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Free keywords:
Granite, Li isotopes, B isotopes, Fractional crystallization, Hydrothermal overprint, Wall-rock interaction, Near-surface alteration
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 δ7Li (−0.52 to 0.75‰) and δ11B (−17.46 to −14.78‰), with surface samples defining the lower end of the δ7Li range. The narrow range in δ7Li suggests that magmatic fractionation and high-temperature overprint have a very minor effect on δ7Li. The B budget of the samples is dominated by tourmaline, which makes δ11B 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 δ11B values. Granite enclaves have δ7Li and δ11B 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 δ7Li (−2.61 to 2.21‰) and δ11B (−21.58 to −9.85‰). These values show that wall-rock interaction via assimilation and external fluids may affect δ7Li and δ11B 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 δ7Li and δ11B 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.
