Role of fractional crystallization, fluid-melt separation, and alteration on the Li and B isotopic composition of a highly evolved composite granite pluton: The case of the Eibenstock granite, Erzgebirge, Germany

Abstract

We present a comprehensive Li and B isotope study of granites, aplites, and igneous enclaves from the multi-phase Eibenstock granite in the Western Erzgebirge-Vogtland metallogenic province of Germany. The studied samples cover the entire compositional range of the granites from moderately to highly evolved and include variably altered types as obtained by magmatic fractionation, post-magmatic high- to medium-temperature and near-surface low-temperature alteration. Fractionation and alteration processes are unequivocally documented by the chemical variability of the rocks. Despite the marked imprint of these processes on bulk-rock compositions, our granite samples show only little variation in δ 7 Li (−0.52 to 0.75‰) and δ 11 B (−17.46 to −14.78‰), with surface samples defining the lower end of the δ 7 Li range. The narrow range in δ 7 Li suggests that magmatic fractionation and high-temperature overprint have a very minor effect on δ 7 Li. The B budget of the samples is dominated by tourmaline, which makes δ 11 B values insensitive for later high- to medium-temperature overprint or surficial low-temperature alteration. Depending on whether tourmaline crystallized before or after exsolution and loss of magmatic fluids, whole-rock samples have higher or lower δ 11 B values. Granite enclaves have δ 7 Li and δ 11 B values ranging from −1.51 to −0.81‰ and − 14.55 to −13.89‰, respectively. Some samples have chemical and mineralogical evidence for wall-rock interaction during emplacement or later overprint by external fluids. These samples show broader ranges in δ 7 Li (−2.61 to 2.21‰) and δ 11 B (−21.58 to −9.85‰). These values show that wall-rock interaction via assimilation and external fluids may affect δ 7 Li and δ 11 B to a larger extent than intra-magmatic processes, such as fractional crystallization, fluid-mediated autometasomatic overprinting, or exsolution of fluids from the melt. The offset of δ 7 Li and δ 11 B values towards the compositions of the wall rocks reflects the contrasting composition of granite and country rock and the addition of country-rock material to the granite. The magnitude of the offset reflects both the relative contribution of wall-rock derived Li and B to the granite and the magnitude of the difference in the Li and B isotopic compositions between them. • Whole rock δ 7 Li and δ 11 B are essentially unaffected by magmatic fractionation. • Fluid-melt separation may reduce δ 7 Li and δ 11 B in residual melt. • Effect of intra-granitic alteration on δ 7 Li and δ 11 B of granitic rocks is small. • Effects on δ 7 Li and δ 11 B during alteration depend on fluid source.