INTERNAL EVOLUTION OF MIAROLITIC GRANITIC PEGMATITES AT THE LITTLE THREE MINE, RAMONA, CALIFORNIA, USA

Miarolitic granitic pegmatites at the Little Three mine property near Ramona, San Diego County, California, USA, possess a bulk composition that closely matches that of a hydrous peraluminous granitic liquid that is saturated with respect to tourmaline at a temperature of similar to 450 degrees C. As such, the pegmatites appear to represent the compositions of the silicate liquid from which they crystallized, which contained <1 wt% B2O3 and minor amounts of Li and F. They do not appear to have contained phenocrysts upon emplacement, and they are neither partially cumulate nor hydrothermal in nature. The Little Three pegmatites are, however, sharply zoned. Chemical zonation across one small dike matches the patterns expected from crystallization of undercooled granitic liquids, in which the far-field diffusion of alkalis and local constitutional zone refining of fluxing and incompatible elements contribute to the chemical and textural changes from the margins inward. Feldspar thermometry records nearly isothermal crystallization at similar to 420 degrees-430 degrees C for dikes from 1 to 2.5 m in thickness. Temperatures recorded by feldspars fall toward the miarolitic center of the thicker (Main) dike, but increase to similar to 500 degrees C in the thinner (Swamp) dike. Fluid inclusions within quartz and topaz from miarolitic cavities of the Main dike contain cryolite, arsenides, arsenates, pollucite, and borates, including Cs borate, the latter of which indicates a higher degree of chemical fractionation than is present in the minerals that line the cavities. Extrapolation along isochores based on the homogenization (200 degrees-225 degrees C) of the low-salinity aqueous fluid (<1 equivalent wt% NaCl) to the temperature of feldspar equilibration (240 degrees-270 degrees C) with the miarolitic cavity points to hydrostatic entrapment pressures of similar to 50 to 100 MPa, which are unrealistically low. If the isochores are extrapolated to 370 degrees +/- 20 degrees C, the temperatures recorded by feldspars in the pegmatite that surrounds the miarolitic pockets, then the entrapment pressure would be similar to 200-250 MPa. Formation of the miarolitic cavities at similar to 370 degrees C and 200-250 MPa is plausible, as the cavities contain zeolites; therefore, the feldspars within the pockets have recrystallized (re-equilibrated) to an extent that they no longer preserve their original compositions.