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

摘要

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 ～450°C. As such, the pegmatites appear to represent the compositions of the silicate liquid from which they crystallized, which contained <1 wt% B_2O_3 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 ～420°-430°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～500°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°-225°C) of the low-salinity aqueous fluid (<1 equivalent wt% NaCl) to the temperature of feldspar equilibration (240°-270°C) with the miarolitic cavity points to hydrostatic entrapment pressures of ～50 to 100 MPa, which are unrealistically low. If the isochores are extrapolated to 370° ± 20°C, the temperatures recorded by feldspars in the pegmatite that surrounds the miarolitic pockets, then the entrapment pressure would be ～200-250 MPa. Formation of the miarolitic cavities at ～370°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.