New discoveries of Silurian and Devonian vertebrates in China and their palaeogeographical significance are reviewed. The Middle Paleozoic fish assemblages of the Tarim basin are similar to those of South China. Fossil vertebrate evidence shows the appearance of the Chinese platform in Middle Paleozoic and its close relation with East Gondwana at least in Early Devonian.
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.
Mesozoic tectonism in Korea is characterized by polycyclic and reiterated intracratonic deformations, which include (a) strike-slip movement of ductile or brittle-ductile shearings, (b) compressive phase of folding, thrusting and transpression of strike-slip movement and (c) extensional phase of basin formation by normal or domino fault system and transtension of strike-slip movement. It occurred during three events; Songnim, Daebo and Bulgugsa orogeneis. Late Permian to Early Triassic Songnim orogeny can be divided into two stages. In the early stage, ductile shearing has affected the southwestern part of the Ogcheon Belt and northwest-trending folds developed in the Paleozoic sedimentary sequences. During the late stage, folding and thrust movement have affected the Paleozoic rocks and produced northwest-trending folds and thrusts which moved toward the southeast. When the Songmin orogeny was diminished, post-tectonic granites were emplaced and the Daedong basins were formed by extensional tectonic fields. The Daebo orogeny follows the deposition of the Later Triassic to Early Jurassic Daedong Group, which inclues northeast-trending folds and thrusts. This phase was continued on the Songnim phase, in which folds and thrust faults had similar trends and transport directions. The Songnim phase was then associated with high angle thrust faults. During the late stage, brittle-ductile strike-slip faults predominated, and produced pull-apart basins along the contact boundary of the Ogcheon belt. S-type granites were emplaced with the northeast trend. From the Late Cretaceous to Early Tertiary periods, the Bulgugsa orogeny produced east-west trending folds and thrusts. Thrust movements prevailed ini the northeastern part of the Ogcheon belt, whereas other areas were affected by east-west trending folds. Sedimentary rocks in the pull-apart basins were affected by transpression and produced en-echelon arrays of folds and flower structures. Mesozoic tectonism in Korea can be correlated to Indosinian, Yanshanian, and Sichuanian tectonisms in China.
Apatite fission-track and organic maturity data from the central region of Sumatra suggest that Tertiary sediments exposed in the Ombilin Basin, in the northern and western margins of the South Sumatra Basin, have low to medium thermal maturities (Ro-average 0.39–0.50%). Apatite fission-track central ages and length data indicate that, at most, low levels of track annealing have occurred and the sediments have only experienced temperatures lower than 120°C. Many samples show a strong component of inherited tracks. In the Ombilin Basin we suggest that the western Talawi sub-basin was not as deeply buried as the eastern Sinamar sub-basin, and probably never received any Miocene sediments. The apatite fission-track age of one basement granite along an intra-basement high, and forward modelling of fission-track length data from the basin, tentatively suggest that cooling (?and uplift) of the western Talawi sub-basin occurred in the late Oligocene to early Miocene. The cause of the sub-division of the basin is ascribed to strike-slip tectonics. Late Eocene rocks exposed along the northern and western margins of the South Sumatra Basin, in the present-day back-arc area, are undermature. Similarly aged and younger rocks in the subsurface of the Central and South Sumatra Basins are reported to be mature/overmature. This suggests that the areas of outcrop studied were not part of the main Paleogene-Neogene graben system that was subsequently inverted. Rather the studied areas have only experienced shallow burial depths and are likely to represent marginal, rift shoulder, areas of sedimentation surrounding areas of deep rift sedimentation.
Radiolarians are abundant in mid-Paleozoic tuffs of the Kurosegawa terrane of southwest Japan. Well preserved Silurian radiolarian faunas recovered from several localities are described herein and include four new species: Ceratoikiscum kurosegum n. sp., Fusalfanus? konomoriensis n. sp., Futobari? jingamoriensis n. sp., and Futobari? tosaensis n. sp. The absence, within the same strata, of fossils belonging to other taxonomic groups has presented difficulties in determining the precise ages of the Kurosegawa radiolarian faunas. U/Pb SHRIMP ages of pyroclastic zircons from tuff units within the succession indicate Wenlockian [427.2 ± 7.6] and Pridolian [408.9 ± 7.6] ages and remove ambiguity regarding the Silurian age of tuffaceous rocks in the Kurosegawa sequence.
The Arena Gneisses (AG), which is considered to form an eastern portion of the Wanni Complex (WC), is exposed at five doubly plunging synforms (arenas) in central Sri Lanka, near the boundary between the WC and the Highland Complex (HC). Structural investigation based on detailed field work on two of the arenas classified the following structural sequences from the earliest to the latest: Dn-2, Dn-1, Dn, Dn + 1, and Dn + 2 deformations. The Dn-2 structures include the major compositional banding, pinch-and-swell and boudinaged structures formed by compression normal to the banding. Mesoscopic intrafolial isoclinal to tight folds (Fn-2) were formed synchronologically with Dn-2 deformation. During the Dn-2 stage, the AG thrusted over the HC with top-to-the-southward movement. The asymmetric structures such as asymmetric foliation boudinaged structure, E-W-trending asymmetric folds (Fn-1b) and asymmetric megacrysts, and NS-stretching lineations were formed by this simple shear deformation. This transport of the AG caused the N-S shortening and produced macroscopic gentle folds (Fn-1a) with mostly E-W-trending upright axial surfaces. During the Dn-1 stage, the eastward movement of the AG took place. This movement was associated with N-S-trending asymmetric folds (Fn-1c) and microfabrics observed in the AG, as well as the eastward vergences of related folds in the HC. N-S-trending upright linear folds (Fn) and related structures such as parasitic folds and axial surface foliations were formed by E-W shortening, which is normal to the axial surface of the macroscopic Fn fold, during the Dn deformational stage. Dextral shear deformation with vertical shear planes took place during Dn + 1 deformation. ENE-WSW- or NE-SE-trending folds (Fn + 1) were developed during Dn + 2 deformation. The southward thrusting of the AG indicates that the WC has displaced over the HC with top-to-the-southward movement. The eastward transportation of the AG and the HC may be due to the collision of the Vijayan Complex (VC) with the HC. Considering the above-mentioned tectonic elements with Sri Lankan geochronological data (e.g. Holzl et al., 1994), the first thrusting event might have taken place around 700 Ma and the second displacement probably occurred around 570 Ma. Copyright (C) 1996 Elsevier Science Ltd
The Bhopalpatnam and Kondagaon granulite belts (BGB, KGB) occur surrounding the Bastar craton of central India. This paper deals with the geology, mineralogy and the fluid characteristics of these two belts. The geology of the two belts indicate that they abound in metasedimentary swathes. The metamorphic P-T conditions of the BGB range from 6 to 9 kbar at temperatures of 750°C, whilst those of the KGB vary from 4 to 6 kbar at temperatures of 700°C. The BGB shows an IBC path, while the KGB exhibits a dominant ITD path. These trends, based on mineral chemistry, are corroborated by fluid-inclusion studies. The lithologies of the BGB exhibit high-density CO2 inclusions that represent the remnants of peak metamorphism. In contrast, the CO2 inclusions of the KGB are of low density, indicative of post-peak conditions. The studies suggest that the CO2-rich fluids may not have come from the supracrustal sediments. An external source, possibly underplated basalt, could have supplied the heat and the supercritical fluids. Based on lithological similarities, it can be stated that the BGB is an extension of the late Archaean Karimnagar granulite belt. The important problem which is still to be solved is the junction of the late Archaean BGB with the Eastern Ghats granulite belt.
The Eastern Ghats on the east coast of India is a largely granulite terrain but also exposes granites, migmatites, anorthosites and alkaline rocks. This granulite belt has had a prolonged history of mountain building from late Archaean to late Proterozoic. During this long period the Eastern Ghats mobile belt witnessed repeated folding and possibly polycyclic metamorphism. Some recent findings suggest breaks between orogenic cycles and a proterozoic reworking of Archaean granulites. Extreme-temperature crustal metamorphism under fluid-absent conditions and crustal anatexis in huge thickness of pelitic to psammitic protoliths producing leptynites are some of the important results of recent investigations of the Eastern Ghats mobile belt. Different generation of charnockites are present in the Eastern Ghats belt, but charnockitisation of granitic gneisses is yet to be documented. Some apparently nascent growths, the patchy charnockites in the Chilka area are shown to be relict of older charnockitic rocks that suffered granulite-facies metamorphism and attendant migmatisation.
Leptynites occur with minor bands of basic granulites, charnockites, calc-silicate rocks, quartzites and khondalites in the Visakhapatnam area in the Eastern Ghats granulite terrane (EGGT), India. Interbedded grey and cream leptynite types are distinguished based on field relations and geochemistry. The mineralogical and geochemical evidences favour a metasedimentary origin for leptynites. The total REE, LREE/HREE and Eu-Eu∗ data indicate their derivation from the post-Archaean granodioritic upper crust comprising large proportions of impure greywackes with K-rich granitic components.
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.
A reconnaissance ground magnetic survey was conducted in the Baguio Mineral District in northern Luzon to investigate the geophysical signatures of the different lithologic units and areas of mineralization. The magnetic-intensity contours match the varying rock types. Magnetic lows were found to correspond to the sedimentary rocks. Magnetic highs coincide with presence of intrusive bodies and the skarn/porphyry-copper deposits. This is to be expected since igneous rocks are more magnetic than sedimentary rocks. Futhermore, the skarn/porphyry copper deposits consist of an ore assemblage of which magnetite is a component. An anomalouos zone, coincident with the sedimentary and igneous rock boundary, was defined from the magnetic survey results. This sedimentary and igneous rock boundary, inferred to be a zone of high permeability, is the probable host to mineable mineral deposits. The geophysically defined anomalous area, characterized by magnetic lows, is consistent with a previously defined prospective zone of mineralization.
Orissa possesses an excellent record of geological history spanning most of the geologic time from Archaean to the Quaternary. It has most of the typical lithologies and many tectonothermal events preserved in the various rock groups. Three distinct crustal blocks could be identified: two cratonic blocks and a mobile belt separated from each other by deep-seated regional fault boundaries. These are the north Orissa craton (NOC), the west Orissa craton (WOC) and the Eastern Ghats granulite belt (EGB). The fault boundaries separating them are identified as the north Orissa boundary fault (NOBF) and the west Orissa boundary fault (WOBF). The NOBF fault running along Mahanadi Valley could be termed the ‘Mahanadi rift’. The NOC contains extensive occurrences of low-grade folded banded iron formations (BIFs), granite intrusives and undeformed volcano-sedimentary assemblages belonging to the Archaean to early Proterozoic times. They are succeeded by medium-grade folded Proterozoic limestone-bearing sequences. The WOC craton is underlain by extensive occurrences of Archaean granites and undeformed Proterozoic limestone-bearing platform sediments. Small occurrences of Archaean BIFs and greenstones have also been noticed. The EGB consists of high-grade granulite-facies rocks such as the khondalites, charnockites, basic granulites, migmatites and augen gneisses. It has been considered as a ‘mobile belt’ during the middle Proterozoic Era. A distinct early Neoproterozoic (ca. 1000 Ma) charnockite event has been recorded in this belt. Close to the faulted boundaries of the crustal blocks are seen occurrences of anorthosites, alkaline rocks and chromite-bearing ultramafics. The rocks in each crustal block show evidence of multiphase tectonothermal history. Similarity of lithology, tectonothermal events and major rift features, e.g. the Mahanadi rift, place Orissa close to Eastern Antarctica.
The Merbok estuary area was formerly a bay with an elongate bay mouth bar. Regression of sea level during the Late Holocene had caused the disappearance of the bay. Evidence from seismic studies show that the Merbok River was not a river till more recent times. Soil survey and field mapping delineated the former beach inland. An emergent shoreline development model is proposed for the geomorphological development. Historical records and archaeological evidence indicate that the present shoreline configuration was almost in place by about 1400. The regression of the sea level had caused the decrease in importance of early Kedah which was sited in the estuary area. In addition several other problems are also explained or resolved by the emergent shoreline model such as the early geography in relation to early Kedah and identification of Kedah in early records from improved knowledge of the palaeogeography. Considering several lines of evidence from field data to historical and archaeological evidence, it is interpreted that the fall in sea level from 2–3 m to the present level was relatively rapid (200 years or less) compared with periods of still-stand. The reconstruction of the palaeo-shoreline also helps to clarify geographical features described in the ancient Kedah Annals, which appear to contain eye-witness accounts of regression of the sea level and not fairy tales as believed by several eminent commentators.
Marine Jurassic rocks of Thailand are well-exposed in the Mae Sot and Umphang areas and less extensively near Mae Hong Son, Kanchanaburi, Chumphon and Nakhon Si Thammarat, in the north, west, and south respectively. They are generally underlain unconformably by Triassic and overlain by Quaternary strata. Based mainly on five measured sections, fourteen new lithostratigraphic units are established: (in ascending order) Pa Lan, Mai Hung and Kong Mu Formations of the Huai Pong Group in the Mae Hong Son area; Khun Huai, Doi Yot and Pha De Formations of the Hua Fai Group in the Mae Sot area; Klo Tho, Ta Sue Kho, Pu Khloe Khi and Lu Kloc Tu Formations of the Umphang Group in the Umphang area; and the Khao Lak Formation in the Chumphon area. Mudstone, siltstone, sandstone, limestone and marl are the dominant lithologies. Mudstones, siltstones and sandstones are widespread; limestones are confined to the Mae Sot, Umphang, Kanchanaburi and Mae Hong Son areas; marls are found only in Mae Sot. The sequences are approximately 900 m thick in Mae Sot and 450 m thick in Umphang and are rather thinner in the other areas, particularly in the south. Based on ammonites, with additional data from bivalves and foraminifera, the marine Jurassic is largely Toarcian-Aalenian plus some Bajocian. Late Jurassic ages given previously for strata in the Mae Sot and Umphang areas have not been confirmed.
East Gondwana incorporates a collage of polymetamorphic terrains with long-lived tectonic histories from the Early Archaean to the Neoproterozoic. The oldest cratonic areas have been identified in South India (north of the Palghat-Cauvery shear zone) and East Antarctica (the Napier Complex). These terrains are remnants of an East Gondwana craton that underwent initial crustal growth during the Early Archaean and granulite-facies metamorphism at ∼2.5 Ga. Both were virtually unaffected by the Pan-African orogeny (1.1-0.5 Ga). In contrast, Proterozoic terrains were subject to high-grade metamorphism during the Pan-African event. On the basis of published Nd model ages, a direct correlation can be made between southern Madagascar (south of the Ranotsara shear zone), southern India (the Madurai Block and Kerala Khondalite Belt) and the Highland/Southwestern Complex of Sri Lanka, which comprise a Later Archaean-Palaeoproterozoic (3.2-2.0 Ga) mobile belt that may extend eastwards into East Antarctica. The youngest period of crustal growth in East Gondwana has been recognised at 1.5-0.8 Ga from isotopic studies of the Mozambique Belt of East Africa, the Vijayan Complex of Sri Lanka and the Yamato-Belgica Complex/Sør Rondane Mountains of East Antarctica. Small slivers of terrain of intermediate age (1.9-1.2 Ga) have been recognised in South India (Achankovil metasediments) and Sri Lanka (Wanni Complex) that may represent mixed-age contributions to clastic sedimentary basins.
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 KAr 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).
A wide variety of Mesozoic basaltic and alkaline igneous rocks crop out on the eastern margin of the Indian shield. We present new KAr isotopic age data on whole rock samples and mineral separates from several igneous rock suites from the Meghalaya-Nagaland region of northeast India. The KAr 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.
Between the Siberian and north China platforms, within the Mongol-Okhotsk fold belt there are a series of ancient massifs (microcontinents), whose origin is not yet known. Combined with available data, the author's studies support the conclusion that the Siberian and north China continents, as well as the Tuvin-Mongolian and central Mongolian microcontinents constituted a unified supercontinent in Early Riphean. The formation of the Paleoasian and Mongol-Okhotsk oceans resulted in its disintegration in Late Riphean. Other Precambrian massifs of this region such as Near Argun, Jiamusi-Bureya, South Gobi and Xingkai are allochthonous terranes of an unknown origin and age. The structures of their basement and sedimentary cover are not similar to those of the Siberian and north China platforms.
The unusual rarity of Fusulinella species is one of the most interesting characteristics among the known Moscovian foraminiferal faunas in Thailand. Earlier an occurrence of Fusulinella bocki von Moller was documented in Northeast Thailand but with neither description nor illustration. For the first time, we have recently discovered a small foraminiferal fauna characterized by two Fusulinella species (F. bocki von Moller and F. praebocki Rauser-Chernoussova) from a limestone collected at Huai Luang, about 5 km east of Wang Saphung, Changwat Loei, Northeast Thailand, where the Fusulina pulchella fauna was reported previously. Geological evidence suggests that our foraminiferal fauna most certainly came from the strata referable to the Fusulina pulchella Zone although F. pulchella Gryzlova is absent from our fauna. The present discovery of a Fusulinella-bearing foraminiferal fauna from Huai Luang, indicative of a Late Moscovian age (most probably to the Myachkovsky Horizon in the Russian standard scheme), provides important data to clarify the faunal composition of the Fusulina pulchella Zone and to compare the Late Moscovian foraminiferal faunal affinity in the Loei-Wang Saphung area with those of the type and reference sections. In this paper, six species of fusulinaceans are described, and coexisting smaller foraminifers are listed and illustrated. Copyright (C) 1996 Elsevier Science Ltd
Orthopyroxene granulites (charnockites, enderbites and associated rocks) form one of the major lithological units in Rayagada, in the north-central sector of the Eastern Ghats Granulite Belt of Peninsular India. Petrographic features show common occurrences of clinopyroxene lamella within orthopyroxene host, although reverse relations have also been observed in some cases. Two types of coronal garnet with different chemical compositions have been noted: one at the interface of orthopyroxene and plagioclase, and the other rimming magnetite, also at the contact of plagioclase. Garnet-orthopyroxene thermometry from these rocks show temperature variations from 840 to 700°C, whereas two-pyroxene and garnet-biotite thermometry yield temperatures around 750 and 700°C, respectively. Garnet-orthopyroxene-plagioclase-quartz barometry ranges from 8.1 to 6.8 kbar. A near-isobaric cooling from a thermal maxima of ∼840°C, followed by decompression is characteristic of the deduced pressure-temperature (PT) path. This shows significant similarities with the PT path derived from associated metapelites. Field and petrographic features collectively suggest that these orthopyroxene granulites have suffered at least the second deformation event (D2), which is the dominant ductile deformation of the area. This event presumably took place around 1000 Ma and might be coeval to the mid-Proterozoic events of East Antarctica and other fragments of Gondwanaland.
