Age and Geochemical Characteristics of a Mafic Dyke Swarm in the Archaean Vestfold Block, Antarctica: Inferences about Proterozoic Dyke Emplacement in Gondwana   总被引：1，自引：0，他引：1  

COLLERSON  KENNETH D.; SHERATON  JOHN W. 《Journal of Petrology》1986,27(4):853-886

Archaean gneisses in the Vestfold Block, Antarctica are cutby abundant tholeiite and rare alkaline dykes. At least fivegenerations of dykes have been recognized on the basis of intrusiverelationships, petrography and geochemistry. Rb-Sr isotopicdata indicate that intrusion of the tholeiites occurred overa period of c. 1000 Ma, during three clearly defined events(viz. c. 2400, 1800, and 1400 Ma). Dykes in the southwesternpart of the Vestfold Block were recrystallized during a lateProterozoic (c. 1000-1100 Ma) high-grade event. Mineral-wholerock Rb-Sr isotopic data show that the terrain was also variablyaffected by an even younger thermal event c. 500 Ma ago whichis correlated with the Pan-African Orogeny. Isotopic, major element and trace element data suggest thatthe tholeiite dyke suites were derived by varying degrees ofpartial melting of compositionally distinct, in some cases ratherheterogeneous subcontinental mantle source regions, combinedwith variations in the extent and nature of crystal fractionation.Extensive crustal contamination does not appear to have playeda significant role in determining compositional trends withinthe different suites, although minor contamination cannot bediscounted. The oldest dykes, a diverse group of high-Mg tholeiites,and the youngest, c. 1400 Ma tholeiites, appear to have beenderived from isotopically similar source regions, which werenevertheless characterized by quite different abundances ofhighly incompatible elements (Pb, Rb, Ba, Th, K, La, and Ce).1800 Ma tholeiites were derived from a more radiogenic mantlesource region, characterized by long-term enrichment in incompatibleelements. Such enrichment is interpreted to be a consequenceof metasomatism of their mantle source region. 2400 Ma high-Mg dykes in Enderby Land have virtually identicalchemical and isotopic compositions to those in the VestfoldBlock, indicating generation from a similar source region; however,1200 Ma tholeiites were apparently derived from a more radiogenicsource region like that of the c. 1800 Ma Vestfold Block dykes.Hence, both Sr isotopic and geochemical data indicate that theancient subcontinental lithospheric mantle beneath Gondwanawas extremely heterogeneous.  相似文献   

Petrogenesis of the Kinsman Intrusive Suite: Peraluminous Granitoids of Western New Hampshire   总被引：1，自引：0，他引：1  

CLARK  RUSSEL G.  JR.; LYONS  JOHN B. 《Journal of Petrology》1986,27(6):1365-1393

The Kinsman Intrusive Suite occurs in six major plutons of westernNew Hampshire, covering a total area of 2240 km². It is an Acadian-agesyntectonic gneissic S-type peraluminous granitoid, rangingin composition from quartz diorite to granite. Much of the Kinsmanis characterized by very large (up to 120 mm in maximum dimension)megacrysts of alkali feldspar, but the bulk chemistry of therocks indicates that these cannot be phenocrysts crystallizedfrom initially homogeneous melts. Locally, there is abundant(20 per cent) almandine-rich garnet, and graphite is a commonaccessory.In contrast to the unannealed orthoclase in surroundingmetapelites, the alkali feldspar of the Kinsman has, for themost part, inverted to maximum microcline. The garnets havecore temperatures in the range of 800 to 900 ?C, and are pseudomorphedby, or show reaction rims to, biotite. Plagioclase commonlyshows zoning, some of it oscillatory. These features are magmaticin nature, and argue against the conclusions of previous investigatorsthat the mineralogy and textures of the rock are due to regionalmetamorphism of a previously-crystallized two-mica granitoidwhich has undergone prograde reactions such as:muse + bio +3 qtz 2 Kfs + gar + 2H₂O.The intrusives have also producedrecognizable contact-metamorphic features in the wallrocks andare probably coeval with the dominant M2 Acadian metamorphism.Majorelement analytical data for the Kinsman suite has been examinedby least squares mixing-model and extended Q-mode factor analysis.These calculations, supported by consideration of REE data,suggest that the most likely origin for the Kinsman magmas isby deep-crustal anatexis of slightly calcareous metapelites,and involves a reaction such as:bio + Al₂SiO₅ + qtz + feldspars gar + cord + Kfs + plag + melt.In non-calcareous pelites thisreaction produces a water-undersaturated peraluminous melt attemperatures above 700 ?C, and allows for the early crystallizationor recrystallization of K-feldspar, plagioclase, and garnetin a crystal-liquid mush or migma. Geochemically, garnet + plagioclaseare treated as restite, and a minimum-melt granite as the magmain the Q-mode and mixing-model calculations. The variabilityin chemistry of the Kinsman Intrusive Suite is best explainedon the basis of mixing of leucogranitic anatectic melts withgarnet-plagioclase restitic material and a quartz-feldspar-sillimanite-biotiterock, but only very slightly affected by crystal settling.  相似文献   

An early Holocene caribou antler from northern Ellesmere Island, Canadian Arctic Archipelago   总被引：1，自引：0，他引：1  

THOMAS G. STEWART  JOHN ENGLAND 《Boreas: An International Journal of Quaternary Research》1986,15(1):25-31

During the last glaciation of northern Ellesmere Island many areas remained ice-free. A caribou antler from deglacial-marine sediments in Clements Markham Inlet dates 8,415 ± 135 B.P. (S-2501). If locally derived it places caribou at the northern limit of their contemporary range at the onset of deglaciation in this area. Immediately to the south, on the Hazen Plateau, ice remained at its limit until c . 8,000 B.P. Therefore, this antler may indicate the presence of caribou during full glacial time.  相似文献   

Lake shoreline development, frost weathering and rock platform erosion in an alpine periglacial environment, Jotunheimen, southern Norway   总被引：2，自引：0，他引：2  

JOHN A. MATTHEWS  ALASTAIR G. DAWSON  RICHARD A. SHAKESBY 《Boreas: An International Journal of Quaternary Research》1986,15(1):33-50

Ice-dammed lake Boverbrevatnet existed for 75–125 years in the 'Little Ice Age'. After about A.D. 1826, glacier retreat led to a fall in lake level and to exposure of the former shoreline, which includes well-developed platforms cut in metamorphic bedrock. The rock platforms, up to 5.3 m wide and backed by cliffs up to 1.55 m high, are partially covered by large angular boulders which form pavements. Accurate levelling has permitted correlation of platform fragments, overflow cols and related features of the shoreline, such as benches eroded in moraines, ice-push ridges, a perched delta, vegetation trim-lines, lichen limits and a 'lichen-kill' zone. The evolution of the lake, the chronology of deglaciation and the period of formation of the rock platforms have been dated by lichenometry, supported by ¹⁴C dating, Schmidt hammer 'R'-values and historical data. The morphology of the rock platforms, together with estimates of their rate of erosion ranging from 1.4 to 7.1 cm/year, indicate the importance of frost shattering (frost riving, frost wedging or macrogelivation) at the lake margin under a periglacial climate, while the permanence of such platforms as landscape features suggests their use in the reconstruction of former periglacial environments. A semi-quantitative model is outlined for the development of rock platforms which emphasises deep penetration of the annual freeze-thaw cycle, the movement of unfrozen lake water towards the freezing plane, and the growth of segregation ice in fissures and cracks at the interface between lake ice and bedrock. Ice-push and ice-pull processes are involved primarily as transporting agents in the formation of boulder pavements and in the removal of debris from the platforms. Analogous processes may occur on polar coasts producing coastal rock platforms.  相似文献   

Monitoring of Saharan dust fallout on Crete and its contribution to soil formation   总被引：3，自引：0，他引：3  

TOMAS NIHLÉN  JAN O. MATTSSON  ERS RAPP  CHRISOULA GAGAOUDAKI  GEORGES KORNAROS  JOHN PAPAGEORGIOU 《Tellus. Series B, Chemical and physical meteorology》1995,47(3):365-374

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Glacial sedimentary processes and environmental reconstruction based on lithofacies     

JOHN SHAW 《Sedimentology》1987,34(1):103-116

Glacigenic sediments exposed in pits around Villeneuve, near Edmonton, Alberta, are subdivided into facies based on grain size, sedimentary structure, glacially-induced deformation and faulting, and groove marks. Two diamicton facies are recognised, one of which is interpreted as a primary till, deposited directly from glacier ice, and the other as a product of mass-movement. The diamicton facies are closely associated with current bedded facies interpreted as fluvioglacial deposits. The stratigraphic sedimentological and tectonic aspects of these fluvial deposits suggest subglacial deposition in channels and cavities. At any one place the glacier appears to have alternated between being attached to the bed, causing thrusting and sole marking, and being separated from the bed by a cavity in which fluvial and mass-movement sediments accumulated. The net result is a highly complex and laterally variable stratigraphy produced by a single glacial advance. The correct interpretation of such sequences is essential if lithostratigraphy is to be used to establish glacial history. In addition, the interpretations presented here have implications regarding the formation of soft zones in ‘till’. They indicate that the soft zones are beds of sorted sediment redeposited by mass-movement.  相似文献   

Report of the Editors     

JOHN BRIDGE  PAUL BRIDGES  MAURICE TUCKER 《Sedimentology》1987,34(1):190-191

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Coarse-grained sediment gravity flow facies in a large supraglacial lake     

NICHOLAS EYLES  BRYAN M. CLARK  JOHN J. CLAGUE 《Sedimentology》1987,34(2):193-216

Near Williams Lake, in the central interior of British Columbia, the Fraser River exposes long sections of late Pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough. The dominant facies types are thick, normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep. Channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice. Fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi-continuous ‘surging’ turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice-cored sediments. Large-scale post-depositional deformation structures, such as synclinal folds, normal faults, sedimentary dyke swarms and dewatering structures, record gravitational foundering of sediment and pore-water expulsion caused by the melt of underlying glacier ice. Melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub-basins within the overlying sediment pile. An idealized model of ‘supraglacial’ lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography.  相似文献   

A numerical technique for analysing pebble-form variations, applied in Loch Earn, Scotland     

JOHN MCMANUS  ALI H. A. ALRASOUL 《Sedimentology》1987,34(4):707-713

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Burial diagenesis and crystal diminution. The origin of crystal diminution in some limestones from South Wales     

JOHN DIXON  V. PAUL WRIGHT 《Sedimentology》1983,30(4):537-546

ABSTRACT Carboniferous speleothem calcites in un-metamorphosed limestones from South Wales exhibit crystal diminution (degrading neomorphism) textures. These textures result from processes which have been considered only important under metamorphic conditions. The textures are interpreted as the result of recrystallization associated with dislocation processes. These processes relate to low temperature creep associated with low strain rates extended over long periods of time. The textures described here occur in unusually large crystals but similar textures have been described in limestones from deep boreholes. Dislocation processes, recovery and recrystallization are likely to be important processes operating during deep burial diagenesis.  相似文献