Petrogenesis of a Stromatic Migmatite (Nelaug, Southern Norway)     

GUPTA  L. N.; JOHANNES  W. 《Journal of Petrology》1982,23(4):548-567

The stromatic migmatites of Nelaug (Tvedestrand area, SouthernNorway) are investigated in detail. They show well developedlayers of leucosomes, mesosomes and melanosomes. It is establishedthat the mesosomes and leucosomes of these migmatites are differentfrom each other texturally, mineralogically, and chemically.Also combinations of leucosome plus adjacent melanosome portionsare chemically different from those of the mesosomes. Theseobservations do not agree with the findings of Mehnert (1971)and do not fit into his genetic model. The mesosome layers and the leucosome + melanosome combinationsare taken to represent the chemical compositions of the countryrock, a metagraywacke with relicts of primary rhythmic layering(Touret, 1965). The mineralogical composition of the layersvaries from granitic to tonalitic. Relict textures indicatethat the leucosome portions were initially occupied by layersof granitic composition relatively rich in K-feldspar, whereasthe mesosomes are the representatives of those metagraywackelayers which were relatively rich in plagioclase. An almostisochemical transformation of a paragneiss into the investigatedstromatic migmatite is established. Melting experiments performed at PH₂O= 5 Kb yielded solidustemperatures of 640±7 °C for all layers. The Composition of plagioclases present in the different layersis explained by isochemical partial melting and in situ crystallization.The chemical, mineralogical, and textural findings support themodel of almost isochemical transformation already establishedfor the Arvika migmatites (Johannes & Gupta, 1982).  相似文献   

Geochemistry and SHRIMP U‐Pb Zircon Dating of Mafic Rocks North of Zunhua City,Eastern Hebei,North China Craton: Paleoproterozoic Gabbro rather than Neoarchean Ophiolite     

XIE Hangqiang  DONG Chunyan  Alfred KR?NER  LIU Shoujie  BAI Wenqian  LIU Dunyi  WAN Yusheng 《《地质学报》英文版》2018,92(3):1024-1040

Abundant mafic-ultramafic blocks and dikes occur in the area north of Zunhua City, eastern Hebei Province, and were previously suggested to be part of a late Archean ophiolitic assemblage. We employed SHRIMP zircon dating and a geochemical study on these mafic and surrounding rocks to test the ophiolite hypothesis. The SHRIMP data suggest that three metagabbro samples were metamorphosed at ～1.8 Ga. Numerous ～2.5 Ga zircons display strong oscillatory zoning, characteristic of zircons from granitoid rocks but not from gabbro, so we suggest that these are xenocrystic grains. The age of these xenocrystic zircons and their metamorpbic rims suggests that these mafic blocks formed in Paleoproterozoic. The surrounding gneiss of intermediate composition also contains 2.5 Ga zircons with oscillatory zoning and 1.8 Ga metamorphic rims. Fractionated REE patterns and Nb, Ta, Zr, Hf negative anomalies to variable extent were observed in the mafic blocks and surrounding rocks, also supporting a significant difference in the chemistry of ophiolitic rocks. Our data suggest that many mafic blocks in northern Zunhua are not part of a late Archean ophiolite complex but part of a tectonically dismembered Paleoproterozoic intrusive gabbro complex. This study shows that late Paleoproterozoic metamorphism occurred in the western part of eastern Hebei Province.  相似文献   

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)   总被引：1，自引：0，他引：1  

JOHANNES SCHOENHERR  LARS REUNING†  PETER A. KUKLA†  RALF LITTKE‡  JANOS L. URAI  MICHAEL SIEMANN§  ZUWENA RAWAHI¶ 《Sedimentology》2009,56(2):567-589

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.  相似文献   

Age and Origin of Paleogene Granitoids from Western Yunnan Province, China: Geochemistry, SHRIMP Zircon Ages, and Hf-in-Zircon Isotopic Compositions     

SHI Yuruo  WU Zhonghai  Alfred KR?NER  FAN Taoyuan  YANG Zhenyu 《《地质学报》英文版》2015,89(5):1601-1615

We report geochemical data, SHRIMP zircon ages and Hf-in-zircon isotopic compositions for Cenozoic granitoids from major fault systems in the Tethyan belt in western Yunnan Province, southwestern China.Four magmatic pulses occurred in the Paleogene, namely at ca.57 Ma, ca.50 Ma, 45–40 Ma, and 38–34 Ma.Early magmatism of this episode(57–50 Ma) produced S-type granites whose zircons yielded εHf(t) values of-5.0 to-0.3.In contrast, late magmatism of this episode reflects heterogeneous sources.Zircons from a granite porphyry along the Ailaoshan-Red River fault system have slightly positive εHf(t) values suggesting derivation from relatively young crust and/or a juvenile source.However, zircons from a granite along the Gaoligong fault system have strongly negative εHf(t) values and suggest derivation from a Paleoproterozoic crustal source.The composition of the granitoids varies with age(from ca.57 Ma to ca.34 Ma) from peraluminous to metaluminous and also suggests a change from syn-collisional to late-orogenic tectonic setting.A new tectonic model, impacting lithospheric wedge(ILW) is shown for the origin of Paleogene granitoids in this paper.  相似文献   

Early Palaeozoic Deep Subduction of Continental Crust in the Kyrgyz North Tianshan: Evidence from Field Relationships and Geochronology     

KR?NER A  ROJAS-AGRAMONTE Y  HERWARTZ D  CORSINI M  ALEXEIEV D.V. 《《地质学报》英文版》2015,89(Z2):36-36

<正>High-and ultrahigh-pressure(UHP)eclogite-bearing metamorphic assemblages in the North Tianshan of Kyrgyzstan contain eclogite-bearing garnet amphibolites interlayered with phengite-beearing granitoid gneisses.The mafic rocks were previously interpreted as remnants of a Precambrianm ophiolite,but field relationships clearly  相似文献   

INQUA report on glacial deposits     

JOHANNES KRÜGER 《Boreas: An International Journal of Quaternary Research》1989,18(3):236-236

  相似文献   

Moraine ridges formed from subglacial frozen-on sediment slabs and their differentiation from push moraines     

JOHANNES KRÜGER 《Boreas: An International Journal of Quaternary Research》1996,25(1):57-64

Throughout the 1980s the annual cycle of ice-front activity along the stationary north margin of the ice-cap Myrdalsjökull, southern Iceland, produced a complex ridge, 4 m high, composed of imbricately stacked slabs of frozen, clast-paved lodgement till dipping up-glacier. Further observations in 1994 revealed that glaciofluvial processes and associated deposits may be involved in the final stage of ridge production depending on local climate and meltwater drainage pattern. It is concluded that at the margin of Myrdalsjökull the progressive stacking of subglacial frozen-on sediment slabs to form a moraine ridge is a fundamentally similar mechanism to that involved in the incremental double-layer model reported from Styggesdalsbreen, southern Norway. This study has also identified internal characteristics which are of potential use for distinguishing between moraine ridges formed by this mechanism and push moraines formed by proglacial thrusting.  相似文献   

EN LANDBRUGSGEOGRAFISK ANALYSE AF GADBJERG OG GIVSKUD SOGNE     

KR. M. JENSEN M.Sc.  RUTH HELKIÆR JENSEN M.Sc. 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):34-51

Jensen, Kr. M. and Jensen R. Helkiær: En landbrugsgeografisk analyse af Gadbjerg og Givskud sogne. Geografisk Tidsskrift 76: 34—51. København juni 1. 1977.

The agricultural development in two parishes in a former heath region in Central Jylland has been investigated. The expansion of farmland up to the turn of the century was especially in Givskud replaced by a regression period with afforestation of marginal areas. The farmers are now increasing the size of their holdings and the specialization on few crops involves a more uniform land-use.  相似文献   

Rates of Holocene isostatic uplift and relative sea-level lowering of the Baltic in SW Finland based on studies of isolation contacts     

MATTI ERONEN  GUNNAR GLUCKERT  LASSI HATAKKA  ORSON VAN DE PLASSCHE  JOHANNES VAN DER PLICHT  PASI RANTALA 《Boreas: An International Journal of Quaternary Research》2001,30(1):17-30

Southwestern Finland was covered by the Weichselian ice sheet and experienced rapid glacio-isostatic rebound after early Holocene deglaciation. The present mean overall apparent uplift rate is of the order of 4-5 mm/yr, but immediately after deglaciation the rate of crustal rebound was several times higher. Concurrently with land uplift, relative sea level in the Baltic basin during the past more than 8000 years was also strongly affected by the eustatic changes in sea level. There is ample evidence from earlier studies that during the early Litorina Sea stage on the southeastern coast of Finland around 7000 yr BP (7800 cal. yr BP), the rise in sea level exceeded the rate of land uplift, resulting in a short-lived transgression. Because of a higher rate of uplift, the transgression was even more short-lived or of negligible magnitude in the southwestern part of coastal Finland, but even in this latter case a slowing down in the rate of regression can still be detected. We used evidence from isolation basins to obtain a set of 71 ¹⁴C dates, and over 30 new sea-level index points. The age-elevation data, obtained from lakes in two different areas and located between c. 64 m and 1.5 m above present sea level, display a high degree of internal consistency. This suggests that the dates are reliable, even though most of them were based on bulk sediment samples. The two relative sea-level curves confirm the established model of relatively gradually decreasing rates of relative sea-level lowering since c. 6100 yr BP (7000 cal. yr BP) and clearly indicate that the more northerly of the two study areas experienced the higher rate of glacio-isostatic recovery. In the southerly study area, changes in diatom assemblages and lithostratigraphy suggest that during the early Litorina Sea stage (8300-7600 cal. yr BP) eustatic sea-level rise exceeded land uplift for hundreds of years. Evidence for this transgression was discovered in a lake with a basin threshold at an elevation of 41 m above sea level, which is markedly higher than any previously known site with evidence for the Litorina transgression in Finland. We also discuss evidence for subsequent short-term fluctuations superimposed on the main trends of relative sea-level changes.  相似文献   

What controls partial melting in migmatites?   总被引：4，自引：0，他引：4  

W. JOHANNES 《Journal of Metamorphic Geology》1988,6(4):451-465

Abstract The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.
The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K-feldspar, if K-feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.  相似文献