main outcrop belt of the Julian Schist in order to determine if volume loss by chemical mass transfer operated as a space-making mechanism for the emplacement of the La Posta pluton. A weighted least-squares analysis was performed on geochemical data from 20 samples of the inclusion and 22 samples of the main outcrop belt in order to assess mobility patterns and mass changes of chemical components within the structural aureole. Forty-one out of 42 components analyzed have statistically similar concentrations within and outside of the structural aureole, and therefore are considered immobile. SiO2 shows a statistically lower concentration within the structural aureole, and therefore is considered mobile. The immobile components were used as a frame of reference for calculating mass and volume change within the structural aureole. Resulting data indicate that a loss of 9.7 ± 6.0 gm of SiO2 for every 100 gm of rock resulted in an average volume loss of 8.2 ± 2.0% within the structural aureole of the La Posta pluton. Furthermore, the results of the work discussed herein suggest that volume loss by the transfer of SiO2 within the structural aureole provided some of the space for the La Posta pluton. Chondrite-normalized REE patterns of the inclusion and the main outcrop belt of the Julian Schist suggest that the REE are enriched within the structural aureole, and therefore, in terms of mass balance relationships, may reflect the loss of SiO2. Observations that possibly support SiO2 mobility within the inclusion include: the occurrence of quartz veins and segregations; and films of opaque material along quartz grain boundaries, indicating dissolution of quartz grains by intergranular diffusion. Mobile SiO2 within the inclusion of the Julian Schist may have been produced during metasomatic conditions by mineral reactions that include quartz grain dissolution and the replacement of cordierite and K-feldspar by fibrolite and mica phases.
