FLUID INCLUSIONS IN THE HARNEY-PEAK GRANITE, BLACK-HILLS, SOUTH-DAKOTA, USA - IMPLICATIONS FOR SOLUBILITY AND EVOLUTION OF MAGMATIC VOLATILES AND CRYSTALLIZATION OF LEUKOGRANITE MAGMAS

Experimental and theoretical studies have shown that magmatic fluids h ave strong influence on phase equilibria of granitic systems. A microt hermometric study of fluid inclusions in the Proterozoic Harney Peak G ranite, Black Hills, South Dakota, USA, was undertaken to obtain direc t evidence for the composition and evolution of magmatic fluids in a l eucogranitic system and to evaluate their effect on liquid-lines-of-de scent of leucogranite magmas. A previous stable isotopic study has dem onstrated that the granite has not interacted with nonmagmatic fluids since its crystallization. Therefore, fluids found in the inclusions a re of magmatic origin and present an opportunity to directly sample ma gmatic fluids trapped during crystallization of a granitic magma and d uring subsolidus conditions. The Harney Peak Granite was emplaced as m ultiple sills, dikes. and small intrusions, many of which differentiat ed into coupled aplite-pegmatite layers. Three types of fluid inclusio ns are found in the granite. Type 1 consist of variable saline H2O-CO2 mixtures with minor CH4 or other gases. Inclusions of this type are f ound in tourmaline and quartz in aplite layers, are generally isolated , and have negative crystal shapes. They are interpreted to be primary . There is a general increase in salinity and decrease in isochore tem perature with decreasing CO2/H2O ratio of the inclusions. The trend is ascribed to progressive change in fluid composition with crystallizat ion due to the differential solubility of CO2 and H2O in the high-sili ca magma. Isochores, coupled with independently determined crystalliza tion temperatures, suggest entrapment pressure similar to 3.5 kbar, co nsistent with geobarometry in the wallrocks. Type 2 inclusions are car bonic fluids dominated by CO2, whereas type 3 inclusions are saline aq ueous fluids without a detected carbonic component. Both types are mos tly found along healed fractures in quartz. They are interpreted to be secondary inclusions that Formed as a result of unmixing of type 1 in clusions at subsolidus conditions during uplift and cooling of the Har ney Peak Granite dome. Freezing-point depressions, birefringent daught er crystals, phase-equilibria considerations, and ct-track mapping of B and Li distribution in thin sections, all indicate that both type 1 and type 3 inclusions contain solutes such as K, Li, and B in addition to NaCl. Using published solubility models for H2O and CO2 in graniti c melts and the composition of the most CO2-rich type 1 inclusions (XC O2 = 0.55), the initial H2O and CO2 concentrations in the Harney Peak Granite magma are deduced to have been similar to 3.5 wt% and 1500 ppm , respectively. The relatively low water concentration is consistent w ith temperatures of dehydration-melting reactions that produced the ma gma. It is suggested that the aplite-pegmatite segregations found in p arts of the Harney Peak Granite are the result of initial rapid near l iquidus crystallization of minerals with high Na/K ratios, including p lagioclase and tourmaline, along a wide range of increasing aH(2)O. Ap lite crystallization was followed by crystallization of relatively pot assic residual liquid enriched in fluxing elements under conditions of high aH(2)O. Copyright (C) 1997 Elsevier Science Ltd.