Anil Kumar

SmNd ages of Archaean metavolcanics of the Dharwar craton, South India

Abstract Well constrained Smue5f8Nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the Dharwar volcano-sedimentary sequence of south India. If interpreted as isochrons, these arrays will correspond to the following ages and initial Nd ratios relative to CHUR: 2.911±0.049 Ga and −0.20 ± 0.40 for the Kalasapura mafic basal metavolcanics of the Bababudan greenstone belt; 2.848±0.070 Ga and −0.88 ± 0.82 for the Santaveri Formation comprising mafic to felsic metavolcanics overlying the Kalasapura rocks; and 2.747±0.015 Ga and +1.72 ± 0.10 for the Ingaldhal mafic-felsic metavolcanic suite of the Chitradurga greenstone belt. The possibility that these Smue5f8Nd arrays are mixing lines between unrelated components depleted and enriched, respectively, in light rare earth elements has been discussed, leading to the conclusion that the ages closely correspond to their time of eruption: the older suites from the Bababudan belt from a mantle source of CHUR-like composition, the younger suite from the Chitradurga belt from one with a prior history of LREE depletion.

The Karnataka Late Cretaceous Dykes as products of the Marion Hot Spot at the Madagascar - India Breakup Event: Evidence from 40Ar-39Ar geochronology and geochemistry

Two late Cretaceous mafic dykes with an ENE strike that is orthogonal to the west coast of India and located nearly 200 km inland around Huliyardurga, Karnataka state, yield 40Ar-39Ar plateau ages of 90.0±1.0 and 87.5±0.9 Ma. These Fe-Ti-enriched tholeiites are essentially co-eval with at least four other igneous suites widely scattered in southern India, namely; the south and north Kerala dykes, the Agali-Anaikatti dykes of central Kerala-Tamil Nadu and lavas of St Mary islands off the west coast of India. The Karnataka Cretaceous dykes are also co-eval and compositionally very similar to the Fe-Ti-enriched tholeiitic lavas and dykes around Mananjary, a major phase of Late Cretaceous magmatism along the eastern rifted margin of Madagascar which are believed to be products of the Marion hot spot that extruded at ∼88 Ma, synchronous with the India-Madagascar break up event. The age and compositional similarities between these Late Cretaceous magmatic rocks from India and Madagascar constitute a clear evidence for extension of the thermal manifestations deep into the Indian peninsula.

A long-lived enriched mantle source for two Proterozoic carbonatite complexes from Tamil Nadu, southern India

We report new neodymium and strontium isotopic data for two Proterozoic carbonatites and related alkalic rocks, at Hogenakal and Sevathur in southern India. These complexes were emplaced into the crust at 2.4 Ga (Hogenakal) and 0.77 Ga (Sevathur). Their initial strontium and neodymium isotopic compositions, together with oxygen isotope data, suggest the involvement of a single long-lived enriched mantle source in their origin. The isotopic evolution of this source indicates that it formed approximately contemporaneously with the accretion and metamorphism of the overlying crust at the southern margin of the Dharwar craton and survived convective disruption in the mantle from early Proterozoic until at least 770 Ma ago. The older of the two carbonatites was intruded into young crust that was not older than about 150 Ma at the time of emplacement. The isotopic data contrast with those from carbonatites of the Canadian Shield, for which isotopic evidence also suggests origin from a long-lived lithospheric source, but one with a depleted chemical signature. They, therefore, indicate that there is no geochemically unique lithospheric source for carbonatites.

Sr and 87Sr/86Sr in the Yamuna River System in the Himalaya: sources, fluxes, and controls on sr isotope composition

Abstract Sr and 87 Sr/ 86 Sr have been measured in the Yamuna river headwaters and many of its tributaries (YRS) in the Himalaya. These results, with those available for major ions in YRS rivers and in various lithologies of their basin, have been used to determine their contributions to riverine Sr and its isotopic budget. Sr in the YRS ranges from 120 to 13,400 nM, and 87 Sr/ 86 Sr from 0.7142 to 0.7932. Streams in the upper reaches, draining predominantly silicates, have low Sr and high 87 Sr/ 86 Sr whereas those draining the lower reaches exhibit the opposite resulting from differences in drainage lithology. 87 Sr/ 86 Sr shows significant co-variation with SiO 2 /TDS and (Na * + K)/TZ + (indices of silicate weathering) in YRS waters, suggesting the dominant role of silicate weathering in contributing to high radiogenic Sr. This is also consistent with the observation that streams draining largely silicate terrains have the highest 87 Sr/ 86 Sr, analogous to that reported for the Ganga headwaters. Evaluation of the significance of other sources such as calc-silicates and trace calcites in regulating Sr budget of these rivers and their high 87 Sr/ 86 Sr needs detailed work on their Sr and 87 Sr/ 86 Sr. Preliminary calculations, however, indicate that they can be a significant source to some of the rivers. It is estimated that on an average, ∼25% of Sr in the YRS is derived from silicate weathering. In the lower reaches, the streams receive ∼15% of their Sr from carbonate weathering whereas in the upper reaches, calc-silicates can contribute significantly (∼50%) to the Sr budget of rivers. These calculations reveal the need for additional sources for rivers in the lower reaches to balance their Sr budget. Evaporites and phosphorites are potential candidates as judged from their occurrence in the drainage basin. In general, Precambrian carbonates, evaporites, and phosphorites “dilute” the high 87 Sr/ 86 Sr supplied by silicates, thus making Sr isotope distribution in YRS an overall two end member mixing. Major constraints in quantifying contributions of Sr and 87 Sr/ 86 Sr from different sources to YRS rivers are the wide range in Sr and 87 Sr/ 86 Sr of major lithologies, limited data on Sr and 87 Sr/ 86 Sr in minor phases and on the behavior of Sr, Na, and Ca during weathering and transport. The Ganga and the Yamuna together transport ∼0.1% of the global Sr flux at the foothills of the Himalaya which is in the same proportion as their contribution to global water discharge. Dissolved Sr flux from the Yamuna and its mobilization rate in the YRS basin is higher than those in the Ganga basin in the Himalaya, a result consistent with higher physical and chemical erosion rates in the YRS.

A Paleoproterozoic giant radiating dyke swarm in the Dharwar Craton, southern India

[1]xa0Identical high precision U-Pb baddeleyite ages, together with paleomagnetic and geochemical data, on mafic dykes occurring over an area of 140,000 km2, define a Paleoproterozoic giant dyke swarm at ca. 2.367 Ga in the Dharwar craton, south India, referred to here as the Dharwar giant dyke swarm. All six U-Pb ages on these dykes are identical within error and suggest emplacement of this swarm within a geologically short time span of ∼5 Myr. A systematic southward progression in the trend of dykes from N48°E to N90°E, defines a fan angle of about 40° with convergence to a focal point about 300 km west of the present-day Dharwar craton boundary, resulting in a spectacular radiating dyke swarm extending across the entire eastern Dharwar craton. The large areal extent, radiating dyke pattern and short duration imply a mantle plume origin for the Dharwar giant dyke swarm. Despite their large areal distribution, all dykes in this swarm are geochemically coherent and have similar primitive mantle-normalized trace element patterns and rare earth element characteristics. Although the NE part of the swarm is magnetically overprinted, a remanence survives that has the same direction as primary magnetizations from dykes in the southern part of the swarm.

40Ar–39Ar age of Siberian basaltic volcanism

Abstract Five of six samples representing the entire stratigraphic sequence (∼3700 m thick) of basaltic flows in the Norilsk section in the northwestern part of Siberian flood basalt province, Russia yield 40 Ar– 39 Ar plateau ages between 248.3±1.7 Ma and 246.9±2.5 Ma relative to an age of 520.4 Ma for the hornblende standard, MMhb-1. These results clarify some discrepancies in existing data and confirm that eruption of the bulk of Siberian basalts was close to 248 Ma and possibly lasted less than 2 Ma. Although the violence of this volcanism may have caused the massive extinctions at the Permo–Triassic boundary, the onset of changes across this boundary seems to have been distinctly earlier.

Palaeolatitudes and the age of Indian laterites

Abstract The problem of the age of Indian laterites is examined on the basis of global models of weather patterns, palaeoclimatology, palaeomagnetism and continental drift. These models suggest that the climate suitable for laterite formation began during the Late Cretaceous and that these conditions became pronounced during the Tertiary when optimum lateritization occurred on the Indian Peninsula.

Sr and 87Sr/86Sr in rivers draining the Deccan Traps (India): Implications to weathering, Sr fluxes, and the marine 87Sr/86Sr record around K/T

The concentration of dissolved Sr and its 87Sr/86Sr has been measured in the headwaters of the Krishna River System and west flowing Western Ghat rivers, all of which have their drainage almost entirely in the Deccan Traps. The Sr concentration follows that of Ca and Mg with Sr/Ca and Sr/Mg ratios similar to that of Deccan basalts, suggesting that all these alkaline earths are released to waters nearly congruently from the Deccan basalts during chemical weathering. The 87Sr/86Sr range from 0.70614 to 0.70986, within that reported for the Deccan basalts. The dissolved Sr flux from the Deccan calculated from the measured data is ∼1.35 × 108 moles yr−1, ∼0.4% of the global riverine flux, which is nearly the same as the proportion of area covered by Deccan basalts relative to the global drainage. The flux, however, is a factor of ∼3 lower than that reported for the Narmada-Tapti-Wainganga (NTW) rivers draining the northern Deccan. This difference in Sr flux among rivers draining the various regions of Deccan could be natural spatial/temporal variations and/or due to supply of Sr to NTW rivers from nonbasaltic sources. Model calculations on the role of emplacement and weathering of Deccan on marine Sr isotope evolution around KTB and late Tertiary show that Deccan basalts can be an important contributor to the decline in 87Sr/86Sr during these periods. It is shown that to account for the pre-KTB dip in 87Sr/86Sr, the flux requirement from Deccan at that time would have to be several times the contemporary flux with 87Sr/86Sr of 0.705–0.708. Considering that the area of Deccan at KTB was about thrice the present area and that the weathering rate of Deccan basalts may have been much higher in early stages following eruption, such high fluxes at about KTB seem feasible. The disproportionately higher flux requirement for Sr is similar to that invoked to explain the pre-KTB decrease in 187Os/188Os. The calculations also show that the supply of unradiogenic Sr from basalts can have significant control on the long-term, ∼66 to ∼55 Ma, decline of marine 87Sr/86Sr.

Kimberlite from Rajmahal magmatic province: Sr-Nd-Pb isotopic evidence for Kerguelen plume derived magmas

[1]xa0Previous studies showed that the Rajmahal-Sylhet-Bengal (RSB) flood basalt province (117 ± 2 Ma) in eastern India was spatially close to the active Kerguelen hotspot about 118 Ma ago. Yet, it could not be unequivocally correlated to this hotspot due to wide variation in isotopic compositions of both the RSB and Kerguelen plateau basalts. However, we report Sr-Nd-Pb isotopic compositions (87Sr/86Sri: 0.70535 to 0.70561; eNd(T): −2.6 to −3.2; 206Pb/204Pbi: 17.88 to 18.07) of a co-eval (116 ± 2 Ma) Group II kimberlite from this flood basalt province that is identical to recently identified pristine Kerguelen plume basalts from the Kerguelen Plateau/Archipelago and Broken Ridge. This suggests that the Kerguelen hotspot could indeed be responsible for the ∼117 Ma magmatic activity in Eastern India.

2.0 GA OLD PYROXENITE-CARBONATITE COMPLEX OF HOGENAKAL, TAMIL-NADU, SOUTH-INDIA

The Hogenakal carbonatites form a series of discontinuous bodies within two pyroxenite dykes. Each body forms veins and lenses emplaced in pyroxenite. Based on field relationships, the sequence of formation is inferred to be pyroxenite first, syenite later, and carbonatites last. The carbonatitic rocks are classified into three types: mica-apatite-calcite (MAC) carbonatite, mica-pyroxene-apatite-calcite (MPAC) carbonatite, and carbonate mica (CM) pyroxenite. Two whole-rock mineral Rbue5f8Sr isochrons for MPAC carbonatites yield ages of 1.984±0.078 Ga and 1.994±0.076 Ga, respectively, which makes this carbonatite not only the oldest yet known in the Indian subcontinent but also one of the few oldest carbonatites known in the world. n nThe carbonatites are poor in alkalies and rich in Sr and to a certain extent in Ba. REE abundances and LaYb ratios vary in the order pyroxenite < CM pyroxenite < MPAC carbonatite < MAC carbonatite; all rocks show LREE-enriched patterns with steep slopes. It is suggested that the pyroxenites represent intrusions of crystal mush formed by separation of pyroxenes from an ijolitic magma. Increase in the concentration of CO2 in such a magma led to the separation of a carbonatitic melt which subsequently intruded the pyroxenite. Mixing of this carbonatite melt with fragmented pyroxenite led to the formation of the MPAC carbonatite and the CM pyroxenite in which pyroxenite dominates over carbonatite. In the closing stages a second pulse of carbonatite was intruded which formed the MAC carbonatite. K-metasomatism is restricted essentially to phlogopitization of pyroxenes and K-feldspar. The association of albite/oligoclase-rich fenites with the carbonatite complex is indicative of Naue5f8K metasomatism. n nThe near equality of the initial Sr ratios of the three samples analysed, together with their high Sr contents, argue against any significant crustal contamination. The initial ratio of 0.70169 must therefore represent the Sr composition of the mantle source of the ijolitic magma. This ratio corresponds to an ϵSr of −6.3 ± 0.6, which implies that the subcontinental mantle which gave rise to the Hogenakal pyroxenite/carbonatite was depleted in incompatible elements even prior to 2 Ga ago, presumably due to an earlier event of crustal formation.