Browsing by Author "Anup K. Sinha"

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A precise U-Pb zircon/baddeleyite age for the Jasra igneous complex, Karbi-Analong District, Assam, NE India
(2002) Larry M. Heaman; Rajesh K. Srivastava; Anup K. Sinha
Five Cretaceous alkaline-carbonatite igneous complexes are reported from the Assam-Meghalaya Plateau. These alkaline intrusions have been interpreted to be coeval and associated with the 117-105 Ma Rajmahal-Sylhet flood basalt province. With the existing age information it is possible that this alkaline magmatism may be a late magmatic stage of the Rajmahal-Sylhet large igneous province. Therefore, it is essential to determine high-precision ages for these alkaline complexes in order to understand the detailed temporal evolution and genesis of this basaltic and alkaline magmatism. Out of five igneous complexes, Sung Valley, Swangkre and Samchampi have been dated, but the emplacement ages of the other two, i.e. Jasra and Barpung, are poorly constrained. The present communication reports a new, high-precision U-Pb zircon/baddeleyite age for a differentiated portion of gabbro phase of the Jasra igneous complex.
Cretaceous potassic intrusives with affinities to aillikites from Jharia area: Magmatic expression of metasomatically veined and thinned lithospheric mantle beneath Singhbhum Craton, Eastern India
(2009) Rajesh K. Srivastava; N.V. Chalapathi Rao; Anup K. Sinha
Cretaceous potassic dykes and sills at the Jharia area intrude the Permo-carboniferous coal-bearing Gondwana sediments of the Eastern Damodar Valley, Singhbhum craton. These intrusives are widely regarded as a part of the Mesozoic alkaline and Rajmahal flood basalt magmatism in the Eastern Indian shield. Jharia intrusives display a wide petrographic diversity; olivine, phlogopite and carbonate are the predominant phases whereas apatite and rutile constitute important accessories. Impoverishment in sodium, silica and alumina and enrichment in potassium, titanium and phosphorous are the hallmark of these rocks and in this aspect they are strikingly similar to the rift-related aillikites (ultramafic lamprophyres) of Aillik Bay, Labrador. Crustal contamination of the Jharia magmas is minimal and the incompatible trace element ratios demonstrate (i) their generation by greater degrees of partial melting of a sub-continental lithospheric mantle (SCLM) source similar to that of the kimberlites of Dharwar craton, southern India, and (ii) retention of long-term memories of ancient (Archaean) subduction experienced by their source regions. We infer that a metasomatically veined and thinned lithosphere located at the margin of the Singhbhum craton and the inheritance of an ancient (Archaean) subducted component has played a significant role in deciding the diverging petrological and geochemical characters displayed by the Jharia potassic intrusives: those of kimberlites (orangeites) and lamproites (cratonic signature) and those of aillikites (rift-related signature). A substantial melt component of Jharia potassic intrusives was derived from the SCLM and the melt contribution of the Kerguelen plume is inferred to be minimal. © 2009 Elsevier B.V. All rights reserved.
Emplacement age and isotope geochemistry of Sung Valley alkaline-carbonatite complex, Shillong Plateau, northeastern India: Implications for primary carbonate melt and genesis of the associated silicate rocks
(2005) Rajesh K. Srivastava; Larry M. Heaman; Anup K. Sinha; Sun Shihua
The early Cretaceous (Albian-Aptian) Sung Valley ultramafic-alkaline-carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite εNd (+0.7 to +1.8) and εSr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate-carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U-Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (∼0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite. © 2004 Elsevier B.V. All rights reserved.
Evidence of sub-continental lithospheric mantle sources and open-system crystallization processes from in-situ U–Pb ages and Nd–Sr–Hf isotope geochemistry of the Cretaceous ultramafic-alkaline-(carbonatite) intrusions from the Shillong Plateau, north-eastern India
(Elsevier B.V., 2019) Rajesh K. Srivastava; Vincenza Guarino; Fu-Yuan Wu; Leone Melluso; Anup K. Sinha
New in-situ U–Pb ages and Sr–Nd–Hf isotopic data on mineral phases of the Sung Valley and Jasra ultramafic-alkaline-(carbonatite) intrusions (Shillong Plateau, India) shed new light on the petrogenetic processes of volcanism in north-eastern India during the Cretaceous. Perovskites of Sung Valley dunite, ijolite and uncompahgrite yielded U–Pb ages of 109.1 ± 1.6, 104.0 ± 1.3 and 101.7 ± 3.6 Ma, respectively. A U–Pb age of 106.8 ± 1.5 Ma was obtained on zircons of a Sung Valley nepheline syenite. Perovskite of a Jasra clinopyroxenite yielded an age of 101.6 ± 1.2 Ma, different from the U–Pb age of 106.8 ± 0.8 Ma on zircon of Jasra syenites. The variation in Sr-Nd-Hf isotopic compositions [initial 87 Sr/ 86 Sr = 0.70472 to 0.71080; ε Nd i = −10.85 to +0.86; ε Hf i = −7.43 to +1.52] matches the bulk-rock isotopic composition of the different rock units of Sung Valley and Jasra. Calcite and apatite in the carbonatites, the perovskite in a dunite, and the bulk-rock carbonatites of Sung Valley intrusion have the lowest initial 87 Sr/ 86 Sr and ε Nd , taken to be the best proxies of the mantle source composition, which is dominated by components derived from the lithospheric mantle. The alkaline intrusions of north-eastern India are significantly younger than the Sylhet tholeiitic magmatism. The silicate rocks of both intrusions have isotopic composition trending to that of the underlying Shillong crust, indicating the effects of fractional crystallization and low-pressure crustal contamination during the emplacement of the various intrusive magma pulses. © 2019 Elsevier B.V.
Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil, East Garo Hills, Shillong plateau, northeast India
(Indian Academy of Sciences, 2004) Rajesh K. Srivastava; Anup K. Sinha
Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic dykes from this area are presented. These mafic dykes show very sharp contact with the host rocks and do not show any signature of assimilation with them. Petrographically these mafic dykes vary from fine-grained basalt (samples from the dyke margin) to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classified as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest that these are genetically related to each other and probably derived from the same parental magma. Although, the high-field strength element (+rare-earth elements) compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group II) mafic dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible for these mafic dykes had undergone extreme differentiation (∼ 50%) before its emplacement. The basaltic-andesite nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼ 10-15% melting of the mantle source. The mafic dyke samples of the present investigation show very close geochemical similarities with the mafic rocks derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived from the Kerguelen mantle plume.© Printed in India.
Geochemistry of Early Cretaceous alkaline ultramafic-mafic complex from Jasra, Karbi Anglong, Shillong plateau, Northeastern India
(Elsevier Inc., 2004) Rajesh K. Srivastava; Anup K. Sinha
An early Cretaceous alkaline ultramafic-mafic complex is emplaced within the Proterozoic rocks of Shillong plateau at Jasra, Karbi Anglong district of Assam. It is associated to the fracture system of Barapani-Tyrsad shear zone, Kopali faults, and Um Ngot lineaments and mainly comprises pyroxenite, gabbro and nepheline syenite. Few small mafic dykes, emplaced within pyroxenitic and granitic plutons, are also reported. No such dyke is reported to cut gabbros or nepheline syenites. Nepheline syenites occur either in the form of small dykes in pyroxenites or as differentiated bodies in the gabbros. Mineralogical and chemical composition of pyroxenite and gabbro clearly indicate their affinity to the alkaline magmatism. Syenitic samples show miaskitic character (agpaitic index < 1), also indicates affinity with alkaline-carbonatite magmatism. Calcite is encountered in a number of pyroxenite samples. From the presented petrological and geochemical data it is difficult to establish any significant genetic relationship through simple differentiation process between these rocks. These data probably suggest that these rocks are derived from a primary carbonatite magma, generated by the low-degree melting of a metasomatized mantle peridotite. CO2 released by this process also progressively metasomatizes the lherzolite to an alkaline wehrlite and melts derived from alkaline wehrlite (ultrabasic alkaline silicate magma) may be responsible for crystallization of Jasra alkaline ultramafic-mafic rocks. © 2004 International Association for Gondwana Research, Japan.
K-rich titanate from the Jharia ultrapotassic rock, Gondwana coal fields, eastern India, and its petrological significance
(2013) N.V. Chalapathi Rao; Anup K. Sinha; Suresh Kumar; Rajesh K. Srivastava
We report a rare accessory groundmass mineral of K-rich titanate, having a composition close to that of potassium triskaidecatitanate (K2Ti 13O27), from an underground drill-core sample of ultrapotassic rock from southwestern part of the Jharia coal field in the Damodar valley, at the northern margin of the Singhbhum craton, Eastern India. Potassium triskaidecatitanate is regarded as a typomorphic mineral of orangeites (Group II kimberlites) of Kaapvaal craton, southern Africa, and its occurrence in the Jharia ultrapotassic rock is significant since ultrapotassic suite of rocks elsewhere from the Damodar valley have been recently suggested to be peralkaline lamproites based on mineral-genetic classification. The important role played by a unique geodynamic setting (involving a thinned metasomatised lithospheric mantle and inheritance of an Archaean subduction component) at the northern margin of the Singhbhum craton in deciding the petrological diversity of the early Cretaceous ultrapotassic intrusives from the Damodar valley is highlighted in this study. © 2013 Geological Society of India.
Mafic dykes swarms from the Chhotanagpur Gneiess Complex, Singhbhum craton, eastern India
(2016) Anup K. Sinha; Rajesh K. Srivastava
[No abstract available]
Mineral compositions and petrogenetic evolution of the ultramafic-alkaline - carbonatitic complex of sung valley, Northeastern India
(2010) Leone Melluso; Rajesh K. Srivastava; Vincenza Guarino; Alberto Zanetti; Anup K. Sinha
The Sung Valley alkaline complex is a relatively small intrusion of Lower Cretaceous age emplaced slightly before or during the India-Antarctica break-up. It consists of ultramafic rocks (dunites, wehrlites, clinopyroxenites, uncompahgrites), mafic rocks (ijolites sensu lato), felsic rocks (nepheline syenites) and carbonatites. The chemical composition of the mafic minerals indicates the expected enrichment in iron toward the felsic rocks. On the other hand, carbonatites feature very Mg-rich minerals, generally Cr-rich, indicating that their genesis is completely unrelated to that of mafic and felsic rocks (ijolites and nepheline syenites). The parageneses indicate that this complex was formed by batches of primitive magmas with a distinct magmatic affinity (olivine melilitites and olivine nephelinites, basanites, and possibly also carbonatites) which evolved independently, generating the observed spectrum of intrusive rocks. Clinopyroxenites have interstitial alkali feldspar and titanite, indicating that they formed from evolved feldspar-normative (phonotephritic, tephriphonolitic) magmas. The sequence perovskite-titanite and titanite-garnet noted in some ijolitic rocks indicates changes in the chemical composition of coexisting silicate melts and, most likely, an increasing(O 2). The trace-element profiles of coexisting phases in interesting associations in a sample of ijolite were documented by means of LA-ICP-MS analyses.
Nature of Cretaceous dolerite dikes with two distinct trends in the Damodar Valley, eastern India: Constraints on their linkage to mantle plumes and large igneous provinces from 40Ar/39Ar geochronology and geochemistry
(GEOSCIENCE WORLD, 2020) Rajesh K. Srivastava; Fei Wang; Wenbei Shi; Anup K. Sinha; Kenneth L. Buchan
Two distinct sets of Cretaceous dolerite dikes intrude the Chhotanagpur gneissic complex of eastern India, mostly within the Damodar Valley Gondwanan sedimentary basins. One dike set trends NNE to ENE, whereas the other set, which includes the prominent Salma dike, trends NW to NNW. One dike from each set in the Raniganj Basin was dated using the 40Ar/39Ar method in order to resolve a controversy concerning the emplacement age of the Salma dike. The NE-trending dike yielded a plateau age of 70.5 ± 0.9 Ma, whereas the NNWtrending Salma dike is much older, with a plateau age of 116.0 ± 1.4 Ma. These results demonstrate that the Salma dike was emplaced at ca. 116 Ma and not at ca. 65 Ma, as suggested in an earlier study. Geochemical characteristics of the two dikes are also distinct and indicate that they belong to previously identified high-Ti and low-Ti dolerite groups, respectively. The observed geochemical characteristics of both dike sets are comparable with the geochemistry of basalts of the Kerguelen Plateau, Bunbury Island, and Rajmahal Group I and suggest a connection to mantle plumes. The new age data presented herein indicate that these two magmatic episodes in the eastern Indian Shield were related to the ca. 120-100 Ma Kerguelen mantle plume and its associated Greater Kerguelen large igneous province and the ca. 70-65 Ma Reunion plume and its associated Deccan large igneous province, respectively. © 2019 The Authors.
Nd and Sr isotope systematics and geochemistry of a plume-related Early Cretaceous alkaline-mafic-ultramafic igneous complex from Jasra, Shillong plateau, northeastern India
(Geological Society of America, 2007) Rajesh K. Srivastava; Anup K. Sinha
This article describes an Early Cretaceous Jasra alkaline-mafic-ultramafic igneous complex related to the Kerguelen hotspot-mantle plume system of the Indian Ocean. This complex, emplaced in the Shillong plateau, consists mainly of pyroxenite, gabbro, and nepheline syenite and is closely associated with the Barapani-Tyrsad shear zone, Kopali faults, and Um Ngot Lineaments. Pyroxenite and gabbro occur as separate plutons, whereas nepheline syenites occur either in the form of small dikes in pyroxenites or as differentiated bodies in the gabbros. A few mafic dikes, contemporaneous with gabbro, cut pyroxenite and granite bodies. Mineral compositions classify the pyroxenites into pyroxenite and alkali pyroxenite and the gabbros into essexite and olivine gabbro. The chemical and normative compositions of mafic-ultramafic rocks also show their alkaline nature. Nepheline syenite samples are miaskitic in character (agpaitic index < 1), suggesting the involvement of a CO 2 -related phase in their genesis. Chemical data do not support any simple genetic relationship between the different rock units of the complex. Field data also support this conclusion and suggest that these rock units have different genetic histories. Major-element-based discrimination function diagrams suggest the ocean island basalt (OIB) affinity of the Jasra samples. The OIB nature of these samples is further corroborated by their radiogenic isotopic compositions. Sr and Nd isotopic compositions ( 87 Sr/ 86 Sr initial between 0.706523 and 0.708891 and 143 Nd/ 144 Nd initial between 0.512258 and 0.512464) suggest that these rocks were derived from a mixing of mantle components such as HIMU (mantle with a high U/Pb ratio) and EM (enriched mantle) components, which show an isotopic composition similar to FOZO (focal zone) composition. Lherzolite mantle was metasomatized into an alkaline wehrlite by CO 2 , released by low-degree melting of a carbonated mantle peridotite. Melting of such a metasomatized mantle source may produce ultrabasic alkaline silicate magma, from which the different rock units of the Jasra Complex were crystallized. The geological, geochemical, geochronological, and isotopic data also suggest a spatial and temporal association with the Kerguelen plume activity. © 2007 The Geological Society of America.
Petrogenesis of an early Cretaceous potassic lamprophyre dyke from Rongjeng, East Garo Hills, Shillong plateau, north-eastern India
(Indian Academy of Sciences, 2016) Rajesh K. Srivastava; Leone Melluso; Anup K. Sinha
An early Cretaceous potassic lamprophyre dyke, exposed near Rongjeng, East Garo Hills, Shillong plateau, north-eastern India, is a highly porphyritic rock with large phenocrysts of clinopyroxene, phlogopite, amphibole and olivine. Reversely zoned phlogopite and clinopyroxene grains indicate that some degree of interaction between magma batches of variable composition took place somewhere during the crystallization of the lamprophyre. Mineral composi-tions indicate its derivation from an alkaline magma comparable with those that filled the nearby Jasra potassic intrusion. Moreover, the geochemistry of the Rongjeng lamprophyre is distinctly different from that of the Damodar Valley lamproites, the Sung Val-ley carbonatitic-ijolitic intrusion, and the Antarctic ultramafic lamprophyres. The contrasting geochemi-cal affinity is suggestive of heterogenous lithospheric mantle sources, rather than input of plume-related magmatism.
Petrogenesis of Kerguelen mantle plume-linked Early Cretaceous ultrapotassic intrusive rocks from the Gondwana sedimentary basins, Damodar Valley, Eastern India
(Elsevier, 2014) N.V. Chalapathi Rao; Rajesh K. Srivastava; Anup K. Sinha; V. Ravikant
Mineral chemistry, bulk-rock geochemistry and radiogenic isotope (Sr and Nd) data of surface and sub-surface samples of early-Cretaceous Kerguelen mantle plume linked ultrapotassic intrusive rocks from previously unstudied localities of Raniganj and Jharia Gondwana sedimentary basins, Damodar Valley, eastern India, are presented. Despite considerable textural diversity and variable mineralogy these rocks display broadly similar geochemistry highlighting their co-genetic nature. Their bulk-geochemical and petrographic characteristics are similar to those of ultramafic lamprophyres and liquidus mineral composition are closer to that of lamproites and are strikingly comparable to ultrapotassic rocks reported from the Denizli region (Western Anatolia, Turkey), Karinya Syncline and Mt. Bundey (Australia) and the Polayapalle, eastern Dharwar craton (southern India). Incompatible trace element concentrations (e.g., Sr, Zr, Nb, Ta etc.) and their ratios (Ce/Pb, Nb/U, Nb/Yb, Th/Yb) reveal limited influence of crustal contamination and involvement of a predominantly within-plate (plume) and minor subduction-derived components in their magmas. Initial Sr-Nd isotopic ratios of the Damodar Valley ultrapotassic intrusives suggest their derivation from source regions with long term incompatible element enrichment relative to that of Bulk Earth which are very different from those of (i) kimberlites and orangeites from India and southern Africa and (ii) primitive Kerguelen plume component but indistinguishable from those of the pristine Kerguelen mantle plume derived basalts. The depleted mantle (TDM) model ages (0.95-1.4Ga) of the Damodar Valley ultrapotassic rocks are strikingly similar to (i) those of the Deccan-age orangeites from the Bastar craton, central India, and (ii) the emplacement ages (1.1-1.4Ga) of kimberlites and lamproites from the eastern Dharwar craton, southern India. The Gondwana ultrapotassic rocks represent small degree-partial melts derived from a depleted harzburgitic source subsequently metasomatised by carbonate- and rutile-rich fluids/melts, presumably derived from Kerguelen plume, within the garnet stability field at lithospheric depths. A temporal difference of ~500Ma in the source enrichment of Jharia and Raniganj ultrapotassic intrusives, coupled with their distinct incompatible element signatures, bring out involvement of a heterogeneous mantle in their genesis. © 2014 Elsevier B.V.
Petrology and geochemistry of high-titanium and low-titanium mafic dykes from the Damodar valley, Chhotanagpur Gneissic Terrain, eastern India and their relation to Cretaceous mantle plume(s)
(Elsevier Ltd, 2014) Rajesh K. Srivastava; Suresh Kumar; Anup K. Sinha; N.V. Chalapathi Rao
The Damodar valley within the Chhotanagpur Gneissic terrain at the northern-most margin of the Singhbhum craton, eastern India, is perhaps the only geological domain in the entire Indian shield which hosts the early Cretaceous Rajmahal as well as the late Cretaceous Deccan igneous activities. A number of Cretaceous mafic dykes intrude the Gondwana sedimentary formations and are focus of the present study. One set of these dykes strike NNE to ENE, are very fresh and mainly exposed within the Jharia, Bokaro and Karanpura basins; whereas the other set of dykes (including the well-known Salma mega dyke) trend NW to NNW, intrude mainly the Raniganj basin and show meagre hydrothermal alteration. Majority of the samples from both these dyke groups display ophitic or sub-ophitic textures and are essentially composed of augite/titan augite and plagioclase. On the basis of petrographic and geochemical characteristics the NNE to ENE dykes are identified as high-Ti dolerite (HTD) dykes and the NW to NNW dykes are referred to as low-Ti dolerite (LTD) dykes. Apart from the first-order distinction on their titanium contents, both these groups also show conspicuous geochemical differences. The HTD dykes contain relatively high values of iron, and high-field strength elements than those from the LTD dykes with an overlapping MgO contents.Although available field, paleomagnetic and limited geochronological data for most of the studied dykes suggests their emplacement during early Cretaceous period (110-115. Ma), the Salma dyke, dated to be of Deccan-age at ~65. Ma, is an exception. Geochemically all the studied samples show an undoubted plume-derived character but their unequivocal affinity to either the early Cretaceous Kerguelen (Rajmahal) or the late-Cretaceous Reunion (Deccan) plume is not straightforward since they share bulk-rock characteristics of rocks derived from both these plumes. Even though, the spatial and temporal association of the mafic dykes of present study with the Rajmahal Traps are suggestive of their linkage to the Kerguelen plume activity, robust geochronological and paleomagnetic constraints are clearly required to understand the relative contributions of the two Cretaceous mantle plumes in the genesis of the mafic igneous activity in this interesting domain. © 2013 Elsevier Ltd.