Stable isotope compositions of tourmalines from granites and related hydrothermal rocks of the Karagwe-Ankolean belt, northwest Tanzania

Abstract

Tourmaline is a ubiquitous mineral in the Mid-Proterozoic, peraluminous, syn- to post-tectonic granites and aplites and the related hydrothermal rocks of the Karagwe-Ankolean belt in northwest Tanzania. Electron microprobe analysis indicates that tourmalines from all of the intrusive and hydrothermal lithologies: (1) belong to the schorl-dravite solid-solution series; and (2) plot within the field occupied by tourmaline from Li-poor granitoids on the FeAlMg classification diagram. Oxygen isotope compositions range from +12.2 to +11.6‰ (SMOW) for magmatic tourmalines and from +10.8 to +9.8‰ for those of hydrothermal origin. Hydrogen isotope compositions vary from −79 to −65‰ (SMOW ) for magmatic tourmalines and from −99 to −84‰ for hydrothermal tourmalines. Water contents measured by manometry are constant at 3.0–3.2 wt.%. Within the broad grouping there arc systematic variations in both chemical [particularly $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$] and isotopic composition that relate to evolving magmatic and hydrothermal conditions. Igneous differentiation [increasing $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$ in magmatic tourmaline] has produced trends with higher δ¹⁸O in quartz, lower δ¹⁸O in tourmaline, and larger Δ_QTZ.-TOUR.-values, that reflect a combination of a reduction of crystallization temperature and an increase of $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$ in the residual melt. Subsequent cooling and interaction of an exsolved, B-rich magmatic fluid with the pelitic country rocks, resulted in the deposition of hydrothermal tourmaline with increasing $\text{Fe}{}_{\mathrm{tot}}\text{(Fe}{}_{\mathrm{tot}}\text{+ Mg ratio)}$ ratios, and progressively lower δ¹⁸O and δD -values.