American Mineralogist, Volume 96, pages 752–767, 2011 0003-004X/11/0506–752$05.00/DOI: 10.2138/am.2011.3560 752 Aluminous and alkali-deicient tourmaline from the Singhbhum Shear Zone, East Indian shield: Insight for polyphase boron iniltration during regional metamorphism NaNdiNi SeNgupta, 1, * pulak SeNgupta, 2 aNd HimaNSHu kumar SacHaN 3 1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700019, India 2 Department of Geological Sciences, Jadavpur University, Kolkata 700032, India 3 Wadia Institute of Himalayan Geology, 33, Gen. Mahadev Singh Road, Dehra Dun 248001, India abStract In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing veins of variable thickness (tens of micrometers to 1 m thick) are hosted in kyanite-quartzite and kyanite-mica schist. The veins have been classified into three types, which are, from oldest to young- est, generation I (tourmaline), II (dumortierite + tourmaline), and III (tourmaline) veins. Alkali- and Mg-rich tourmaline [X Mg = Mg/(Mg + Fe) = 0.68 ± 0.09; X = Na, Ca, K, o (vacancy) = 0.40 ± 0.12] is the sole borosilicate in generation I veins, which have been folded in response to regional deformation. Generation II veins were emplaced along shear bands (1 mm to 1 m thick) developed parallel to the axial planes of these folds. Long axes of fibrous dumortierite and prismatic tourmaline of generation II veins are oriented along the shear bands and have been bent around lenticular remnants of host kyanite-quartzite. Generation III veins have a dendritic pattern, crosscut generation II veins and show aggregates of fibrous to acicular tourmaline. Prismatic tourmaline in generation II veins is optically zoned with a green tourmaline core that is variably replaced and rimmed by blue tourmaline. Fibrous to acicular tourmaline in generation III veins is comprised up of blue tourmaline with compositions similar to the rim composition of prismatic tourmaline in generation II veins. Green and blue tour- maline is aluminous (Al total >7 apfu) and alkali-deficient (X = 0.71 ± 0.08). High Y Al content, high X, low X Mg (0.19 ± 0.10), and excess cation charge indicate tourmaline in generation II veins is rich in an “oxy-foitite” component. Foitite-rich tourmaline in generation III veins has tetrahedral Al and a slightly lower Mg-content and X than those of generation II veins. Optical zoning in prismatic tour- maline corresponds to an abrupt compositional change with paragenetically older green tourmaline having higher Al and X Mg , but lower alkali content in the X-site than the blue tourmaline rim. The compositional variation in green and blue tourmaline can be explained by a combination of coupled substitutions represented by AlO[R(OH)] –1 and Al(NaR) –1 , where R = (Fe 2+ + Mg). Pseudosections in the system Na 2 O-K 2 O-Al 2 O 3 -SiO 2 -H 2 O constructed from bulk chemical compositions of the studied rocks and the P-T slopes of two isochors computed from brine-rich inclusions trapped in quartz grains indicate that borosilicate formation in generation II and III veins occurred within 4.1 ± 0.5 kbar and 377 ± 21 °C. The mineral assemblages and textures suggest that the borosilicate-bearing veins formed from infiltration-driven alteration of host kyanite-quartzite and kyanite-mica schist along structurally controlled conduits by more than one batch of chemically distinct boron-rich aqueous fluids. Keywords: Singhbhum shear zone, kyanite, foitite, “oxy-foitite,” boron-infiltration iNtroductioN Rocks in shear zones commonly show enhanced permeability compared to wall rocks and hence, serve as conduits for fluid flow in orogenic belts (Ferry and Gerdes 1998). Fluids in shear zones are not necessarily in equilibrium with the wall rocks; to achieve equilibrium, infiltrating fluids exchange mass and en- ergy with rocks with which they come in contact. This process of fluid-rock interaction stabilizes exotic assemblages including boron-bearing minerals and Cu-Fe-U-Au ores (Slack 1996). Therefore, the minerals present in and around the network of veins within sheared country rock provide a wealth of informa- tion about hydrological regimes of orogenic belts. Tourmaline has the general structural formula, XY 3 Z 6 (T 6 O 18 ) (BO 3 ) 3 V 3 W, where X = Na, Ca, K, o (vacancy); Y = Li, Mg, Fe 2+ , Mn 2+ , Al, Cr 3+ , V 3+ , Fe 3+ , Ti 4+ ; Z = Mg, Al, Fe 3+ , V 3+ , Cr 3+ ; T = Si, Al; V = OH, O; W = OH, F, O. There is extensive substitution within the cation and anion sites in response to P, T, and the compositions of the host rock and fluid with which the tourmaline equilibrated (e.g., Hawthorne and Henry 1999; Dutrow and Henry 2000; Henry and Dutrow 2001; Wodara and Schreyer 2001; von Goerne et al. 2001; Henry et al. 2003; Van den Bleeken et al. 2007; Torres-Ruiz et al. 2003; Bacik et al. 2008; Pesquera et al. 2005, 2009). Published data on natural Li-poor tourmaline compositions suggest that tourmaline with * E-mail: nandinisg@yahoo.com