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21.
Y. Najman R. Allen E. A. F. Willett A. Carter D. Barfod E. Garzanti J. Wijbrans M. J. Bickle G. Vezzoli S. Ando G. Oliver M. J. Uddin 《Basin Research》2012,24(5):499-519
The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west‐propagating Indo‐Burmese wedge. We present a basin‐wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15‐NN16 (ca. 2.5–3.9 Ma), MS2 to NN19‐NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan‐derived, with a subordinate arc‐derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans‐Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west‐propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already‐uplifted Shillong Plateau that caused diversion of the palaeo‐Brahmaputra to the west of the plateau as the north‐east drainage route closed. 相似文献
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The Eastern Alps in Austria have been interpreted as a pile of thrust sheets resulting from the collision of two continental masses. The only remains of the ocean-floor which may once have separated these continents could be the highly deformed greenschists, metasediments and serpentinites found in the lower thrust sheets. To test this hypothesis, a total of sixty mafic rocks from the Großglockner, Mooserboden, Fusch, Hochtor, Matrei Zone and Strobl localities have been analysed for the stable trace elements, Ti, Zr, Y, Nb and Cr, and the less stable elements K, Rb, and Sr. Visual and statistical comparison of the stable elements with known magma types reveals that five of the sample groups classify clearly as tholeiitic ocean-floor basalts, while one group, the Fusch locality, classifies as within-plate (probably ocean island) basalts. It is suggested that the tectonic units containing such rocks comprise a mélange of disrupted oceanic crust, upper mantle and seamounts, pelagic sediments and continental margin sediments. The rocks may have formed in a large ocean basin, rather than a marginal basin behind an island arc. 相似文献
24.
The age and origin of younger granitic plutons of the Shaw Batholith in the Archaean Pilbara Block,Western Australia 总被引：1，自引：0，他引：1
M. J. Bickle L. F. Bettenay H. J. Chapman D. I. Groves N. J. McNaughton I. H. Campbell J. R. de Laeter 《Contributions to Mineralogy and Petrology》1989,101(3):361-376
The whole-rock Pb-Pb method has been used to date four of the younger, mainly adamellite, late-tectonic plutonic phases within the ca. 3.5 Ga Shaw Batholith of the Archaean east Pilbara Block. Three suites give ages within error of 2966 Ma (Porphyritic Granites at 2948±50 Ma, Leuco-adamellites at 2943±46 Ma and Garden Creek Adamellite at 3007±48 Ma). The post-tectonic Cooglegong Adamellite gives an age of 2847±34 Ma. The Sm-Nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca. 3200 and 3600 Ma in age. The sources for these four younger plutonic phases were heterogeneous and, although exhibiting some isotopic characteristics of the older (3.5–3.3 Ga) calc-alkali plutonic suites, were more depleted in the LIL elements Rb, U and Th. In addition, the Garden Creek Adamellite and the Cooglegong Adamellite lack the very fractionated and HREE-depleted REE patterns characteristic of both the older calc-alkali plutonic rocks and the Porphyritic Granites and Leuco-adamellites. The crust underlying the Shaw Batholith at ca. 2950 Ma must have been both markedly heterogeneous and variably depleted, a conclusion consistent with the complex tectonic and plutonic evolution of this region. 相似文献
25.
Systematic shifts of oxygen isotopic compositions in the higher grade parts of the high temperature-low pressure Hercynian metamorphic sequence, exposed in the Trois Seigneurs Massif, have previously been explained as a result of an influx of surface-derived water during the prograde part of the metamorphic cycle. It has been suggested that this caused a regional lowering of 87Sr/86Sr in the metamorphic sequence. Mapping of strontium isotopic compositions across a 15 m meta-carbonate horizon in the higher grade pelite-psammite sequence shows that strontium isotopic compositions were homogenised over length scales of metres or less during the Hercynian metamorphism, which brought the carbonate and pelite-psammite to oxygen isotopic equilibrium with a common fluid. Comparison of model pre-Hercynian 87Sr/86Sr profiles across the carbonate (based on a depositional/diagenetic age of 450 Ma and initial 87Sr/86Sr ratio of 0.7086 given by 10 m length scale averaging) with the post-Hercynian 87Sr/86Sr profile (calculated from analysed 87Sr/86Sr and Rb/Sr compositions) implies strontium isotopic diffusion distances of ca. 0.4 m in the carbonate and ca. 7 m in the pelite-psammite. The limited Sr-isotopic diffusion distance of 0.4–0.7 m within the carbonate is compatible with pervasive oxygen-isotopic exchange over distances restricted to 4–15 m if fluid strontium concentrations were between 4 and 50 ppm. The strontium isotopic transport distances are not compatible with pervasive oxygen isotopic alteration over the observed 5 km regional scale. Either the flow was perfectly layer-parallel or, more probably, the regional-scale alteration of oxygen took place by fluid circulation in the brittle regime early in, or prior to, the Hercynian metamorphic event. Flow along cracks with incomplete diffusive exchange between fluid and wall rock would allow greater decoupling of oxygen and strontium isotopic transport than pervasive advective transport with local fluid-solid equilibrium. 相似文献
26.
M.J. Bickle 《Earth and Planetary Science Letters》1986,80(3-4):314-324
The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures. 相似文献
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M.J Bickle H.J Chapman L.F Bettenay D.I Groves J.R de Laeter 《Geochimica et cosmochimica acta》1983,47(5):907-914
The Pb-Pb whole-rock geochronology of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgarn granite-greenstone terrain provides important constraints on crustal evolution. The regionally extensive banded gneisses, previously considered as candidates for basement to the greenstones give a Pb-Pb whole-rock age of 2700 ± 97 Ma (2σ errors). This is within error of previously published Rb-Sr and Sm-Nd gneiss ages and also within error of the Sm-Nd ages on the greenstones in the Eastern Goldfields Province. Two synkinematic plutons give Pb-Pb whole-rock ages (2737 ± 62 Ma and 2700 ± 100 Ma) and Pb isotopic compositions consistent with the hypothesis, based on field and geochemical relations, that these plutons were derived by partial melting of the precursors to the banded gneisses. Assuming this, the combined data date the melting event at 2723 ± 25 Ma with a model source μ value of 8.18 ± 0.02. This source μ value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than ~200 Ma. A post-kinematic pluton with a whole-rock Pb-Pb age of 2685 ± 26 Ma and μ value of 8.26 ± 0.02 puts a younger limit on this relatively short lived crustal accretion-differentiation event.Comparison of Pb-Pb and Rb-Sr whole-rock dates for the plutons suggests that the latter became closed systems up to 200 Ma after the Pb-Pb ages, and that the plutons gained or lost Rb or Sr at this time. 相似文献
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Reductive deposition of graphite at lithological margins in East Central Vermont: a Sr, C and O isotope study 总被引：3，自引：0，他引：3
K. A. Evans M. J. Bickle A. D. L. Skelton M. Hall H. Chapman 《Journal of Metamorphic Geology》2002,20(8):781-798
O, Sr and C isotopes from east‐central Vermont are used to provide information on the timing and volume of metamorphic fluid flow. The results are then used to assess the evidence for redox transformations between C species. Oxygen profiles are homogenised on a metre scale; comparison with Sr isotopes suggest that O alteration may have occurred over a significantly larger timescale than that of Sr, possibly because O was modified during dewatering and diagenesis in addition to the high temperature alteration recorded by strontium. Sr isotope distributions are consistent with cross‐layer fluid fluxes of 104?106 moles m?2; absolute values depend on the Sr fluid‐rock distribution coefficient which is poorly known; however, reaction progress constraints suggest that fluxes were towards the lower end of this range. High δ13C values observed at lithological boundaries cannot be explained by volume loss or closed system processes and are taken to indicate reductive precipitation of graphite as a result of mixing between CO2 and CH4‐bearing fluids. Mass balance calculations indicate that redox reactions occurring under metamorphic conditions convert a minimum of 10% of the CO2 released from limestones into graphite, thus providing a potentially important control on the average residence time of C within the crust with implications for C cycling models. 相似文献
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John A. Becker Mike J. Bickle Albert Galy Tim J.B. Holland 《Earth and Planetary Science Letters》2008,265(3-4):616-629
Hot springs in the Marsyandi Valley, Nepal, vent CO2 sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earth's surface. The δ13C of spring waters ranges up to + 13‰, while that of the coexisting free gas phase is close to ? 4‰. Empirical and thermodynamic modelling of this isotopic fractionation suggests > 97 ± 1% CO2 degassing. The calculated minimum total CO2 degassing in the Marsyandi catchment is 5.4 × 109 mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO2 degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO2 flux of 0.9 × 1012 mol/yr, or ～ 13% of solid Earth CO2 degassing. The calculated flux is a factor of three greater than the estimated CO2 drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO2 flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability. 相似文献
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The stromatolites of the Belingwe Greenstone Belt (approximately 2700 Ma old) are perhaps the best-developed Archaean stromatolites yet found. Exposures occur on two stratigraphic levels, both part of the “Bulawayan” in Rhodesian stratigraphic terminology (Wilson et al., 1978). The extensive outcrops show a wide variety of stromatolites, including forms similar to Baicalia, Conophyton, Irregularia and Stratifera. Many stromatolites occur in cyclic units, possibly reflecting periodic changes in lagoonal conditions. Associated sedimentary rocks were deposited in a very shallow-water environment and some display well-developed desiccation features. Currently held concepts concerning the evolution of stromatolites and their usefulness in biostratigraphy do not appear to be supported by the evidence from Belingwe. 相似文献