Field and geochemical characteristics of the Mesoarchean ( 3075 Ma) Ivisaartoq greenstone belt, southern West Greenland: Evidence for seafloor hydrothermal alteration in supra-subduction oceanic crust

The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075 Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside) + quartz + plagioclase ± hornblende ± scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside + garnet + amphibole + plagioclase + quartz ± vesuvianite ± scapolite ± epidote ± titanite ± calcite ± scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial ε_Nd values of + 1.3 to + 5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents Mesoarchean supra-subduction zone oceanic crust.     

Colluvial deposits in northwest Deccan,India: their significance in the interpretation of Late Quaternary history

The Deccan Trap region exhibits an erosional landscape over a relatively ancient and stable Deccan shield. The Quaternary history of the area has been reconstructed on the basis of evidence from alluvial deposits occurring along the major rivers. However, recent investigations have revealed that evidence for geo-environmental change during the Quaternary Period is also contained in the colluvial deposits that occur in the foothill zones. The colluvial deposits, ranging in thickness from 1 to 10 m, invariably occupy gently inclined pediment slopes. The sediments are presently deeply dissected by gullies, and the process of colluviation has almost ceased. These deposits are best preserved in the semi-arid parts of the region. Detailed textural, geochemical and stratigraphical studies at four different sites reveal similar input processes, the slight variations being attributed to local environmental factors. Scanning electron microscopy studies of some grains indicate marginal contribution of aeolian processes at the time of deposition. Mesolithic artefacts and a few U/Th dates indicate that the colluviation took place during the Late Quaternary. The properties of the deposits suggest relatively high energy conditions as well as a remarkable variability in the intensity of hillslope processes. These properties are indicative of semi-arid conditions during which the regolith was stripped from devegetated hillslopes and was deposited on the pediments. A variety of evidence indicates that the period of colluviation coincided with arid conditions during the Last Glacial Maximum. The geomorphological and archaeological evidence also indicates that incision by gully systems was initiated during the early Holocene humid phase. The environmental conditions deduced for the study area are similar to those reported for other parts of the intertropical zone. © 1997 John Wiley & Sons, Ltd.     

The Early Palaeozoic Break-up of Northern Gondwana: Sedimentology, Physical Volcanology and Geochemistry of a Submarine Volcanic Complex in the Bavarian Facies Association, Saxothuringian Basin, Germany

Ordovician volcano-sedimentary successions of the Bavarian facies association in the Saxothuringian basin record the continental rift phase of the separation of the Saxothuringian Terrane from Gondwana. An 80 m succession from the Vogtendorf beds and Randschiefer Series (Arenig-Middle Ordovician), exposed along the northern margin of the Münchberg Gneiss Massif in northeast Bavaria, were subjected to a study of their sedimentology, physical volcanology and geochemistry. The Randschiefer series previously has been interpreted as lavas, tuffs, sandstones and turbidites, but the studied Ordovician units include four main lithological associations: mature sandstones and slates, pillowed alkali-basalts and derivative mass flow deposits, trachyandesitic lavas and submarine pyroclastic flow deposits interbedded with turbidites. Eight lithofacies have been distinguished based on relict sedimentary structures and textures, which indicate deposition on a continental shelf below wave base. The explosive phase that generated the pyroclastic succession was associated with the intrusion of dykes and sills, and was succeeded by the eruption of pillowed basalts. Debris flow deposits overlie the basalts. Ordovician volcanism in this region, therefore, alternated between effusive and explosive phases of submarine intermediate to mafic volcanism.

Based on geochemical data, the volcanic and pyroclastic rocks are classified as basalts and trachyandesites. According to their geochemical characteristics, especially to their variable concentrations of incompatible elements such as the High Field Strength Elements (HFSE), they can be divided into three groups. Group I, which is formed by massive lavas at the base of the succession, has extraordinarily high contents of HFSE. The magmas of this group were probably derived from a mantle source in the garnet stability field by low (ca. 1%) degrees of partial melting and subsequent fractionation. Group II, which comprises the pillow lavas at the top of the sequence, displays moderate enrichment of HFSE. This can be explained by a slightly higher degree of melting (ca. 1.6%) for the primary magma. Group I and II melts fractionated from their parental magmas in different magma chambers. The eruption centres of Groups I and II, therefore, cannot be the same, and the volcanic rocks must have originated from different vents. The sills and pyroclastic flow deposits of Group III stem at least partly from the same source as Group I. Rocks of Group I most likely mixed together with Group II components during the formation of the Group III flows, which became hybridised during eruption, transportation and emplacement.

The sedimentological and geochemical data best support a rift as the tectonic setting of this volcanism, analogous to modern continental rift zones. Hence, the rift-associated volcanic activity preserved in the Vogtendorf beds and Randschiefer Series represents an early Ordovician stage of rift volcanism when the separation of the Saxothuringian Terrane from Gondwana had just commenced.     

Oiled birds' bibliography

Mussels collected from hydrocarbon polluted areas of the Lagoon of Venice were transferred to the relatively unpolluted port entrance of Malamocco. Hydrocarbon elimination was quite rapid in the first 10–15 days but then slow and incomplete: 12% of the initial content persisted in the tissues for 8 weeks. Elimination of hydrocarbons was almost independent of water temperature between 7.5–26.0°C but virtually ceased in one lot when the temperature abruptly dropped from about 11°C to 4.5–6.0°C. The biological half-life of hydrocarbons in the mussels was calculated to be a little over $\text{3}\text{1}\text{2}$ days, but this relates only to the initial rapid phase of elimination.     

Turam response of a permeable inhomogeneous cylindrical conductor surrounded by a thin shell

Summary Electromagnetic response characteristics of an inhomogeneous permeable cylinder surrounded by a thin insulated concentric dissipative shell is obtained in presence of a line-source of current. The 3-layer boundary value problem is solved by assuming the thickness of the shell to be much smaller than the skin-depth in the shell. Numerically computed results are presented in the form of argand diagrams to examine the influence of the shell parameters (conductivity and its proximity with the core) and magnetic contrast. The multi-frequency response of the system is also given which resolves the two conducting layers and analyzes the frequency dependence of screening behavior of the shell.N.G.R.I. Contribution No. 282.     

INDUCTION ANOMALY DUE TO AN ELONGATED COVERED ORE-ZONE EXCITED BY A LONG CURRENT CARRYING CABLE*

A detailed analysis of the electromagnetic anomaly due to an elongated mineralized zone of low resistivity in presence of a fixed-transmitter (a long cable carrying an, alternating current Ie^Iωt) is presented where (i) the host rock is not highly resistive and might contain some disseminated mineralization, (ii) the ore-body has an inhomogeneous conductivity, (iii) there is a contrast in magnetic properties of the ore-body and the host rocks, and (iv) source is close to the conducting system i.e., anomalous zone is lying shallow. The numerical computation of the generalized analytical expressions have been made. The selective screening behaviour of the cover is delineated and suitable frequency ranges for maximum detectability of a covered conductor have been obtained. A paradoxical decrease of the in-phase component of the response function with the increase of the core conductivity has been found for large conductivities of the cover. For moderate values of response parameter the inhomogeneity in the conductivity is found to significantly affect the induced field. The present analysis of the various features of the secondary field will aid to the existing interpretation sensitivity of the induction prospecting data for porphyry conducting ore deposits with zonal wall-rock alteration and sulphide distributions.     

Schwerestörungen im östlichen Mittelmeer nach Messungen mit einem Askania-Seegravimeter

Zusammenfasung Das Gebiet der großen Schwereanomalien um Kreta wurde Ende 1961 durch kontinuierlich vermessene Gravimeterprofile genauer untersucht. Zusammen mit den Ergebnissen früherer Pendelmessungen im U-Boot konnte ein detaillierter Isanomalenplan der Freiluftschwere des gesamten östlichen Mittelmeeres (Tafel 3) gewonnen werden. Daraus wurden die Bougueranomalien und die isostatischen Anomalien berechnet und in farbigen Kartendarstellungen (Tafel 4 und 5) wiedergegeben.Der Vergleich zwischen zwei gleichzeitig eingesetzten Askania-Seegravimetern (Abb. 3) ergibt eine Instrumentengenauigkeit von etwa 2 mgal für die in Fahrt gemessenen Schwerewerte. Hierzu kommen mittlere Fehler von etwa 5–6 mgal aus Unsicherheiten in der Positionsbestimmung (Eötvöseffekt), der Geländekorrektion und der isostatischen Kompensation. Somit sind die Freiluftanomalien auf etwa ±6 mgal, die Bougueranomalien auf etwa ±8 mgal und die isostatischen Anomalien auf etwa ±10, mgal genau anzugeben. Bei der Größe der Anomalien, die positive und negative Werte bis zu 200 mgal annehmen können ist diese Genauigkeit durchaus zufriedenstellend.Die gravimetrische Vermessung zeigt, daß das östliche Mittelmeer zum großen Teil den Gleichgewichtszustand noch nicht erreicht hat. Die Ausgleichsbewegungen sind in den einzelnen Teilgebieten verschieden weit fortgeschritten, am wenigsten in der Umgebung von Kreta.Die heutige Morphologie dieses Gebietes hat sich wahrscheinlich durch Aufsteigen des Meeresbodens im Ägäisraum und Absinken außerhalb des Inselbogens Kythera-Kreta-Rhodos entwickelt. Wenn man annimmt, daß vor Einsetzen dieser Bewegung Isostasie herrschte, müßte das Meer im Mittel etwa 1500 m tief gewesen sein.Aus der Kombination der Schwereanomalien mit der heutigen Morphologie in der dem Inselbogen vorgelagerten Ringzone (blau II auf Tafel 6) ergibt sich eine mittlere Gesteinsdichte von etwa 3,0 g/cm³. Zwischen diesem Gebiet und dem in fast vollkommenem Gleichgewichtszustand befindlichen südöstlichen Levantemeer (grün IV, Tafel 6) liegt eine Übergangszone (rot III, Tafel 6), die sich — ebenso wie die Ägäis — durch einen nicht ausgeglichenen Dichteüberschuß von etwa 2,1 g/cm³ auszeichnet.

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Gravity anomalies in the Eastern Mediterranean as measured with the Graf-Askania sea gravimeter

Summary At the end of 1961, the area round the isle of Crete distinguished by large gravity anomalies was subjected to thorough investigations and for this purpose continuous gravity profiles were run. The material of the above research and the results of former submarine pendulum measurements were included in a detailed isogam map of free-air anomalies of the whole Eastern Mediterranean (plate 3). From this map the Bouguer anomalies and the isostatic anomalies were derived and represented on coloured maps (plates 4 and 5).A comparison of two Askania sea gravimeters (fig. 3) operated simultaneously, demonstrates an instrumental accuracy of about 2 mgals for the gravity values measured from the ship underway. In addition, there resulted mean errors of about 5–6 mgals from uncertainties in the determination of the position (Eötvös effect), in the terrain correction, and in the isostatic compensation. The freeair anomalies are thus to be stated with an accuracy of about ±6 mgals, the Bouguer anomalies with an accuracy of about ±8 mgals and the isostatic anomalies with about ±10 mgals. Considering the fact, that the anomalies can assume positive and negative values of up to 200 mgals, this accuracy is completely sufficient.The measurement of gravity shows that the Eastern Mediterranean has, to a great extent, not yet reached the state, of equilibrium. The movements of compensation have developed differently in the individual parts of the area; the least advance is found in the area of the isle of Crete.The present morphology of this area has probably developed from an uplifting of the sea bottom of the Aegean Sea and a depression outside the island arc Kithira-Crete-Rhodes. Taking into account that before the beginning of this movement isostasy prevalied, the medium depth of the water must have been about 1500 metres.The combination of gravity anomalies with the present morphology of the ring zone extending in front of the three islands of kithira, Crete and Rhodes (bule II on plate 6) results in a mean density of the substratum of about 3,0 g/cm³. The extreme south-east of the Mediterranean, however, is in almost complete equilibrium (green IV, on plate 6). Between these two zones (blue II and green IV on plate 6) lies a transition zone (red III on plate 6) showing — like the Aegean Sea — an unbalanced density contrast of about 2,1 g/cm³.

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Anomalies de presanteur en Méditerranée orientale, suivantles enregistrements avec le gravimètre de mer Askania

Résumé Vers la fin de 1961, les parages autour de la Candie, marqués de grandes anomalies de pesanteur, ont été l'objet des enregistrements précis et continus de l'intensité de gravité, exécutés à bord d'un navire en route. Les résultats de ces recherches ainsi que ceux des mesures pendulaires antérieures faites à bord d'un sous-marin, pouvaient être réunis sur une carte montrant en détail les courbes iso-anomales des anomalies totales mesurées de toute la Méditerranée orientale (planche 3). On se servait de cette carte pour en déduire les anomalies d'après Bouguer et les anomalies isostatiques pour les représenter sur des cartes multicolores (planches 4 et 5).La comparaison de deux gravimètres de mer Askania (fig. 3), actionnés simultanément, faisait la preuve d'une précision d'enregistrement de 2 mglas environ pour les valeurs gravimétriques mesurées à bord d'une navire en route. De plus, il faut tenir compte des erreurs moyennes d'environ 5–6 mgals dues aux déterminations un peu insécures de la position (effet d'Eötvös) et aux insécurités de la correction du terrain et de la compensation isostatique. On doit, donc, indiquer les anomalies totales mesurées à ±6 mgals environ, près, les anomalies de Bouguer à ±8 mgals environ près et les anomalies isostatiques à ±10mgals environ près. Tenant compte du fait que les anoamlies peuvent atteindre de valeurs positives ou négatives jusqu' à 200 mgals, cette précision est bien satisfaisante.Les enregistrements gravimétriques montrent que la Méditerranée orientale est encore loin d'avoir atteint son état d'équilibre. Les mouvements de compensation ont fait des progrès inégaux dans les diverses parties de la Méditerranée orientale; le moindre progrès a été constaté aux alentours de la Candie.La morphologie actuelle de cette région paraît être le résultat d'un soulevèment du fond de la mer Egée et d'un affaisement du fond au dehors, du cordon des îles de Cythérée, de Candie et de Rhodes. Lorsqu'on suppose que le commencement de ces mouvements était précédé d'un état d'isostasie, la profondeur moyenne de la mer aurait été de 1500 mètres environ.De l'association des anomalies de pesanteur avec la morphologie actuelle de la région circulaire, qui s'étend devant, le cordon des îles de Cythérée, de Candie et de Rhodes (no. II, bleu, planche 6) résulte une densité moyenne du substratum d'environ 3,0 g/cm³. La partie de l'extrème sud-est de la Mediterranée (no. IV, vert, planche 6) se trouve, cependant, dans un état d'istostasie presque parfait. Entre ces deux zones (nos. II, bleu, et IV, vert, planche 6) s'étend une zone de transition (no. III, rouge, planche 6) qui est, comme la mer Egée, caractérisée par un excès non-compensé de densité de 2,1 g/cm³ environ.

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Inhalt eines Vortrages, gehalten auf der 25. Jahrestagung der Deutschen Geophysikalischen Gesellschaft im April 1963 in Bochum.     

Spectral observations of the symbiotic Mira variable V407 Cyg in 1993–2002

We present high-and low-resolution spectral observations of the symbiotic Mira variable V407 Cyg obtained in 1993–2002. Both emission and absorption spectra of the star are analyzed in detail. We present the basic spectral parameters for three states of the hot component of V407 Cyg—its passive and quiescent states and an outburst in 1998—and study the evolution of the emission-line profiles during transitions from one state to another.