Origin and geochemical characterization of the glauconites in the Upper Cretaceous Lameta Formation,Narmada Basin,central India

This study presents geochemical characteristics of glauconites in estuarine deposits within the Maastrichtian Lameta Formation in central India. Resting conformably over the Bagh Group, the Lameta Formation consists of ~4-5 m thick arenaceous, argillaceous and calcareous green sandstones underlying the Deccan Traps. The sandstone is friable, medium-to coarse-grained, well-sorted and thoroughly crossstratified, and contains marine fossils. Detailed petrography, spectroscopy and mineral chemistry indicates unique chemical composition of glauconite with high K_2O, MgO, Al_2O_3 and moderate TFe_2O_3. Glauconite is formed by the replacement of K-feldspars, initially as stringers in the cleavages and fractures of feldspars. Incipient glauconite subsequently evolves fully, appearing as pellets. Fully-evolved glauconite pellets often leave tiny relics of K-feldspar. XRD exhibits characteristic peak of 10A from basal(001)reflection of glauconite, indicating the "evolved" character. The K_2O content of glauconites in the Lameta Formation varies from 5.51% to 8.29%, corroborating the "evolved" to "highly-evolved" maturation stage.The TFe_2O_3 content of glauconite varies from 12.56% to 18.90%. The PASS-normalized-REE patterns of glauconite exhibit a "hat-shape" confirming the authigenic origin of glauconites. The slightly-negative to slightly-positive Ce anomaly value and the moderate TFe_2O_3 content of glauconite agree well with a suboxic,estuarine condition. The replacement of K-feldspar by the glauconite contributes towards the high K_2O content. Compositional evolution of glauconites in the Lameta Formation is similar to those observed in many Precambrian sedimentary sequences.     

The influence of a mature cyclonic eddy on particle export in the lee of Hawaii

Mesoscale eddies may enhance primary production (PP) in the open ocean by bringing nutrient-rich deep waters into the euphotic zone, potentially leading to increased transport of particles to depth. This hypothesis remains controversial, however, due to a paucity of direct particle export measurements. In this study, we investigated particle dynamics using ²³⁴Th–²³⁸U disequilibria within a mesoscale cold-core eddy, Cyclone Opal, which formed in the lee of the Hawaiian Islands. ²³⁴Th samples were collected along two transects across Cyclone Opal as well as during a time-series within the eddy core during a decaying diatom bloom. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO₂) fluxes at 150 m varied spatially and temporally within the eddy and strongly depended on the ²³⁴Th model formulation used (e.g., steady state versus non-steady state, inclusion of upwelling, etc.). Particle fluxes estimated from a steady state model assuming an upwelling rate of 2 m day⁻¹ yielded the best fit to sediment-trap data. These ²³⁴Th-derived particle fluxes ranged from 332±14 to 1719±53 μmol C m⁻² day⁻¹, 27±3 to 114±12 μmol N m⁻² day⁻¹, and 33±20 to 309±73 μmol Si m⁻² day⁻¹. Although PP rates within Cyclone Opal were elevated by a factor of 2–3, PC and PN fluxes were the same, within error, inside and outside of Cyclone Opal. The ratio of PC export to PP remained surprisingly low at <0.03 and similar to those measured in surrounding waters. In contrast, bSiO₂ fluxes within the eddy core were three times higher. Detailed analyses of ²³⁴Th depth profiles consistently showed excess ²³⁴Th at 100–175 m, associated with the remineralization and possible accumulation of suspended and dissolved organic matter from the surface. We suggest that strong microzooplankton grazing facilitated particulate organic matter recycling and resulted in the export of empty diatom frustules. Thus, while eddies may increase PP, they do not necessarily increase PC and PN export to deep waters. This may be a general characteristic of wind-driven cyclonic eddies of the North Pacific Subtropical Gyre and suggests that eddies may preferentially act as a silica pump, thereby playing an important role in promoting silicic-acid limitation in the region.     

40Ar-39Ar age of carbonatite-alkaline magmatism in Sung valley, Meghalaya, India

⁴⁰Ar-³⁹Ar analyses of one alkali pyroxenite whole rock and two phlogopite separates of calcite carbonatites from the Sung Valley carbonatite-alkaline complex, which is believed to be a part of the Rajmahal-Bengal-Sylhet (RBS) flood basalt province, yielded indistinguishable plateau ages of 108.8 ± 2.0Ma, 106.4 ± 1.3Ma and 107.5 ± 1.4Ma, respectively. The weighted mean of these ages, 107.2 ± 0.8 Ma, is the time of emplacement of this complex. This implies that Sung Valley complex and probably other such complexes in the Assam-Meghalaya Plateau postdate the main flood basalt event (i.e., the eruption of tholeiites) in the RBS province by ∼10Ma.     

Age and geochemistry of the Newania dolomite carbonatites, India: implications for the source of primary carbonatite magma

The Newania carbonatite complex of India is one of the few dolomite-dominated carbonatites of the world. Intruding into Archean basement gneisses, the rocks of the complex have undergone limited diversification and are not associated with any alkaline silicate rock. Although the magmatic nature of the complex was generally accepted, its age of emplacement had remained equivocal because of the disturbed nature of radioisotope systems. Many questions about the nature of its mantle source and mode of origin had remained unanswered because of lack of geochemical and isotopic data. Here, we present results of our effort to date the complex using ¹⁴⁷Sm–¹⁴³Nd, ²⁰⁷Pb–²⁰⁶Pb and ⁴⁰Ar–³⁹Ar dating techniques. We also present mineral chemistry, major and trace element geochemistry and Sr–Nd isotopic ratio data for these carbonatites. Our age data reveal that the complex was emplaced at ~1,473 Ma and parts of it were affected by a thermal event at ~904 Ma. The older ²⁰⁷Pb–²⁰⁶Pb ages reported here (~2.4 Ga) and by one earlier study (~2.3 Ga; Schleicher et al. Chem Geol 140:261–273, 1997) are deemed to be a result of heterogeneous incorporation of crustal Pb during the post-emplacement thermal event. The thermal event had little effect on many magmatic signatures of these rocks, such as its dolomite–magnesite–ankerite–Cr-rich magnetite–magnesio-arfvedsonite–pyrochlore assemblage, mantle like δ¹³C and δ¹⁸O and typical carbonatitic trace element patterns. Newania carbonatites show fractional crystallization trend from high-Mg to high-Fe through high-Ca compositions. The least fractionated dolomite carbonatites of the complex possess very high Mg# (≥80) and have similar major element oxide contents as that of primary carbonatite melts experimentally produced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calcio-carbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbonatite melt and that it was derived from a carbonate bearing mantle. The Sr–Nd isotopic data suggest that the primary magma originated from a metasomatized lithospheric mantle. Trace element modelling confirms such an inference and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone.     

Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution

Determination of the peak thermal condition is vital in order to understand tectono-thermal evolution of the Himalayan belt. The Lesser Himalayan Sequence (LHS) in the Western Arunachal Pradesh, being rich in carbonaceous material (CM), facilitates the determination of peak metamorphic temperature based on Raman spectroscopy of carbonaceous material (RSCM). In this study, we have used RSCM method of Beyssac et al. (J Metamorph Geol 20:859–871, 2002a) and Rahl et al. (Earth Planet Sci Lett 240:339–354, 2005) to estimate the thermal history of LHS and Siwalik foreland from the western Arunachal Pradesh. The study indicates that the temperature of 700–800 °C in the Greater Himalayan Sequence (GHS) decreases to 650–700 °C in the main central thrust zone (MCTZ) and decreases further to <200 °C in the Mio-Pliocene sequence of Siwaliks. The work demonstrates greater reliability of Rahl et al.’s (Earth Planet Sci Lett 240:339–354, 2005) RSCM method for temperatures >600 and <340 °C. We show that the higher and lower zones of Bomdila Gneiss (BG) experienced temperature of ~600 °C and exhumed at different stages along the Bomdila Thrust (BT) and Upper Main Boundary Thrust (U.MBT). Pyrolysis analysis of the CM together with the Fission Track ages from upper Siwaliks corroborates the RSCM thermometry estimate of ~240 °C. The results indicate that the Permian sequence north of Lower MBT was deposited at greater depths (>12 km) than the upper Siwalik sediments to its south at depths <8 km before they were exhumed. The ⁴⁰Ar/³⁹Ar ages suggest that the upper zones of Se La evolved ~13–15 Ma. The middle zone exhumed at ~11 Ma and lower zone close to ~8 Ma indicating erosional unroofing of the MCT sheet. The footwall of MCTZ cooled between 6 and 8 Ma. Analyses of P–T path imply that LHS between MCT and U.MBT zone falls within the kyanite stability field with near isobaric condition. At higher structural level, the temperatures increase gradually with P–T conditions in the sillimanite stability field. The near isothermal (700–800 °C) condition in the GHS, isobaric condition in the MCTZ together with T–t path evidence of GHS that experienced relatively longer duration of near peak temperatures and rapid cooling towards MCTZ, compares the evolution of GHS and inverted metamorphic gradient closely to channel flow predictions.     

~(40)Ar-~(39)Ar GEOCHRONOLGY OF THE OPHIOLITE OF INDUS SUTURE ZONE,LADAKH,INDIA:IMPLICATION FOR THE TIMING OF INITIATION OF THE COLLISION

~(40)Ar-~(39)Ar GEOCHRONOLGY OF THE OPHIOLITE OF INDUS SUTURE ZONE,LADAKH,INDIA:IMPLICATION FOR THE TIMING OF INITIATION OF THE COLLISION1　BeckRA ,BurbankDW ,etal.Nature,1995,373( 55) . 2　DeweyJF ,etal.EclogaegeolHelv ,1989,82 ( 717) . 3　RowleyDB .EarthandPlanetaryScienceLetters,1996 ,14 5( 1) . 4　SharmaKK .PhysicsandChemistryoftheEarth ,1990 ,17. 5　VenketasanTR ,PandeK ,GopalanK .EarthandPlanataryScienceLetters,1993…     

<Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar age of a lava flow from the Bhimashankar Formation,Giravali Ghat,Deccan Traps

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.