Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al₂O₃ and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of ¹⁸⁷Os/¹⁸⁸Os values. The most unradiogenic ¹⁸⁷Os/¹⁸⁸Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.     

Medium pressure crystallization of a monchiquitic magma — evidence from megacrysts of Drever''s block, Ubekendt Ejland, West Greenland

Megacrysts and polymineralic fragments of extraordinary diversity from a Tertiary monchiquitic dyke of Ubekendt Ejland comprise three groups: (1) Cr-diopside-fassaitic diopside + olivine, Fo_90.5?81.5 + CrAl spinels. (II) Fassaitic salite-ferrisalite + KTi-pargasite-ferropargasite + apatite + AlTi-magnetite, (III) Scapolite + hyalophane + potassium feldspar + nepheline + analcime. By comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in H₂O and CO₂ at 5–11 kb. Group I megacrysts formed at temperatures of 1300-1150°C and group II between ? bar at the latter temperature. High P_co2 may have stabilized the scapolite in the more evolved liquid and K-feldspar and nepheline began to crystallize at ca. 800°C possibly together with the ferrisalite.     

The lamprophyres of Ubekendt Ejland,West Greenland: Products of renewed partial melting or extreme differentiation?

The late tholeiitic lavas of southwestern Ubekendt Ejland (63-54 Ma) are intruded by lamprophyre dykes (～33 Ma). The compositional and temporal differences would seem to preclude any genetic relationship between the two igneous episodes. Competing models of renewed partial melting versus extended batch fractionation are considered to explain the origin of the lamprophyres. Mathematically modelled lamprophyres produced by batch fractionation are very close to observed lamprophyre compositions and this, combined with other temporal and spatial evidence, suggests that renewed partial melting of the mantle is not the best way to explain such bimodal suites.     

Isolation basin stratigraphy and Holocene relative sea-level change on Arveprinsen Ejland,Disko Bugt,West Greenland

This paper presents the results of an investigation into Holocene relative sea-level (RSL) change, isostatic rebound and ice sheet dynamics in Disko Bugt, West Greenland. Data collected from nine isolation basins on Arveprinsen Ejland, east Disko Bugt, show that mean sea level fell continuously from ca. 70 m at 9.9 ka cal. yr BP (8.9 ka ¹⁴C yr BP) to reach a minimum of ca. −5 m at 2.8 ka cal. yr BP (2.5 ka ¹⁴C yr BP), before rising to the present day. A west–east gradient in isostatic uplift across Disko Bugt is confirmed, with reduced rebound observed in east Disko Bugt. However, RSL differences (up to 20 m at 7.8 ka to 6.8 ka cal. yr BP (7 ka to 6 ka ¹⁴C yr BP)) also exist within east Disko Bugt, suggesting a significant north–south component to the area’s isostatic history. The observed magnitude and timing of late Holocene RSL rise is not compatible with regional forebulge collapse. Instead, RSL rise began first in the eastern part of the bay, as might be expected under a scenario of crustal subsidence caused by neoglacial ice sheet readvance. The results of this study demonstrate the potential of isolation basin data for local and regional RSL studies in Greenland, and the importance of avoiding data compilations from areas where the isobase orientation is uncertain. Copyright © 1999 John Wiley & Sons, Ltd.     

The origin of a calcareous amphibolite from the Nagssugtoqidian, west Greenland

On the basis of chemical charactiristics (major elements) it is concluded that the investigated amphibolite was formed as a result of high-grade regional metamorphism of a calcareous sediment mixed with basic igneous material, probably in the form of tuff. In view of the many recent publications on the origin of amphibolites it seems justified to propose a new definition of the term ‘para-amphibolite‘.     

Petrogenetic Significance of Initial 87Sr/86Sr Ratios in the North Atlantic Tertiary Igneous Province in the Light of Rb-Sr, K-Ar and 18O-abundance Studies of the Sarqata qaqa Intrusive Complex Ubekendt Ejland, West Greenland

A single Rb-Sr whole rock ‘isochron’ from threegabbroic and six granitic samples from the Sarqâta qáqâComplex, Ubekendt Ejland, yields an age and initial ⁸⁷Sr/⁸⁶Srratio of 65±5 m.y. (ß = 1.47x10^-11 yr^-1) and0.7045±0.0003. K-Ar measurements on two biotite separatesfrom the gabbro give ages of 55.4±1.5 and 54.0±1.5m.y. Determinations of ¹⁶O/¹⁸O ratios in two gabbro and fourgranite samples yield average values of ±5.53±0.04and +6.74± 0.37% respectively, relative to SMOW. The following conclusions are drawn: 1. Since the balance of field evidence indicates that the graniteshortly post-dates the gabbro, the Rb-Sr and K-Ar ages mustbe highly discordant, rather than dating separate intrusiveevents. It is suggested that the Sarqâta qáqâComplex cooled very slowly (ca. 50–200 °C per m.y.). 2. The extrusion of the plateau basalts in West Greenland musthave occurred before 65 m.y. This appears to be slightly olderthan the Tertiary igneous activity in East Greenland, the Faeroes,NE Ireland, NW Scotland and Lundy; an age pattern which accordswith recent reconstructions of the formation of the North Atlantic. 3. The strontium and oxygen isotope data both support an hypothesisthat the Sarqâta qáqâ granite is comagmaticwith the mantle-derived basic lavas and plutonic rocks of UbekendtEjland. This in turn gives indirect support to recent suggestionsthat the Eocene granites of Skye, NW Scotland, may have a similarorigin and that their relative enrichment in ⁸⁷Sr may not indicategenesis by crustal melting in the aureoles of large basic plutons,but rather trace element equilibration with surrounding ⁸⁷Sr-richcountry rocks during an episode of influx of meteoric groundwater which has left them severely depleted in ¹⁸O.     

Alkali Picrites Formed by Melting of Old Metasomatized Lithospheric Mantle: Manitdlat Member, Vaigat Formation, Palaeocene of West Greenland

Alkaline picrites and basalts constitute 20–200 m of lavaflows and hyaloclastites in the middle part of an     

An early Holocene Greenland whale from Melville Bugt, Greenland

Radiocarbon age determination of a Greenland whale (Balaena mysticetus) vertebra from Melville Bugt in northwestern Greenland yields an age of 9259-8989 cal yr BP. The margin of the Greenland Ice Sheet in Melville Bugt was situated behind its AD 1950-2000 position in the early Holocene, at a similar position to that being reached following rapid retreat in recent years. Such an early deglaciation of areas close to the Greenland Ice Sheet is unusual. This probably reflects the unique glaciological setting resulting from the narrow fringe of ice-free islands and peninsulas and offshore waters with deep areas that characterize this part of Greenland. The timing of Greenland Ice Sheet retreat to its present margin varies significantly around Greenland.     

Controls on the sedimentology of an ice-contact jökulhlaup-dominated delta,Kangerlussuaq, west Greenland

This paper characterises the sedimentary impact of a glacial outburst flood or ‘jökulhlaup’ on an ice-contact delta topset at Russell Glacier, Kangerlussuaq, west Greenland. Rapid drainage of an ice-dammed lake in July 1987 generated a jökulhlaup with a peak discharge of ∼ 1300 m³ s^− 1, which drained across a 500-m-wide, 200-m-long, delta top into a proglacial lake. The delta topset comprises boulder clusters, ice block obstacle marks with relief of up to 4 m, and is graded to lake levels up to 6 m higher than those during typical non-jökulhlaup conditions. The delta top was dissected by the 1987 jökulhlaup causing a fan-shaped extension of the delta front by 30 m. Surface grain size on the delta decreases rapidly away from the main flood flow direction, reflecting rapid downstream reduction in sediment transport capacity. The 1987 jökulhlaup was predominantly fluidal and turbulent and had peak stream powers of 2846 W m^− 2 proximally and < 400 W m^− 2 distally. Delta topset sedimentation can be characterised by four lithofacies associations in order of decreasing flow energy: (A) coarse-grained deposits related to a flow expansion; (B) finer-grained peripheral deposits located at the margins of the main flow; (C) lobate bars and delta fronts deposited within distal locations and (D) fine-grained deposits at distance from the delta front associated with slackwater conditions. Jökulhlaup-dominated delta topsets are controlled by the geometry of the channel expansion into the proglacial lake, jökulhlaup hydrograph form, the sediment availability and character, proglacial lake basin depth and surface area, lake outflow spillway erodibility and cross-sectional area, and history of previous jökulhlaups.     

Late Quaternary glacial history of the central west coast of Jameson Land, East Greenland

The Late Quaternary ( c . 130,000–10,000 BP) glacial history of the central west coast of Jameson Land, East Greenland, is reconstructed through glacial stratigraphical studies. Seven major sedimentary units are described and defined. They represent two interglacial events (where one is the Holocene). one interstadial event and two glacial events. The older interglacial event comprises marine and fluvial sediments, and is correlated to the Langelandselv interglacial, corresponding to oxygen isotope sub-stage 5e. It is followed by an Early Weichselian major glaciation during the Aucellaelv stade, and subsequently by an Early Weichselian interstadial marine and deltaic event (the Hugin Sø interstade). Sediments relating to the Middle Weichselian have not been recognized in the area. The Hugin Sø interstade deposits have been overrun by a Late Weichselian ice advance, during the Flakkerhuk stade, when the glacier, which probably was a thin, low gradient fjord glacier in Scoresby Sund, draped older sediments and landforms with a thin till. Subsequent to the final deglaciation, some time before 10,000BP, the sea reached the marine limit around 70 m a.s.l., and early Holocene marine, fluvial and littoral sediments were deposited in the coastal areas.     

Molluscan assemblage from the marine Palaeocene sequence in southwestern Kachchh,Gujarat

Presence of oldest marine Tertiary rocks in Kachchh for a long time has remained debatable and controversial. To resolve the issue in better way authors have worked out a section along tributary of Kakdi Nadi about 0.5 km west of village Nareda in southwestern Kachchh and collected molluscan fauna. Systematic study of these fauna reveals presence of 15 taxa of which 12 are of bivalves and 3 of gastropods. This molluscan assemblage leads to the presence of marine Palaeocene sequence around Nareda. Recorded assemblage hitherto not known earlier from Kachchh.     

Little Ice Age and neoglacial landforms at the Inland Ice margin, Isunguata Sermia, Kangerlussuaq, west Greenland

Neoglacial and Little Ice Age (LIA) limits occur within 2km of the Inland Ice margin in the Kangerlussuaq area on west Greenland. The LIA limit is clearly demarcated by ice-cored and non-ice-cored moraines, out-wash surfaces and trimlines. Rhizocarpon sp. thalli of ≥16mm on these landforms indicate a 1-2km retreat of the Inland Ice in the past c. 100 years, coincident with peripheral thinning of the ice. An older neoglacial moraine host of Rhizocarpon sp. thalli <40 mm indicates a minimum limiting age of <400 BP, whereas Optically Stimulated Luminescence (OSL) ages on aeolian silt capping the moraine yield close limiting ages of c. 2000 BP. Aeolian silt deposition beyond neoglacial limits yields OSL ages of c. 3000 BP, potentially coeval with advance of the Inland Ice. Aeolian sedimentation and the inferred age of the moraine are coincident with pronounced cooling inferred from palaeolimnological records from west and south Greenland. This neoglacial event at c. 2000 BP is probably of similar extent to the LIA maximum, because of the paucity of preserved moraine remnants.     

Zirconium-bearing aegirines from Motzfeldt,South Greenland

Aegirines with almost 7.0 wt.% ZrO₂ have been discovered in nepheline syenites from the Motzfeldt Centre, South Greenland. The analyses require the postulate of a new endmember pyroxene composition with the formula NaFM_0.5Zr_0.5Si₂O₆.A possible acronym is FM-NAZ. Aegirines rich in this component occur in rocks in which there is no other zirconium-bearing phase such as eudialyte.These zirconian pyroxenes have crystallised from magmas which were peralkaline, low in lime and silica and relatively low in oxygen fugacity compared with other nepheline syenites. These factors have combined to prevent the usual crystallisation of such Zr-phases as eudialyte, zircon or baddeleyite.     

Zirconium-bearing aegirines from Motzfeldt,South Greenland

Aegirines with almost 7.0 wt.% ZrO₂ have been discovered in nepheline syenites from the Motzfeldt Centre, South Greenland. The analyses require the postulate of a new endmember pyroxene composition with the formula NaFM_0.5Zr_0.5Si₂O₆.A possible acronym is FM-NAZ. Aegirines rich in this component occur in rocks in which there is no other zirconium-bearing phase such as eudialyte.These zirconian pyroxenes have crystallised from magmas which were peralkaline, low in lime and silica and relatively low in oxygen fugacity compared with other nepheline syenites. These factors have combined to prevent the usual crystallisation of such Zr-phases as eudialyte, zircon or baddeleyite.     

Isotopic characteristics and petrogenesis of the lamproites and kimberlites of central west Greenland

Kimberlites which intruded the Sisimiut (formerly Holsteinsborg) region of central west Greenland during the Early Palaeozoic have initial ⁸⁷Sr/⁸⁶Sr between 0.7028 and 0.7033 and ε_Nd between + 1.3 and + 3.9. Mid-Proterozoic potassic lamproites from the same region have initial ⁸⁷Sr/⁸⁶Sr between 0.7045 and 0.7060, ε_Nd between −13 and −10 and unradiogenic initial Pb isotopic compositions. The isotopic data favour an asthenospheric mantle source for the kimberlite magmas, in common with “basaltic” kimberlites from other localities, whereas the lamproite magma sources evolved in isolation from the convecting mantle for > 1000 Ma, probably within the subcontinental lithospheric mantle of the Greenland craton, prior to emplacement of the lamproites.     

The volatile component of quaternary ignimbrite magmas from the North Island,New Zealand

Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO₂. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of using pyrrhotite and thence , . The assemblage biotite-sanidine can be used to estimate and thence . Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction . It is high and approaches P _total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma.     

Late Holocene environmental change in Disko Bugt, west Greenland: interaction between climate, ocean circulation and Jakobshavn Isbrae

Foraminiferal assemblages and the sedimentology of two cores (POR20 and POR21) from eastern Disko Bugt, west Greenland, are used to identify environmental changes in the area over the past c. 2200 years. Changes in the sediment flux supplied to the core sites from Jakobshavn Isbrae are used to assess the relative position of the calving margin. An Atlantic water influence as strong as, or slightly stronger than, present prevailed at c. 2200 cal. yr BP. A trend of increasing Atlantic water influence then culminated in peak warm and saline hydrographic conditions c. 1664-474 cal. yr BP encompassing the 'Medieval Warm Period'. This period was marked by a retreat of the calving front of Jakobshavn Isbrae and was followed by a marked cooling in hydrographic conditions relating to an increase in the influence of the East Greenland Current in the West Greenland Current corresponding to the climatic episode the 'Little Ice Age'. A rise in sedimentation rate over this period relates to the well-documented advance of Jakobshavn Isbrae. The record from Disko Bugt shows good agreement with the temperature record from the Greenland ice cores and other climatic and oceanographic reconstructions in the region.     

西藏岗巴古新世地层及构造作用的影响

西藏岗巴地区出露有完整的晚白垩世至古近纪地层，地层中化石丰富，根据化石研究准确地进行了地层时代的划分，本文认为该区白垩系－古代系界线位于宗山组和基堵拉组之间，以底栖有大孔虫Orbitoides-Omphalocyclus动物群的消亡和Rotalia-Smoutina-Lockhartia动物群的始现为标志。界线上下岩层为假整合接触，期间有一短暂的暴露面。古新世的砂砾岩直接覆于晚白晋世的陆棚碳酸盐岩沉积之上，沉积的重大转变代表一次构造运动，印度大陆北缘与冈底斯南缘直至白垩纪末均具有明显的浅海生物地理分区现象，期间被深海盆地地所阻隔。古新世开始浅海相动物群在该地显示同一生物地理区系特征，说明两大陆间深水盆地的阻隔已消失，南北生物地理区同归于一残留海盆，沉积类型转变和地层古生物特征为印度－亚洲板块的起始碰撞时间研究提供了基础资料，据此推测大陆早期碰撞发生在白晋系－古近系的界线时期（约65Ma)，古新世中一晚期碳酸盐台地遭受不断的挤压与变形，进一步说明大陆的碰撞在古新世之初就已发生。沉积地层的破碎变形和滑塌堆积是持续碰撞与挤压的结果。     

Alfred Wegener,from research in Greenland to plate tectonics

GeoJournal - The life of the most famous German Polar researcher is described. The investigations during his four journeys to Greenland are considered in detail. Through his hypothesis concerning...