Effect of metamorphism and partial melting of host rocks on zircons     

L. N. GUPTA  W. JOHANNES 《Journal of Metamorphic Geology》1985,3(3):311-323

Abstract Zircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock-forming processes of migmatites.
The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence of characteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial melting.  相似文献   

Lateglacial vegetation and palaeoenvironment in W Norway,with new pollen data from the Sunnmøre region     

LINN CECILIE KRÜGER  AAGE PAUS  JOHN INGE SVENDSEN  ANNE E. BJUNE 《Boreas: An International Journal of Quaternary Research》2011,40(4):616-635

Krüger, L. C., Paus, A., Svendsen, J. I. & Bjune, A. E. 2011: Lateglacial vegetation and palaeoenvironment in W Norway, with new pollen data from the Sunnmøre region. Boreas, 10.1111/j.1502‐3885.2011.00213.x. ISSN 0300‐9483. Two sediment sequences from Sunnmøre, northern W Norway, were pollen‐analytically studied to reconstruct the Lateglacial vegetation history and climate. The coastal Dimnamyra was deglaciated around 15.3 ka BP, whereas Løkjingsmyra, further inland, became ice‐free around 14 ka BP. The pioneer vegetation dominated by snow‐bed communities was gradually replaced by grassland and sparse heath vegetation. A pronounced peak in Poaceae around 12.9 ka BP may reflect warmer and/or drier conditions. The Younger Dryas (YD) cooling phase shows increasing snow‐bed vegetation and the local establishment of Artemisia norvegica. A subsequent vegetation closure from grassland to heath signals the Holocene warming. Birch forests were established 500–600 years after the YD–Holocene transition. This development follows the pattern of the Sunnmøre region, which is clearly different from the Empetrum dominance in the Lateglacial interstadial further south in W Norway. The Lateglacial oscillations GI‐1d (Older Dryas) and GI‐1b (Gerzensee) are hardly traceable in the north, in contrast to southern W Norway. The southern vegetation was probably closer to an ecotone and more susceptible to climate changes.  相似文献   

Moraine-ridge formation along a stationary ice front in Iceland   总被引：1，自引：0，他引：1  

JOHANNES KRÜGER 《Boreas: An International Journal of Quaternary Research》1993,22(2):101-109

At present the north margin of the temperate ice-cap Myrdalsjökull is stationary: the ice edge retreats slowly during summer and readvances during winter to much the same position as the previous winter. Although the ice margin in this way has been stationary since around 1984. a frontal moraine ridge. 1.5-2.5 m high. was under formation in 1986. and in 1989 it was 3–4 m high. The interior of the ridge appeared as imbricately stacked slabs of frozen, clast-paved lodgement till dipping up-glacier. At least five to seven slabs were identified in the 1989 ridge. The most proximal one was frozen to the up-arched glacier sole and dipped beneath the glacier at about 30. In 1989 the volume of lodgement till sediments within the ridge represented a horizontal shortening of the ground moraine of roughly 60–90 m. On the other hand. between 1984 and 1989 the lateral displacement of the ridge toe amounted to only 10 m. It is concluded that the frontal ridge is formed progressively. not like conventional push moraines by thrusting of contemporaneous proglacial or ice-contact sediments. but chiefly by a combination of basal freezing beneath the thin. clast-loaded glacier toe each winter and recurrent superposition of frozen lodgement till slabs during small winter readvances.  相似文献   

Genetic Model for the Stromatic Migmatites of the Rantasalmi-Sulkava Area, Finland     

GUPTA  L. N.; JOHANNES  W. 《Journal of Petrology》1986,27(2):521-539

Metasediments of the Rantasalmi-Sulkava area (Finland) showprogressive regional metamorphism with migmatization. The metasedimentsare represented by various types of metapsammites (plagioclase-rich,quartz-rich, and layers of granitic compositions—somerich in microcline and others in plagioclase) and metapelites(dark and light layers). The migmatites of this area are of stromatic type. They consistof leucosomes, mesosomes, and light-coloured plagioclase-richlayers which do not fit the definition of leucosome. Melanosomes,which usually separate leucosomes and mesosomes in stromaticmigmatites, are almost absent. The leucosomes are of three types: (i) quartz-rich; (ii) cordierite-rich;and (iii) granitic. The quartz-rich leucosomes formed firstat subsolidus temperatures through recrystallization. The graniticleucosomes are considered to have developed via partial melting.The cordierite-rich leucosomes are formed—like the graniticones—at supersolidus conditions, but the role of partialmelting is not clear. The mesosomes are the metamorphic portions of the migmatiteswhich are not transformed into leucosomes. They include metapsammiticlayers and light-coloured metapelitic layers, both rich in plagioclase. Besides mineral reactions resulting in new assemblages duringregional metamorphism, the main process changing the protolithsinto migmatites is the conversion of some of the protolith layersinto leucosomes, through (as we believe) an almost isochemicalpartial melting. The migmatites of the Rantasalmi-Sulkava area differ from othermigmatites investigated by the authors in having two differentgenetic types of leucosomes: one formed via partial meltingand the other through subsolidus recrystallization as mentionedabove. The process of migmatization is described and modelledin three steps. Reprint requests to W. Johannes  相似文献   

High-resolution stratigraphy of the northernmost concentric raised bog in Europe: Sellevollmyra, Andøya, northern Norway     

KARL-DAG VORREN  MAARTEN BLAAUW  STEFAN WASTEGÅRD  JOHANNES VAN DER PLICHT  CHRISTIN JENSEN 《Boreas: An International Journal of Quaternary Research》2007,36(3):253-277

From the Sellevollmyra bog at Andøya, northern Norway, a 440‐cm long peat core covering the last c. 7000 calendar years was examined for humification, loss‐on‐ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle‐match of 35 ¹⁴C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410–6380, 6230–6050, 5730–5640, 5470–5430, 5340–5310, 5270–5100, 4790–4710, 4890–4820, 4380–4320, 4220–4120, 4000–3810, 3610–3580, 3370–3340 (regionally 2850–2750; in Sellevollmyra a hiatus between 2960–2520), 2330–2220, 1950, 1530–1450, 1150–840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla‐4 tephra layer started some decades before the eruption event.  相似文献   

Modern and Holocene hydrographic characteristics of the shallow Kara Sea shelf (Siberia) as reflected by stable isotopes of bivalves and benthic foraminifera     

JOHANNES SIMSTICH  HELMUT ERLENKEUSER  INGO HARMS  ROBERT F. SPIELHAGEN  VLADIMIR STANOVOY 《Boreas: An International Journal of Quaternary Research》2005,34(3):252-263

River discharge of Ob and Yenisei to the Kara Sea is highly variable on seasonal and interannual time scales. River water dominates the shallow bottom water near the river mouths, making it warmer and less saline but seasonally and interannually more changeable than bottom water on the deeper shelf. This hydrographic pattern shows up in measurements and modelling, and in stable isotope records (δ¹⁸O, δ¹³C) along the growth axis of bivalve shells and in multiple analyses of single benthic foraminiferal shells. Average isotope ratios increase, but sample-internal variability decreases with water depth and distance from river mouths. However, isotope records of bivalves and foraminifera of a sediment core from a former submarine channel of Yenisei River reveal a different pattern. The retreat of the river mouth from this site due to early Holocene sea level rise led to increasing average isotope values up core, but not to the expected decrease of the in-sample isotope variability. Southward advection of cold saline water along the palaeo-river channel probably obscured the hydrographic variability during the early Holocene. Later, when sediment filled the channel, the hydrographic variability at the core location remained low, because the shallowing proceeded synchronously with the retreat of the river mouth.  相似文献   

Time‐transgressive tunnel‐valley infill revealed by a three‐dimensional sedimentary model,Hamburg, north‐west Germany     

ADRIAAN JANSZEN  JULIEN MOREAU  ANDREA MOSCARIELLO  JÜRGEN EHLERS  JENS KRÖGER 《Sedimentology》2013,60(3):693-719

Deep, elongated incisions, often referred to as tunnel valleys, are among the most characteristic landforms of formerly glaciated terrains. It is commonly thought that tunnel valleys were formed by meltwater flowing underneath large ice sheets. The sedimentary infill of these features is often highly intricate and therefore difficult to predict. This study intends to improve the comprehension of the sedimentology and to establish a conceptual model of tunnel‐valley infill, which can be used as a predictive tool. To this end, the densely sampled, Pleistocene tunnel valleys in Hamburg (north‐west Germany) were investigated using a dataset of 1057 deep wells containing lithological and geophysical data. The stratigraphic correlations and the resulting three‐dimensional lithological model were used to assess the spatial lithological distributions and sedimentary architecture. The sedimentary succession filling the Hamburg area tunnel valleys can be subdivided into three distinct units, which are distinguished by their inferred depositional proximity to the ice margin. The overall trend of the succession shows a progressive decrease in transport energy and glacial influence through time. The rate of glacial recession appears to have been an important control on the sedimentary architecture of the tunnel‐valley fill. During periods of stagnation, thick ice‐proximal deposits accumulated at the ice margin, while during rapid recession, only a thin veneer of such coarse‐grained sediments was deposited. Ice‐distal and non‐glaciogenic deposits (i.e. lacustrine, marine and terrestrial) fill the remaining part of the incision. The infill architecture suggests formation and subsequent infill of the tunnel valleys at the outer margin of the Elsterian ice sheet during its punctuated northwards recession. The proposed model shows how the history of ice‐sheet recession determines the position of coarse‐grained depocentres, while the post‐glacial history controls the deposition of fines through a progressive infill of remnant depressions.  相似文献   

MODELLING LONG‐TERM SUMMER AND WINTER BALANCES AND THE CLIMATE SENSITIVITY OF STORBREEN,NORWAY     

LISS M. ANDREASSEN  JOHANNES OERLEMANS 《Geografiska Annaler: Series A, Physical Geography》2009,91(4):233-251

Measurements of winter balance (b_w) and summer balance (b_s) have been carried out at Storbreen since 1949. Here we apply a simple mass balance model to study the climate sensitivity and to reconstruct the mass balance series priorto 1949. The model is calibrated and validated with data from an automatic weather station (AWS) operating in the ablation zone of Storbreen since 2001. Regression analysis revealed that b_w was best modelled using precipitation data southwest of the glacier. Results from the model compared well with reported mass balance values for the period 1949–2006, obtained correlations (r) for b_w and b_s varied between 0.83 and 0.87 depending on model set up. Reconstruction of the mass balance series for the period 1924/1925–1948/1949 suggested a cumulative mass deficit of c. 30 m w.e. mainly due to highly negative summer balances, but also lower b_wthan the average for 1949–2006. Calculated change in specific mass balance for a ±1°C change in air temperature was ±0.55 m w.e., whereas a ±10 % increase in precipitation represented a change of ±0.20 m w.e. Model results further indicated that for a 2°C warming, the ablation season will be extended by c. 30 days and that the period of ice melt at the AWS location will increase from c. 40 to c. 80 days.  相似文献   

Structures and textures in till indicating subglacial deposition   总被引：1，自引：0，他引：1  

JOHANNES KRÜGER 《Boreas: An International Journal of Quaternary Research》1979,8(3):323-340

Five structural and textural features are discussed: (1) small lenses of sorted material, (2) smudges, (3) small-scale deformations of till matrix and smudges by clasts, (4) clasts consistently striated, and (5) clasts with stoss-and-lee sides. Analyses suggest that these features may be produced by subglacial processes acting in the ice-bed interface. Long axes of small sand lenses and smudges as well as the striation on the upper surface of scattered clasts in lodgement till have a strong preferred orientation in good agreement with the glacier flow direction as indicated by clast fabrics, bedrock striation, and surface fluting of ground moraine. When in traction against the till bed, clasts may plough up till banks. Clasts with stoss-and-lee sides development were also very distinctly oriented as their stoss sides faced significantly up-glacier.
It is concluded (1) that each of the five features discussed is useful as a criterion for subglacial deposition by lodgement, (2) that they indicate important differential movement along the ice-bed interface and therefore suggest a temperate regime in this part of the glacier during the till deposition, (3) that very few orientation measurements of one or more of these features signify the ice movement direction; i.e. a time-saving method to find the paleoflow direction of Pleistocene glaciers, and (4) that taken together with till preconsolidation, mechanical composition, and clast fabric, they may support each other and give good indications of the genesis of Pleistocene tills.  相似文献   

THE SORS EXPERIMENT IN THE CORONAS-I PROJECT: SOME RESULTS     

FOMICHEV  V. V.  ORAEVSKY  V. N.  PULINETS  S. A.  PRUTENSKY  I. S.  GORGUTSA  R. V.  KLOS  Z.  KIRAGA  A.  ROTHKAEHL  H.  KRÜGER  A.  HILDEBRANDT  J.  AURASS  H.  KLASSEN  A.  MANN  G. 《Solar physics》1997,171(1):221-224

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