Sr isotopic evidence on the spilitic degradation of the Deccan basalt

Similar Sr isotopic ratios (∼ 0.7055) for the tholeiite-spilite flow unit and the associated mineral phases, of Bombay (Deccan Traps) provide a direct evidence for the spilitic degradation of tholeiite. In contrast, a dramatic increase in the rare earth elements (REE) from basalt to spilite is rather puzzling as rare earths are considered to be relatively immobile. The geochemistry thus suggests that the process of spilitization is due to the reaction with a complex fluid having identical Sr-isotopic composition as that of the basaltic magma—thereby masking the details of the mixing process.     

Continental flood basalt magmatism contemporaneous with Deccan traps in the Mannar basin,offshore Sri Lanka

The Gulf of Mannar and adjoining Cauvery basin to the north between India and Sri Lanka are associated with a failed rift, which initiated during the late Jurassic to early Cretaceous as a precursor to the breakup of East Gondwana. Despite the occurrence of igneous rocks that can be noted in seismic profiles, offshore, and deep seated occurrence of those have lead only to the limited understanding of igneous activity in the Mannar basin. Rock cuttings recovered in the Barracuda exploratory well in the Mannar basin shows approximately 700 m thick basalt rock sequence interlayered with sediments at a depth of 3500–4200 m below mean sea level. Here, we analyzed samples recovered from the Barracuda well for major and trace element composition. Major and trace element data suggest that the basalts were crystallized from two different degrees of partial melts from a similar source. Chondrite normalized rare earth element (REE) patterns indicate that the basalts are similar to continental flood basalt, though they show a distinct Ba positive anomaly. Importantly, supported with previously available K–Ar data, we decipher that these basalts are contemporaneous with the Deccan traps. Rifting between Seychelles and India which had occurred at ~62 Ma approximately 3.5 Ma after the main Deccan eruption is synchronous with the Barracuda volcanism suggesting coeval rifting between Seychelles–India and India–Sri Lanka. Thus, our data suggest simultaneous rifting between Seychelles–India and India–Sri Lanka. Large plate reorganizations that took place during this time period in the Indian Ocean have likely caused consequent passive rifting in the Mannar basin.     

Clinopyroxene compositions in the Deccan and Rajmahal Traps and their bearing on magma types and evolution

Electron probe analyses of clinopyroxenes from several areas of the Deccan and Rajmahal Traps consisting mostly of subalkalic and alkalic basalts, picritic basalts and a few dolerite dykes have been obtained. Evaluation of the data indicate the absence of pigeonite from subalkalic basalts that occur in close spatial association with mild or strongly alkalic basalts in areas such as Rajpipla, Navagam and central Kachchh. Co-existence of augite and pigeonite, however, has been noticed in subalkalic basalts/dykes and picritic basalts from a number of Deccan localities such as Sagar, Igatpuri, Kalsubai, Triambak, Pavagarh and Girnar besides the one sample from Rajmahal. Diopside, salite, and wollastonite-rich compositions dominate the basanites and foidites of Kachchh whereas chrome-diopside and salite are the main types in the picrite basalt samples from Anila, Botad and Paliyad in Saurashtra akin to those found in contiguous areas in the east from borehole flows at Dhandhuka and Wadhwan studied in detail previously. Compositional variations in zoned clinopyroxenes indicate differentiation of the parental magma and also mixing of different magma types (subalkalic and alkalic) from areas such as Igatpuri, Rajpipla and Kachchh. Based on host-rock chemistry, total alkalis-silica plot, CIPW norms, estimated temperatures of eruption and augite – pigeonite thermometry, it has been inferred that clinopyroxene compositions, especially the incidence of pigeonite, appear to be very sensitive to bulk chemistry of host rocks, especially their Na₂O, K₂O, SiO₂, total iron and TiO₂ contents. Non-quadrilateral cationic components in the clinopyroxenes, such as Al in tetrahedral and octahedral positions together with Si, Na, Ti and Cr abundances have been found to be useful to discriminate clinopyroxenes from alkalic and subalkalic basalt types besides inferences on the ferric iron component in them. Evaluation of host-rock compositions in the ternary olivine–clinopyroxene-quartz plot indicate polybaric conditions of crystallization and evolution especially in samples that are picritic (e.g. Pavagarh, Anila and Kachchh) and which could also breach the olivine–clinopyroxene-plagioclase thermal divide that exists in part between alkalic and subalkalic basalts under atmospheric conditions.     

Mapping the sub-trappean Mesozoic sediments in the western part of Narmada–Tapti region of Deccan Volcanic Province,India

Deccan Traps spread over large parts of south, west and central India, possibly hiding underneath sediments with hydrocarbon potential. Here, we present the results of seismic refraction and wide-angle reflection experiments along three profiles, and analyze them together the results from all other refraction profiles executed earlier in the western part of Narmada–Tapti region of the Deccan Volcanic Province (DVP). We employ travel time modelling to derive the granitic basement configuration, including the overlying Trap and sub-trappean sediment thickness, if any. Travel time skips and amplitude decay in the first arrival refraction data are indicative of the presence of low velocity sediments (Mesozoic), which are the low velocity zones (LVZ) underneath the Traps. Reflection data from the top of LVZ and basement along with the basement refraction data have been used to derive the Mesozoic sediment thickness.In the middle and eastern parts of the study region between Narmada and Tapti, the Mesozoic sediment thickness varies between 0.5 and 2.0 km and reaches more than 2.5 km south of Sendhwa between Narmada and Tapti Rivers. Thick Mesozoic sediments in the eastern parts are also accompanied by thick Traps. The Mesozoic sediments along the present three profiles may not be much prospective in terms of its thickness, except inside the Cambay basin, where the subtrappean sediment thickness is about 1000–1500 m. In the eastern part of the study area, the deepest section (>4 km) has thick (∼2 km) Mesozoic sediments, but with almost equally thick Deccan Trap cover. Results of the present study provide important inputs for future planning for hydrocarbon exploration in this region.     

http://www.sciencedirect.com/science/article/pii/S1674987113000893

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.     

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan     

Hydrological-drainage analysis in watershed-programme planning: a case from the Deccan basalt,India

Watershed development in India is being adopted increasingly as an integrated mechanism of addressing ecological concerns, particularly in dryland areas. Increasing groundwater recharge constitutes one of the principal objectives of watershed-development programmes because many parts of India face acute shortages of groundwater resources on which rural livelihood depends. A combination of hydrogeological mapping and drainage analysis can form an important tool for planning of watershed-development programmes. Studies on the Kurzadi watershed from the Deccan volcanic province in west-central India illustrate how this technique is useful in selecting sites for artificial recharge of groundwater.The Kurzadi river basin includes three third-order sub-basins that are compared for relative variability in surface-material permeability. Domains of high surface permeability typically indicate relatively higher length ratios and lower drainage density and stream frequency. In the Kurzadi watershed, drainage parameters reveal areas for recharge-related measures and areas where surface-water augmentation measures can be undertaken, even on lower, second-order streams.

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Resumen En la India, se está adoptando el desarrollo de cuenca de forma creciente como un mecanismo integrado para abordar las inquietudes ecológicas, particularmente en zonas áridas. El aumento de la recarga a los acuíferos constituye uno de los objetivos principales de los programas de desarrollo de cuenca, porque muchas zonas de la India deben afrontar períodos acusados de escasez de recursos de aguas subterráneas, de los que dependen los asentamientos rurales. La combinación de cartografía hidrogeológico y de análisis del drenaje puede significar una herramienta importante para la planificación de los programas de desarrollo de cuenca. Los estudios efectuados en la cuenca de Kurzadi, en la Provincia Volcánica de Deccan (región central-occidental de India) ilustran la utilidad de esta técnica con el objeto de seleccionar emplazamientos adecuados para la recarga artificial de acuíferos.La cuenca del río Kurzidi incluye tres subcuencas de tercer orden que han sido comparadas con respecto a la variabilidad relativa de la permeabilidad en superficie. Los dominios de alta permeabilidad superficial son indicadores típicos de relaciones de longitud relativamente mayores y de menor densidad de drenaje y frecuencia de curso fluvial. En la cuenca de Kurzadi, los parámetros de drenaje revelan áreas de medidas relacionadas con la recarga y áreas en las que se puede emprender medidas de aumento de agua superficial, incluso en cursos fluviales menores, de segundo orden.

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Résumé En Inde, la mise en valeur de bassins versants est en train d'être pris en compte de plus en plus comme un mécanisme intégrateur de problèmes concernant l'écologie, en particulier dans les régions sèches. L'accroissement de la recharge des nappes constitue l'un des principaux objectifs des programmes de mise en valeur des bassins versants, parce que de nombreuses régions de l'Inde doivent faire face à une grave pénurie de ressources en eau souterraine, dont dépendent les moyens d'existence en zones rurales. Une combinaison de cartographie hydrogéologique et d'analyse du drainage peut constituer un outil important de planification des programmes de développement des bassins. Des études sur le bassin de Kurzadi, dans la province volcanique du Deccan, dans le centre-ouest de l'Inde, illustre combien cette technique est utile pour décider des sites de recharge artificielle de nappe.Le bassin de la rivière Kurzadi est formé de trois sous-bassins de troisième ordre dont on a comparé la variabilité relative de la perméabilité de surface. Des domaines de forte perméabilité de surface dénotent bien des rapports de longueurs relativement plus forts et une densité de drainage et une fréquence de cours d'eau plus faibles. Dans le bassin de la rivière Kurzadi, les paramètres du drainage révèlent des zones convenant à des mesures relatives à la recharge et des zones où des mesures d'augmentation de l'eau de surface peuvent être entreprises, même sur des cours d'eau plus petits, de second ordre.

     

玄武岩内非均质性对地震资料的影响:印度德干圈闭的一个研究实例

Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves （P to S and vice-versa at the top and bottom basalt interfaces） for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit.     

Plant pathogen Protocolletotrichum from a Deccan intertrappean bed (Maastrichtian), India

A fossil fungus related to Colletotrichum Corda, which causes leaf spot and red rot in plants, was recovered from an intertrappean bed intersected by a well at Mohgaon-Kalan village, Chhindwara District, Madhya Pradesh. The intertrappean bed is sandwiched between the two basaltic flows and is dated as Maastrichtian on the basis of plant and animal fossils. Radiometric dating of the volcanic rocks yielded ages of 67.8–61.6 Ma. The specimens are preserved on a leaf cuticle and are named Protocolletotrichum deccanensis gen. et sp. nov. They are strongly constructed, 1–2 septate, dark brown setae, each with a slightly swollen base and pointed tip.