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      Volume 113, Issue 4

      December 2004,   pages  517-852

    • Preface

      Hetu Sheth Kanchan Pande

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    • Trace element geochemistry of Amba Dongar carbonatite complex, India: Evidence for fractional crystallization and silicate-carbonate melt immiscibility

      Jyotiranjan S Ray P N Shukla

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      Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas or from secondary melts derived from carbonated silicate magmas through liquid immiscibility or from residual melts of fractional crystallization of silicate magmas. Although the observed coexistence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in many, and overlapping initial87Sr/86Sr and143Nd/144Nd ratios are supportive of their cogenesis; there have been few efforts to devise a quantitative method to identify the magmatic processes. In the present study we have made an attempt to accomplish this by modeling the trace element contents of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element data suggest that the carbonatites and alkaline silicate rocks of this complex are products of fractional crystallization of two separate parental melts. Using the available silicate melt-carbonate melt partition coefficients for various trace elements, and the observed data from carbonatites, we have tried to simulate trace element distribution pattern for the parental silicate melt. The results of the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for the evolution of Amba Dongar but also establish a procedure to test the above hypothesis in such complexes.

    • A preliminary geochemical study of zircons and monazites from Deccan felsic dikes, Rajula, Gujarat, India: Implications for crustal melting

      Nilanjan Chatterjee Somdev Bhattacharji

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      Zircons of 10–100μm size and monazites of up to 10μm size are present in rhyolite and trachyte dikes associated with Deccan basalts around Rajula in the southern Saurashtra Peninsula of Gujarat. On the basis of structural conformity of the felsic and basaltic dikes, K-Ar ages and trace element considerations, a previous study concluded that the felsic rocks are coeval with the Deccan Volcanics and originated by crustal anatexis. The felsic rocks contain two populations of zircons and monazites, one that crystallized from the felsic melt and the other that contains inherited crustal material. Trace element variations in the rhyolites and trachytes indicate that zircons and monazites crystallized from the felsic melts, but compositional analysis of a zircon indicates the presence of a small core possibly inherited from the crust. Hf compositional zoning profile of this zircon indicates that it grew from the host rhyolitic melt while the melt differentiated, and Y and LREE contents suggest that this zircon crystallized from the host melt. Pb contents of some monazites also suggest the presence of inherited crustal cores. Hence, any age determination by the U-Th-Pb isotopic method should be interpreted with due consideration to crustal inheritance. Temperatures estimated from zircon and monazite saturation thermometry indicate that the crust around Rajula may have been heated to a maximum of approximately 900°C by the intruding Deccan magma. Crustal melting models of other workers indicate that a 1–2 million year emplacement time for the Deccan Traps may be appropriate for crustal melting characteristics observed in the Rajula area through the felsic dikes.

    • Petrogenesis of granitoid rocks at the northern margin of the Eastern Ghats Mobile Belt and evidence of syn-collisional magmatism

      S Bhattacharya Rajib Kar S Moitra

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      The northern margin of the Eastern Ghats Mobile belt against the Singhbhum craton exposes granitic rocks with enclaves from both the high-grade and low-grade belts. A shear cleavage developed in the boundary region is also observed in these granitoids. Field features and petrography indicate syn-tectonic emplacement of these granitoids. Petrology-mineralogy and geochemistry indicate that some of the granitoids are derived from the high-grade protoliths by dehydration melting. Others could have been derived from low-grade protoliths. Moreover, microstructural signatures in these granitoids attest to their syn-collisional emplacement.

    • Charnockitic magmatism in southern India

      H M Rajesh M Santosh

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      Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The older (late Archaean to early Proterozoic) charnockites occur in the northern part and the younger (late Proterozoic) charnockites occur in the southern part of this high-grade terrain. Among these, the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate charnockites, with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from northern Kerala and Cardamom hill show spatial association of intermediate and felsic charnockites, with the youngest Nagercoil massif consisting of felsic charnockites. Their igneous parentage is evident from a combination of features including field relations, mineralogy, petrography, thermobarometry, as well as distinct chemical features. The southern Indian charnockite massifs show similarity with high-Ba-Sr granitoids, with the tonalitic intermediate charnockites showing similarity with high-Ba-Sr granitoids with low K2O/Na2O ratios, and the felsic charnockites showing similarity with high-Ba-Sr granitoids with high K2O/Na2O ratios. A two-stage model is suggested for the formation of these charnockites. During the first stage there was a period of basalt underplating, with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed the charnockitic magmas. Here emplacement of basalt with low water content would lead to dehydration melting of the lower crust forming intermediate charnockites. Conversely, emplacement of hydrous basalt would result in melting at higher {ie565-01} favoring production of more siliceous felsic charnockites. This model is correlated with two crustal thickening phases in southern India, one related to the accretion of the older crustal blocks on to the Archaean craton to the north and the other probably related to the collision between crustal fragments of East and West Gondwana in a supercontinent framework.

    • Chemical evolution, petrogenesis, and regional chemical correlations of the flood basalt sequence in the central Deccan Traps, India

      L Melluso M Barbieri L Beccaluva

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      The lava sequence of the central-western Deccan Traps (from Jalgaon towards Mumbai) is formed by basalts and basaltic andesites having a significant variation in TiO2 (from 1.2 to 3.3 wt%), Zr (from 84 to 253 ppm), Nb (from 5 to 16ppm) and Ba (from 63 to 407 ppm), at MgO ranging from 10 to 4.2 wt%. Most of these basalts follow a liquid line of descent dominated by low pressure fractionation of clinopyroxene, plagioclase and olivine, starting from the most mafic compositions, in a temperature range from 1220° to 1125°C. These rocks resemble those belonging to the lower-most formations of the Deccan Traps in the Western Ghats (Jawhar, Igatpuri and Thakurvadi) as well as those of the Poladpur formation. Samples analyzed for87Sr/86Sr give a range of initial ratios from 0.70558 to 0.70621. A group of flows of the Dhule area has low TiO2 (1.2–1.5 wt%) and Zr (84–105 ppm) at moderate MgO (5.2–6.2 wt%), matching the composition of low-Ti basalts of Gujarat, low-Ti dykes of the Tapti swarm and Toranmal basalts, just north of the study area. This allows chemical correlations between the lavas of central Deccan, the Tapti dykes and the north-western outcrops. The mildly enriched high field strength element contents of the samples with TiO2 > 1.5 wt% make them products of mantle sources broadly similar to those which generated the Ambenali basalts, but their high La/Nb and Ba/Nb, negative Nb anomalies in the mantle normalized diagrams, and relatively high87Sr/86Sr, make evident a crustal input with crustally derived materials at less differentiated stages than those represented in this sample set, or even within the sub-Indian lithospheric mantle.

    • High-Ti type N-MORB parentage of basalts from the south Andaman ophiolite suite, India

      Rajesh K Srivastava R Chandra Anant Shastry

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      A complete dismembered sequence of ophiolite is well exposed in the south Andaman region that mainly comprises ultramafic cumulates, serpentinite mafic plutonic and dyke rocks, pillow lava, radiolarian chert, and plagiogranite. Pillow lavas of basaltic composition occupy a major part of the Andaman ophiolite suite (AOS). These basalts are well exposed all along the east coast of southern part of the south AOS. Although these basalts are altered due to low-grade metamorphism and late hydrothermal processes, their igneous textures are still preserved. These basalts are mostly either aphyric or phyric in nature. Aphyric type exhibits intersertal or variolitic textures, whereas phyric variety shows porphyritic or sub-ophitic textures. The content of alkalies and silica classify these basalts as sub-alkaline basalts and alkaline basalts. A few samples show basaltic andesite, trachy-basalt, or basanitic chemical composition. High-field strength element (HFSE) geochemistry suggests that studied basalt samples are probably derived from similar parental magmas. Al2O3/TiO2 and CaO/TiO2 ratios classify these basalts as high-Ti type basalt. On the basis of these ratios and many discriminant functions and diagrams, it is suggested that the studied basalts, associated with Andaman ophiolite suite, were derived from magma similar to N-MORB and emplaced in the mid-oceanic ridge tectonic setting.

    • Relative contributions of crust and mantle to the origin of the Bijli Rhyolite in a palaeoproterozoic bimodal volcanic sequence (Dongargarh Group), central India

      S Sensarma S Hoernes D Mukhopadhyay

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      New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal volcanic sequence (Dongargarh Group) in central India, and one of the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic K-feldspar + albite ± anorthoclase set in fine-grained micro-fragmental matrix of quartz-feldspar-sericite-chlorite-iron-oxide ± calcite. The rocks are largely metaluminous with high SiO2, Na2O + K2O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to an ‘A-type granite’ melt. The rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible element concentrations 2–3 times more than the upper continental crust (UCC). LREE is high (La/Yb ∼ 20) and HREE 20–30 times chondritic. δ18Owhole-rock varies between 4.4 and 7.8‰ (mean 5.87±1.26‰).

      The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component with low CaO and Al2O3 to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ∼ 30% partial melting of AAG under anhydrous condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ18Owhole-rock values. The fast eruption of high temperature (∼ 900°C) rhyolitic melts suggests a rapid drop in pressure of melting related to decompression in an extensional setting.

    • Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

      A Roy A Sarkar S Jeyakumar S K Aggrawal M Ebihara H Satoh

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      Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.

    • Geochemistry and petrogenesis of anorogenic basic volcanic-plutonic rocks of the Kundal area, Malani Igneous Suite, western Rajasthan, India

      A Krishnakanta Singh G Vallinayagam

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      The Kundal area of Malani Igneous Suite consists of volcano-plutonic rocks. Basalt flows and gabbro intrusives are associated with rhyolite. Both the basic rocks consist of similar mineralogy of plagioclase, clinopyroxene as essential and Fe-Ti oxides as accessories. Basalt displays sub-ophitic and glomeroporphyritic textures whereas gabbro exhibits sub-ophitic, porphyritic and intergrannular textures. They show comparable chemistry and are enriched in Fe, Ti and incompatible elements as compared to MORB/CFB. Samples are enriched in LREE and slightly depleted HREE patterns with least significant positive Eu anomalies. Petrographical study and petrogenetic modeling of [Mg]-[Fe], trace and REE suggest cogenetic origin of these basic rocks and they probably derived from Fe-enriched source with higher Fe/Mg ratio than primitive mantle source. Thus, it is concluded that the basic volcano-plutonic rocks of Kundal area are the result of a low to moderate degree (< 30%) partial melting of source similar to picrite/komatiitic composition. Within plate, anorogenic setting for the basic rocks of Kundal area is suggested, which is in conformity with the similar setting for Malani Igneous Suite.

    • Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil, East Garo Hills, Shillong plateau, northeast India

      Rajesh K Srivastava Anup K Sinha

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      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.

    • Geodynamic evolution and crustal growth of the central Indian Shield: Evidence from geochemistry of gneisses and granitoids

      M Faruque Hussain M E A Mondal T Ahmad

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      The rare earth element patterns of the gneisses of Bastar and Bundelkhand are marked by LREE enrichment and HREE depletion with or without Eu anomaly. The spidergram patterns for the gneisses are characterized by marked enrichment in LILE with negative anomalies for Ba, P and Ti. The geochemical characteristics exhibited by the gneisses are generally interpreted as melts generated by partial melting of a subducting slab. The style of subduction was flat subduction, which was most common in the Archean. The rare earth patterns and the multi-element diagrams with marked enrichment in LILE and negative anomalies for Ba, P and Ti of the granitoids of both the cratons indicate interaction between slab derived melts and the mantle wedge. The subduction angle was high in the Proterozoic. Considering the age of emplacement of the gneisses and granitoids that differs by ∼ 1 Ga, it can be assumed that these are linked to two independent subduction events: one during Archaean (flat subduction) that generated the precursor melts for the gneisses and the other during the Proterozoic (high angle subduction) that produced the melts for the granitoids. The high values of Mg #, Ni, Cr, Sr and low values of SiO2 in the granitoids of Bastar and Bundelkhand cratons compared to the gneisses of both the cratons indicate melt-mantle interaction in the generation of the granitoids. The low values of Mg#, Ni, Cr, Sr and high values of SiO2 in the gneisses in turn overrules such melt-mantle interaction.

    • Petrology of the prehistoric lavas and dyke of the Barren Island, Andaman Sea, Indian Ocean

      M A Alam D Chandrasekharam O Vaselli B Capaccioni P Manetti P B Santo

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      Although Barren Island (Andaman Sea, Indian Ocean) witnessed several volcanic eruptions during historic times, the eruptions that led to the formation of this volcanic island occurred mainly during prehistoric times. It is still active and currently in the fumarolic stage. Its volcanic evolution appears to be characterized by a constructive phase with the piling up of lava flows and scoria deposits and Strombolian activities, followed by a sudden collapse of the main cone. Deposits of a possible caldera-forming eruption were not recognized earlier. After a period of peri-calderic hydromagmatic activity, whose deposits presently mantle inner and outer caldera walls, a new phase of intracalderic Vulcanian activities took place. A prominent dyke in the SE inner side of the caldera wall was recognized. Petrographically the lava flows and dyke are similar but they differ in their chemical composition (viz., SiO2, MgO, Ni, Cr) significantly. Similarity in major, minor and trace element composition (viz., K/La, K/Nb, K/Rb, K/Ti ratios) of these rocks together with Chondrite normalized trace element (Rb, Ba, Sr, P, Zr, Ti and Nb) and REE (La, Ce, Nd and Y) patterns of the Barren Island prehistoric lava flows and dyke and low-K lavas of Sunda Arc indicates that Barren Island must have evolved from a source similar to that of Sunda Arc lavas during the Quaternary Period.

    • Silica-poor, mafic alkaline lavas from ocean islands and continents: Petrogenetic constraints from major elements

      Shantanu Keshav Gudmundur H Gudfinnsson

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      Strongly silica-poor (ne-normative), mafic alkaline lavas generally represented by olivine nephelinites, nephelinites, melilitites, and olivine melilitites have erupted at various locations during Earth's history. On the basis of bulk-rock Mg#, high concentrations of Na2O, TiO2, and K2O, and trace element geochemistry, it has been suggested that these lavas represent low-degree melts that have undergone little crystal fractionationen route to the surface. Many of these lavas also carry high-pressure mantle material in the form of harzburgite, spinel lherzolite, and variants of websterite xenoliths, and rare garnet-bearing xenoliths. However, phenocryst phases instead indicate that these magmas cooled to variable extents during their passage. We note subtle, yet important, differences in terms of CaO, Al2O3, CaO/AlP2O3, and CaO/MgO. High-pressure experimental melting studies in CMAS-CO2 (3-8 GPa) and natural lherzolitic systems (3GPa) demonstrate that at an isobar increasing F leads to a moderate decrease in CaO + MgO, whereas CaO/MgO and CaO/Al2O3 sharply decrease. Relatively high CaO/Al2O3 indicates melting in the presence of garnet (>- 85 km). Studies also demonstrate that CO2-bearing lherzolitic systems, when compared with anhydrous ones, also have higher CaO content in the coexisting melt at a given P and T. Comparison of the bulk-rock major-element chemistry of silica-poor, mafic alkaline lavas with experimentally determined high-pressure melts indicates that melting of anhydrous mantle lherzolite or garnet pyroxenite is not able to explain many of the major element systematics of the lavas. However, high-pressure partial melts of carbonated lherzolite have the right major element trends. Among ocean islands, lavas from Samoa and Hawaii are perhaps the products of very low degree of partial melting. Lavas from Gran Canaria and Polynesia represent products of more advanced partial melting. On continents, lavas from South Africa and certain localities in Germany are the products of a very low degree of partial melting, and those from Texas and certain other localities in Germany are products of a slightly more advanced degree of partial melting of a carbonated lherzolite. Lavas from Deccan, Czech Republic, and Freemans Cove are the products of even more advanced degree of partial melting. The mere presence of mantle xenoliths in some of these lavas does not necessarily mean that the erupted lavas represent direct mantle melts.

    • Tectono-thermal evolution of the India-Asia collision zone based on40Ar-39Ar thermochronology in Ladakh, India

      Rajneesh Bhutani Kanchan Pande T R Venkatesan

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      New40Ar-39Ar thermochronological results from the Ladakh region in the India-Asia collision zone provide a tectono-thermal evolutionary scenario. The characteristic granodiorite of the Ladakh batholith near Leh yielded a plateau age of 46.3 ± 0.6 Ma (2σ). Biotite from the same rock yielded a plateau age of 44.6 ± 0.3 Ma (2σ). The youngest phase of the Ladakh batholith, the leucogranite near Himya, yielded a cooling pattern with a plateau-like age of ∼ 36 Ma. The plateau age of muscovite from the same rock is 29.8 ±0.2 Ma (2σ). These ages indicate post-collision tectono-thermal activity, which may have been responsible for partial melting within the Ladakh batholith. Two basalt samples from Sumdo Nala have also recorded the post-collision tectono-thermal event, which lasted at least for 8 MY in the suture zone since the collision, whereas in the western part of the Indus Suture, pillow lava of Chiktan showed no effect of this event and yielded an age of emplacement of 128.2 ±2.6 Ma (2σ). The available data indicate that post-collision deformation led to the crustal thickening causing an increase in temperature, which may have caused partial melting at the base of the thickened crust. The high thermal regime propagated away from the suture with time.

    • 40Ar-39Ar age of a lava flow from the Bhimashankar Formation, Giravali Ghat, Deccan Traps

      Kanchan Pande S K Pattanayak K V Subbarao P Navaneethakrishnan T R Venkatesan

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      We report here a40Ar-39Ar age of 66.0 ± 0.9 Ma (2σ) for a reversely magnetised tholeiitic lava flow from the Bhimashankar Formation (Fm.), Giravali Ghat, western Deccan province, India. This age is consistent with the view that the 1.8–2 km thick bottom part of the exposed basalt flow sequence in the Western Ghats was extruded very close to 67.4 Ma.

    • Magmatic underplating beneath the Rajmahal Traps: Gravity signature and derived 3-D configuration

      A P Singh Niraj Kumar Bijendra Singh

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      The early Cretaceous thermal perturbation beneath the eastern continental margin of the Indian shield resulted in the eruption of the Rajmahal Traps. To understand the impact of the magmatic process that originated in the deep mantle on the lower crustal level of the eastern Indian shield and adjoining Bengal basin the conspicuous gravity anomalies observed over the region have been modelled integrating with available geophysical information. The 3-D gravity modelling has delineated 10–15 km thick high-density (ρ = 3.02 g/cm3) accreted igneous layer at the base of the crust beneath the Rajmahal Traps. Thickness of this layer varies from 16 km to the west of the Rajmahal towards north to about 12 km near Kharagpur towards south and about 18 km to the east of the Raniganj in the central part of the region. The greater thickness of the magmatic body beneath the central part of the region presents itself as the locus of the potential feeder channel for the Rajmahal Traps. It is suggested that the crustal accretion is the imprint of the mantle thermal perturbation, over which the eastern margin of the eastern Indian shield opened around 117 Ma ago. The nosing of the crustal accretion in the down south suggests the possible imprint of the subsequent magmatic intrusion along the plume path.

    • Two- and three-dimensional gravity modeling along western continental margin and intraplate Narmada-Tapti rifts: Its relevance to Deccan flood basalt volcanism

      Somdev Bhattacharji Rajesh Sharma Nilanjan Chatterjee

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      The western continental margin and the intraplate Narmada-Tapti rifts are primarily covered by Deccan flood basalts. Three-dimensional gravity modeling of +70mgal Bouguer gravity highs extending in the north-south direction along the western continental margin rift indicates the presence of a subsurface high density, mafic-ultramafic type, elongated, roughly ellipsoidal body. It is approximately 12.0 ±1.2 km thick with its upper surface at an approximate depth of 6.0 ±0.6 km, and its average density is {dy2935} kg/m3. Calculated dimension of the high density body in the upper crust is 300 ±30 km in length and 25 ±2.5 to 40 ±4 km in width. Three-dimensional gravity modeling of +10mgal to -30mgal Bouguer gravity highs along the intraplate Narmada-Tapti rift indicates the presence of eight small isolated high density mafic bodies with an average density of {dy2961} kg/m3. These mafic bodies are convex upward and their top surface is estimated at an average depth of 6.5 ±0.6 (between 6 and 8km). These isolated mafic bodies have an average length of 23.8 ±2.4km and width of 15.9 ±1.5km. Estimated average thickness of these mafic bodies is 12.4±1.2km. The difference in shape, length and width of these high density mafic bodies along the western continental margin and the intraplate Narmada-Tapti rifts suggests that the migration and concentration of high density magma in the upper lithosphere was much more dominant along the western continental margin rift. Based on the three-dimensional gravity modeling, it is conjectured that the emplacement of large, ellipsoidal high density mafic bodies along the western continental margin and small, isolated mafic bodies along the Narmada-Tapti rift are related to lineament-reactivation and subsequent rifting due to interaction of hot mantle plume with the lithospheric weaknesses (lineaments) along the path of Indian plate motion over the Réunion hotspot. Mafic bodies formed in the upper lithosphere as magma chambers along the western continental margin and the intraplate Narmada-Tapti rifts at estimated depths between 6 and 8 km from the surface (consistent with geological, petrological and geochemical models) appear to be the major reservoirs for Deccan flood basalt volcanism at approximately 65 Ma.

    • Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, west of Khariar, NW Orissa: Evidence from meso- and microstructures

      T K Biswal Harish Ahuja Himansu Sekhar Sahu

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      Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.

    • The Neoproterozoic Malani magmatism of the northwestern Indian shield: implications for crust-building processes

      Kamal K Sharma

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      Malani is the largest event of anorogenic felsic magmatism (covering ∼50, 000 km2) in India. This magmatic activity took place at ∼750 Ma post-dating the Erinpura granite (850 Ma) and ended prior to Marwar Supergroup (680 Ma) sedimentation. Malani eruptions occurred mostly on land, but locally sub-aqueous conditions are shown by the presence of conglomerate, grits and pillow lava. The Malani rocks do not show any type of regional deformation effects. The Malanis are characterised by bimodal volcanism with a dominant felsic component, followed by granitic plutonism and a terminal dyke phase. An angular unconformity between Malani lavas and basement is observed, with the presence of conglomerate at Sindreth, Diri, and Kankani. This indicates that the crust was quite stable and peneplained prior to the Malani activity. Similarly, the absence of any thrust zone, tectonic mélange and tectonised contact of the Malanis with the basement goes against a plate subduction setting for their genesis. After the closure of orogenic cycles in the Aravalli craton of the northwestern shield, this anorogenic intraplate magmatic activity took place in a cratonic rift setting under an extensional tectonic regime.

    • A brief comparison of lava flows from the Deccan Volcanic Province and the Columbia-Oregon Plateau Flood Basalts: Implications for models of flood basalt emplacement

      Ninad R Bondre Raymond A Duraiswami Gauri Dole

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      The nature and style of emplacement of Continental Flood Basalt (CFB) lava flows has been a matter of great interest as well as considerable controversy in the recent past. However, even a cursory review of published literature reveals that the Columbia River Basalt Group (CRBG) and Hawaiian volcanoes provide most of the data relevant to this topic. It is interesting to note, however, that the CRBG lava flows and their palaeotopographic control is atypical of other CFB provinces in the world. In this paper, we first present a short overview of important studies pertaining to the emplacement of flood basalt flows. We then briefly review the morphology of lava flows from the Deccan Volcanic Province (DVP) and the Columbia-Oregon Plateau flood basalts. The review underscores the existence of significant variations in lava flow morphology between different provinces, and even within the same province. It is quite likely that there were more than one way of emplacing the voluminous and extensive CFB lava flows. We argue that the establishment of general models of emplacement must be based on a comprehensive documentation of lava flow morphology from all CFB provinces.

    • Possible lava tube system in a hummocky lava flow at Daund, western Deccan Volcanic Province, India

      Raymond A Duraiswami Ninad R Bondre Gauri Dole

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      A hummocky flow characterised by the presence of toes, lobes, tumuli and possible lava tube system is exposed near Daund, western Deccan Volcanic Province, India. The lava tube system is exposed as several exhumed outcrops and is composed of complex branching and discontinuous segments. The roof of the lava tube has collapsed but original lava tube walls and fragments of the tube roof are seen at numerous places along the tube. At some places the tube walls exhibit a single layer of lava lining, whereas, at other places it shows an additional layer characterised by smooth surface and polygonal cracks. The presence of a branching and meandering lava tube system in the Daund flow, which represents the terminal parts of Thakurwadi Formation, shows that the hummocky flow developed at a low local volumetric flow rate. This tube system developed in the thinner parts of the flow sequence; and tumuli developed in areas where the tube clogged temporarily in the sluggish flow.

    • Cones and craters on Mount Pavagadh, Deccan Traps: Rootless cones?

      Hetu C Sheth George Mathew Kanchan Pande Soumen Mallick Balaram Jena

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      Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Mývatn in northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism.

    • Subject Index

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    • Acknowledgements

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