Tourmaline growth and evolution in S-type granites and pegmatites: constraints from textural, chemical and B-isotopic study from the Gangpur Schist Belt granitoids, eastern India

Abstract

AbstractAn extensive dataset of major- and trace-element and B-isotope composition related to the compositional variation of the tourmalines from the S-type granite and pegmatite from the Upper Bonai and Gangpur Group granitoids, eastern India, is provided. The Gangpur Schist Belt, consisting of the Upper Bonai and Gangpur Group, lies at the eastern end of the Proterozoic Central Indian Tectonic Zone. An attempt to constrain the source of the boron isotope and describe the magmatic–hydrothermal evolution of the granite–pegmatite system is made. The tourmalines record generally low trace-element concentrations, with the exception of some elements such as Zn, Li, Ga, Mn and Ti. The Zn and Li concentration increases from the tourmalines in the granites to the tourmalines in the pegmatite. Trace-element compositions also vary slightly from the core tourmaline to the rim tourmaline. Tourmalines in granites/pegmatites are mostly Fe-rich schorl, and have S-type B-isotopic signature (δ11B = −10‰ to −12‰). Their chemistries reflect changing salinities of the granitic melts as well as vapour exsolution during progressive crystallization. This study indicates that the tourmalines in the pegmatites crystallized from evolved flux-element-rich (F, B, Li) melt, probably after the crystallization of granite-forming melt.