Fluid inclusions in the Harney Peak Granite, Black Hills, South Dakota, USA: Implications for solubility and evolution of magmatic volatiles and crystallization of leucogranite magmas

Abstract

Experimental and theoretical studies have shown that magmatic fluids have strong influence on phase equilibria of granitic systems. A microthermometric study of fluid inclusions in the Proterozoic Harney Peak Granite, Black Hills, South Dakota, USA, was undertaken to obtain direct evidence for the composition and evolution of magmatic fluids in a leucogranitic system and to evaluate their effect on liquid-lines-of-descent of leucogranite magmas. A previous stable isotopic study has demonstrated that the granite has not interacted with nonmagmatic fluids since its crystallization. Therefore, fluids found in the inclusions are of magmatic origin and present an opportunity to directly sample magmatic fluids trapped during crystallization of a granitic magma and during subsolidus conditions. The Harney Peak Granite was emplaced as multiple sills, dikes, and small intrusions, many of which differentiated into coupled aplite-pegmatite layers. Three types of fluid inclusions are found in the granite. Type 1 consist of variable saline H 2OC0 2 mixtures with minor CH 4 or other gases. Inclusions of this type are found 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 temperature with decreasing CO 2/H 2O ratio of the inclusions. The trend is ascribed to progressive change in fluid composition with crystallization due to the differential solubility of CO 2 and H 2O in the high-silica magma. Isochores, coupled with independently determined crystallization temperatures, suggest entrapment pressure ∼3.5 kbar, consistent with geobarometry in the wallrocks. Type 2 inclusions are carbonic fluids dominated by CO 2, whereas type 3 inclusions are saline aqueous fluids without a detected carbonic component. Both types are mostly found along healed fractures in quartz. They are interpreted to be secondary inclusions that formed as a result of unmixing of type 1 inclusions at subsolidus conditions during uplift and cooling of the Harney Peak Granite dome. Freezing-point depressions, birefringent daughter crystals, phase-equilibria considerations, and α-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 H 2O and CO 2 in granitic melts and the composition of the most CO 2-rich type 1 inclusions ( X CO 2 = 0.55), the initial H 2O and CO 2 concentrations in the Harney Peak Granite magma are deduced to have been ∼3.5 wt% and 1500 ppm, respectively. The relatively low water concentration is consistent with temperatures of dehydration-melting reactions that produced the magma. It is suggested that the aplite-pegmatite segregations found in parts of the Harney Peak Granite are the result of initial rapid near liquidus crystallization of minerals with high Na/K ratios, including plagioclase and tourmaline, along a wide range of increasing αH 2O. Aplite crystallization was followed by crystallization of relatively potassic residual liquid enriched in fluxing elements under conditions of high αH 2O.