Chemical and boron isotopic composition of tourmaline from the Conadong leucogranite-pegmatite system in South Tibet

Abstract

Tourmalines occur widely within leucogranites and pegmatites that intrude the leucogranites in the south Tibetan plateau. Morphological and geochemical (elemental and boron-isotopic) studies on these tourmalines are very limited. Furthermore, the tourmaline genetic correlation between the leucogranite and the pegmatite is also unknown. In this contribution, two types of tourmaline occurrences have been identified in the Conadong area, south Tibet, including randomly disseminated tourmaline in leucogranite (GT type) and radial, dendritic or massive tourmaline aggregates in pegmatite dikes that intrude the leucogranite (PT type). We performed major element and boron isotopic analyses of tourmaline using electron microprobe (EMPA) and laser ablation multi-collector inductively coupled plasma mass spectrometry, respectively. The two types of tourmalines are enriched in Fe, Na and Al but relatively depleted in Ca and Mg elements, with compositions close to alkali group tourmaline and schorl. All the tourmalines are likely the products of equilibrium crystallization and their dominance of Na was controlled by the magmatic compositions of their host rocks. The GT type tourmalines show a narrow range of δ11B values between −9.78 ± 0.81‰ and − 8.53 ± 0.68‰ (with a mean of −8.91 ± 0.18‰); whereas the PT type tourmalines have significantly lower δ11B values than the GT type, ranging from −14.02 ± 0.85‰ to −11.83 ± 0.57‰ (with a mean of −13.31 ± 0.49‰). These data together with their morphology and petrography indicate that the Conadong granitic tourmalines were derived by the melts at the early magmatic stage, whereas the pegmatitic tourmalines precipitated from magmatic–hydrothermal fluids at the late magmatic stage. The variation in boron isotopic compositions (Δ11B = 4.4‰) between the PT and the GT type tourmalines is most likely correlated with their different melt/fluid source regions that had distinct Sr-Nd-B isotopic compositions.