P–T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the Rhodope UHP Metamorphic Complex     

M. JANÁK  N. FROITZHEIM  N. GEORGIEV  T. J. NAGEL  S. SAROV 《Journal of Metamorphic Geology》2011,29(3):317-332

A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak–pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarometry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg‐rich cores (X_Mg = 0.44–0.46) record a prograde increase in P–T conditions from ～2.5 GPa and 650 °C to ～3 GPa and 700–750 °C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak–pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplectitic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (X_Mg = 0.4–0.45) + corundum + anorthite assemblage in the kyanite‐bearing domains at ～1.1 GPa and 800–850 °C suggests a thermal overprint in the high‐pressure granulite facies stability field. This thermal event was followed by cooling at ～0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre‐Alpine orogenic events, and that two independent HP/UHP provinces of different age overlap in this area.  相似文献   

Morphology of Younger Dryas subglacial and ice-proximal submarine landforms, inner Vestfjorden, northern Norway     

KAI ROGER FLØISTAD  JAN SVERRE LABERG  TORE O. VORREN 《Boreas: An International Journal of Quaternary Research》2009,38(3):610-619

The sea-floor morphology of two pronounced across-fjord bedrock thresholds located at the mouths of Ofotfjorden and Tysfjorden, northern Norway, has been analysed based on swath bathymetry and seismic data. The Younger Dryas ice front was located here during the recession of one of the large palaeo-ice streams of the Fennoscandian Ice Sheet. The thresholds are several kilometres long and wide, rising to several hundred metres above the adjacent sea floor, and the slopes are steep, up to 25°. The Ofotfjorden threshold is draped by acoustically discontinuous to chaotic sediments partly infilling the bedrock relief. A pattern of well-developed, subglacial bedforms (e.g. crag-and-tail formations, drumlins and glacial lineations) on top of both thresholds suggests fast-flowing ice. A series of smaller transverse ridges is identified on both thresholds and probably records ice-front oscillations during the final deglaciation. The distal parts of the sediments have been remobilized by slides that occurred after glacial retreat from the thresholds. Earthquake activity due to the isostatic rebound following ice retreat from this area was the most likely triggering mechanism for the slides. The location of the ice front on a prominent bedrock threshold indicates that the basin configuration was important in locating the maximum position of the climatically induced re-advance, i.e. a topographic control on the maximum Younger Dryas position in the Ofotfjorden and Tysfjorden area is suggested.  相似文献   

Magnetic mineralogy of the sediments in Lake Ådran, eastern Sweden, and an interpretation of early Holocene water level changes     

PER SANDGREN  JAN RISBERG 《Boreas: An International Journal of Quaternary Research》1990,19(1):57-68

A number of magnetic parameters have been measured on samples from a master sequence in Lake Adran, eastern Sweden. Based on the results, frequency dependent magnetic susceptibility, high isothermal rem-anence magnetization and different magnetic ratios have been calculated. The results from the master sequence have also been compared with magnetically analysed soil samples from the lake catchment. The results of the magnetic analyses show that the sediments in Lake Ådran originate from two sources, one regional and one local. The different sources can be related to water level changes during the Ancylus and Litorina stages. Sediments originating from the regional source are characterized by high ferrimagnetic to antiferromagnetic ratios (high S-ratios) and are assumed to have been deposited when the water level was well above the threshold. Sediments derived from the catchment, having lower S-ratios, are assumed to have been deposited when the basin was cut off from the regional water circulation. On the basis of the magnetic results, combined with pollen and diatom analyses, tentative water level changes for the Lake Adran basin are compiled.  相似文献   

Diagenetic formation of gypsum and dolomite in a cold‐water coral mound in the Porcupine Seabight,off Ireland     

HANS PIRLET  LAURA M. WEHRMANN  BENJAMIN BRUNNER  NORBERT FRANK  JAN DEWANCKELE  DAVID VAN ROOIJ  ANNELEEN FOUBERT  RUDY SWENNEN  LIEVEN NAUDTS  MATTHIEU BOONE  VEERLE CNUDDE  JEAN‐PIERRE HENRIET 《Sedimentology》2010,57(3):786-805

Authigenic gypsum was found in a gravity core, retrieved from the top of Mound Perseverance, a giant cold‐water coral mound in the Porcupine Basin, off Ireland. The occurrence of gypsum in such an environment is intriguing, because gypsum, a classic evaporitic mineral, is undersaturated with respect to sea water. Sedimentological, petrographic and isotopic evidence point to diagenetic formation of the gypsum, tied to oxidation of sedimentary sulphide minerals (i.e. pyrite). This oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments. The oxidation of pyrite produced acidity, causing carbonate dissolution and subsequently leading to pore‐water oversaturation with respect to gypsum and dolomite. Calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21·6% and 28·6% of the sulphate incorporated into the gypsum derived from pyrite oxidation. The dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment‐water interface. Thus, authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate‐rich sediments. These observations demonstrate that fluid flow, steered by environmental factors, has an important effect on the diagenesis of coral mounds.  相似文献   

Drilling Blake Nose: the search for evidence of extreme Palaeogene–Cretaceous climates and extraterrestrial events     

KROON  NORRIS  KLAUS  IAN T. ALEXANDER  LEON PAUL BARDOT  CHARLES E. BARKER  JEAN-PIERRE BELLIER  CHARLES D. BLOME  LEON J. CLARKE  JOCHEN ERBACHER  KRISTINA L. FAUL  MARY ANNE HOLMES  BRIAN T. HUBER  MIRIAM E. KATZ  KENNETH G. MACLEOD  SANDRA MARCA  FRANCISA C. MARTINEZ-RUIZ  ISAO MITA  MUTSUMI NAKAI  JAMES G. OGG  DOROTHY K. PAK  THOMAS K. PLETSCH  JEAN M. SELF-TRIAL  NICHOLAS J. SHACKLETON  JAN SMIT  WILLIAM USSLERIII  DAVID K. WATKINS  JOEN WIDMARK  & PAUL A. WILSON 《Geology Today》1998,14(6):222-226

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Infrared stimulated luminescence dating of an Eemian (MIS 5e) site in Denmark using K‐feldspar     

JAN‐PIETER BUYLAERT  SÉBASTIEN HUOT  ANDREW S. MURRAY  PETER VAN DEN HAUTE 《Boreas: An International Journal of Quaternary Research》2011,40(1):46-56

Buylaert, J.‐P., Huot, S., Murray, A.S. & Van den haute, P.: Infrared stimulated luminescence dating of an Eemian (MIS 5e) site in Denmark using K‐feldspar. Boreas, 10.1111/j.1502‐3885.2010.00156.x. ISSN 0300‐9483. Infrared stimulated luminescence (IRSL) dating of K‐feldspars may be an alternative to quartz optically stimulated luminescence (OSL) dating when the quartz OSL signal is too close to saturation or when the quartz luminescence characteristics are unsuitable. In this paper, Eemian (MIS 5e) coastal marine sands exposed in a cliff section on the coast of southern Jutland (Denmark) are used to test the accuracy and precision of IRSL dating using K‐feldspars. This material has been used previously to test quartz OSL dating ( Murray & Funder 2003 ): a small systematic underestimation of <10% compared to the expected age of ～130 ka was reported. In our study, a single‐aliquot regenerative‐dose (SAR) IRSL protocol is used to determine values of equivalent dose (D_e) and the corresponding fading rates (g values). A significant age underestimation (of up to ～35%) is observed; this is attributed to anomalous fading. Using a single site‐average fading rate of 3.66 ± 0.09%/decade to correct the IRSL ages for all samples provides good agreement between the average fading‐corrected K‐feldspar age (119 ± 6 ka) and the independent age control (132–125 ka). This is despite the reservations of Huntley & Lamothe (2001) that their fading correction method is not expected to work on samples older than ～20–50 ka. This fading‐corrected feldspar result is not significantly different from the overall revised quartz age (114 ± 7 ka) also presented here. We conclude that fading‐corrected IRSL ages measured using K‐feldspar may be both precise and accurate over a greater age range than might be otherwise expected.  相似文献   

Palaeohydrology and global continental changes - panta rhei?     

JAN PIOTROWSKI 《Boreas: An International Journal of Quaternary Research》1997,26(2):157-158

Branson, J., Brown, A. G. & Gregory, K. J. (cds.) 1996: Glubai Continental Changes: The Context of Palaeohydrology  相似文献   

Validation of a skill prediction method     

JAN BARKMEIJER  PETER HOUTEKAMER  XUELI WANG 《地球，A辑:动力气象学与海洋学》1993,45(5):424-434

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The effect of fractional cloudiness on the oxidation of SO₂     

JAN MATTHIJSEN  KARSTEN SUHRE  PETER BECHTOLD  ROBERT ROSSET 《Tellus. Series B, Chemical and physical meteorology》1997,49(4):343-356

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Weichselian in Sweden before 15,000 B.P.     

JAN LUNDØLVIST 《Boreas: An International Journal of Quaternary Research》1981,10(4):395-402

A brief summary is given of the present state of knowledge about the Weichselian glaciations and interstadials in sweden. The following stages are discussed: (1) The first Weichselian glaciation (W1). This glaciation has not been identified. Probably only northern Sweden was ice-covered. (2) The Jämtland Interstadial , dated at > 50,000 B.P. and correlated with the Finnish Peräpohjola and the Danish Brörup Interstadials. - (3) The second Weichselian glaciation (W II). There are several uncertainteis concerning this glaciation. Sweden was probably ice-covered down to the latitude of Stockholm. – (4) A Middle Weichsedlian interstadial , or complex of interstadials. Some radiocarbon dates indicate, although very uncertain, that most of Sweden may have been free of ice some time rather well known. – Some main problems which have to be investigated are also identified.  相似文献