CRYSTALLIZATION TEMPERATURES OF THE TIN MOUNTAIN PEGMATITE, BLACK HILLS, SOUTH DAKOTA: INSIGHTS FROM COMPOSITIONS OF COEXISTING FELDSPARS

Coexisting K-rich and Na-rich feldspars from a zoned, Li-bearing pegmatite were investigated in order to better constrain the crystallization temperature of rare-element pegmatites. Water, alkalis, and volatile elements are thought to act as fluxes, depressing the granitic solidus, thus leading to low temperatures of crystallization of granitic magmas. However, the temperature at which rare-element pegmatites crystallize remains an area of controversy. The Tin Mountain pegmatite is located in the Precambrian core of the Black Hills, South Dakota, within a rare-element pegmatite field surrounding the 1.715 B.y. Harney Peak Granite. Its semi-concentric zones are comprised of variable proportions of quartz, K-rich feldspar, Na-rich feldspar, micas, spodumene, amblygonite, beryl, and minor amounts of other Li, Cs, B, and P minerals.

The methods used included petrographic and textural analysis of feldspars from the wall and intermediate pegmatite zones, electron microprobe analysis, and feldspar geothermometry. We observed a lack of perthitic textures in the areas where both K- and Na-feldspars where present. Instead, the pegmatite contains randomly oriented, euhedral to subhedral crystals of coexisting feldspars. This suggests that the feldspars formed at equilibrium below the alkali-feldspar solvus curve. In addition, the K-feldspar lacked tartan twinning, indicating either that the K-feldspar is not microcline or that the microcline formed at a temperature below its inversion point. The three-component feldspar thermometer of Elkins and Grove (1990, Am. Mineral. v. 75) applied to preliminary microprobe data collected on coexisting feldspars yielded temperatures ranging between 200 and 400 °C. The lowest temperatures probably indicate deuteric re-equilibration of the pegmatite, while the higher temperatures may represent magmatic temperatures of crystallization of the fluxed silicate melt.