STABLE ISOTOPE COMPOSITIONS OF TOURMALINES FROM GRANITES AND RELATED HYDROTHERMAL ROCKS OF THE KARAGWE-ANKOLEAN BELT, NORTHWEST TANZANIA

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 parts per thousand (SMOW) for magmatic tourmalines and from +10.8 to +9.8 parts per thousand for those of hydrothermal origin. Hydrogen isotope compositions vary from -79 to -65 parts per thousand (SMOW) for magmatic tourmalines and from -99 to -84 parts per thousand for hydrothermal tourmalines. Water contents measured by manometry are constant at 3.0-3.2 wt.%. Within the broad grouping there are systematic variations in both chemical [particularly Fe(tot)/(Fe(tot) + Mg ratio)] and isotopic composition that relate to evolving magmatic and hydrothermal conditions. Igneous differentiation [increasing Fe(tot)/(Fe(tot) + Mg) in magmatic tourmaline] has produced trends with higher delta-O-18 in quartz, lower delta-O-18 in tourmaline, and larger DELTA(QTZ.-TOUR.-values, that reflects a combination of a reduction of crystallization temperature and an increase of Fe(tot)/(Fe(tot) + Mg) 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 Fe(tot)/(Fe(tot) + Mg) ratios, and progressively lower delta-O-18 and delta-D-values.