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Author(s): Kumar Akhilesh [1]; Shanti Pappu (corresponding author) [1]; Haresh M. Rajapara [2, 3]; Yanni Gunnell [4]; Anil D. Shukla [5]; Ashok K. Singhvi [3] Luminescence dating at the stratified [...]

The loess-paleosol proxy records from mid-latitude Asia have been instrumental in the reconstruction of regional paleoclimate evolution and its relationship with global climatic changes. The present study explores the discrete occurrence of a loess and paleosol sequence (LPS) in the Southern Zanskar Range (SZR), NW Himalaya. Stratigraphic variations in sediment texture, supplemented with geochemical, organic and magnetic proxies, indicate two broad phases of loess accretion (L-1) and paleosol formation (PS-1). The older phase of loess accretion (L-1) is dated between 2.5 ± 0.3 ka and >2.0 ka, whereas the L-2, which fully transformed into the modern soil (MS) probably deposited during the Little Ice Age (LIA) phase. The textural attributes (high sand content) of loess indicate source proximal deposition, whereas the geochemical and magnetic proxies point towards the pre-depositional weathering (in the source area). The PS-1 dated to 2189 ± 296 cal yr BP indicates improved moisture conditions, whereas the MS is assigned to post LIA warm phase. These findings provide an important step towards better understanding the sensitivity of loess accretion and paleosol formation in the SZR linked to late Holocene climate variability. [ABSTRACT FROM AUTHOR]

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 [sup.147]Sm-[sup.143]Nd, [sup.207]Pb-[sup.206]Pb and [sup.40]Ar-[sup.39]Ar 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 [sup.207]Pb-[sup.206]Pb 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 [δ.sup.13]C and [δ.sup.18]O 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# ([greater than or equal to]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. Keywords Carbonatites * Geochronology * Geochemistry * Continental lithospheric mantle * Newania * India, Introduction Carbonatitic melts are one of the agents of carbon removal from the Earth's interior, and their extraction forms part of the deep carbon cycle, which is responsible for the [...]

Byline: Hetu C. Sheth (1), Jyotiranjan S. Ray (2), Ranjini Ray (1), LoA?c Vanderkluysen (3), John J. Mahoney (3), Alok Kumar (2), Anil D. Shukla (2), Partha Das (1), Subhrashis Adhikari (1), Bikashkali Jana (1) Keywords: Volcanism; Continental flood basalt; Dyke swarm; Deccan Traps; Gondwana Abstract: Many tholeiitic dyke-sill intrusions of the Late Cretaceous Deccan Traps continental flood basalt province are exposed in the Satpura Gondwana Basin around Pachmarhi, central India. We present field, petrographic, major and trace element, and Sr--Nd--Pb isotope data on these intrusions and identify individual dykes and sills that chemically closely match several stratigraphically defined formations in the southwestern Deccan (Western Ghats). Some of these formations have also been identified more recently in the northern and northeastern Deccan. However, the Pachmarhi intrusions are significantly more evolved (lower Mg numbers and higher TiO.sub.2 contents) than many Deccan basalts, with isotopic signatures generally different from those of the chemically similar lava formations, indicating that most are not feeders to previously characterized flows. They appear to be products of mixing between Deccan basalt magmas and partial melts of Precambrian Indian amphibolites, as proposed previously for several Deccan basalt lavas of the lower Western Ghats stratigraphy. Broad chemical and isotopic similarities of several Pachmarhi intrusions to the northern and northeastern Deccan lavas indicate petrogenetic relationships. Distances these lava flows would have had to cover, if they originated in the Pachmarhi area, range from 150 to 350 km. The Pachmarhi data enlarge the hitherto known chemical and isotopic range of the Deccan flood basalt magmas. This study highlights the problems and ambiguities in dyke-sill-flow correlations even with extensive geochemical fingerprinting. Author Affiliation: (1) Department of Earth Sciences, Indian Institute of Technology Bombay (IITB), Powai, Mumbai, 400076, India (2) Planetary and Geosciences Division, Physical Research Laboratory (PRL), Navrangpura, Ahmedabad, 380009, India (3) Department of Geology and Geophysics, School of Ocean and Earth Science and Technology (SOEST), University of Hawaii, Honolulu, HI, 96822, USA Article History: Registration Date: 22/01/2009 Received Date: 08/09/2008 Accepted Date: 22/01/2009 Online Date: 14/02/2009 Article note: Communicated by T. L. Grove.

The Indo‐Myanmar Range is thought to represent a relict eastward‐dipping subduction zone that runs from the eastern edge of the Himalayan Range. In the Indo‐Myanmar Ophiolite (IMO) Belt rodingite is present as discontinuous lenses, dikes, and small‐ to medium‐sized blocks in the serpentinized zones of ultramafic rocks. Petrological and geochronological studies of rodingite from the IMO Belt have been carried out to characterize and elucidate the timing and genesis of the rodingite. Petrography and mineral chemistry indicate that rodingites are composed of grossular garnet, vesuvianite, diopside, tremolite, actinolite, chlorite, albite, prehnite, apatite, and sphene (titanite). The investigated rodingite samples show variability in mineralogical assemblage and geochemical composition. Two mineral assemblages have been identified: (a) grossular, diopside, vesuvianite, apatite, and titanite, observed in veins and dikes; (b) chlorite, epidote, prehnite, diopside, apatite, anatase, and sphene, observed in the samples exposed as blocks. Rodingites are classified into different categories based on their occurrence, and mineralogical and chemical composition. Subtype 1B rodingite is enriched in Al2O3 and incompatible trace elements, most likely formed during seawater infiltration as it contains minerals typically formed during ocean floor alteration. Subtypes 1A and Type 2 rodingite are highly enriched in CaO, Fe2O3T, MgO, and compatible trace elements. The formation of these types of rodingites probably followed serpentinization of the oceanic lithosphere as they contain minerals characteristic of high‐Ca, low‐Si fluid infiltration. These variations might have resulted due to infiltration of a metasomatic fluid that interacted with two different protoliths or infiltration of different fluids that may have interacted (at different times) with the same protolith. Complex variations in mineral assemblages and chemical compositions of Subtype 1 and Type 2 rodingites are the result of compositional variations in their protoliths and the degree of rodingitization. Our data indicate that the T of formation for the investigated rodingites varied from ~435 to ~200°C, corresponding to greenschist facies. The T estimates deduced from the mineralogy of rodingites are consistent with the ambient T during cooling of an oceanic lithospheric slab. Development of a low‐ to medium‐T oceanic environment may be a result of exothermic reactions concurrent to serpentinization. The rodingitization in the study area most likely happened within a rift‐produced oceanic setting during a late period of cooling between 114 and 137 Ma as evidenced by zircon geochronological data. [ABSTRACT FROM AUTHOR]

The article offers information on the need to declare the Higher Himalaya as an eco-sensitive zone due to anthropogenic impact, climate change and unstable paraglacial sediments. It emphasizing the importance of safeguarding the region's ecosystem, life and infrastructure; recent disasters in Uttarakhand and Himachal Pradesh, India, highlight the potential dangers in the Higher Himalayan valleys; and necessitating protective measures similar to eco-sensitive zones established in other areas.

The Vindhyan Basin of western India is a part of the largest Proterozoic sedimentary basin of the Indian subcontinent. Deposited in a largely shallow marine environment, the sedimentary sequence of the basin, known as the Vindhyan Supergroup, is believed to hold clues to our understanding of the evolution of life and climate during the Proterozoic. Inspite of multiple important paleobiological discoveries from these rocks, our knowledge about the development of the basin and responsible regional tectonics had remained rudimentary. Here, we propose a model for the evolutionary history of the Vindhyan Basin based on quantitative provenance analysis through geochemical and isotopic compositions of siliciclastic sediments. According to our model, the basin opened up in a foreland setting during an event that was contemporaneous with the amalgamation of the supercontinent Columbia. In the Lower Vindhyans, most of the sediments were derived from the basement gneisses (BGC-I & II) and a younger differentiated magmatic arc, whereas the >1.7 Ga rocks of the Aravalli and Delhi supergroups formed the source area for the Upper Vindhyans. In comparison to their counterparts in the eastern sector in the Son Valley, the Semri and Kaimur groups of the western sector had a relatively younger provenance; whereas, the Rewa and Bhander groups of both the sectors appear to have shared geochemically similar provenances. Unlike its counterpart in the eastern sector, the provenance of the sediments in the western sector had remained constant across the supposedly 400 million years break in sedimentation between the Lower and the Upper Vindhyans. However, a change in the provenance is observed during the transition from the Kaimur to Rewa group in the western sector which we infer to be a break in the deposition. [ABSTRACT FROM AUTHOR]