Journal of the Geological Society
2041-479X
0016-7649
Anh Quốc
Cơ quản chủ quản: Geological Society of London , GEOLOGICAL SOC PUBL HOUSE
Lĩnh vực:
Geology
Phân tích ảnh hưởng
Thông tin về tạp chí
JGS publishes topical, high-quality recent research across the full range of Earth Sciences. Papers are interdisciplinary in nature and emphasize the development of an understanding of fundamental geological processes. Broad interest articles that refer to regional studies, but which extend beyond their geographical context are also welcomed.
Các bài báo tiêu biểu
The mineralogy and metamorphic geology of low-grade metasediments, Northern Range, Trinidad Forty-one low-grade metasediments from the Northern Range of Trinidad and from the neighbouring island of Tobago, West Indies, were investigated by microscopic, X-ray powder diffraction, Guinier camera technique and electron microprobe methods. The following minerals were encountered (in order of decreasing abundance): quartz, muscovite, chlorite, paragonite/muscovite mixed-layer, paragonite, organic material, calcite, pyrophyllite, dolomite, albite, hematite, chloritoid, pyrite, rutile, titanite, rectorite, stilpnomelane.
The chemical composition of muscovite is dependent on the mineral assemblage, with Si = 6.1–6.2 per formula unit in two high alumina assemblages, versus Si = 6.6 in a low alumina assemblage. Microprobe analyses indicate that muscovite, paragonite and pyrophyllite are sometimes intergrown on a micrometre scale. Chlorite composition estimated from X-ray data shows good correspondence with microprobe data for two Al-poor samples but is less satisfactory for an Al-rich sample.
Observed mineral assemblages in AKNa are indicative of the pyrophyllite-paragonite-albite fades. For the first time, chemical data for coexisting pyrophyllite, chlorite (M = MgO/(MgO+ FeO) = 0.377) and chloritoid (M = 0.110) are given. This assemblage is indicative of the low or medium pressure range of the lower greenschist facies.
Pressure conditions during metamorphism are not well constrained but a minimum pressure of 2 kbar, corresponding to 5–6 km of overburden, can be estimated from the Na
M4
content of actinolite. Temperatures derived from the calcite-dolomite geothermometer range between 300 and 350°C and are consistent with the presence of the assemblage pyrophyllite + calcite + quartz.
Tập 145 Số 4 - Trang 563-575 - 1988
Molecular taphonomy of graptolites
Graptolites are important fossils in Early Palaeozoic assemblages. Preserved graptolite periderm consists dominantly of an aliphatic polymer, immune to base hydrolysis. It contains no protein even though its structure, and chemical analyses of the periderm of the living relative
Rhabdopleura
, indicate that it was originally collagen. This anomaly was previously interpreted as the result of replacement by macromolecular material from the surrounding sediment. New analyses suggest that the aliphatic composition of graptolite periderm reflects direct incorporation of lipids from the organism itself by
in situ
polymerization. A similar process may account for the preservation of most organic fossils.
Tập 163 Số 6 - Trang 897-900 - 2006
The geochemistry of young volcanism throughout western Panama and southeastern Costa Rica: an overview Oblique aseismic subduction below western Panama and southeastern Costa Rica has produced Recent arc-related volcanism. The aseismicity is probably related to the subduction of relatively hot oceanic lithosphere. The volcanism throughout this region over the past 2 Ma has been quite distinct, consisting of felsic magmas (andesites to rhyolites but mainly dacites) with geochemical signatures suggesting a metamorphosed basaltic source. It is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge. Rocks with slab-melt geochemistries and associated with young subducted crust have been termed adakites elsewhere. The young adakite melts are sometimes associated with a few rare young high-Nb basalts, but there is no obvious genetic link between them through differentiation. High-Nb basalts may also be derived from the partial melting of the subducted oceanic crust. High-Nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the Catalina Schist, California. Alternatively, the high-Nb basalts may be partial melts of phlogopite-rich mantle that has previously reacted with adakite magmas.
Eruption of adakites and high-Nb basalts was preceded by a 2-3 Ma period of relative quiescence. Prior to this, there was a 7 Ma period of calc-alkaline volcanism typical of the present-day magmatism (associated with a distinct Benioff zone) found throughout the Central American arc. The abrupt transition in volcanism with time from an early calc-alkaline sequence to a later adakite-high-Nb basalt sequence may record a change in the tectonic setting of western Panama and southeastern Costa Rica over the past 12 Ma.
Tập 149 Số 4 - Trang 569-579 - 1992
Sulphur isotope compositions of sedimentary phosphorites from the basal Cambrian of China: implications for Neoproterozoic-Cambrian biogeochemical cycling
The Meishucun Section (Yunnan Province, South China) is considered to be an important Precambrian–Cambrian boundary section, primarily because of its rich small shelly fossil record. In this article, we report the results of a sulphur isotope study of phosphate-bound sulphate from the Meishucun Section and several correlative sections in South China. Forty clastic, granular phosphorites from Meishucun yield tightly grouped δ
34
S values averaging 33‰ (CDT), which agree well with published evaporite data for the lower Cambrian of Siberia and elsewhere. We argue that these strongly positive values reflect the sulphur isotopic composition of ambient seawater, confirming further the existence of uniquely high δ
34
S values in the earliest Cambrian oceans. This novel use of trace-sulphate in phosphate to constrain seawater δ
34
S represents the first time that sulphate δ
34
S data for this period have been given precise biostratigraphic assignments. Superimposed on the overall trend are short-term, stratigraphic variations, which might reflect local variations in the sedimentary and early diagenetic environment. Our data, together with other published data, indicate that seawater sulphate δ
34
S rose from low values (15–20‰) during the pre-750 Ma Proterozoic to possibly all-time high values (>32‰) by the earliest Cambrian. We argue that this rise may, in part, relate to increases in the amount of sulphur isotopic discrimination during microbially mediated sulphate reduction as a result of increased sulphide reoxi-dation. On the other hand, the Neoproterozoic trend to high δ
34
S values appears to mirror a trend to decreasing seawater δ
13
C towards the Proterozoic–Phanerozoic transition, implying progressive increases in the efficiency of organic carbon recycling, which would normally be coupled with real increases in sulphate reduction on the global scale. We consider that both these changes in biogeochemical cycling derive ultimately from the introduction of macrofauna around this time and, in particular, from the influence of bioturbation on early diagenesis. Precise constraints on S-isotopic evolution during the Neoproterozoic require additional trace sulphate studies.
Tập 156 Số 5 - Trang 943-955 - 1999
Geology of Nepal and its regional frame Since the opening of Nepal in 1950, a wealth of new information on the geology of the Himalaya has emanated from this country.
The sedimentary history of the Range is most reliably recorded in the richly fossiliferous ‘Tethyan’ or ‘Tibetan’ zone, which extends to the N from the summit region and has revealed an epicontinental to miogeosynclinal sequence, over 10 km thick, ranging from Cambrian to Cretaceous. Included are minor volcanic and glacial deposits and a
Glossopteris
flora of Permo-Carboniferous age, suggesting close palaeogeographic links with India and Gondwana-land. The absence of significant unconformities refutes allegations about a Hercynian or Caledonian orogenic prehistory for the Himalaya. The Mesozoic portion of the sequence passes northwards into the Indus-Tsangpo eugeosynclinal zone, where deep-sea sedimentation commenced in Triassic times and continued to the early Tertiary, with emplacement of ophiolites in the Cretaceous and thick flysch deposits in the Cretaceous-Eocene. Subduction of Tethyan oceanic crust and collision of India with Eurasia along the Indus-Tsangpo ophiolitic suture is a current hypothesis.
The Central crystalline zone, which forms the High Range, appears at first sight to be the Precambrian crystalline basement of the Tibetan sediments. But the crystalline rocks (which in addition to gneisses and granites contain high- and low-grade metasediments) show a transitional relationship with the sediments of the Tibetan zone. There is an intimate relationship of metamorphism and granitization with late Tertiary deformation and thrusting, particularly with the Main Central Thrust (MCT) which separates the High Range from the Lesser Himalaya. Thrusting along the MCT is equated by many with continental subduction along a rupture in the Indian continental plate which was primarily responsible for the deformation, metamorphism and granitization in the Himalaya, but this is hardly conceivable without assuming prior consolidation of the plate fragments involved. Radiometric dating of gneisses and granites from the Central Crystalline zone has indicated Precambrian-Cambrian in addition to the predominant late Tertiary ages; together with stratigraphic data from the Lesser Himalaya they suggest that Indian shield elements are present in the crystalline masses of the Himalaya but have been largely obliterated by the Himalayan orogeny.
Fundamental problems remain in the Lesser Himalaya. Stratigraphic work in the thick, slightly metamorphosed argillo-arenaceous and calcareous deposits is hampered by the almost total lack of palaeontological control. The sporadic and partly controversial discoveries of organic traces point to Tethyan affinities and a range from Precambrian to Tertiary, with a predominance of late Precambrian-early Palaeozoic and Permo-Carboniferous deposits, a widespread Middle Palaeozoic gap, and restricted Mesozoic-early Tertiary deposition in marginal basins in the S. The facies suggest continuity of shelf sedimentation from the Indian platform in the S across the Lesser Himalayan zone to the Tibetan zone in the N, with gradual thickening and completion of the section (closing of the Middle Palaeozoic gap) but with considerable differentiation in the Mesozoic.
The ‘Lesser Himalayan crystallines’, which overlie the low-grade metasediments as klippen-like isolated masses or, in eastern Nepal, as extensive sheets merging with the Central Crystalline zone, pose the difficult problem of ‘reverse metamorphism’. Heat metamorphism by
in situ
granite intrusions, selected metamorphism and migmatization, inversion of stratigraphy by recumbent folding, block faulting, nappe structure and other explanations have been offered. The crystalline complex of Kathmandu in central Nepal, recently mapped in detail, consists primarily of a right way up sequence of regionally metamorphosed sediments displaying a metamorphic zonation roughly concordant with stratigraphy and a regular decrease in metamorphic grade from highly garnetiferous schists at the base to barely metamorphosed, fossiliferous Palaeozoic sediments on top. Banded gneisses and augen-gneisses have a restricted, laterally and vertically irregular distribution in this sequence, reflecting a superimposed migmatization that disrupts the primary (regional) metamorphic zonation. Small granite bodies are genetically related to the migmatites. The contact of the Kathmandu Crystalline zone with the underlying metasediments is marked by intense shearing and by a stratigraphic, metamorphic and structural discontinuity indicating a thrust plane. The Kathmandu Crystalline zone is interpreted as the remnant of a nappe, rooted in the Central Crystalline zone.
The Himalayan orogeny also involved vast expanses of Trans-himalayan Tibet and Sinkiang. Studies by Chinese geologists show Tibet to be an intensely folded mountain country, forming part of a vast ‘Tethys-Himalayan Domain’ affected by Mesozoic-Tertiary folding and magmatism. It displays striking similarities with central Iran and appears linked with it through the Hindukush-Pamir-Karakorum system, a continuous orogenic belt to the N of the main Alpine-Himalayan ophiolitic suture. The Palaeozoic deposits of Central Tibet have the epicontinental facies of their Himalayan counterparts. The Sungpan-Kantze and Sankiang fold systems of northern and eastern Tibet are distinguished as a broad ‘Indosinian’ belt of intense late Triassic folding. Its axial zone, the Chinshakiang fault zone, is characterized by thick flysch deposits associated with basic and acid volcanic material of the Variscan-Indosinian cycle and accompanied by late Triassic-early Jurassic granite intrusions. This fold belt links the late Triassic (‘late Hercynian’) fold belt of northern Afghanistan and the northern Pamir with the classical Indosinian (late Triassic) fold belt of Yunnan and SE Asia.
The mountains of Sinkiang, part of the ‘Pal-Asiatic Domain’ N of the Chinshakiang fault zone, bear the stamp of the Caledonian and Hercynian orogenies; however, the late Tertiary Himalayan movements strongly remoulded them as far N as the Tienshan Range, 1500 km N of the Himalaya.
A southward migration of the centres of orogenic activity from the mountains of Sinkiang in Palaeozoic time to northern Tibet in late Triassic and to the Indus-Tsangpo line in Cretaceous-early Tertiary time, and further to the Himalayan Main Central Thrust in Middle Tertiary and to the Main Boundary Thrust and the Himalayan front in Pliocene-Pleistocene time, can be clearly recognized. It is tentatively explained in terms of continental drift by the breakaway of two large continental fragments—Tibet and India—from Gondwanaland and their successive collision with, and accretion to, Eurasia.
Tập 137 Số 1 - Trang 1-34 - 1980
Age and biostratigraphy of Early Cambrian tuffs from SE Australia and southern China
Reassessment of faunal discoveries in the Fleurieu Peninsula, South Australia, shows that the Sellick Hill tuff is late Atdabanian or early Botomian in age and is preceded by Tommotian to Atdabanian equivalents. Previous and new sensitive high-resolution ion microprobe (SHRIMP) zircon ages from the tuff have been adjusted for Pb loss in the reference zircon, and mixture-modelling has been applied to resolve inheritance and Pb loss. The revised tuff age is now 522.0 ± 2.0 Ma (
σ
), 0.7% younger than the original estimate. Undocumented SHRIMP ages for zircons from a middle to upper Botomian tuff near Cymbric Vale were cited in 1994 as a single 525 ± 4 Ma population, but reprocessing using the present approach shows instead that the correct age of volcanism is 517.8 ± 2.1 Ma (
σ
). Reprocessed and new SHRIMP ages for zircons from tuff within Bed 5 of the Lower Cambrian Meishucun section, South China, give a revised age for the earlier Tommotian of 538.2 ± 1.5 Ma (
σ
). We agree with US workers that the stratotypic lowest Cambrian sequence of Newfoundland correlates within the Nemakit–Daldynian of Siberia. These results agree broadly with the Early Cambrian time scale based on zircon U–Pb isotope dilution data, but differences remain that arise more from interpretations of biostratigraphy and from the preservation and origin of the sampled zircon grains than from instrumental accuracy. In particular, the increased duration for the Tommotian indicated by our results supports notions of a more gradualistic evolution attending the origins of animal skeletons.
Tập 159 Số 6 - Trang 645-658 - 2002
Tectonic environment of the Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of south Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. The presence of an arc to the south of Cornwall during the Devonian implies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking of flysch–ophiolite nappes.
Tập 144 Số 4 - Trang 531-542 - 1987
The role of the Indian summer monsoon and the mid-latitude westerlies in Himalayan glaciation: review and speculative discussion
New dates for late Quaternary glaciations in the Himalayas show that, during the last glacial cycle, glaciations were not synchronous throughout the region. Rather, in some areas glaciers reached their maxima at the global glacial maximum of
c.
18–20 ka
bp
, whereas in others glaciers were most extensive at
c.
60–30 ka
bp
. Comparison of these data with palaeoclimatic records from adjacent regions suggest that, on millennial timescales, Himalayan glacier fluctuations are controlled by variations in both the South Asian monsoon and the mid-latitude westerlies.
Tập 155 Số 2 - Trang 353-363 - 1998
The geometry and evolution of a transpressional strike-slip system: the Carboneras fault, SE Spain The Carboneras fault system is a 40 km long, 1 km wide Neogene NE-SW-trending left-lateral transpressional strike-slip fault system which is part of the Trans-Alboran shear zone in SE Spain. The Carboneras fault system is an anastomosing array of sub-vertical, individual fault planes or fault zones which surround pods or lenses of less intensely strained rocks. Displacements along the individual fault planes exhibit reverse components of slips and form positive flower structures. Faults have either sharp boundaries or wider bands of gouge, typically a few metres thick and are hundreds of metres in length. Second-order fault splays are well developed and usually exhibit P-shear rather than Riedel-shear orientations. These are interpreted to be related to the transpressional displacement and may also characterize other oblique convergent zones elsewhere. The second-order faults are interpreted to have formed as shear strain increased along the first-order fault and was transferred laterally to branch segments. This process produced pods, or shear lenses, bounded by the fault segments. The length to width aspect ratios of the shear lenses were found to be scale-independent across five orders of magnitude with the most common values between 3:l and 6:l. A model is proposed for the development of the fault zone by incremental finite displacements along the segmented fault surfaces. This model is based on field evidence that displacement switched with time from one fault strand to another.
Tập 152 Số 2 - Trang 339-351 - 1995
Stratigraphy, structure and volcanology of the SE Deccan continental flood basalt province: implications for eruptive extent and volumes
The Deccan Volcanic Province is one of the world's largest continental flood basalt provinces, and derives additional importance because its eruptions (64–67 Ma) straddle the Cretaceous–Tertiary boundary. To better assess the environmental impact of Deccan volcanism, and its possible effect upon Cretaceous–Tertiary boundary biota, it is necessary to document the stratigraphy, chronology and volume of the eruptions. New chemostratigraphical data permit mapping of the SE Deccan. These data strengthen the likelihood that the Rajahmundry Traps of eastern India were originally fed by long-distance flows, and are an extension of the Main Deccan Volcanic Province. An east–west cross-section reveals a depression or ‘moat’ around the SE periphery of the Deccan Volcanic Province. This provided a site in which shallow lakes initially formed, and along which later lava eruptions became channelled and confined. Published palaeomagnetic data indicate that the lavas of the SE Deccan were erupted during Chron 29R, coeval with the Cretaceous–Tertiary boundary, and the chemostratigraphic data place the associated lake sediments (i.e. Lameta Group) beneath and within lavas of the Wai Subgroup. Finally, these new map data are combined with previous work to provide a quantitative estimate for the original Deccan Volcanic Province eruptive volume of
c
. 1.3 × 10
6
km
3
.
Tập 165 Số 1 - Trang 177-188 - 2008