Kanchan Pande

Did Deccan volcanism pre-date the Cretaceous/Tertiary transition?

Nine basalt samples collected from the bottom to the top of a > 2.5 km thick composite section in the western margin of the Deccan Flood basalt province, India, yield40Ar-39Ar plateau ages between 67 and 62.5 Ma relative to an age of 520.4 Ma for the monitor standard MMhb-1. These ages are consistent with the stratigraphy and comparable to earlier results [1]. They indicate that the lower ∼ 2 km thick reversely magnetised lava sequence erupted within ∼ Ma close to 67 Ma ago and the eruption interval even for the exposed and surviving units of the Deccan was not less than 3 Ma. With the Cretaceous/Tertiary boundary as demarcated by Haiti tektites (65.2 ± 0.1 Ma) [2] and melt rock glasses from Chicxulub crater dated precisely at 64.98 ± 0.10 Ma [3] relative to MMhb-1 (age 520.4 Ma), our results on the thick reversely magnetised lava units imply their eruption before the Cretaceous/Tertiary boundary events by more than the 1.0 Ma.

The Indus river system (India-Pakistan): Major-ion chemistry, uranium and strontium isotopes

The Indus River is one of the large river systems draining the Himalaya. We report in this paper the major-ion chemistry, Sr and U isotope systematics of the Indus system, particularly its headwaters. The results show that: (1) on an average about a third of the cations in the waters can be from silicate weathering; however, most of the (Ca + Mg) is likely to be from the weathering of carbonates and evaporites; (2) the 87Sr86Sr of the waters ranges between 0.7085 and 0.7595, the higher values (> 0.72) are typical of the tributaries draining the Precambrian granite/gneissic terrains. The 87Sr86Sr of the Indus main channel, throughout its entire stretch, shows only minor variations, 0.7104–0.7116. The Indus transports ∼8.8 · 106 mol Sr to the Arabian Sea annually with a 87Sr86Sr of 0.7111 if our results of Sr concentration and 87Sr86Sr measured at Thatta can be considered typical of the Indus throughout the year; and (3) the U concentration in the Indus and its tributaries is generally high, 0.37–10.3 μg 1−1. The source for the high uranium can be the weathering of granites, zones of uranium mineralisation and black shales. The Indus results when compared with our earlier data on the Ganga-Brahmaputra show that in all these three river systems carbonate weathering is the dominant source of (Ca + Mg) and HCO3 and that U concentrations are high. In the case of 87Sr86Sr, the Indus waters are less radiogenic; however, its tributaries draining the Precambrian granites/gneisses have 87Sr86Sr in excess of 0.72, similar to that in the rivers of the Ganga system. The low 87Sr86Sr of the Indus, 0.7111, suggests that its contribution to the Sr isotope evolution of oceans since the Cenozoic is less significant than that of the Ganga-Brahmaputra.

Chemical and Strontium, Oxygen, and Carbon Isotopic Compositions of Carbonates from the Lesser Himalaya: Implications to the Strontium Isotope Composition of the Source Waters of the Ganga, Ghaghara, and the Indus Rivers

Samples of Precambrian carbonate (mostly dolomite) outcrops collected across the Lesser Himalaya have been analysed for their mineralogy, chemical composition, and isotope ratios of Sr, O, and C to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus. Their Sr concentrations range from 20 to 363 ppm, δ18OPDB −1.4 to −12.8‰ and Mn 11–2036 ppm. The petrography of the samples, their low Sr concentrations, and wide range of δ18O values are suggestive of their postdepositional alteration. The 87Sr/86Sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0.7064 to 0.8935 and are generally more radiogenic than that of contemporaneous seawater. Comparison of the 87Sr/86Sr and Sr/Ca ratios among the carbonates and silicates from the Lesser Himalaya and the source waters of the Ganga, Ghaghara, and the Indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates. An upper limit of carbonate Sr in the various source waters is calculated to be between 6% and 43%, assuming that all the Ca in the rivers is of carbonate origin. The results show that on the average, weathering of the Precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters; however, they can be a dominant supplier of radiogenic Sr to some rivers on a regional scale. The silicate Sr component in some of the source waters of the Ganga (Bhagirathi, Bhilangna, Alaknanda, and Ganga), Ghaghara (Kali and Sarju), and the Indus (Sutlej) was calculated from the Ca/Na, Sr/Na ratios, and strontium isotope compositions of these rivers and the silicate endmember. These calculations suggest that 33–89% of Sr in the Bhagirathi, Bhilangna, Alaknanda, Ganga, and Sarju rivers is of silicate origin, whereas in the Kali and the Sutlej it is much lower, only ∼8%. The remaining Sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites. This study underscores the importance of weathering of silicates, carbonates, and evaporites in contributing to the Sr mass balance and 87Sr/86Sr of the source waters of the Ganga, Ghaghara, and the Indus. The present day silicate and carbonate Sr contributions to the Sr budget of the rivers vary considerably, but among the major source waters of the Ganga, silicate Sr exerts a more dominant control on their Sr abundance and 87Sr/86Sr.

Carbon isotopes in Kerguelen plume-derived carbonatites: evidence for recycled inorganic carbon

Carbonatites form from deep mantle melts that are believed to incorporate recycled crustal carbon. Most of the evidence in favour of this hypothesis is, however, circumstantial and comes from the study of radiogenic (Nd‐Sr‐Pb) isotopes that show HIMU and EM-I mantle signatures. In this work, we present direct evidence for the incorporation of recycled crustal carbon in carbonatites of Eastern India through a study of their stable isotope systematics. The 40 Ar= 39 Ar age of one of these coeval complexes is 107:2 0:8 Ma, which suggests that these carbonatites represent late magmatic pulses of the Rajmahal‐Bengal‐Sylhet flood basalt province. Their age, spatial proximity to the Sylhet traps, HIMU-EM I isotopic signatures, and Sr-isotopic similarity to the 115‐105 Ma old Kerguelen Plateau basalts are consistent with the hypothesis of their Kerguelen plume origin. The carbon and oxygen isotope compositions of three of these carbonatite complexes are homogeneous, unlike most of the carbonatites world-wide, and is suggestive of batch crystallization of these rocks under plutonic conditions. The d 18 O values of all the complexes are consistent with their derivation in equilibrium with mantle silicates, whereas d 13 C shows higher values than a ‘normal’ mantle ( d 13 CD 5.0 to 8.0‰). The homogeneity of isotope compositions, absence of 18 O enrichments, co-precipitation of calcite and dolomite in isotopic equilibrium and absence of any crustal contamination effects, preclude the possibility of any change in d 13 C of the carbonatite magmas=rocks by magmatic or secondary fractionation process. Therefore, the d 13 C values of these carbonatites directly reflect the d 13 C values of their source regions. As all these complexes probably belonged to a single magmatic episode, the higher d 13 C of the parent magma (average for all the complexes D 3.2‰) than that of a ‘normal’ mantle is clear evidence for incorporation of recycled inorganic carbon. We suggest that this incorporation is a result of entrainment of a subcontinental lithospheric mantle, which was already enriched in 13 C derived from subducted ancient oceanic crusts through mantle metasomatism.

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.

40Ar–39Ar age of the St. Mary’s Islands volcanics, southern India: record of India–Madagascar break-up on the Indian subcontinent

Abstract The felsic volcanics (rhyolites and rhyodacites) of the St. Mary’s Islands (SMI), southern India (∼13°N), were originally interpreted as a distant outlier of the ∼65 Ma Deccan volcanic province of west–central India, comprising dominantly flood basalts. Later the SMI volcanics were dated at ∼93 Ma by the K–Ar technique. However, this K–Ar ‘age’ was dubious, being merely an average of five out of six widely varying dates and arbitrary data selectivity being involved in this averaging. Our first 40Ar–39Ar dating of the SMI volcanics yields excellent plateau and isochron ages, and their weighted mean isochron age is 85.6±0.9 Ma (2σ). Interestingly, the southern Indian Precambrian terrain is intruded by numerous mafic–doleritic dyke swarms ranging in age from Proterozoic to the latest Cretaceous (69–65 Ma, Deccan-related), and indeed, two regional dykes (a leucograbbro and a felsite) from the Kerala region of southwestern India remain previously dated at ∼85 Ma, but again with the K–Ar technique. However, this age for the SMI volcanics also corresponds excellently with 40Ar–39Ar ages of ∼89–85 Ma (weighted mean isochron age 87.6±1.2 Ma, 2σ: equivalent to 88.1±1.2 Ma corresponding to MMhb-1 age of 523.1±2.6 Ma) for the Madagascar flood basalt province. Together, therefore, the Madagascar flood basalt province, the SMI volcanics, and possibly the Kerala dykes could represent volcanic activity associated with the break-up of Greater India (India plus Seychelles) and Madagascar, thought to have occurred in the Upper Cretaceous at ∼88 Ma.

40Ar‐39Ar ages of Bombay trachytes: Evidence for a Palaeocene phase of Deccan volcanism

We present 40Ar-39Ar ages of 60.4±0.6 Ma and 61.8±0.6 Ma (2σ) for Deccan Trap trachytes from Manori and Saki Naka, Bombay, situated in the tectonized Panvel flexure zone along the western Indian rifted continental margin. These ages provide clear evidence that (i) these trachytes are of Palaeocene age and therefore substantially younger than the lower part of the main flood basalt sequence exposed in the Western Ghats, which precedes the K-T Boundary in age and (ii) the formation of the Panvel flexure along the west coast must have been subsequent to ∼60 Ma. Considering early alkaline Deccan rocks previously dated at ∼68.5 Ma, the total duration of Deccan volcanism was at least ∼8 MY.

Luminescence studies on neotectonic events in south-central Kumaun Himalaya — a feasibility study

Abstract Results of a feasibility study on luminescence dating of neotectonic events as recorded in fault gouges and buried soils formed on landslide debris are reported. It is suggested that clay mineralogical data on illite and chlorite can provide a reasonable estimate on the thermal excursion (and hence on resetting of the luminescence clock) during faulting. The thermoluminescence ages are consistent with the available radiocarbon ages and have been used to infer that Naini Lake was formed at ca. 40–50 ka.

Carbonatite alkaline magmatism associated with continental flood basalts at stratigraphic boundaries: Cause for mass extinctions

The debate between the impact scenario and the hypothesis of the Deccan volcanism leading to Cretaceous/Tertiary (K/T) extinctions is far from settled. We add a new dimension to this debate by introducing an overlooked aspect of the Deccan volcanism, the emplacement and eruption of associated carbonatite-alkaline complexes, which are capable of rapidly injecting catastrophic amounts of CO2 and SO2 into the atmosphere. 40Ar-39Ar dating of one of the several coeval late-Deccan alkaline pulses has revealed that this alkaline volcanism took place at the K/T boundary. A conservative estimate shows that these activities can indeed emit a substantial amount of CO2 and SO2, comparable to that of an impact scenario, which could have enhanced the catastrophic effects of the ongoing basaltic eruptions. Other major Phanerozoic extinctions also show contemporaneity with the late carbonatite-alkaline pulses of associated flood basalt provinces, suggesting a causal relationship between such carbonatite-alkaline magmatisms and the mass extinctions.

Isotope and rare earth element chemistry of carbonatite–alkaline complexes of Deccan volcanic province: implications to magmatic and alteration processes

Abstract Results of different isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0±0.3 Ma by the 40 Ar– 39 Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (∼5%) by the lower crustal material, which is clearly reflected in the initial 87 Sr/ 86 Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13 C-enriched subcontinental lithospheric mantle by the plume.