APPLICATIONS OF FELDSPAR SOLVUS THERMOMETRY TO THE CRYSTALLIZATION OF MIAROLITIC GRANITIC PEGMATITES

Much of the textural complexity that defines granitic pegmatites arises from the feldspars (Fsp). Within a given body, plagioclase (Pl) and K-feldspar (Kfs) range in size from millimeters to meters, are equant to highly elongate, in fabrics that are isotropic (granitic and aplitic) to highly anisotropic (unidirectional solidification perpendicular to contacts), with habits that are skeletal, graphic, radial, and euhedral. Though the pegmatitic Fsp preserve their igneous habits, they have not been utilized for Fsp solvus thermometry because of a widely-held belief that Kfs does not retain its magmatic composition upon recrystallization to microcline perthite. The K/Cs ratio of the Kfs, which is a sensitive monitor of post-crystallization alteration in an open hydrothermal system, universally records an igneous evolution and hence preservation of composition in Kfs from primitive granites to complex pegmatites, from the margins of pegmatites to their centers, and from base to termination of individual crystals. Not all Kfs is microcline perthite. Crystals in outer zones are braid-twinned, non-perthitic, and retain much of their original monoclinic disorder. The An content of primary Pl falls from border (An_35-09) to core (An_05-00) as if following a cooling path, but the trend is a fictive result of isothermal crystallization at a highly undercooled and supersaturated starting state. Quartz-Pl granophyre along the border attests to extreme liquidus undercooling upon emplacement of pegmatite-forming melts far from their source into cold rocks at 6-8 km depth. Feldspar thermometry of thin pegmatite dikes indicates their crystallization with the inward advance of an isotherm of 425°± 50°C. That magnitude of liquidus undercooling (~ 250°C) induces a crystallization response that produces all of the textural and chemical attributes of pegmatites. Feldspars in gem-bearing and clay-packed miarolitic cavities at the centers of dikes record crystallization from ~ 450°C down to ~ 380°C at rims. Gem minerals crystallize within this interval, which lasts only months in thin dikes. Glassy rims of euhedral Kfs in cavities are non-perthitic because they crystallize close to the closure temperature of perthite exsolution (~ 320°C). Feldspars are succeeded by zeolites below ~ 280°C and clays at < 200°C.