Tourmaline as a recorder of the magmatic-hydrothermal evolution in the formation of pegmatite: In-situ elemental and boron isotopic compositions of tourmaline from the Qinghe pegmatite, Chinese Altay orogen

Abstract

• Tourmalines record the magma-hydrothermal evolution of pegmatite-forming magma . • Extremely low REEs of tourmaline indicate direct melting of metapelite to form pegmatite . • Boron isotopic variation reflects the scale of fluid exsolution during pegmatitic evolution. We conducted elemental and boron isotope studies on tourmalines from a single pegmatite dyke of the Altay orogen to investigate its origin and evolution. The pegmatite dyke is composed of border, intermediate and core zones. The border and intermediate zone tourmalines mostly belong to schorl, while the core zone and magmatic hydrothermal tourmalines to dravite. Most tourmalines follow the (Na + Mg) (Al + X vac ) -1 , FeMg -1 and MnMg -1 exchange vectors. The core zone tourmalines show positively correlated FeO t and MgO, and negatively correlated FeO t and Al 2 O 3 , suggesting a rise of f O2 in this zone. Moreover, they show higher B, Mg, Sr and Eu than other tourmalines due to magmatic evolution. Tourmaline from the Altay pegmatite is characterized by extremely low REE abundances (<3 ppm) when compared with those from granites, suggesting an origin from direct melting of metapelitic rocks. The border and intermediate zone tourmalines have comparable boron isotopic compositions with δ 11 B = −14.0 to −12.2‰, which are similar to the mica schist (−13.2 ‰ to −12.2‰), but slightly heavier than the core zone and magmatic hydrothermal tourmalines (−14.4‰ to −13.3‰). This suggests derivation of the pegmatite from melting of the metapelite without causing boron isotope fractionation, whereas late-stage fluid exsolution caused somewhat boron fractionation. This may be indicative of only small amounts of fluids being released from the barren pegmatite-forming magma. Tourmalines from mineralized pegmatites basically exhibit heavier boron isotope compositions than those from barren pegmatites, due probably to more involvement of evaporate in the source of the former.