Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic

Abstract

Abundant tourmaline occurs in the Lavicky leucogranite, Czech Republic as spherical to ovoid quartz-tourmaline orbicules, typically 5 to 7 cm in diameter. The tourmalines also occur as fine-grained quartz-tourmaline veins (<1 cm thick) that cut the orbicule-rich granite. Electron microprobe analyses reveal that tourmaline from the quartz-tourmaline orbicules is Fe-rich schorl with a range of Fe/(Fe + Mg) ratio 0.62 to 0.77 and Na/(Na + Ca) ratio 0.82 to 0.95. In contrast, tourmaline from quartz-tourmaline veins is Mg-rich dravite with a range of Fe/(Fe + Mg) ratio 0.23 to 0.45 and Na/(Na + Ca) ratio 0.67 to 0.90. Very low δ11B values of −37.3 to −32.1‰ are found in the tourmalines from the orbicules, whereas tourmalines from the veins display relatively higher δ11B values of −28.2 to −21.3‰. The overall large δ11B variation is suggested to reflect mixing of different boron sources and boron isotope fractionation during magmatic degassing and magmatic-hydrothermal evolution at late solidus to early subsolidus stages of granite crystallization. Only non-marine evaporites show very negative δ11B values (<−20‰) in all natural boron reservoirs, hence, the very low δ11B values of the quartz-tourmaline orbicules likely indicate a major contribution of boron from non-marine evaporites that probably exist in the magma source regions or assimilated into the magma during its ascent. Quartz-tourmaline orbicules may have formed during a transition from magmatic to hydrothermal processes, whereas the vein tourmalines formed by mixing of the exsolved magmatic-hydrothermal fluids with an external fluid rich in Ca and Mg and having higher δ11B than the exsolved magmatic fluids.