亚洲3个大火成岩省(峨眉山、西伯利亚、德干)对比研究

峨眉山(～260Ma)、西伯利亚(～250Ma)和德干(～66Ma)大陆溢流玄武岩是世界上3个重要的大火成岩省。大火成岩省至少具有4个通常被用于识别古地幔柱的标志:(1)先于岩浆作用的地表隆升;(2)与大陆裂谷化和裂解事件相伴;(3)与生物灭绝事件联系密切;(4)地幔柱源玄武岩的化学特征。虽然这3个大火成岩省都是来源于原始地幔柱,但是它们的地球化学特征有本质上的差异,反映其地幔柱曾与不同的上地幔库相互作用。(1)峨眉山和西伯利亚大陆溢流玄武岩的母岩浆,在上升过程中经受了与地球化学上和古老克拉通岩石圈地幔相同的上地幔库(EM1型幔源)的相互作用;(2)而德干大火成岩省没有受到地壳(或岩石圈)混染的原生玄武岩则显示地幔柱和EM2之间的Sr-Nd同位素变化。这种差异有可能制约了3个大火成岩省的成矿潜力。峨眉山和西伯利亚大火成岩省含有世界级岩浆矿床,而德干大火成岩省则不含矿。     

Geochemistry and petrogenesis of basalts from the West Siberian Basin: an extension of the Permo–Triassic Siberian Traps, Russia

New major and trace element data for the Permo–Triassic basalts from the West Siberian Basin (WSB) indicate that they are strikingly similar to the Nadezhdinsky suite of the Siberian Trap basalts. The WSB basalts exhibit low Ti/Zr (50) and low high-field-strength element abundances combined with other elemental characteristics (e.g., low Mg#, and negative Nb and Ti anomalies on mantle-normalised plots) typical of fractionated, crustally contaminated continental flood basalts (CFBs). The major and trace element data are consistent with a process of fractional crystallisation coupled with assimilation of incompatible-element-enriched lower crust. Relatively low rates of assimilation to fractional crystallisation (0.2) are required to generate the elemental distribution observed in the WSB basalts. The magmas parental to the basalts may have been derived from source regions similar to primitive mantle (OIB source) or to the Ontong Java Plateau source. Trace element modelling suggests that the majority of the analysed WSB basalts were derived by large degrees of partial melting at pressures less than 3 GPa, and therefore within the garnet-spinel transition zone or the spinel stability field.

It seems unlikely that large-scale melting in the WSB was induced through lithospheric extension alone, and additional heating, probably from a mantle plume, would have been required. We argue that the WSB basalts are chemically and therefore genetically related to the Siberian Traps basalts, especially the Nadezhdinsky suite found at Noril'sk. This suite immediately preceded the main pulse of volcanism that extruded lava over large areas of the Siberian Craton. Magma volume and timing constraints strongly suggest that a mantle plume was involved in the formation of the Earth's largest continental flood basalt province.     

Multiple factors in the origin of the Cretaceous/Tertiary boundary: the role of environmental stress and Deccan Trap volcanism

A review of the scenarios for the Cretaceous/Tertiary (K/T) boundary event is presented and a coherent hypothesis for the origin of the event is formulated. Many scientists now accept that the event was caused by a meteorite impact at Chicxulub in the Yucatan Peninsula, Mexico. Our investigations show that the oceans were already stressed by the end of the Late Cretaceous as a result of the long-term drop in atmospheric CO₂, the long-term drop in sea level and the frequent development of oceanic anoxia. Extinction of some marine species was already occurring several million years prior to the K/T boundary. The biota were therefore susceptible to change. The eruption of the Deccan Traps, which began at 66.2 Ma, coincides with the K/T boundary events. It erupted huge quantities of H₂SO₄, HCl, CO₂, dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature. The result was a major reduction in oceanic productivity and the creation of an almost dead ocean. The volcanism lasted almost 0.7 m.y.. Extinction of biological species was graded and appeared to correlate with the main eruptive events. Elements such as Ir were incorporated into the volcanic ash, possibly on soot particles. This horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the K/T boundary. An oxidation front penetrated this horizon leading to the redistribution of elements. The eruption of the Deccan Traps is the largest volcanic event since the Permian-Triassic event at 245 Ma. It followed a period of 36 m.y. in which the earth’s magnetic field failed to reverse. Instabilities in the mantle are thought to be responsible for this eruption and therefore for the K/T event. We therefore believe that the K/T event can be explained in terms of the effects of the Deccan volcanism on an already stressed biosphere. The meteorite impact at Chicxulub took place after the onset of Deccan volcanism. It probably played a regional, rather than a global, role in the K/T extinctions.     

Ichthyofauna (Chondrichthyes,Osteichthyes) from the Upper Cretaceous intertrappean beds of Piplanarayanwar,Chhindwara District,Madhya Pradesh,India

A new fossiliferous intertrappean section is found 92 km southwest of the extensively studied intertrappean beds of Mohgaonkalan and Jhilmili in Chhindwara District, Central India. Application of the bulk screen‐washing method led to the recovery of a rich microvertebrate fauna represented by fish, amphibians, crocodiles, snakes and dinosaurs. In this paper, the ichthyofauna recovered from the new intertrappean section consisting of Igdabatis indicus Prasad and Cappetta, 1993, Lepisosteus indicus Woodward, 1908, Osteoglossidae gen. et sp. indet., Pycnodontidae gen. et sp. indet. and Siluriformes indet. is described. The ichthyofauna dominantly represented by the teeth of Igdabatis indicus is suggestive of a nearshore, deltaic or estuarine palaeoenvironment and Late Cretaceous (Maastrichtian) age for the intertrappean beds. The remarkable similarity of the new intertrappean fauna to that of Asifabad and the infratrappean beds of Marepalli also is suggestive of coastal‐plain conditions all along the Godavari lineament in the Late Cretaceous. Although the presence of planktonic foraminifera in the intertrappean beds of Jhilmili in the Mandla Lobe of Deccan volcanic province has been inferred in terms of a short term marine incursion from the west coast along the Narmada lineament in the Early Palaeocene, currently there is no definitive evidence for the prevalence of marine or coastal‐plain conditions along the Narmada lineament at least in the latest Cretaceous.     

Hydrocarbon prospects across Narmada-Tapti rift in Deccan trap, central India: Inferences from integrated interpretation of magnetotelluric and geochemical prospecting studies

As the Mesozoic sediments contribute most of the oil and gas reserves of the world, we present an integrated interpretation approach using magnetotellurics (MT) and surface geochemical prospecting studies to demarcate hydrocarbon prospective Gondwana (Mesozoic) formations underneath the Deccan flood basalts of Late Cretaceous age across Narmada-Tapti rift (between Bhusawal and Barwah) in Central India. The MT interpretation shows deep (∼5 km) basement structure between southern and central part of the MT profile however, it gradually becomes shallower to either ends of the profile with a predominant basement depth reduction in the northern end compared to the southern end. The geophysical results suggest thick (2-3.5 km) Mesozoic sediments in the area characterized by deep basement structure. The geochemical analysis of the near surface soil samples indicate higher concentrations of light gaseous hydrocarbons constituents over the area marked with thick sub-basalt Mesozoic formations. Analyses of the geochemical data imply that these hydrocarbons are genetically related, generated from a thermogenic source and these samples fall in the oil-producing zone. The temperature-depth estimations in the region supports favorable temperature conditions (80-120 °C) for oil generation at basement depths.     

First observation of microspherule from the infratrappean Gondwana sediments below Killari region of Deccan LIP,Maharashtra (India) and possible implications

A rare occurrence of a microspherule has been found in the infratrappean sediments, encountered below 338 m thick Deccan volcanic cover in KLR-1 scientific borehole, drilled in the epicentral zone of the 1993 Killari earthquake (Maharashtra, India). Palynological studies of the sediments indicate their age as Early Permian (Asselian, 298–295 Ma) for deposition. Transmission electron microscope studies reveal that the spherule from the infratrappeans, is having a similar composition to that of the Neoarchean amphibolite to granulite facies mid crustal basement. The spherule is non-spherical in nature, containing mostly FeO (10.70 ± 0.20 wt.%), CaO (13.8 ± 0.5 wt.%), Al₂O₃ (7.78 ± 0.30 wt.%), MgO (6.47 ± 0.3 wt.%), SiO₂ (47.46 ± 0.50 wt.%), TiO₂ (2.47 ± 0.3 wt.%), K₂O (1.89 ± 0.20 wt.%), and Cl (0.33 ± 0.05 wt.%). Since the Fe composition of the spherule is almost same as the basement rock (10.5 wt.%), and the chlorine content is also in the same range as the basement (0.04–0.24 wt.%), it would suggest possibility of an extraterrestrial impact over the Indian terrain during the erstwhile Gondwana sedimentation period that may be associated with the Permian–Triassic mass extinction, the most severe one in the Earth's history.     

Cones and craters on Mount Pavagadh,Deccan Traps: Rootless cones?

Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Myvatn in northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism.     

Increased Particle Fluxes at the INDEX Site Attributable to Simulated Benthic Disturbance

Particle fluxes were measured 7 m above the sea bottom during the predisturbance, disturbance, and postdisturbance periods by using time series sediment traps attached to seven deep-sea moorings deployed in the INDEX experiment site in the Central Indian Basin. The predisturbance particle fluxes varied between 22.3 to 55.1 mg m -2 day -1 . Increased and variable particle fluxes were recorded by the sediment traps during the disturbance period. The increase observed was 0.5 to 4 times more than the background predisturbance fluxes. The increases in particle fluxes (~4 times) recorded by the sediment trap located in the southwestern direction (DMS-1) were the greatest, which could be the result of preferential movement of resuspended particles generated during the deep-sea benthic disturbance along the general current direction prevailing in this area during the experimental period. Also, the traps located closer to the disturbance area recorded greater fluxes than did the traps far away, across the Deep Sea Sediment Resuspension System path. This variability in recorded particle fluxes by the traps around the disturbance area clearly indicates that physical characteristics such as grain size and density of the resuspended particles produced during the disturbance had an important effect on particle movement. The postdisturbance measurements during ~5 days showed a reduction in particle fluxes of ~50%, indicating rapid particle settlement.     

A mantle plume origin for the Siberian traps: uplift and extension in the West Siberian Basin, Russia

The West Siberian Basin (WSB) records a detailed history of Permo-Triassic rifting, extension and volcanism, followed by Mesozoic and Cenozoic sedimentation in a thermally subsiding basin. Sedimentary deposits of Permian age are absent from much of the basin, suggesting that large areas of the nascent basin were elevated and exposed at that time. Industrial seismic and well log data from the basin have enabled extension and subsidence modelling of parts of the basin. Crustal extension (β) factors are calculated to be in excess of 1.6 in the northern part of the basin across the deep Urengoy graben. 1-D backstripping of the Triassic to Cenozoic sedimentary sequences in this region indicates a period of delayed subsidence during the early Mesozoic. The combination of elevation, rifting and volcanism is consistent with sublithospheric support, such as a hot mantle plume.

This interpretation accords with the geochemical data for basalts from the Siberian Traps and the West Siberian Basin, which are considered to be part of the same large igneous province. Whilst early suites from Noril'sk indicate moderate pressures of melting (mostly within the garnet stability field), later suites (and those from the West Siberian Basin) indicate shallow average depths of melting. The main region of magma production was therefore beneath the relatively thin (ca. 50–100 km) lithosphere of the basin, and not the craton on which the present-day exposure of the Traps occurs. The indicated uplift, widespread occurrence of basalts, and short duration of the volcanic province as a whole are entirely consistent with published models involving a mantle plume. The main argument against the plume model, namely lack of any associated uplift, appears to be untenable.