REE-Depleted Leucogranites, Black Hills, South Dakota: a Consequence of Disequilibrium Melting of Monazite-Bearing Schists

Abstract

Abstract Two isotopically distinct but otherwise chemically similar leucogranite suites in the Proterozoic Horney Peak Granite, Black Hills, South Dakota, have contrasting light rare earth element (LREE) concentrations. Most samples of a relatively 18O-depleted suite have LREE- enriched, chondrite-normalized patterns, typical of melts derived from metasedimentary protoliths, whereas all samples of the regionally significant, relatively 18O-enriched suite have LREE-depleted patterns. The latter patterns are interpreted to have resulted from disequilibrium melting of schists. Monazite and perhaps other accessory minerals remained armored by biotite and garnet which did not partake in the muscovite dehydration-melting reaction that produced LREE-depleted melts. The REE concentrations in the LREE-depleted samples are below saturation levels for monazite at reasonable melting temperatures and melt water contents, whereas the REE concentrations in the LREE-enriched samples yield 700–800°C monazite saturation temperatures, reasonable for biotite dehydration-melting reactions. LREE depletions, analogous to those in the LREE-depleted granites, are also found in leucosomes of partially molten schists, thought to be the protolith for the granite. In contrast, the melanosomes hold the accessory minerals and bulk of the LREEs.