Journal of Southeast Asian earth sciences最新文献

Mesozoic circum-Tethys and circum-Pacific basins, which are genetically related to the Tethys and circum-Pacific tectonic zones, are the regions with the most abundant petroleum and natural gas. These two tectonic systems intersect in China and most of the Mesozoic basins in China are closely associated with these two systems. Circum-Tethys basins developed against a background of crustal compression. They are the result of compressional thrusting and the resultant sinking of the crust. Warm-moderately hot foreland basins are the most representative. Hydrocarbon source rock evolved slowly. Threshold depth of oil-generation is great and petroliferous beds have relatively older ages and occur at greater depths. Circum-(west) Pacific basins were formed against a background of extension and thinning of the crust and frequent geothermal activities. Hot rift basins are the most representative. Hydrocarbon source rock evolved rapidly. Threshold depth of oil-generation is small and petroliferous beds have relatively younger ages and occur at shallower depths.

Oil and gas-bearing basins in China are grouped into three types: the tensional, compressional and transitional. The first type is dominant in the east, with the Songliao and the Bohai Gulf basin as its representative; the second in the west, exemplified by the Tarim and Junggar basins; and the last in central China, represented by the Sichuan and Ordos basins. These petroliferous provinces are polycyclic superimposed and composite basins, with complex traps of various origins and megastructural oil-gas belts.

A new palaeomagnetic study of mid Cretaceous lavas from 23 localities in the northern part of the Rajmahal Hills gives a mean virtual geomagnetic pole at 10.4°N, 296.6°E, confirming the results of earlier workers. Stepwise thermal and alternating field demagnetisation indicates that 21 out of 23 sites are normally magnetised, whereas two sites in the northwest of the area have recorded two different transitional directions. We have measured the strong-field thermomagnetic behaviour, DC and AC hysteresis parameters, and the variation at low temperature of low-field susceptibility of these and other samples from the Rajmahal Volcanics. These rock magnetic studies reveal that the basaltic lavas contain titanomagnetites which have hardly been oxidised at high temperature, but have undergone low temperature oxidation. The reddened lavas of Gandeswari Hill, on the other hand, show evidence of extensive high temperature deuteric oxidation of titanomagnetite.

This thematic set results from a conference entitled “Mesozoic magmatism of the eastern margin of India” convened by N. C. Ghose at Patna University, India, 28–29 February 1992. The meeting was designed as a forum for discussion of recent work on the Rajmahal and Rajahmundry flood basalts of eastern India, and studies of Early Cretaceous alkaline igneous rocks from the Shillong Plateau, Meghalaya.

Bentonite is distributed widely in the Rajmahal Hills of eastern India, usually in association with volcaniclastic deposits. Although bentonite occurs between the successive flow units, it always forms pockets, rather than a continuous bed. The occurrence of bentonite in the lower part of the Rajmahal lava pile (characterised by fossiliferous sedimentary interbeds) is consistent with its formation in a subaerial or lacustrine environment. The bentonite appears to have formed as a result of alteration of basaltic pyroclastic rocks. The Rajmahal bentonite is of non-swelling type. The exchangeable base is Ca, which is substituted by Na. Petrographic studies reveal that montmorillonite is the predominant mineral in the bentonite, together with minor kaolinite and illite. Cristobalite constitutes the main non-clay mineral. Nontronite, saponite, dickite, nacrite, anauxite and beidellite have also been identified. The bentonite is completely devoid of attapulgite, dolomite and organic material. Rajmahal bentonite with high $\text{Fe}{}_2\text{O}{}_3\text{CaO}$ ratio is gritty in texture and low plasticity, whereas bentonite with high $\text{Al}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ and $\text{CaO}\text{Al}{}_2\text{O}{}_3$ and low $\text{Fe}{}_2\text{O}{}_3\text{CaO}$ ratios is grit-free and of high plasticity. The latter material is superior in quality when compared to the former. Based on surface and drill core data, a total economic reserve of ∼177,000 t of bentonite is estimated in the Rajmahal Hills. Of this total, 82,300 t is Grade I quality and 94,400 t is Grade II quality.

A wide variety of Mesozoic basaltic and alkaline igneous rocks crop out on the eastern margin of the Indian shield. We present new KAr isotopic age data on whole rock samples and mineral separates from several igneous rock suites from the Meghalaya-Nagaland region of northeast India. The KAr isotopic ages of these samples lie between 149-107 Ma, i.e., Late Jurassic to Early Cretaceous. Published data on the tectonic setting and petrology of the basaltic and alkaline rocks suggest that their emplacement was controlled by major fault systems. Active mantle upwelling in an extensional tectonic régime related to the fragmentation of the indian plate from Australia-Antarctica in the Early Cretaceous, appears to have been the main trigger for the magmatic activity.

The Rajmahal Traps of Early Cretaceous age (117 ± 1 Ma) are medium/fine-grained rocks of quartz-tholeiitic composition. Plagioclase phenocrysts in the basalts show compositional zoning from An₅₃ to An₆₈, whereas groundmass plagioclase compositions range from An₅₀ to An₅₈. Clinopyroxene phenocrysts range in composition from En_46–48Wo_35–39Fs_14–19, with groundmass clinopyroxene compositions in the range En_32–46Wo_33–37Fs_18–34. Titanomagnetite occurs as small specks, prismatic grains and elongate needles. Secondary minerals include smectite-chlorite replacing groundmass plagioclase and augite, suggesting that the basalts have undergone hydrothermal alteration subsequent to their emplacement. Major, trace and rare-earth element data suggest two distinct catergories of basalts; Group I, characterised by high MgO, K₂O, Sr, Rb, Cr, Ba and Nb; and Group II, characterised by high Fe₂O₃, TiO₂ and rare-earth element contents. Both basalt groups appear to have been derived by melting of the same mantle source. Rajmahal basalts collected from three sites show very good grouping in their natural remanent magnetic vectors, with normal and reverce polarities. Detailed AF and thermal demagnetisation treatment of these rocks reveals characteristic components at all three sites. Sites A and B exhibit normal magnetisation, and site C exhibits both normal and reverse magnetisations. These results are critically evaluated in relation to models for widespread basaltic magmatism during the break-up of eastern Gondwana.

The Shillong Plateau of northeast India is identified as an alkaline province in view of the development of several carbonatite complexes e.g. the Sung Valley (Jaintia Hills), Jasra (Karbi-Anglong), Samchampi and Barpung (Mikir Hills) and lamprophyre dyke swarms (Swangkre, Garo-Khasi Hills). On the basis of limited KAr data, magmatic activity appears to have taken place over a protracted period, ranging from the Late Jurassic to the Early Cretaceous. The carbonatite complexes of the Shillong Plateau share several common traits: they are emplaced along rift zones, either within Archaean gneisses or Proterozoic metasediments and granites, and exhibit enrichment in the light rare-earth elements, U, Th, Nb, Zr, Ti, K and Na. The enrichment in incompatible trace elements can best be accounted for if the parental magmas were of alkali basaltic type (e.g. mela-nephelinite or carbonate-rich alkali picrite).

Closely jointed, veined and locally sheared basalts some 15–20 m thick are exposed in quarries at Gauripatna and Kateru on the banks of the Godavari River, near Rajahmundry, Andhra Pradesh, India (17°N 21°E). The clinopyroxene in these basalts is variably replaced by saponite, which locally preserves the primary phenocryst shape. At Gauripatna, the amygdales have dolomite in the centre and saponite in the rim; opaque phases occur only as dendritic aggregates within saponite. At Kateru, dolomite is absent and large and hypidiomorphic magnetite mantles fresh clinopyroxene. Measured rare-earth element (REE) concentrations indicate the occurrence of light REE-enriched basalts, most probably derived from a single mantle source region by different degrees of partial melting. Comparison with published data indicates that the Rajahmundry basalts have REE abundances similar to lavas from the Deccan Traps, western India. Fairly intense brittle deformation of the Rajahmundry lavas, at both the scale of outcrop and hand specimen, is evident in the deeper parts of the quarries. A pilot study of the magnetic fabric shows a large apparent spread in azimuth. Palaeopole positions derived from such deformed lavas are likely to be unreliable, due to replacement of primary iron oxides.

An approximately 220-m thick sequence of Early Cretaceous flood basalts (the Rajmahal Basalt Group) crop out over some 4300 km² in Bihar, eastern India, forming the leading edge of a seaward-dipping reflector sequence emplaced during the break-up of India and Australia/East-Antarctica. Geochemical data support a division of the basalts and associated dykes into high-Ca and low-Ca magma types. High-Ca tholeiites have CaO contents >10.0 wt%, mg# 50.3–59.6 and K₂O 0.11–0.55 wt%. $\text{La}\text{Nb}$ ranges from 1.29 to 3.62. Rocks of the low-Ca magma type have ≤ 10.5 wt% CaO, mg# 52.1–70.7 and K₂O 0.26–1.1 wt%. $\text{La}\text{Nb}$ is between 1.6 and 3.29. These element abundances and ratios are similar to those of Cretaceous tholeiites from the central Kerguelen Plateau (ODP Site 120–749). Plate reconstructions indicate that the plateau lay adjacent to the Indian continental margin during Early Cretaceous times. It is shown that certain of the Rajmahal basalts (low-Ca magma type) have been contaminated by Indian upper crust, whilst others (high-Ca lavas) retain the near-flat mantle-normalized trace element patterns of oceanic plateau tholeiites.