... Mantle xenolith bearing alkali basalts are fairly common in the central part (Karmalkar et al... more ... Mantle xenolith bearing alkali basalts are fairly common in the central part (Karmalkar et al., 2000, Karmalkar and Sarma, 2003 and Krishnamurthy et al., 1989) while tholeiites have been reported from the southern part (Biswas, 1993). ...
Journal of Volcanology and Geothermal Research, Feb 1, 2016
Abstract Kutch rift basin of northwestern India is characterized by a topography that is controll... more Abstract Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo> 90), phenocrysts (euhedral, smaller, and less forsteritic ~ Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37 km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.
Abstract Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC)... more Abstract Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC), India occur as massive conformable lenses within a gabbronorite sill. The occurrence, lithological association, mineralogy, and geochemistry of the Fe-Ti oxide ore bodies are significantly different from the typical Fe-Ti oxide deposits associated with anorthosite complexes and mafic- ultramafic complexes. Textural evidence and mineral chemistry suggest mostly simultaneous fractional crystallization of the comprising minerals e.g. titanomagnetite, ilmenite and magnesio-hercynite. The temperature and fO2 conditions of re-equilibration are calculated using the compositions of ilmenite and pre-exsolved titanomagnetite pairs. Extrapolation of the data along ulvospinel isopleths reveals the probable original magmatic conditions e.g. temperature ∼900°C and fO2 ∼ ΔFMQ +2. The ore forming melt calculated by ‘equilibrium distribution method’ shows no crustal contamination and has a trace elemental pattern similar to typical enriched basaltic melts e.g. ferropicrites, alkali basalts, OIB and EMORB. High Ti and Zr contents of the melt suggest a possible within plate tectonic setting and enrichment in HFSEs and LREEs suggest that the parent melt could be generated from an enriched mantle source by low degree partial melting of garnet-peridotite layer of the upper mantle. The ‘quantitative fractional crystallization modelling’ indicates that early crystallization of olivine, pyroxenes, plagioclase and ilmenite from an enriched basaltic melt similar to EMORB (most fit melt) led to a condition of high oxygen fugacity and iron enrichment in the residual melt. The residual Fe-Ti rich melt moved to low pressure zones within the gabbronorite sill during late stage of D2 deformation, and later crystallized in the form of present ore bodies.
A layered gabbro suite was emplaced around Kuliana (lat 22°3ʹN and long 86°38ʹE), Orissa, eastern... more A layered gabbro suite was emplaced around Kuliana (lat 22°3ʹN and long 86°38ʹE), Orissa, eastern India, within deformed metasedimentary rocks of the north Singhbhum mobile belt. This magmatic suite does not have any exposures of feeder dikes, chilled margins, or other evidence for an intrusive character. Because the cumulate layers do not provide information about the parent magma of this suite, the equilibrium distribution method has been utilized in this study to estimate trace element concentrations of melts in equilibrium with the cumulate solid products. The mineralogical and chemical evolution of the layered suite has been carried out on the basis of both calculated parent magmas and chemical data of cumulate layers. The magmatic differentiation of the layered suite was guided by fractional crystallization and punctuated by an event of magma chamber replenishment. The calculated parent melts exhibit enriched light rare earth elements and large ion lithophile elements, marked depletion of Ti and Y, and successive total rare earth element depletion through progressive differentiation. Plotting several immobile trace element diagrams—Nb/Yb versus Th/Yb, La/Nb versus Th/Nb, Nb/La versus Ba/La, and Zr versus Zr/Y—reveals their chemical affinity to enriched mid-ocean ridge basalt–like magmas. The La versus La/Sm plot of mantle sources affected by partial melting shows a relatively low degree (8%–30%) of melting and indicates a deeper (garnet facies) mantle as the possible source region of the parent magmas of the Kuliana layered mafic suite.
Abstract Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed me... more Abstract Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed metasedimentary rocks of Singhbhum Group belonging to North Singhbhum Mobile Belt of eastern India. Detailed mineralogical and geochemical studies reveal that these rocks have suffered a two-stage alteration involving a deeper level modal and cryptic metasomatism and a subsequent shallower depth pervasive hydrothermal alteration. Cryptic metasomatism is defined by elevated LREE contents of the wehrlite and its clinopyroxne grains. Metasomatism induced changes in the modal mineralogy of the rocks include the absence of primary orthopyroxene grains, presence of secondary diopside-phlogopite (now present as vermiculite) defining disequilibrium reaction textures and secondary orthopyroxene rims around serpentinized olivine. The mineralogical and geochemical changes due to the metasomatic event present a contrasting picture in regard to the metasomatic history of the rocks. Possible scenarios involving a single stage or multiple stage metasomatism events have been discussed while explaining the metasomatic reactions that took place. An attempt has been made to estimate the REE concentrations of the final equilibrating melt from REE contents of clinopyroxene grains of the wehrlite. The possibility of the LREE-enriched equilibrating melt of the wehrlite rocks (the deeper level metasomatic agent) being similar to residual melts from the E-MORB type parental melts of nearby gabbro suite has been ruled out by geochemical modeling. REE abundance patterns of several natural enriched melts have been compared with REE pattern of calculated LREE-enriched equilibrating melt of the wehrlite and most resemblance has been observed with calcic and potassic melts. It is therefore suggested that the Cr-spinel bearing wehrlite rocks of Bangriposi has been affected by a calcio-potassic melt in deeper level, prior to the shallow level serpentinization event.
Journal of Volcanology and Geothermal Research, 2016
Abstract Kutch rift basin of northwestern India is characterized by a topography that is controll... more Abstract Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo> 90), phenocrysts (euhedral, smaller, and less forsteritic ~ Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37 km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.
Abstract Evidences of Mesoarchean crustal reworking are not very common in geological records, be... more Abstract Evidences of Mesoarchean crustal reworking are not very common in geological records, being only found from Precambrian terrains like Kaapvaal and Pilbara Cratons where it is preserved as anorogenic/post-collisional granitic activity of 3–3.2 Ga age. The present study focuses on a granitoid unit of similar age, found near the eastern margin of the Precambrian Singhbhum Craton of eastern India. This ellipsoidal, N-S trending, well-foliated granite-gneiss body is found within the polydeformed, metasedimenatry rocks of Singhbhum Group, belonging to the North Singhbhum Mobile Belt. LA-ICP MS U-Pb analysis of zircon grains from one sample give crystallization ages ranging from 3079.4 ± 6.8 Ma to 3115 ± 10 Ma. These rocks hereby dubbed as “Bangriposi Granite Gneiss”, are composed of quartz, alkali feldspar, ferroan biotite (Fe/Fe + Mg: 0.7–0.9), titanite, illmenite, hastingsite, apatite, and numerous U-Th-REE- bearing accessory phases. They have high SiO2 (67–77 wt%) and Na2O + K2O (8.19–9.01 wt%), low CaO (0.35–1.5 wt%), MgO (0.05–0.36 wt%) and shows enrichment of Nb, Rb, Zr, Y, Th, and REEs, and depletion of Cr, Ni, U, with high FeOt/FeOt + MgO (~0.9), Ga/Al (2.04–3.51), (La/Yb)N (6.5–13), and low Eu/Eu* (0.11–0.47). Geochemically and mineralogically they are categorized as metaluminous to weakly peraluminous (A/CNK: 0.9–1.1) ferro-potassic alkali feldspar granites. These rocks show unequivocal affinities towards ‘A-type” granites and from geochemical evidences it is suggested that the parent magma was produced in response to crustal anatexis under low fO2 with P-T estimates of ~900 °C and 7–8 kbar. Geochemical modelling has revealed that the probable source was lower crustal amphibolites belonging to the Paleoarchean Older Metamorphic Group, which suffered low degrees (5–15%) of melting. Negative ƐNd values (−0.5 to −1.5) are also in favour of reworking of older crust and their Nd isotopic signature bears similarities with other coeval anatectic granites. Bangriposi Granite-Gneiss, along with Mayurbhanj Granite and Bonai Granite, represent a major phase of Mesoarchean anorogenic/post-collisional granitic activity in Singhbhum Craton, indicating the onset of its stabilization. Similar Mesoarchean crustally reworked felsic units are encountered in Pilbara and Kaapvaal cratons suggesting a possibility of correlation with the hypothesized “Vaalbara” supercontinent.
ABSTRACT The Proterozoic Chhotanagpur Granite Gneissic Complex (CGGC) is one of the major geologi... more ABSTRACT The Proterozoic Chhotanagpur Granite Gneissic Complex (CGGC) is one of the major geological components of Peninsular India. It covers approximately 80,000 sq.km area. CGGC is dominantly made up of granite gneiss, granitoids, enclaves of meta-sediments and meta-basics. Anorthosite plutons are found at several places in CGGC and most of them are intruding into the granulite facies country rock. The Bankura anorthosite is the largest anorthosite pluton situated at the easternmost part of CGGC. In this part, the country rocks (gneiss, meta-sediments) exhibit three phases of deformation with the development of folds, foliations and lineations. At the margin with the country rock, the anorthosite body is banded. It is massive in the remaining major part. To find out whether the massive anorthosite body is at all affected by deformation, Anisotropy of Magnetic Susceptibility (AMS) study was carried out with oriented samples. From the AMS study of four samples collected from different parts of massive anorthosite, two phases of deformations are detected. The magnetic foliations from three samples are mostly east-west striking with moderate northerly dip. This magnetic foliation is parallel to the axial plane foliation of 2nd phase of deformation within the country rock. The magnetic lineations are also parallel to the lineations of 2nd phase of deformation in country rock. The fourth sample shows that the magnetic foliations and lineations are parallel to the 3rd phase of deformation of the country rock. The massive Bankura anorthosite body lacks any deformational feature in the field. But with the help of AMS study we can conclude that it endures the 2nd and 3rd phases of deformation and emplaced during the 1st phase of deformation of CGGC.
... Mantle xenolith bearing alkali basalts are fairly common in the central part (Karmalkar et al... more ... Mantle xenolith bearing alkali basalts are fairly common in the central part (Karmalkar et al., 2000, Karmalkar and Sarma, 2003 and Krishnamurthy et al., 1989) while tholeiites have been reported from the southern part (Biswas, 1993). ...
Journal of Volcanology and Geothermal Research, Feb 1, 2016
Abstract Kutch rift basin of northwestern India is characterized by a topography that is controll... more Abstract Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo> 90), phenocrysts (euhedral, smaller, and less forsteritic ~ Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37 km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.
Abstract Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC)... more Abstract Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC), India occur as massive conformable lenses within a gabbronorite sill. The occurrence, lithological association, mineralogy, and geochemistry of the Fe-Ti oxide ore bodies are significantly different from the typical Fe-Ti oxide deposits associated with anorthosite complexes and mafic- ultramafic complexes. Textural evidence and mineral chemistry suggest mostly simultaneous fractional crystallization of the comprising minerals e.g. titanomagnetite, ilmenite and magnesio-hercynite. The temperature and fO2 conditions of re-equilibration are calculated using the compositions of ilmenite and pre-exsolved titanomagnetite pairs. Extrapolation of the data along ulvospinel isopleths reveals the probable original magmatic conditions e.g. temperature ∼900°C and fO2 ∼ ΔFMQ +2. The ore forming melt calculated by ‘equilibrium distribution method’ shows no crustal contamination and has a trace elemental pattern similar to typical enriched basaltic melts e.g. ferropicrites, alkali basalts, OIB and EMORB. High Ti and Zr contents of the melt suggest a possible within plate tectonic setting and enrichment in HFSEs and LREEs suggest that the parent melt could be generated from an enriched mantle source by low degree partial melting of garnet-peridotite layer of the upper mantle. The ‘quantitative fractional crystallization modelling’ indicates that early crystallization of olivine, pyroxenes, plagioclase and ilmenite from an enriched basaltic melt similar to EMORB (most fit melt) led to a condition of high oxygen fugacity and iron enrichment in the residual melt. The residual Fe-Ti rich melt moved to low pressure zones within the gabbronorite sill during late stage of D2 deformation, and later crystallized in the form of present ore bodies.
A layered gabbro suite was emplaced around Kuliana (lat 22°3ʹN and long 86°38ʹE), Orissa, eastern... more A layered gabbro suite was emplaced around Kuliana (lat 22°3ʹN and long 86°38ʹE), Orissa, eastern India, within deformed metasedimentary rocks of the north Singhbhum mobile belt. This magmatic suite does not have any exposures of feeder dikes, chilled margins, or other evidence for an intrusive character. Because the cumulate layers do not provide information about the parent magma of this suite, the equilibrium distribution method has been utilized in this study to estimate trace element concentrations of melts in equilibrium with the cumulate solid products. The mineralogical and chemical evolution of the layered suite has been carried out on the basis of both calculated parent magmas and chemical data of cumulate layers. The magmatic differentiation of the layered suite was guided by fractional crystallization and punctuated by an event of magma chamber replenishment. The calculated parent melts exhibit enriched light rare earth elements and large ion lithophile elements, marked depletion of Ti and Y, and successive total rare earth element depletion through progressive differentiation. Plotting several immobile trace element diagrams—Nb/Yb versus Th/Yb, La/Nb versus Th/Nb, Nb/La versus Ba/La, and Zr versus Zr/Y—reveals their chemical affinity to enriched mid-ocean ridge basalt–like magmas. The La versus La/Sm plot of mantle sources affected by partial melting shows a relatively low degree (8%–30%) of melting and indicates a deeper (garnet facies) mantle as the possible source region of the parent magmas of the Kuliana layered mafic suite.
Abstract Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed me... more Abstract Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed metasedimentary rocks of Singhbhum Group belonging to North Singhbhum Mobile Belt of eastern India. Detailed mineralogical and geochemical studies reveal that these rocks have suffered a two-stage alteration involving a deeper level modal and cryptic metasomatism and a subsequent shallower depth pervasive hydrothermal alteration. Cryptic metasomatism is defined by elevated LREE contents of the wehrlite and its clinopyroxne grains. Metasomatism induced changes in the modal mineralogy of the rocks include the absence of primary orthopyroxene grains, presence of secondary diopside-phlogopite (now present as vermiculite) defining disequilibrium reaction textures and secondary orthopyroxene rims around serpentinized olivine. The mineralogical and geochemical changes due to the metasomatic event present a contrasting picture in regard to the metasomatic history of the rocks. Possible scenarios involving a single stage or multiple stage metasomatism events have been discussed while explaining the metasomatic reactions that took place. An attempt has been made to estimate the REE concentrations of the final equilibrating melt from REE contents of clinopyroxene grains of the wehrlite. The possibility of the LREE-enriched equilibrating melt of the wehrlite rocks (the deeper level metasomatic agent) being similar to residual melts from the E-MORB type parental melts of nearby gabbro suite has been ruled out by geochemical modeling. REE abundance patterns of several natural enriched melts have been compared with REE pattern of calculated LREE-enriched equilibrating melt of the wehrlite and most resemblance has been observed with calcic and potassic melts. It is therefore suggested that the Cr-spinel bearing wehrlite rocks of Bangriposi has been affected by a calcio-potassic melt in deeper level, prior to the shallow level serpentinization event.
Journal of Volcanology and Geothermal Research, 2016
Abstract Kutch rift basin of northwestern India is characterized by a topography that is controll... more Abstract Kutch rift basin of northwestern India is characterized by a topography that is controlled by a number of fault controlled uplifted blocks. Kutch Mainland Uplift, the largest uplifted block in the central part of the basin, contains alkali basalt plugs and tholeiitic basalt flows of the Deccan age. Alkali plugs often contain small, discoidal mantle xenoliths of spinel lherzolite and spinel wehrlite composition. Olivine occurs as xenocrysts (coarse, fractured, broken olivine grains with embayed margin; Fo> 90), phenocrysts (euhedral, smaller, and less forsteritic ~ Fo80), and as groundmass grains (small, anhedral, Fo75) in these alkali basalts. In a few cases, the alkali plugs are connected with feeder dykes. Based on the width of feeder dykes, on the sizes of the xenocrysts and xenoliths, thickness of alteration rim around olivine xenocryst, we estimate that the alkali magmas erupted at a minimum speed of 0.37 km per hour. The speed was likely greater because of the fact that the xenoliths broke up into smaller fragments as their host magma ascended through the lithosphere.
Abstract Evidences of Mesoarchean crustal reworking are not very common in geological records, be... more Abstract Evidences of Mesoarchean crustal reworking are not very common in geological records, being only found from Precambrian terrains like Kaapvaal and Pilbara Cratons where it is preserved as anorogenic/post-collisional granitic activity of 3–3.2 Ga age. The present study focuses on a granitoid unit of similar age, found near the eastern margin of the Precambrian Singhbhum Craton of eastern India. This ellipsoidal, N-S trending, well-foliated granite-gneiss body is found within the polydeformed, metasedimenatry rocks of Singhbhum Group, belonging to the North Singhbhum Mobile Belt. LA-ICP MS U-Pb analysis of zircon grains from one sample give crystallization ages ranging from 3079.4 ± 6.8 Ma to 3115 ± 10 Ma. These rocks hereby dubbed as “Bangriposi Granite Gneiss”, are composed of quartz, alkali feldspar, ferroan biotite (Fe/Fe + Mg: 0.7–0.9), titanite, illmenite, hastingsite, apatite, and numerous U-Th-REE- bearing accessory phases. They have high SiO2 (67–77 wt%) and Na2O + K2O (8.19–9.01 wt%), low CaO (0.35–1.5 wt%), MgO (0.05–0.36 wt%) and shows enrichment of Nb, Rb, Zr, Y, Th, and REEs, and depletion of Cr, Ni, U, with high FeOt/FeOt + MgO (~0.9), Ga/Al (2.04–3.51), (La/Yb)N (6.5–13), and low Eu/Eu* (0.11–0.47). Geochemically and mineralogically they are categorized as metaluminous to weakly peraluminous (A/CNK: 0.9–1.1) ferro-potassic alkali feldspar granites. These rocks show unequivocal affinities towards ‘A-type” granites and from geochemical evidences it is suggested that the parent magma was produced in response to crustal anatexis under low fO2 with P-T estimates of ~900 °C and 7–8 kbar. Geochemical modelling has revealed that the probable source was lower crustal amphibolites belonging to the Paleoarchean Older Metamorphic Group, which suffered low degrees (5–15%) of melting. Negative ƐNd values (−0.5 to −1.5) are also in favour of reworking of older crust and their Nd isotopic signature bears similarities with other coeval anatectic granites. Bangriposi Granite-Gneiss, along with Mayurbhanj Granite and Bonai Granite, represent a major phase of Mesoarchean anorogenic/post-collisional granitic activity in Singhbhum Craton, indicating the onset of its stabilization. Similar Mesoarchean crustally reworked felsic units are encountered in Pilbara and Kaapvaal cratons suggesting a possibility of correlation with the hypothesized “Vaalbara” supercontinent.
ABSTRACT The Proterozoic Chhotanagpur Granite Gneissic Complex (CGGC) is one of the major geologi... more ABSTRACT The Proterozoic Chhotanagpur Granite Gneissic Complex (CGGC) is one of the major geological components of Peninsular India. It covers approximately 80,000 sq.km area. CGGC is dominantly made up of granite gneiss, granitoids, enclaves of meta-sediments and meta-basics. Anorthosite plutons are found at several places in CGGC and most of them are intruding into the granulite facies country rock. The Bankura anorthosite is the largest anorthosite pluton situated at the easternmost part of CGGC. In this part, the country rocks (gneiss, meta-sediments) exhibit three phases of deformation with the development of folds, foliations and lineations. At the margin with the country rock, the anorthosite body is banded. It is massive in the remaining major part. To find out whether the massive anorthosite body is at all affected by deformation, Anisotropy of Magnetic Susceptibility (AMS) study was carried out with oriented samples. From the AMS study of four samples collected from different parts of massive anorthosite, two phases of deformations are detected. The magnetic foliations from three samples are mostly east-west striking with moderate northerly dip. This magnetic foliation is parallel to the axial plane foliation of 2nd phase of deformation within the country rock. The magnetic lineations are also parallel to the lineations of 2nd phase of deformation in country rock. The fourth sample shows that the magnetic foliations and lineations are parallel to the 3rd phase of deformation of the country rock. The massive Bankura anorthosite body lacks any deformational feature in the field. But with the help of AMS study we can conclude that it endures the 2nd and 3rd phases of deformation and emplaced during the 1st phase of deformation of CGGC.
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