Boron B is a quintessentially crustal element Marschall and

Boron (B) is a quintessentially crustal adenosine receptor agonist (Marschall and Jiang, 2011). Its average abundance in the mantle is relatively low, whereas rocks of the continental crust show relatively high average abundances, containing a large fraction of Earth\'s B budget (Dutrow and Henry, 2011; Chaussidon and Albarède, 1992). The primitive mantle had a relatively low δ11B (−10 ± 2‰) (Chaussidon and Marty, 1995). In general, marine sedimentary rocks and magmatic rocks undergoing seawater alteration, or tourmaline originating from these rocks or fluids, are enriched in 11B but depleted in 10B relative to the mantle and continental crust (Leeman and Sisson, 1996; Marschall et al., 2006; Marschall and Jiang, 2011). Consequently, relatively high 11B/10B ratios in rocks and minerals imply derivation of boron from average value of B in seawater, whereas lower 11B/10B ratio suggests a continental source of the boron (Marschall and Jiang, 2011). Therefore, this distinctive geochemical characteristic of B isotopes is widely used to assess several important geological processes, e.g., oceanic slab subduction or evolution of continental crust (e.g., Chaussidon and Albarède, 1992; Ishikawa and Nakamura, 1994; Bebout and Nakamura, 2003; Leeman and Sisson, 1996; Kasemann et al., 2000; Marschall and Jiang, 2011; Martin et al., 2016). For example, in volcanic arcs, continuous dehydration of micas from subducted slabs and boron transport via fluid into the mantle wedge is generally considered to have been responsible for the boron isotopic signature (e.g., Wunder et al., 2005). As a borosilicate containing ∼3 wt.% B, tourmaline has significant chemical variability because of the variety of sites in the structure and the ease with which several of these sites can incorporate a wide variety of chemical species (Hawthorne and Dirlam, 2011). Once formed, tourmaline is highly stable in a variety of rock types over an exceptionally large P–T range, extending from near surface conditions to pressures in the diamond stability field and to temperatures above 700 °C (Marschall et al., 2009a; Dutrow and Henry, 2011; Marschall and Jiang, 2011; Van Hinsberg et al., 2011a,b). Because of this compositional sensitivity, tourmaline is an excellent indicator of conditions in its host environment (Van Hinsberg et al., 2011a,b). With the development of in situ analytical techniques, in situ B isotopic analyses of tourmalines have been widely applied to provide valuable information on fluid–rock interaction, fluid or magma origin and evolution and the sources of ore deposits (e.g., Jiang and Palmer, 1998; Jiang et al., 1999, 2002, 2008; Marschall and Jiang, 2011; van Hinsberg et al., 2011a,b; Yang and Jiang, 2012). Tourmaline is a very common mineral in peraluminous granites and rhyolites, which often occur in collisional orogenic belts, e.g., Himalayan–Tibetan Orogenic Belt. Neogene tourmaline-bearing two-mica granites occur widely in the Himalayan Block on the southern margin of the Tibetan Plateau (e.g., Le Fort et al., 1987; Inger and Harris, 1993; Harris and Massey, 1994; Guillot and Le Fort, 1995; Harris et al., 1995; Patiño Douce and Harris, 1998; Knesel and Davidson, 2002; Zhang et al., 2004, 2012; Liao et al., 2007; Guo and Wilson, 2012; Gao and Zeng, 2014; Liu et al., 2014, 2016a,b; Wu et al., 2015), but only minor Miocene–Quaternary tourmaline-bearing two-mica rhyolites are distributed along the northern margins of the Tibetan Plateau (Burchfiel et al., 1989; McKenna and Walker, 1990; Wang et al., 2012, 2016). Some researchers suggest that the leucogranites in the Himalayan Block crystallized from highly fractionated magmas, which originated from undetermined or weakly constrained sources, and underwent assimilation of crustal materials during their ascent (Wu et al., 2015; Liu et al., 2016a,b; Zheng et al., 2016). However, most researchers believe that these Neogene tourmaline-bearing two-mica granites or rhyolites on the northern and southern margins of the Himalayan–Tibetan Orogenic Belt were generated by dehydration melting of metasedimentary rocks involving biotite or muscovite or both (e.g., Le Fort et al., 1987; Burchfiel et al., 1989; McKenna and Walker, 1990; Inger and Harris, 1993; Harris and Massey, 1994; Guillot and Le Fort, 1995; Harris et al., 1995; Patiño Douce and Harris, 1998; Knesel and Davidson, 2002; Zhang et al., 2004, 2012; Liao et al., 2007; Guo and Wilson, 2012; Wang et al., 2012; Gao and Zeng, 2014). Therefore, the issue of source rock components needs to be further constrained. In particular, the question of whether the sedimentary rocks formed in either marine or continental settings, which has not ever been addressed before, needs to be resolved.