Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Pal... more Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo- to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7–11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (~49 mol%) with significant La (~30 mol%) and Nd (~21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencite-kyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks sugges...
In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing v... more 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 youngest, 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 tourmaline 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 tourmaline 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.
Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic ass... more Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about ß uid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic
ABSTRACT The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern... more ABSTRACT The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66-1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (XMg &gt; 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions: 1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
Albitization of K-feldspar in syntectonic granitoid rocks provides valuable insight about the syn... more Albitization of K-feldspar in syntectonic granitoid rocks provides valuable insight about the synergic interplay of physical and chemical forces (Passchier 1985, Pryer and Robin 1995, 1996, Vernon 1999). Closed system reworking that involve local decomposition of Na-bearing minerals (e.g. plagioclase or primary perthite) and 'flame perthite' formation during deformation and metamorphism has been documented by these existing studies. In this communication, I present textural features of 'flame perthite' from a granitoid exposed in parts of the Palaeoproterozoic Mahakoshal Group (MG). Detailed geological and geochronological information are reviewed in Roy et al. (2002). In the study area (near the city of Renukoot, 24° 12' N, 83° 2' E), MG exposes an ensemble of lithologies including andalusite bearing mica schist, banded psammopelite, phyllite and quartz reef. These rocks show evidences of poly-phase deformation and develop roughly E-W trending pervasive foliation. A suite of porphyritic granitoid syntectonically intrudes the supracrustal rocks of the MG (Fig 1). The porphyritic granitoid rocks contain the xenoliths of mica schist and show the regional E-W fabric. Though a porphyritic structure defined by eu-and subhedral crystals of alkali feldspar and a fine matrix of mica, quartz and feldspar, intense crystalloplastic deformation are evident in narrow zones (Fig. 2a). In these zones of intense deformation, phenocrysts of feldspar are stretched parallel to the fabric with increase in phyllosilicate content. The latter minerals define the mylonitic foliation (regional E-W fabric) that bends around the feldspar augen (Fig. 2b). Under microscope, feldspar grains show Carlsbad-and Tartar (microcline) and lamellar (plagioclase) twins and are swerved by thin folia made up of mica and elongated quartz grains that commonly form ribbon texture. Extensive replacement of K-feldspar and plagioclase (An 02) by mica is a common feature of this rock (Fig. 2b, 3b). In most places, K-feldspar grains are variably replaced with albite. Albite lamellae replace K-feldspar grains along the Murchison plane and develop the conspicuous flame perthite (Fig. 2c, d). The 'flame perthite' has the maximum thickness near the rim and tapers away into the inner part of the grain of host K-feldspar (Fig. 2d). The albite 'flames' are preferentially aligned parallel to the direction of maximum compression. In places, myrmekitic lobe develops on the surface (which is normal to the principal compression direction) of K-feldspar augen and protrudes inward to the porphyroclasts (Fig. 3a). Microstructural features such as (a) 'chessboard' like blocky subgrain texture in quartz (Fig. 3c), (b) grain boundary migration recrystallization of quartz and (c) bending of twin lamellae and recrystallization of plagioclase (Fig. 3d) suggest that the temperature was at least 450 o C during deformation (reviewed in Vernon, 2004). Field features and microstructures together support that shear deformation affecting the granitoid, occurred in a brittle-ductile transitional regime. To understand the chemical evolution of the albitized K-feldspar, textural modeling is performed integrating the observed textures and algebraic analysis of the compositions of involved mineral phases with the help of the computer program C-Space. Modeled balanced chemical reactions are given below: 49.94Kfs+ 45.50Na + + 1.00Ca 2+ = 49.97Pl+ 48.47K + (∆Vs %=-6.37%)-(1)
The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe o... more The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66–1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (XMg > 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions:
(1) 1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na
(2) 1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
Stoichiometry of the balanced chemical reactions suggests that a significant amount of P, Mg, and H2O were added to, and Al and SiO2 were subtracted from, the host kyanite-rich rock to produce augelite and lazulite. Experimental studies in the system Al2O3-SiO2-FeO-MgO-P2O5-H2O and the results of quantitative geothermobarometry suggest that lazulite and augelite were formed in a narrow temperature (440 ± 40°C) and pressure (~6.3 ± 1 kbar) range. Ductile shearing along the SSZ channelized the P- and Mg-rich fluids that metasomatized the kyanite-rich bands and veins to produce lazulite. The inferred P-T conditions can be explained by burial of the studied rock under an ~25 km-thick thrust sheet of NSFB during the Palaeoproterozoic orogenesis.
Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Pal... more Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo-to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7-11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (∼49 mol%) with significant La (∼30 mol%) and Nd (∼21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencitekyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks suggest that florencite was formed during Palaeoproterozoic metamorphism that culminated at the -range of 490 ± 40 ∘ C and 6.3 ± 1 kbar. Integrating all the geological features it is postulated that florencite was formed due to metasomatism of some aluminous protolith by infiltration of acidic fluids charged with PO 4 −3 and LREE.
Behavior of elements during fluid -mediated processes provides crucial information to understand ... more Behavior of elements during fluid -mediated processes provides crucial information to understand the chemical evolution of the continental crust and the upper mantle. Field evidences coupled with the observations from a number of experimental studies have demonstrated that Al and Ti remain virtually immobile during fluid-rock interactions owing to their low solubilities in crustal and mantle fluids. This realization provoked many petrologists to assume Ti-and Al-fixed reference frames to model the chemical changes in rocks that had interacted with chemically active fluids. Contrary to this common perception, we present geological evidence that demonstrates that under certain conditions, Al and Ti can be significantly mobile during crustal metasomatism. The example comes from a kyanitequartzite occurring within the 1.5-1.7 Ga Singhbhum Shear Zone (SSZ) of the East Indian shield. The SSZ exposes an ensemble of lithologies with diverse bulk compositions. These rocks are repeatedly folded and are dissected by ductile shear zone. Original compositions of the rocks in the SSZ were significantly changed due to extensive hydrothermal alteration (sensu lato) that accompanied the shear deformation (Mukhopadhyay and Deb, 1995). Extant petrological information indicate that the metamorphism / metasomatism in the SSZ and the adjoining areas culminated at ~480 o ± 50 o C and 6.5 ± 1 kbar (Sengupta et al. 2005). The studied kyanite quartzite is located about 20km west of the town Tatanagar, in the state of
In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing v... more 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 youngest, 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 tourmaline 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 tourmaline 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.
The arcuate Singhbhum shear zone (SSZ) of East India separates Meso-to Palaeo-Proterozoic supracr... more The arcuate Singhbhum shear zone (SSZ) of East India separates Meso-to Palaeo-Proterozoic supracrustal rocks of North Singhbhum fold belt (NSFB) from the Archaean Singhbhum craton. The SSZ exposes diverse lithologies with high variant mineral assemblages that host economically viable polymetallic deposits (Cu, Fe & U) ore deposits. Detailed geological investigation in this narrow belt has identified superposed folding and accompanying ductile shearing. Laterally continuous to discontinuous outcrops of kyanite-rich rocks occur all along the SSZ close to its hanging wall side and also with in the immediate pelitic rocks of the adjoining NSFB. Field features together with low concentrations of Fe 2 O 3 (0.03-0.61 wt%), TiO 2 (0.55±0.13 Wt%) and Ga (23±12 ppm), high Al 2 O 3 (c.35±10 wt%) and lack of any correlation between the last two chemical constituents are inconsistent with the idea that these rocks represent metamorphosed bauxite deposits or sedimented residual clay. Base leaching and consequent enrichment of Al 2 O 3 in the host rocks by acidic fluids presumably derived from magmatogenic hydrothermal fluids seem to be consistent with the geological and geochemical data and high δ 18 O values of kyanite rich rocks (>60 vol% kyanite, c. 4.7-6.8 o / oo ) including δ 18 O values of two kyanite grains (5, 5.5 o / oo ) from an unaltered massive kyanite deposits. Extensive fluid-rock interaction during and subsequent to the intense shearing of this region converted the anhydrous kyanite quartzite to muscovite schist with variable proportion of chloritoid, chlorite, biotite, and tourmaline. Frequently, kyanite grains are pseudomorphed by muscovite and chloritoid without the development of any Al-rich phases nearby. These features indicate nearly uniform rates of kyanite dissolution and precipitation of less aluminous muscovite and chloritoid, which in turn, support Al mobility during this metasomatic event. Part of the scavenged Al was presumably re-deposited as kyanite (± quartz) bearing veins that dissected the kyanite muscovite schists. Ramifying veins of fine-grained tourmaline and randomly oriented paragonite flakes are also present in muscovite schist indicating that the infiltrating fluid was locally rich in boron and Na. In eastern and south eastern parts of SSZ, physical conditions deduced from the conventional geothermobarometry and stability relation of the pseudomorph forming phases converged at 6.4±0.4 Kbar and 480 ± 40 o C. Extant fluid inclusion data and thermodynamic modeling of aH 2 O computed from mineral fluid equilibria indicate high salinity (>25 wt% NaCl equivalent) of the infiltrated fluids. These fluids were out of equilibrium with the host rocks and triggered the metasomatic alteration of the kyanite rich rocks, transport of Al 3+ and formation of kyanite rich veins. Symposium titles MPN-03 Mineral replacement and mass transfer in hydrothermal systems: From the nanoscale to the megascale Presentation Preference Oral presentation
Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Pal... more Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo-to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7-11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (∼49 mol%) with significant La (∼30 mol%) and Nd (∼21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencitekyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks suggest that florencite was formed during Palaeoproterozoic metamorphism that culminated at the -range of 490 ± 40 ∘ C and 6.3 ± 1 kbar. Integrating all the geological features it is postulated that florencite was formed due to metasomatism of some aluminous protolith by infiltration of acidic fluids charged with PO 4 −3 and LREE.
Fluorite, a fluorine-bearing mineral present in different rock types can cause groundwater contam... more Fluorite, a fluorine-bearing mineral present in different rock types can cause groundwater contamination through its dissolution as fluoride ion into aquifer. Bounded by roughly E-W-trending Son-Narmada north and south faults (SNNF and SNSF respectively), the areas in and around Jabalpur (Madhya Pradesh, central India) expose Palaeoproterozoic Mahakoshal Group of rocks including mica-schist, quartzite, dolomitic marble, minor mafic dykes and large elliptical granite plutons commonly known as Madan Mahal Granite (MMG). Part of the MMG is covered by unmetamorphosed Phanerozoic rock sequence including sandstone, limestone, clay, Deccan basalts and un/semi-consolidated sediments. Aquifers with high fluorine content present in the MMG and in the overlying sandstones are presumed to be the source of dental and skeletal fluorosis of the people of Jabalpur District. Integrating the extant geological and hydrological parameters, a model has been proposed to explain the high fluorine content in groundwater of Jabalpur District. This study shows that fluorite was developed in the MMG during Palaeoproterozoic deformation and metamorphism. Sedimentary rocks that are sourced from the MMG and host the major aquifers also contain fluorite. The proposed model visualises that groundwater became alkaline due to chemical interaction with feldspar. Because of enhanced solubility of F in alkaline water, groundwater that are buffered by feldspathic wall rocks leached large amount of F from fluorite present in the host rocks and become hazardous for human consumption. High heat flow of the region further aggravates F-solubility in ground water.
Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic ass... more Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about ß uid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic to psammopelitic and quartzofeldspasthic rocks, including meta-granite. The rocks were affected by two sets of folding (F 1 and F 2 ) and ductile shearing associated with F 1 during ca. 1.6-1.8 Ga tectonic activity in this belt. Quantitative geothermobarometry and the stability relations of the metamorphic assemblages developed in the pelitic rocks establish the presence of two episodes of metamorphism; a prograde M 1 event that culminated at 480 ± 40 °C, 6.4 ± 0.4 kbar, and an M 2 event that caused retrogression of the M 1 assemblages.
Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Pal... more Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo- to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7–11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (~49 mol%) with significant La (~30 mol%) and Nd (~21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencite-kyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks sugges...
In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing v... more 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 youngest, 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 tourmaline 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 tourmaline 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.
Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic ass... more Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about ß uid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic
ABSTRACT The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern... more ABSTRACT The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66-1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (XMg &gt; 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions: 1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
Albitization of K-feldspar in syntectonic granitoid rocks provides valuable insight about the syn... more Albitization of K-feldspar in syntectonic granitoid rocks provides valuable insight about the synergic interplay of physical and chemical forces (Passchier 1985, Pryer and Robin 1995, 1996, Vernon 1999). Closed system reworking that involve local decomposition of Na-bearing minerals (e.g. plagioclase or primary perthite) and 'flame perthite' formation during deformation and metamorphism has been documented by these existing studies. In this communication, I present textural features of 'flame perthite' from a granitoid exposed in parts of the Palaeoproterozoic Mahakoshal Group (MG). Detailed geological and geochronological information are reviewed in Roy et al. (2002). In the study area (near the city of Renukoot, 24° 12' N, 83° 2' E), MG exposes an ensemble of lithologies including andalusite bearing mica schist, banded psammopelite, phyllite and quartz reef. These rocks show evidences of poly-phase deformation and develop roughly E-W trending pervasive foliation. A suite of porphyritic granitoid syntectonically intrudes the supracrustal rocks of the MG (Fig 1). The porphyritic granitoid rocks contain the xenoliths of mica schist and show the regional E-W fabric. Though a porphyritic structure defined by eu-and subhedral crystals of alkali feldspar and a fine matrix of mica, quartz and feldspar, intense crystalloplastic deformation are evident in narrow zones (Fig. 2a). In these zones of intense deformation, phenocrysts of feldspar are stretched parallel to the fabric with increase in phyllosilicate content. The latter minerals define the mylonitic foliation (regional E-W fabric) that bends around the feldspar augen (Fig. 2b). Under microscope, feldspar grains show Carlsbad-and Tartar (microcline) and lamellar (plagioclase) twins and are swerved by thin folia made up of mica and elongated quartz grains that commonly form ribbon texture. Extensive replacement of K-feldspar and plagioclase (An 02) by mica is a common feature of this rock (Fig. 2b, 3b). In most places, K-feldspar grains are variably replaced with albite. Albite lamellae replace K-feldspar grains along the Murchison plane and develop the conspicuous flame perthite (Fig. 2c, d). The 'flame perthite' has the maximum thickness near the rim and tapers away into the inner part of the grain of host K-feldspar (Fig. 2d). The albite 'flames' are preferentially aligned parallel to the direction of maximum compression. In places, myrmekitic lobe develops on the surface (which is normal to the principal compression direction) of K-feldspar augen and protrudes inward to the porphyroclasts (Fig. 3a). Microstructural features such as (a) 'chessboard' like blocky subgrain texture in quartz (Fig. 3c), (b) grain boundary migration recrystallization of quartz and (c) bending of twin lamellae and recrystallization of plagioclase (Fig. 3d) suggest that the temperature was at least 450 o C during deformation (reviewed in Vernon, 2004). Field features and microstructures together support that shear deformation affecting the granitoid, occurred in a brittle-ductile transitional regime. To understand the chemical evolution of the albitized K-feldspar, textural modeling is performed integrating the observed textures and algebraic analysis of the compositions of involved mineral phases with the help of the computer program C-Space. Modeled balanced chemical reactions are given below: 49.94Kfs+ 45.50Na + + 1.00Ca 2+ = 49.97Pl+ 48.47K + (∆Vs %=-6.37%)-(1)
The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe o... more The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66–1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (XMg > 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions:
(1) 1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na
(2) 1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
Stoichiometry of the balanced chemical reactions suggests that a significant amount of P, Mg, and H2O were added to, and Al and SiO2 were subtracted from, the host kyanite-rich rock to produce augelite and lazulite. Experimental studies in the system Al2O3-SiO2-FeO-MgO-P2O5-H2O and the results of quantitative geothermobarometry suggest that lazulite and augelite were formed in a narrow temperature (440 ± 40°C) and pressure (~6.3 ± 1 kbar) range. Ductile shearing along the SSZ channelized the P- and Mg-rich fluids that metasomatized the kyanite-rich bands and veins to produce lazulite. The inferred P-T conditions can be explained by burial of the studied rock under an ~25 km-thick thrust sheet of NSFB during the Palaeoproterozoic orogenesis.
Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Pal... more Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo-to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7-11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (∼49 mol%) with significant La (∼30 mol%) and Nd (∼21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencitekyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks suggest that florencite was formed during Palaeoproterozoic metamorphism that culminated at the -range of 490 ± 40 ∘ C and 6.3 ± 1 kbar. Integrating all the geological features it is postulated that florencite was formed due to metasomatism of some aluminous protolith by infiltration of acidic fluids charged with PO 4 −3 and LREE.
Behavior of elements during fluid -mediated processes provides crucial information to understand ... more Behavior of elements during fluid -mediated processes provides crucial information to understand the chemical evolution of the continental crust and the upper mantle. Field evidences coupled with the observations from a number of experimental studies have demonstrated that Al and Ti remain virtually immobile during fluid-rock interactions owing to their low solubilities in crustal and mantle fluids. This realization provoked many petrologists to assume Ti-and Al-fixed reference frames to model the chemical changes in rocks that had interacted with chemically active fluids. Contrary to this common perception, we present geological evidence that demonstrates that under certain conditions, Al and Ti can be significantly mobile during crustal metasomatism. The example comes from a kyanitequartzite occurring within the 1.5-1.7 Ga Singhbhum Shear Zone (SSZ) of the East Indian shield. The SSZ exposes an ensemble of lithologies with diverse bulk compositions. These rocks are repeatedly folded and are dissected by ductile shear zone. Original compositions of the rocks in the SSZ were significantly changed due to extensive hydrothermal alteration (sensu lato) that accompanied the shear deformation (Mukhopadhyay and Deb, 1995). Extant petrological information indicate that the metamorphism / metasomatism in the SSZ and the adjoining areas culminated at ~480 o ± 50 o C and 6.5 ± 1 kbar (Sengupta et al. 2005). The studied kyanite quartzite is located about 20km west of the town Tatanagar, in the state of
In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing v... more 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 youngest, 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 tourmaline 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 tourmaline 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.
The arcuate Singhbhum shear zone (SSZ) of East India separates Meso-to Palaeo-Proterozoic supracr... more The arcuate Singhbhum shear zone (SSZ) of East India separates Meso-to Palaeo-Proterozoic supracrustal rocks of North Singhbhum fold belt (NSFB) from the Archaean Singhbhum craton. The SSZ exposes diverse lithologies with high variant mineral assemblages that host economically viable polymetallic deposits (Cu, Fe & U) ore deposits. Detailed geological investigation in this narrow belt has identified superposed folding and accompanying ductile shearing. Laterally continuous to discontinuous outcrops of kyanite-rich rocks occur all along the SSZ close to its hanging wall side and also with in the immediate pelitic rocks of the adjoining NSFB. Field features together with low concentrations of Fe 2 O 3 (0.03-0.61 wt%), TiO 2 (0.55±0.13 Wt%) and Ga (23±12 ppm), high Al 2 O 3 (c.35±10 wt%) and lack of any correlation between the last two chemical constituents are inconsistent with the idea that these rocks represent metamorphosed bauxite deposits or sedimented residual clay. Base leaching and consequent enrichment of Al 2 O 3 in the host rocks by acidic fluids presumably derived from magmatogenic hydrothermal fluids seem to be consistent with the geological and geochemical data and high δ 18 O values of kyanite rich rocks (>60 vol% kyanite, c. 4.7-6.8 o / oo ) including δ 18 O values of two kyanite grains (5, 5.5 o / oo ) from an unaltered massive kyanite deposits. Extensive fluid-rock interaction during and subsequent to the intense shearing of this region converted the anhydrous kyanite quartzite to muscovite schist with variable proportion of chloritoid, chlorite, biotite, and tourmaline. Frequently, kyanite grains are pseudomorphed by muscovite and chloritoid without the development of any Al-rich phases nearby. These features indicate nearly uniform rates of kyanite dissolution and precipitation of less aluminous muscovite and chloritoid, which in turn, support Al mobility during this metasomatic event. Part of the scavenged Al was presumably re-deposited as kyanite (± quartz) bearing veins that dissected the kyanite muscovite schists. Ramifying veins of fine-grained tourmaline and randomly oriented paragonite flakes are also present in muscovite schist indicating that the infiltrating fluid was locally rich in boron and Na. In eastern and south eastern parts of SSZ, physical conditions deduced from the conventional geothermobarometry and stability relation of the pseudomorph forming phases converged at 6.4±0.4 Kbar and 480 ± 40 o C. Extant fluid inclusion data and thermodynamic modeling of aH 2 O computed from mineral fluid equilibria indicate high salinity (>25 wt% NaCl equivalent) of the infiltrated fluids. These fluids were out of equilibrium with the host rocks and triggered the metasomatic alteration of the kyanite rich rocks, transport of Al 3+ and formation of kyanite rich veins. Symposium titles MPN-03 Mineral replacement and mass transfer in hydrothermal systems: From the nanoscale to the megascale Presentation Preference Oral presentation
Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Pal... more Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo-to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7-11.8 mol%) and goyazite (<2 mol%) components. Florencite of this study is dominated by Ce (∼49 mol%) with significant La (∼30 mol%) and Nd (∼21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencitekyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks suggest that florencite was formed during Palaeoproterozoic metamorphism that culminated at the -range of 490 ± 40 ∘ C and 6.3 ± 1 kbar. Integrating all the geological features it is postulated that florencite was formed due to metasomatism of some aluminous protolith by infiltration of acidic fluids charged with PO 4 −3 and LREE.
Fluorite, a fluorine-bearing mineral present in different rock types can cause groundwater contam... more Fluorite, a fluorine-bearing mineral present in different rock types can cause groundwater contamination through its dissolution as fluoride ion into aquifer. Bounded by roughly E-W-trending Son-Narmada north and south faults (SNNF and SNSF respectively), the areas in and around Jabalpur (Madhya Pradesh, central India) expose Palaeoproterozoic Mahakoshal Group of rocks including mica-schist, quartzite, dolomitic marble, minor mafic dykes and large elliptical granite plutons commonly known as Madan Mahal Granite (MMG). Part of the MMG is covered by unmetamorphosed Phanerozoic rock sequence including sandstone, limestone, clay, Deccan basalts and un/semi-consolidated sediments. Aquifers with high fluorine content present in the MMG and in the overlying sandstones are presumed to be the source of dental and skeletal fluorosis of the people of Jabalpur District. Integrating the extant geological and hydrological parameters, a model has been proposed to explain the high fluorine content in groundwater of Jabalpur District. This study shows that fluorite was developed in the MMG during Palaeoproterozoic deformation and metamorphism. Sedimentary rocks that are sourced from the MMG and host the major aquifers also contain fluorite. The proposed model visualises that groundwater became alkaline due to chemical interaction with feldspar. Because of enhanced solubility of F in alkaline water, groundwater that are buffered by feldspathic wall rocks leached large amount of F from fluorite present in the host rocks and become hazardous for human consumption. High heat flow of the region further aggravates F-solubility in ground water.
Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic ass... more Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about ß uid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic to psammopelitic and quartzofeldspasthic rocks, including meta-granite. The rocks were affected by two sets of folding (F 1 and F 2 ) and ductile shearing associated with F 1 during ca. 1.6-1.8 Ga tectonic activity in this belt. Quantitative geothermobarometry and the stability relations of the metamorphic assemblages developed in the pelitic rocks establish the presence of two episodes of metamorphism; a prograde M 1 event that culminated at 480 ± 40 °C, 6.4 ± 0.4 kbar, and an M 2 event that caused retrogression of the M 1 assemblages.
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(1) 1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na
(2) 1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
Stoichiometry of the balanced chemical reactions suggests that a significant amount of P, Mg, and H2O were added to, and Al and SiO2 were subtracted from, the host kyanite-rich rock to produce augelite and lazulite. Experimental studies in the system Al2O3-SiO2-FeO-MgO-P2O5-H2O and the results of quantitative geothermobarometry suggest that lazulite and augelite were formed in a narrow temperature (440 ± 40°C) and pressure (~6.3 ± 1 kbar) range. Ductile shearing along the SSZ channelized the P- and Mg-rich fluids that metasomatized the kyanite-rich bands and veins to produce lazulite. The inferred P-T conditions can be explained by burial of the studied rock under an ~25 km-thick thrust sheet of NSFB during the Palaeoproterozoic orogenesis.
(1) 1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na
(2) 1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
Stoichiometry of the balanced chemical reactions suggests that a significant amount of P, Mg, and H2O were added to, and Al and SiO2 were subtracted from, the host kyanite-rich rock to produce augelite and lazulite. Experimental studies in the system Al2O3-SiO2-FeO-MgO-P2O5-H2O and the results of quantitative geothermobarometry suggest that lazulite and augelite were formed in a narrow temperature (440 ± 40°C) and pressure (~6.3 ± 1 kbar) range. Ductile shearing along the SSZ channelized the P- and Mg-rich fluids that metasomatized the kyanite-rich bands and veins to produce lazulite. The inferred P-T conditions can be explained by burial of the studied rock under an ~25 km-thick thrust sheet of NSFB during the Palaeoproterozoic orogenesis.