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51.
SIWAN M. DAVIES WIM Z. HOEK SJOERD J. P. BOHNCKE J. JOHN LOWE SEAN PYNE O''DONNELL CHRIS S. M. TURNEY 《Boreas: An International Journal of Quaternary Research》2005,34(2):123-135
Three distal tephra layers or cryptotephras have been detected within a sedimentary sequence from the Netherlands that spans the last glacial-interglacial transition. Geochemical analyses identify one as the Vedde Ash, which represents the southernmost discovery of this mid-Younger Dryas tephra so far. This tephra was found as a distinct horizon in three different cores sampled within the basin. The remaining two tephras have not been geochemically 'fingerprinted', partly due to low concentrations and uneven distributions of shards within the sequences sampled. Nevertheless, there is the potential for tracing these tephra layers throughout the Netherlands and into other parts of continental Europe. Accordingly, the possibilities for precise correlation of Dutch palaeoenvironmental records with other continental, marine and ice-core records from the North Atlantic region are highlighted. 相似文献
52.
CHRIS R. STOKES CHRIS D. CLARK 《Boreas: An International Journal of Quaternary Research》2002,31(3):239-249
It has been suggested that extremely long subglacial bedforms (e.g. attenuated drumlins and mega-scale glacial lineations) record former areas of fast-flowing ice and that bedform elongation ratio is a useful proxy for ice velocity. Despite the availability of much data pertaining to the measurement and analysis of subglacial bedforms, these assumptions have rarely been explicitly addressed in detail. In this paper, we demonstrate that long subglacial bedforms (length:width ratios ≥10:1) are indicative of fast ice flow. Using satellite imagery, we mapped over 8000 lineaments associated with a highly convergent flow pattern near Dubawnt Lake, District of Keewatin, Canada. This flow pattern is unusual in that it displays a large zone of convergence feeding into a main 'trunk' and then diverging towards the inferred ice margin. The 'bottleneck' pattern is taken to record an increase and subsequent decrease in ice velocity and we analysed transverse and longitudinal variations in bedform morphometry. The main trunk of the flow pattern (down-ice of the convergent zone) is characterized by mega-scale glacial lineations of great length (up to 13 km) and high elongation ratios (up to 43:1). The down-ice variations in elongation ratio reflect exactly what we would expect from a terrestrial ice stream whose velocity increases in the onset zone passes through a maximum in the main trunk and slows down as the ice diverges at the terminus. It is suggested that any unifying theory of drumlin formation must be able to account for the association between long subglacial bedforms and fast ice flow, although it is not assumed that fast ice flow always produces attenuated bedforms. A further implication of this work is that many more ice streams may be identified on the basis of attenuated subglacial bedforms, radically altering our views on the flow dynamics of former ice sheets. 相似文献
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54.
JAN ALEXANDER STUART J. MCLELLAND† THOMAS E. GRAY CHRIS E. VINCENT MIKE R. LEEDER SUSANNA ELLETT 《Sedimentology》2008,55(4):845-868
The turbulent flow structure, suspended sediment dynamics and deposits of experimental sustained turbidity currents exiting a channel across a break in slope into a wide tank are documented. The data shed light on the flow evolution and deposit geometry of analogous natural channel‐fed submarine fans. Flows generated in a 0·3 m wide, sloping (0°, 6°, 9° or 20°) channel crossed an angular slope break and spread onto a horizontal tank floor. Flow development comprised: (i) channelized phase (unsteady channel flow developing into steady channel flow); (ii) initial lateral expansion phase (unsteady‐spreading wall jet phase); (iii) constant lateral expansion phase (steady‐spreading wall jet phase); and (iv) rapid waning phase. Phases (i) and (iv) are similar to laterally constrained turbidity currents, but phases (ii) and (iii) are considerably different from such two‐dimensional currents. Steeper channel slopes produced greater flow velocities and turbulence intensities, but these effects diminished markedly with distance from the channel mouth. Flow velocity vectors in the tank had similar patterns for all channel slopes, with a central core of faster velocity and narrow vector dispersion and slower flow with larger dispersion at the jet margins. Suspended sediment concentrations were higher within flow heads and dense basal layers in flow bodies. Time‐averaged acoustic backscatter data showed vertical concentration gradients, confirmed by siphon samples. The deposits comprised a thick central ridge, of similar order width to the channel mouth, with abrupt margins and a surrounding, very thin, fan‐like sheet. The ridge was coarser grained and better sorted than the original sediment, with grain‐size fining downstream, particularly over the fan‐like sheet. The formation of a central ridge suggests that, in the tank, vertical turbulent momentum exchange is more significant for sediment dynamics than spanwise momentum exchange due to lateral expansion. The streamwise elongate geometry of the ridge contrasts with conventional fan‐like geometry found with surge‐type turbidity flows, a result that has widespread stratigraphic and economic implications. 相似文献
55.
The Generation of Continental Crust: An Integrated Study of Crust-Forming Processes in the Archaean of Zimbabwe 总被引：3，自引：1，他引：2
The Archaean craton of Zimbabwe includes two major episodesof crust generation at 3.5 and 2.9 Ga recorded in the emplacementof tonalite-gneiss granitoids. A total of 180 samples of representativegneisses and massive tonalites and sills has been collectedfrom three areas in the southern part of the craton, at Mashaba,Chingezi, and Shabani. These rocks have been analysed for major,trace, and rare earth elements to evaluate the effects of thefractional crystallization and partial melting processes inthe generation of this segment of Archaean crust. Three groups are distinguished on the basis of their major andtrace element contents, and they follow two main trends of differentiation:the sodic and the calc-alkaline (sensu stricto) trends. GroupI samples are tonalitic in composition and follow a sodic trendcharacterized by decreasing CaO/Na2O ratios. Y and Sr behaveas compatible elements and are negatively correlated with Rb.REE patterns are moderately fractionated with La/Ybn=4–23.5.The characteristics of this group have been described only inthe Archaean craton from Swaziland. Group II is an intermediateGroup with a marked decrease in Na2O/K2O with increasing differentiation,similar to the Archaean tonalite-trondhjemite-granodiorite suitesfrom Finland or the Pilbara Block, Australia. Samples displaybiotite tonalite and trondhjemite compositions, and Y, Sr, andRb are all incompatible. The REE patterns are strongly fractionated,with La/Ybn=23–44, and with small positive or negativeEu anomalies, as observed in other Archaean tonalite-trondhjemites.Group III is composed mainly of trondhjemites and granites similarto many post-Archaean granitoids: they follow a calc-alkalinetrend (sensu stricto) with decreasing CaO/Na2O and Na2O/K2O.Sr and Y are incompatible, whereas Rb increases with differentiation.REE patterns are variably fractionated, with La/Ybn=6–36,with high REE contents, and marked negative Eu anomalies. The above geochemical features are explained in a three-stagepetrogenetic model. The first stage consists of 6–20%melting of upper-mantle peridotite and the generation of tholeiiticbasalts, as observed in the associated greenstone belts. Thesecond stage involves 4–25% partial melting of metamorphosedbasalts with a Gt amphibolite (15–45% Pl + 30–50%Hb+2–35% Cpx+3– 15% Gt) residue resulting in theGroup I samples, under water-unsaturated conditions at intermediatepressure (16 kbar), or with an eclogite residue to generatethe parental magmas for the Group II rocks. The third stageis lowpressure fractional crystallization (<8 kbar) of liquidsgenerated during this second stage, leaving a 19–20% Qtz+36–42%Pl0–2% HbMt cumulate for the more evolved Group II samples,and 55% fractional crystallization of a 14% Qtz+37.6% Pl (An26)3.3%Bt+0.1% Ilm0.8% Mt cumulate for Group III samples. The highlyfractionated REE patterns of the Group II rocks are inheritedfrom the second stage of partial melting of the metamorphosedbasalt source rocks with an eclogite residue. Thus Group IIand III initial liquids were generated through partial meltingof eclogite and Gt amphibolite, respectively. The genetic relationshipsbetween Group I sodic and Group III calc-alkaline suites areevaluated, with the latter resulting from various stages offractional crystallization processes of parental magmas withinthe sodic suite. 相似文献
56.
HARRIS CHRIS; MARSH JULIAN S.; DUNCAN ANDREW R.; ERLANK ANTHONY J. 《Journal of Petrology》1990,31(2):341-369
The 420 m thick sequence of Kirwan basalt crops out along thesouthernmost 50 km of the Kirwanveggen Escarpment (74?S, 6?W).There is little variation in major element chemistry of thesebasalts (SiO2 49?3–51?6 wt.%; MgO 5?1–6?6 wt.%),but the concentrations of certain incompatible elements (e.g.,Zr) vary by factors of approximately two or more. Most interelementplots show rather poor correlation (r<0?78), but rocks fromopposite ends of the data array can be related by 30% fractionationof plagioclase, clinopyroxene, olivine, and magnetite in theproportions 51:35:11:3. Plagioclase is much more abundant inphenocryst assemblages (85%) and it appears that selective transportof plagioclase to the surface occurred. The range in incompatible element concentrations cannot be explainedby crystal fractionation and is most probably a result of theparent liquids of these basalts being derived by slightly differentdegrees of partial melting of a common source, or alternativelyof open-system (RTF) magma processes. The strontium isotopedata for the freshest rocks (R0=0?7049–0?7065) may beexplained by 7% contamination by crustal material with an R0of 0?709 and bulk Sr of800 ppm, but there is little supportingevidence from other trace element variations for this hypothesis.Oxygen isotope determinations on whole-rock-plagioclase pairsshow that alteration has resulted in a 0?5%o shift in (18O.Alteration also appears to have resulted in a greater spreadof data, particularly for the LIL elements and Sr isotopes.The Sr and Nd isotopic composition of the suite is close tobulk Earth at 172 Ma and this, together with REE and other traceelement data, shows these basalts to be similar in compositionto the more primitive basalts among the Karoo basalt lavas.It is suggested that the Kirwan basalts were derived from asource which was similar to that of the southern Lebombo variantof the Sabie River Basalt Formation of the Karoo Volcanic Province.This part of the Karoo was closest to the Kirwanveggen beforethe break-up of Gondwanaland. 相似文献
57.
Chemical versus Temporal Controls on the Evolution of Tholeiitic and Calc-alkaline Magmas at Two Volcanoes in the Alaska-Aleutian Arc 总被引：1，自引：0，他引：1
GEORGE RHIANNON; TURNER SIMON; HAWKESWORTH CHRIS; BACON CHARLES R.; NYE CHRIS; STELLING PETE; DREHER SCOTT 《Journal of Petrology》2004,45(1):203-219
The Alaska–Aleutian island arc is well known for eruptingboth tholeiitic and calc-alkaline magmas. To investigate therelative roles of chemical and temporal controls in generatingthese contrasting liquid lines of descent we have undertakena detailed study of tholeiitic lavas from Akutan volcano inthe oceanic Aleutian arc and calc-alkaline products from Aniakchakvolcano on the continental Alaskan Peninsula. The differencesdo not appear to be linked to parental magma composition. TheAkutan lavas can be explained by closed-system magmatic evolution,whereas curvilinear trace element trends and a large range in87Sr/86Sr isotope ratios in the Aniakchak data appear to requirethe combined effects of fractional crystallization, assimilationand magma mixing. Both magmatic suites preserve a similar rangein 226Ra–230Th disequilibria, which suggests that thetime scale of crustal residence of magmas beneath both thesevolcanoes was similar, and of the order of several thousandyears. This is consistent with numerical estimates of the timescales for crystallization caused by cooling in convecting crustalmagma chambers. During that time interval the tholeiitic Akutanmagmas underwent restricted, closed-system, compositional evolution.In contrast, the calc-alkaline magmas beneath Aniakchak volcanounderwent significant open-system compositional evolution. Combiningthese results with data from other studies we suggest that differentiationis faster in calc-alkaline and potassic magma series than intholeiitic series, owing to a combination of greater extentsof assimilation, magma mixing and cooling. KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales 相似文献
58.
CHRIS R. STOKES CHRIS D. CLARK 《Boreas: An International Journal of Quaternary Research》2003,32(1):263-279
We report evidence for a major ice stream that operated over the northwestern Canadian Shield in the Keewatin Sector of the Laurentide Ice Sheet during the last deglaciation 9000–8200 (uncalibrated) yr BP. It is reconstructed at 450 km in length, 140 km in width, and had an estimated catchment area of 190000 km2 . Mapping from satellite imagery reveals a suite of bedforms ('flow-set') characterized by a highly convergent onset zone, abrupt lateral margins, and where flow was presumed to have been fastest, a remarkably coherent pattern of mega-scale glacial lineations with lengths approaching 13 km and elongation ratios in excess of 40:1. Spatial variations in bedform elongation within the flow-set match the expected velocity field of a terrestrial ice stream. The flow pattern does not appear to be steered by topography and its location on the hard bedrock of the Canadian Shield is surprising. A soft sedimentary basin may have influenced ice-stream activity by lubricating the bed over the downstream crystalline bedrock, but it is unlikely that it operated over a pervasively deforming till layer. The location of the ice stream challenges the view that they only arise in deep bedrock troughs or over thick deposits of 'soft' fine-grained sediments. We speculate that fast ice flow may have been triggered when a steep ice sheet surface gradient with high driving stresses contacted a proglacial lake. An increase in velocity through calving could have propagated fast ice flow upstream (in the vicinity of the Keewatin Ice Divide) through a series of thermomechanical feedback mechanisms. It exerted a considerable impact on the Laurentide Ice Sheet, forcing the demise of one of the last major ice centres. 相似文献
59.
Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex 总被引：21，自引：7，他引：14
We have analysed sulphides, silicates, and chromites of theMerensky Reef for platinum-group elements (PGEs), Re and Auusing laser ablation-inductively coupled plasma mass spectrometryand synthetic pyrrhotite standards annealed with known quantitiesof noble metals. Os, Ir and Ru reside in solid solution in pyrrhotiteand pentlandite, Rh and part of the Reef’s Pd in pentlandite,whereas Pt, Au, Re and some Pd form discrete phases. Olivineand chromite, often suspected to carry Os, Ir and Ru, are PGEfree. All phases analysed contain noble metals as discrete micro-inclusionswith diameters typically <100 nm. Inclusions in sulphidescommonly have the element combinations Os–Ir–Ptand Pt–Pd–Au. Inclusions in olivine and chromiteare dominated by Pt ± Au–Pd. Few inclusion spectracan be related to discrete noble metal phases, and few inclusionshave formed by sub-solidus exsolution. Rather, some PGE inclusions,notably those in olivine and chromite, are early-magmatic nuggetstrapped when their host phases crystallized. We suggest thatthe silicate melt layer that preceded the Merensky Reef wasPGE oversaturated at early cumulus times. Experiments combinedwith available sulphide–silicate partition coefficientssuggest that a silicate melt in equilibrium with a sulphidemelt containing the PGE spectrum of the Merensky ore would indeedbe oversaturated with respect to the least soluble noble metals.Sulphide melt apparently played little role in enriching thenoble metals in the Merensky Reef; rather, its role was to immobilizea pre-existing in situ stratiform PGE anomaly in the liquid-stratifiedmagma chamber. KEY WORDS: Bushveld Complex; Merensky Reef; laser-ablation ICP-MS; platinum-group mineralization 相似文献
60.
Oxygen and Hydrogen Isotope Stratigraphy of the Rustenburg Layered Suite, Bushveld Complex: Constraints on Crustal Contamination 总被引：5，自引：1，他引：4
HARRIS CHRIS; PRONOST JULIE J. M.; ASHWAL LEWIS D.; CAWTHORN R. GRANT 《Journal of Petrology》2005,46(3):579-601
New 18O values for plagioclase, pyroxene and olivine, and limitedwhole-rock D values are presented for samples from the RustenburgLayered Suite of the Bushveld Complex, South Africa. In combinationwith existing data, these provide a much more complete compositeO-isotope stratigraphy for the intrusion. Throughout the layeredsuite, mineral 18O values indicate that the magmas from whichthey crystallized had 18O values that were about 7·1,that is, 1·4 higher than expected for mantle-derivedmagmas, suggesting extensive crustal contamination. More limitedH-isotope data suggest that the OH present within whole rocks,regardless of the degree of alteration, is of magmatic originand not an alteration phenomenon. There appears to be no systematicchange in 18O value with stratigraphic height and this requiresthe contamination to have taken place in a ‘staging chamber’before emplacement of the magma(s) into the present chamber.Large amounts (30–40%) of contamination by the lower tomiddle crust are needed to explain these 18O values, which isin general agreement with previous estimates based on Sr- andNd-isotope data. Alternatively, smaller amounts of contamination(20%) by sedimentary rocks, or their partial melts, representedby the country rock can explain the data, but it is not apparenthow such material could have been present at the depth of the‘staging chamber’ in the lower to middle crust. KEY WORDS: Bushveld Complex; Rustenburg Layered Suite; oxygen isotopes; hydrogen isotopes; crustal contamination 相似文献