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1.
西藏日喀则地区德村-昂仁蛇绿岩内基性岩的元素与Sr-Nd-Pb同位素地球化学及其揭示的特提斯地幔域特征 总被引：4，自引：7，他引：4
系统研究了西藏雅鲁藏布江蛇绿岩带中部日喀则地区德村、吉丁和昂仁蛇绿岩中基性岩石的元素与 Sr-Nd-Pb 同位素地球化学特征。这些基性岩石,包括玄武岩、辉长岩和辉绿岩,属于低钾拉斑玄武岩系列,球粒陨石标准化稀土元素分配模式为轻稀土元素亏损的 N-MORB 型,(La/Yh)_N=0.31～0.65(除样品 DC993为1.17)。在原始地幔标准化微量元素图上,亏损高度不相容元素,与 N-MORB 配分模式一致。相对于 Th,无 Nb、Ta的亏损,显示样品不是产于 SSZ 环境。经构造环境图解判别,样品落入了 N-MORB 区域内;这些元素成分特征表明样品具有洋中脊环境或成熟的弧后盆地环境属性。Sr、Nd 和 Pb同位素组成特征表明特提斯地幔源区以 DM(亏损地幔)为主,同时存在少量 EMⅡ(富集地幔类型Ⅱ)、Sr,Nd 和 Pb 同位素组成特征还表明特提斯地幔域具有印度洋 MORB 型的 Sr-Nd-Pb 同位素组成特征。本文的结果进一步支持 Zhang et al.(2005)的研究结果,现今印度洋不仅在地理位置上占据了曾经是特提斯洋的大部分,而且它的地幔域还继承了曾经特提斯的地幔域的地球化学特征。 相似文献
2.
内蒙古温都尔庙和巴彦敖包-交其尔蛇绿岩的元素与同位素地球化学：对古亚洲洋东部地幔域特征的限制 总被引：2，自引：5，他引：2
内蒙古中部发育的三条蛇绿岩带是华北板块和西伯利亚板块之间的缝合带。本文系统研究了其中的温都尔庙和巴彦敖包-交其尔两个蛇绿岩带中变质玄武岩的元素和 Sr、Nd、Pb 同位素地球化学。苏右旗温都尔庙碱性玄武岩为轻稀土富集型;岩石具有板内和大陆裂谷区玄武岩的特征,可能代表了600Ma 左右,温都尔庙地区开始发育的新洋盆。采自苏左旗的巴彦敖包-交其尔玄武岩分为两类,一类呈现轻稀土富集型,呈洋岛玄武岩特征;另一类具有明显的 Nb、Ta 负异常,显示大洋岛弧玄武岩特征,洋岛玄武岩的存在表明古亚洲洋曾经发育洋盆,大洋岛弧玄武岩的存在表明古亚洲洋内部有大洋岩石圈之间的俯冲。将本文的古亚洲洋洋岛玄武岩与中国西南地区的特提斯洋岛玄武岩进行系统的元素和同位素地球化学特征对比表明,古亚洲洋的洋岛玄武岩显示高 U/Pb(HU)和北大西洋和太平洋省的特征,而特提斯洋岛玄武岩属于印度洋省。这些说明古亚洲洋地幔域与特提斯地幔域是两个独立的构造域,它们代表了长期演化的两个不同的地幔地球化学域。 相似文献
3.
S. ANGIBOUST R. LANGDON P. AGARD D. WATERS C. CHOPIN 《Journal of Metamorphic Geology》2012,30(1):37-61
To constrain deep (40–100 km) subduction dynamics, extensive P–T data are provided on the eclogitic Monviso ophiolite derived from the subducted Liguro‐Piemontese oceanic lithosphere (which was exhumed, together with associated continental units, before the Alpine collision). The Monviso ophiolite has so far been interpreted either as a fossilized subduction channel, with tectonic blocks detached from the slab at different depths and gathered in a weak serpentinized matrix, or as a more or less continuous portion of oceanic lithosphere. To evaluate potential heterogeneities within and between the various subunits, extensive sampling was undertaken on metasedimentary rocks and Fe–Ti metagabbros. The results indicate that the Monviso ophiolite comprises two main coherent tectonic subunits (the Monviso and Lago Superiore Units) detached during subduction at different depths and later juxtaposed at epidote–blueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of ～50 °C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average P–T estimates using thermocalc in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (～550 °C, 26–27 kbar). Parageneses, peak conditions and tectonic setting are very similar to those of the Zermatt‐Saas ophiolite, 200 km northwards, thus suggesting a common detachment mechanism for the whole Western Alpine belt. 相似文献
4.
M. J. JESSUP J. M. COTTLE M. P. SEARLE R. D. LAW D. L. NEWELL R. J. TRACY D. J. WATERS 《Journal of Metamorphic Geology》2008,26(7):717-739
U(–Th)–Pb geochronology, geothermobarometric estimates and macro‐ and micro‐structural analysis, quantify the pressure–temperature–time–deformation (P–T–t–D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 ° C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 ° C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ～650 ± 50 ° C. P–T estimates record a decrease in pressure from ～6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn‐ to post‐solid‐state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region. 相似文献
5.
A re-evaluation of the threat status of New Zealand's marine invertebrates was undertaken in 2009, following earlier review of New Zealand's Threat Classification System and subsequent refinement of the national criteria for classifying threat of extinction to New Zealand's flora and fauna. Sufficient information was available to enable 295 marine invertebrate taxa to be fully evaluated and assigned to a national threat category. The 10 taxa at most risk of extinction (‘nationally critical’) were the giant seep clam Calyptogena sp., the primitive acorn barnacle Chionelasmus crosnieri, O'Shea's vent barnacle Volcanolepas osheai, the stalked barnacle Ibla idiotica, the four-blotched umbrella octopus Cirroctopus hochbergi, the roughy umbrella octopus Opisthoteuthis chathamensis, the giant squid Idioteuthis cordiformis, the large-egged polychaete Boccardiella magniovata and two gravel maggots, Smeagol climoi and Smeagol manneringi. The key threatening processes identified for marine invertebrates were fishing and land-use associated impacts such as sedimentation. We identified no taxa that had improved in threat status as a result of past or ongoing conservation management action, nor any taxa that had worsened in threat status because of known changes in their distribution, abundance or rate of population decline. We evaluated a small fraction of New Zealand's marine invertebrate fauna for their threat status. Many taxa remain ‘data deficient’ or unlisted. In addition to the most threatened taxa, we recommend these taxa and their habitats as priorities for further survey and monitoring. 相似文献
6.
Metamorphic history of the South Tibetan Detachment System,Mt. Everest region,revealed by RSCM thermometry and phase equilibria modelling 总被引：2，自引：0，他引：2
J. M. COTTLE D. J. WATERS O. BEYSSAC M. J. JESSUP 《Journal of Metamorphic Geology》2011,29(5):561-582
This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history of a well‐exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ～340 °C. Temperatures increase down‐section, reaching ～650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ～310 °C km?1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1‐km‐thick zone of distributed top‐down‐to‐the‐northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo‐isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up‐section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau. 相似文献
7.
Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany 总被引：5，自引：0，他引：5
Grey tuffs of late Pleistocene age form broad fans radiating from the Laacher See basin. They were derived from phreatomagmatic outbursts, and transported in turbulent pyroclastic flows, in contrast with the underlying white pumice tuffs of air fall origin. Flow origin of the grey tuffs is inferred from the well-bedded plane parallel to cross-bedded tephra characteristic of base surge deposits, and a variety of other sedimentary structures, as well as grain size distributions. We recognize a tentative sequence of five main kinds of dune structures or cross-bedded strata. With some reservations these may be compared with the high flow-regime alluvial bedforms produced experimentally in flumes. Most of the cross-bedded structures in the Laacher See deposits resemble antidunes, with steep stoss sides and very low-dipping lee sides. Upcurrent migration of antidune crests is dominant close to the source, but changes to downcurrent migration at greater distances, presumably because of decay in flow energy. The most spectacular cross-bedding is somewhat similar to chute and pool structures formed under experimental condition in alluvial flumes, but not recognized in ancient sedimentary rocks. We suggest that these structures of the Laacher See tuffs formed during deposition from phreatic pyroclastic flows of very high flow energy and high sediment concentration. The antidunes apparently formed at lesser flow velocity than chute and pool structures, although interpretation of velocity conditions by examination of the deposits is difficult because of other factors such as the cohesiveness of wet material erupted by explosive phreatic volcanic activity. The large wave lengths of the dune-like structures, however, suggest unusually high velocities. The Laacher See magmas were of phonolitic to tephritic composition, and may have erupted with greater explosive energy and in greater volume than comparable basaltic eruptions. 相似文献
8.
DJ Karoly JA Cohen GA Meehl JFB Mitchell AH Oort RJ Stouffer RT Wetherald 《Climate Dynamics》1994,10(1-2):97-105
As an example of the technique of fingerprint detection of greenhouse climate change, a multivariate signal or fingerprint of the enhanced greenhouse effect is defined using the zonal mean atmospheric temperature change as a function of height and latitude between equilibrium climate model simulations with control and doubled CO2 concentrations. This signal is compared with observed atmospheric temperature variations over the period 1963 to 1988 from radiosonde-based global analyses. There is a significant increase of this greenhouse signal in the observational data over this period.These results must be treated with caution. Upper air data are available for a short period only, possibly too short to be able to resolve any real greenhouse climate change. The greenhouse fingerprint used in this study may not be unique to the enhanced greenhouse effect and may be due to other forcing mechanisms. However, it is shown that the patterns of atmospheric temperature change associated with uniform global increases of sea surface temperature, with El NinoSouthern Oscillation events and with decreases of stratospheric ozone concentrations individually are different from the greenhouse fingerprint used here. 相似文献
9.
R. M. PALIN M. P. SEARLE D. J. WATERS M. S. A. HORSTWOOD R.R. PARRISH 《Journal of Metamorphic Geology》2012,30(8):793-820
Petrographic analysis of peraluminous metapelites from two separate regions of the Karakoram metamorphic complex, North Pakistan, has produced new insights into the P–T–t evolution of the deep crust along the south Asian margin before and after the India‐Asia collision. Average P–T estimates and pseudosection construction in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (MnNCKFMASHTO) system using THERMOCALC have provided prograde and peak metamorphic conditions and U–Pb geochronology of metamorphic monazite has provided age constraints. Two new events in the tectonothermal evolution of the Hunza Valley have been documented; an andalusite‐grade contact metamorphic event at 105.5 ± 0.8 Ma, at unknown P–T conditions, associated with the widespread subduction‐related granite magmatism before the India‐Asia collision, and a kyanite‐grade overprint of sillimanite‐grade rocks with peak P–T conditions of ～7.8 kbar, 645 °C at 28.2 ± 0.8 Ma associated with the ongoing India‐Asia collision. A kyanite‐grade event observed in the Baltoro region with similar peak P–T conditions (～7.4–8.0 kbar, ～640–660 °C) is interpreted to have occurred sometime after 21.8 ± 0.6 Ma, however, previous studies have suggested that this event commenced in the Baltoro as early as c. 28 Ma. A calculated prograde P–T path for this kyanite‐grade event in the Baltoro indicates that garnet first nucleated on an initially high geothermal gradient (～30 °C km?1) and grew during a significant increase in pressure of ～2.6 kbar over a temperature increase of ～100 °C. This event is thought to represent evidence for conductive heating of the middle crust during early stages of intrusion and lateral migration of the Baltoro batholith, with thermal conditions comparable with tectonic models of magmatic over‐accretion. 相似文献
10.
NEIL F. GLASSER JAMES L. ETIENNE MICHAEL J. HAMBREY JEREMY R. DAVIES RICHARD A. WATERS PHILIP R. WILBY 《Boreas: An International Journal of Quaternary Research》2004,33(3):224-237
This article presents the results of a geomorphological and sedimentological investigation of former glacial meltwater drainage in the region of the lower Afon Teifi, one of the major rivers of southwest Wales. Former drainage characteristics in the region are reconstructed concentrating on palaeo-drainage routes associated with successive Pleistocene glaciations and their role in the Quaternary evolution of the lower Teifi. Mapping of these features throughout a c. 100 km 2 area reveals a complex evolution in the establishment of the present-day drainage system, with evidence for the following surface channel types: (i) type 1 channels of primary subglacial origin cut during the late Devensian (late Wisconsinan/late Weichselian) glaciation; (ii) type 2 channels representing either pre-late Devensian subaerial fluvial run-off, unconnected to the course of the preglacial Afon Teifi, or originating as subglacial chute channels; (iii) type 3 channels developed as subglacially modified pre-late Devensian tributaries of the Afon Teifi. Two further features are also described: (iv) type 4 channels are drift-plugged abandoned preglacial courses of the Afon Teifi, and (v) type 5 channels formed as lateglacial and post-late Devensian gorges which bypass type 4 channels. A relative chronostratigraphy based on channel geomorphology and sedimentology reveals an evolutionary sequence considerably more complicated than identified in previous studies, with extensive modification of the lower Afon Teifi region by glacial meltwater during at least two periods of Pleistocene glaciation. 相似文献