Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

Abstract

To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with δ 7 Li varying from –3.1 to +6.6. The δ 7 Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from –3.1 to +2.5 and overlap with post-Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between δ 7 Li and elements indicative of degrees of granite differentiation (SiO2, Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to res ect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may res ect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100–1100 ppm), quartz (30–140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in δ 7 Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to +21.3), with δ 7 Li showing an inverse correlation with Li concentration. This correlation res ects the mixing of isotopically heavy Li in quartz and lighter Li in s uid inclusions, as documented by s uid inclusion compositions (δ