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

1. Introduction

Tourmaline is the principal boron-bearing mineral associated with S-type granites and pegmatites, and may contain important information pertaining to the petrogenesis of these rocks. The isotopic fractionation among the two isotopes of B (¹¹B and ¹⁰B) in melt-fluid-crystal systems, reflected in the B-isotopic composition of tourmaline, is a powerful tool for constraining magmatic–metamorphic–hydrothermal processes (Henry & Guidotti, 1985; Bebout et al. 1993, 1999; Henry & Dutrow, 1996; Shaw, 1996; Slack, 1996; Jiang, 2001; Pesquera et al. 2005; Marschall et al. 2007; Pal et al. 2010; da Costa et al. 2014; Drivenes et al. 2015; Duchoslav et al. 2017). Boron isotope is also a good tracer to understand the sedimentary source, thus making it extremely efficient to reconstruct the parent source of the S-type granites (Swihart et al. 1986; Ishikawa & Nakamura, 1993). In the past few decades, the tourmalines in different granitic–pegmatitic rocks have been studied extensively and a plethora of different ideas have emerged about their formation in granitoid systems (Smith & Yardley, 1996; Trumbull & Chaussidon, 1999; Roda-Robles et al. 2004; Černý & Ercit, 2005; Maner & London, 2017).

Tourmalines in S-type granitoids/pegmatites can preserve the record of the evolution of boron in a magmatic–hydrothermal system. Previous studies have documented contrasting results on the fractionation of B-isotope in tourmaline during the magmatic–hydrothermal processes. Some studies report strong fractionation of ¹¹B and ¹⁰B between the tourmalines in granite and evolved pegmatites (Trumbull et al. 2013), whereas some report relatively small to no change at all (Tonarini et al. 1998).

In this study, I investigate the chemistry and B-isotope composition of tourmalines in granitoid rocks from the Gangpur Schist Belt (GSB) in eastern India. These chemical and isotopic data for tourmaline in S-type granites and pegmatites associated with the metasediments are used to put constraints on petrological processes operating during magmatic to hydrothermal evolution of the granite–pegmatite system. The GSB rocks are ideally suited for this purpose as tourmaline is a ubiquitous phase in S-granitoids and related pegmatites, which indicates that the rocks have high boron content and may have been derived from B-rich sedimentary sources. The Gangpur Schist Belt has undergone polyphase deformation-metamorphism events (Chakraborty et al. 2019), which may have been responsible for several hydrothermal...