Stable isotope evidence for the petrogenesis and fluid evolution in the Proterozoic Harney Peak leucogranite, Black Hills, South Dakota

Abstract

Oxygen and hydrogen isotope systematics of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in order to constrain its petrogenesis and to examine the role of fluids in a peraluminous granite-pegmatite magmatic system. The leucogranite and its satellite intrusions were emplaced as hundreds of sills and dikes which are texturally heterogeneous, ranging from aplitic to pegmatitic. The dominant ferromagnesian mineral in the core of the granite is biotite while along its perimeter and in satellite intrusions it is tourmaline. Biotite and tourmaline are for the most part mutually exclusive. Oxygen isotopes among minerals in non-pegmatitic rocks from throughout the pluton equilibrated for the most part at magmatic temperatures between >800 to 650°C. In the pegmatitic samples, quartzfeldspar oxygen isotope fractionations point to disequilibrium, probably a result of the sequential crystallization of these minerals. The isotopic composition of most pegmatites suggests local derivation by differentiation of emplaced batches of magma. The whole rock δ 18O values of the granites are heterogeneous, ranging from 10.4 to 14.3‰ However, there is a pronounced difference in the isotopic composition of the biotite-containing granites from the core of the pluton ( 11.5 ± 0.6‰) and the tourmaline-rich granites from its perimeter and the satellite intrusions ( 13.2 ± 0.8‰). The average oxygen isotopic composition of the surrounding schists is identical to that of the latter granites. Biotite-muscovite and tourmaline-muscovite ΔD values vary from −20 to − 10‰ and from 0 to +10‰, respectively. Although these values differ from theoretical values, the narrow ranges suggest that the minerals are in isotopic equilibrium and that their isotopic compositions record the composition of the magmatic fluid. The calculated δD value of the magmatic fluid is −67 to −58‰, well within the magmatic water range. However, somewhat elevated mineral δD values in the southern part of the Harney Peak Granite, where the metasediments reached the second-sillimanite isograd, suggest that limited interaction with fluids derived from the dehydrating metasediments may have occurred. Alternatively, the elevated values could reflect a hydrogen isotope heterogeneity in the source or interaction with another magmatic system to the south of the Harney Peak Granite. It is shown that fractional crystallization or subsolidus interaction of the Harney Peak Granite with the magmatic fluid or a fluid derived from the schist cannot explain the difference between the δ 18O values of the core and perimeter granites. Although some oxygen isotope heterogeneity in the granite could be explained by assimilation of the country rocks, assimilation cannot explain all of the difference between the two granite types. Instead, it is proposed that intrusion of the magma which led to the biotite granites in the core of the pluton at the culmination of regional metamorphism initiated melting of the schists at a depth somewhat greater than the present level of erosion. The melts were emplaced into the overlying schist and differentiated into the many tourmaline-rich granite-pegmatite sills and dikes comprising much of the perimeter of the Harney Peak Granite and its satellite plutons. Alternatively, the different melts may have resulted from melting along an isotopically heterogeneous vertical section of the crust in response to the ascent of a thermal pulse.