Petrology of the Skien basaltic rocks,southwestern Oslo Region,Norway

The pile of Permian continental basaltic rocks near Skien has a minimum thickness of 1 500 m, dipping 40^g towards NE, consisting of several thin lava flows. Most flows are porphyric, though both tuffs and aphyric flows occur to a lesser extent. Amygdules are abundant and a characteristic feature. The flows in the lower 2/3 of the basalt pile are mainly classified as melanite-ankaramite; some in the middle part as melanite-nephelinite; and the upper 1/3 of the flows as basanite. The basaltic rocks evolved from an assumed olivine-nephelinitic magma at great depths principally by clinopyroxene crystal fractionation accompanied by later olivine and maybe melilite reactions at moderate pressure in minor shallower magma chambers.     

Caledonide influences on the Old Red Sandstone fluvial systems of the Oslo Region,Norway

Red or buff‐coloured sandstones and siltstones of fluvial origin comprise approximately 80% of the Ringerike Group, a late Silurian Old Red Sandstone (ORS) sequence that crops out extensively in the Oslo Region of southern Norway. These fluvial sediments are lithostratigraphically ascribed to two laterally equivalent formations—the Stubdal Formation (to the north of Oslo) and the Skien Formation (to the south of Oslo). The fluvial strata of each of the two formations have a distinct style of sandbody geometry, facies, provenance, and palaeocurrent direction. Within the Stubdal Formation, shallow channelized sandbodies, low‐ to upper‐flow regime sedimentary structures, a Caledonide provenance and a palaeoflow toward the southeast are evident. Within the Skien Formation, sandbody geometry is entirely sheet form, with upper‐flow regime sedimentary structures, a provenance from Precambrian rocks to the northern and local parts of the Oslo Region and a palaeoflow toward the east. No stratal contact can be seen between the two fluvial formations, due to a 15 km break in exposure between the southernmost Stubdal Formation and the northernmost Skien Formation. Relationships with adjacent formations indicate that they are diachronous, lateral equivalents. Given the abrupt change in sedimentary style between the two formations, it is proposed that a barrier had developed within the foreland basin, diverting the ORS fluvial systems in southern Norway, from a southward (north of Oslo), to an eastward direction (south of Oslo). This diversion had implications for depositional gradient, fluvial regime and provenance, resulting in the differences visible in the deposits of those rivers. The barrier invoked is arguably a Caledonide blind thrust fault that developed a topographic high, running east–west through the vicinity of Oslo, during the late Silurian. Copyright © 2004 John Wiley & Sons, Ltd.     

The genesis of nodular limestones in the Ordovician and Silurian of the Oslo Region (Norway)

Nodular limestones have been studied from the Lower Chasmops Shale (Middle Ordovician) and the Rytteráker Formation (Lower Silurian). Observations on nodule-host-rock relations and variations of ferroan/non-ferroan calcite cements help explain the role of precipitation, dissolution and redistribution of carbonate. Distribution and frequency of nodules depends on environmental parameters such as carbonate/clay ratio, grain size distribution and bioturbation, though final shapes are the result of pressure-dissolution and cleavage.     

Mildly peraluminous high-silica granites in a continental rift: the Drammen and Finnemarka batholiths,Oslo Rift,Norway

The peraluminous Drammen batholith (650 km²) is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex (125 km²). The emplacement of the Drammen granite, and probably most of the other biotite granite complexes, predate the alkaline syenites and granites. The eight separate petrographic types of the Drammen batholith range in SiO₂ from 70 to 79 wt.% and have experienced variable amounts of fractionation of feldspars, biotite, zircon, apatite, titanite and Fe–Ti-oxides. The initial Sr, Nd and Pb isotopic ratios and a decoupling between the variations in the SiO₂ content and the aluminum saturation index [ASI=Al₂O₃/(CaO+Na₂O +K₂O)] show that the various intrusive phases are not strictly comagmatic. The _Nd values of the southern part of Finnemarka (+3.5 to +4) and the northern part of the Drammen granite (+1 to +1.5) are high and indicate insignificant (for Finnemarka) to minor Precambrian crustal or enriched mantle contributions. The very low _Sr values of all of these samples (–1 to –12, outside the main Oslo Rift magmatic array), point to a time integrated Rb-depleted crustal contaminant or an EM1 mantle component. The earliest extruded alkali basalts along the southwestern margin of the Oslo Rift are the only other samples within this low _Sr area, but their isotopic signature may also be linked to a mantle enrichment event (involving an EM1 component), e.g. associated with the Fen carbonatite magmatism 540 Ma ago. For a given ²⁰⁶Pb/²⁰⁴Pb, the ²⁰⁸Pb/²⁰⁴Pb ratios of the Drammen and Finnemarka batholiths are distinctly lower than those of the Skien alkaline volcanics and all other magmatic Oslo Rift rocks. This may indicate that the lithosphere of the central part of the rift had a time integrated Th-depletion. The samples from the southern part of the Drammen batholith, characterized by the presence of abundant miarolitic cavities, have _Nd near 0 (–0.7 to +0.4) but strongly elevated _Sr of +35 to +67. The combined Pb isotopic ratios of all the samples analyzed indicate that the Precambrian crustal anatectic contribution is in the form of time integrated Th-and U-depleted lower crust, and the high +_Sr of the sourthern part of the Drammen granite results from shallow level wallrock assimilation or magma-fluid interactions. The remarkably low contribution of old crustal components to the Finnemarka and the northernmost Drammen batholiths may result from extensive late Precambrian intracustal differentiation in southwestern Scandinavia, leading to widespread upper crustal granites ( 900 Ma) and a correspondingly dense and refractory lower crust, in particular in a zone intersecting the central part of the rift. Liquidus phase relations and mass-balance constrainst permit derivation of the granites from mildly alkaline to tholeiitic melts by extensive crystal fractionation of clinopyroxene-and amphibole-rich assemblages. It is equally possible to form the granitic magmas by partial melting of Permian gabbros of similar composition. Either scenario is consistent with the isotopic constrainst and with the presence of dense cumulates and/or residues in the lower crust. The lack of igneous rocks of intermediate composition associated with the Drammen and Finnemarka batholiths point to an efficient upper crustal density filtering. Considerable amounts of heat would be accumulated in this region if differentiated, intermediate melts could not escape to shallower levels. Successive magma injections would therefore easily result in partial melting of already solidified mafic to intermediate melts and cumulates, and it is suggested that the peraluminous granites formed mainly by water-undersaturated anatexis of mafic material.     

Crustal contaminants in the Permian Oslo Rift, South Norway: constraints from Precambrian geochemistry

During the late Paleozoic Oslo rifting event, the SW part of the Baltic Shield was penetrated by mantle-derived magmas from a depleted lithospheric or sublithospheric source. Along the way to their final emplacement, these magmas may have interacted with a heterogeneous continental crust, consisting of a mosaic of continental terranes, each with its unique composition and internal crustal history. Information on radiogenic isotope ratios and trace element distributions in the Precambrian terranes surrounding the rift can be used to define characteristic crustal components. These components may be used as endmembers in petrogenetic modelling of the Oslo Rift magmatic system. Based on available data, six endmember components can be identified, and (semi) quantitatively characterized in terms of Sr, Nd and Pb isotopes and selected trace elements. Data on the distribution of rock-types along the rift flanks allow estimates to be made of the relative importance of the components in different parts of the rift. Combining these data with petrological information may allow a realistic understanding of crust–magma interaction in the Oslo Rift magmatic system.     

Geochemical and mineralogical influence of Ordovician Island Arcs on epicontinental clastic sedimentation. A study of Lower Palaeozoic sedimentation in the Oslo Region, Norway

The mineralogy and geochemistry of clastic sediments in the 1000 m thick Lower Palaeozoic marine sequence of the Oslo Region is discussed on the basis of 200 chemical (XRF) and mineralogical (X-ray diffractometer) analyses. The composition of these sediments is considered to largely be a function of the relative supply from land of the continental shield (Baltic Shield) and from island are systems to the NW in the Trondheim Region. The relative abundance of chlorite and illite is discussed in light of recent knowledge about their distribution in modern oceans. Middle Cambrian to Lowermost Ordovician black shales contain only illite as the dominant clay mineral and have a high potassium content. These sediments are thought to be derived from land on the continental shield, which during this period probably had a low relief and a warm climate where chlorite is not stable. Chlorite is introduced first in the Arenigan (L. Ordovician) and this is interpreted as evidence for transport of clastic chlorite from a developing island arc system in the Trondheim Region to the west. The highest chlorite/illite ratio in the Middle and Upper Ordovician sediments of the Oslo Region corresponds to a Taconic orogenic phase. The content of clastic chlorite in the Lower Palaeozoic of the Oslo Region is thus shown to be a sensitive indicator of palaeoenvironment and tectonic evolution of the Caledonian geosyncline. This conclusion is also supported by a systematic increase in Mg, Fe, Ni, and Cr from Lower to Middle and Upper Ordovician beds. High Cr values exceeding 0–1 % Cr in the Middle/Upper Ordovician shales are due to small grains of clastic chromite resembling those in the serpentinites of the Trondheim Region. This also suggests transport from exposed island arcs onto the epicontinental shelf.     

Olivine-rich units in the Fongen–Hyllingen Intrusion, Norway: implications for magma chamber processes

The Fongen–Hyllingen Intrusion (FHI) is considered to have crystallised from stratified magma residing in a bowl-shaped magma chamber. Seven olivine-rich units, representing the most primitive cumulates in the central part of the intrusion, are associated with compositional reversals and are interpreted as having formed at the lowest part of the magma chamber floor. Based on phase-relationships, the crystallisation order is explained in terms of magma mixing and fractional crystallisation. Repeated influxes of small volumes of dense, primitive magma at the base of the chamber had a major impact on the crystallising assemblage on the local floor and a decreasing effect towards the flanks of the chamber. This was due to the small volume of replenishing magma, the geometry of the chamber and the consequent restriction of magma mixing to the deepest part of the chamber where the new magma was emplaced. It is estimated that the chamber floor sloped as little as 1–2°, but this was sufficient to give widely different cumulate sequences near the bottom of the chamber and on the flanks.     

Distribution of rare earth elements in crystalline bedrock groundwaters: Oslo and Bergen regions,Norway

One hundred and fifty Norwegian bedrock groundwater samples, from Bergen and from Vestfold (Oslofjord), have been analysed by ICP-MS techniques at two laboratories for a large suite of trace elements including rare earth elements (REEs) and Y. The bedrock lithologies include granites (dominated by the Permian Drammen Granite) and Permian latites/rhomb porphyries from Oslofjord, and Caledonian/Precambrian granitic and gneissic lithologies in the Bergen area. The REEs show good correlation with each other, with the exception of Eu. REEs generally show a weak negative correlation with pH. REE concentrations are highest in waters in acidic lithologies and generally decrease with increasing atomic weight. Yttrium, La, Ce and Nd are the most abundant REEs in the waters, with median concentrations exceeding 0.1 μg/l. On crustal (PAAS)-normalised plots, distinct geochemical signatures are observed for the different lithologies. Most groundwaters exhibit negative Eu anomalies on such plots, except for latitic waters from the Oslo area which show a positive Eu anomaly. Aquifer host-rock-normalised plots for groundwaters from Vestfold indicate minor enrichment in heavier REEs and depletion in Ce during water–rock interaction.     

The Bjerkreim-Sokndal Layered Intrusion, Rogaland, SW Norway: Evidence from marginal rocks for a jotunite parent magma

The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO₂-rich for hemo-ilmenite to crystallise early, and low in CaO and CaO/Al₂O₃compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An_45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeO^total, high TiO₂, low MgO/MgO + FeO, moderate Al₂O₃ and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance.     

Depositional environments in the Telychian Stage (Silurian) of the central Oslo region,Norway

The Telychian succession in the central Oslo region was previously interpreted as deposited in a relatively deep, calm environment showing a continuous transgressive development. A new analysis of fossil assemblages shows that sediments in the central districts were deposited at varying water depths in an environment marked by intense infaunal activity in soft substrates. Two small-scale fluctuations in sea level are attributed to eustasy. The maximum deepening events occurred during the Monograptus turriculatus and Monoclimacis crenulata graptolite zones. Previous depositional models include development of a trough to the north of the Oslo region in early Telychian time, which formed due to isostatic loading caused by an advancing thrust front of the Caledonian Orogeny. Bathymetric analysis shows a deepening in the Ringerike district in the latest Aeronian, which here is interpreted as a distal effect of the loading to the north. The local deepening led to reversal of the epicontinental slope between the northern and western districts and the central districts and the development of an east–west elongate positive area further south. The thrust front acted as a source of siliciclastic material; and when the front halted in mid-Telychian time the trough was filled in gradually by a prograding coast or delta. The diachronous Vik Formation is viewed as a distal development of this progradation. When the east–west oriented positive area subsided the Vik Formation was deposited in Skien to the south.     

A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

The clinopyroxene–plagioclase–plagioclase dihedralangle, _cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in _cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle     

Intrusion and contamination of high-temperature dunitic magma: the Nordre Bumandsfjord pluton, Seiland, Arctic Norway

The Caledonian Nappe Complex of Arctic Norway provides rare insights into the interaction between mafic–ultramafic magmas and the deep continental crust. The Kalak Nappe Complex contains >25,000 km³ of mafic igneous rocks, mostly layered gabbros, making up the 570–560 Ma Seiland Igneous Complex. The complex has been intruded by a series of ultramafic magmatic rocks, including the Nordre Bumandsfjord pluton. Field relationships in this pluton show that extremely fluid, dry, relatively Fe-rich (Fo₈₁) dunite magmas intruded a pile of cumulate gabbros, with block stopping and intrusive brecciation. Diking on scales from mm to metres and extensive melting and assimilation of the gabbros attest to high temperatures, consistent with a 2-km-wide granulite-facies contact aureole. Major- and trace-element trends show that the dunites were progressively contaminated by a cpx-rich partial melt of the gabbros, producing a range of lithologies from dunite through lherzolites to wehrlite. Experimental studies of natural samples at 0.8–1 GPa define the dunite solidus at 1,650–1,700 °C. In the average peridotite, contamination has produced a crystallisation interval of ca 400 °C (1,600–1,200 °C); this would provide large amounts of heat for melting and metamorphism and would maintain the fluidity of the magmas to relatively low T, consistent with field relationships. Thermochemical and dynamic modelling demonstrates that the dunitic primary magmas may represent the last melting of a rapidly ascending diapir of previously depleted subducted oceanic lithosphere. The mafic rocks of the Seiland Complex may already have been extracted from this diapir, and the late dikes of the province may reflect melting of the asthenosphere as the diapir spread out beneath the lithosphere. Ultramafic magmas, abundant in the Archean, may still be more common than usually assumed. However, they would only penetrate to the shallow crust under unusually extensional conditions, where ascent could outpace assimilation.     

Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation

New eclogite localities and new ⁴⁰Ar/³⁹Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km²; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. ⁴⁰Ar/³⁹Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New ⁴⁰Ar/³⁹Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. ⁴⁰Ar/³⁹Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr^?1), perhaps because of continued tectonic unroofing.     

Stagnant lids and mantle overturns:Implications for Archaean tectonics,magmagenesis,crustal growth,mantle evolution,and the start of plate tectonics

The lower plate is the dominant agent in modern convergent margins characterized by active subduction,as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight.This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle.As geological and geochemical data seem inconsistent with the existence of modernstyle ridges and arcs in the Archaean,a periodically-destabilized stagnant-lid crust system is proposed instead.Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle,perturbing Earth's heat generation/loss balance,eventually triggering mantle overturns.Archaean basalts were derived from fertile mantle in overturn upwelling zones(OUZOs),which were larger and longer-lived than post-Archaean plumes.Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods,allowing basal crustal cannibalism,garnetiferous crustal restite delamination,and coupled development of continental crust and sub-continental lithospheric mantle.Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB(mid-ocean ridge basalt)mantle.Only after the start of true subduction did sequestration of subducted slabs at the coremantle boundary lead to the development of the depleted MORB mantle source.During Archaean mantle overturns,pre-existing continents located above OUZOs would be strongly reworked;whereas OUZOdistal continents would drift in response to mantle currents.The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion,imbrication,subcretion and anatexis of unsubductable oceanic lithosphere.As Earth cooled and the background oceanic lithosphere became denser and stiffer,there would be an increasing probability that oceanic crustal segments could founder in an organized way,producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga.Plate tectonics today is constituted of:(1)a continental drift system that started in the Early Archaean,driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons;(2)a subduction-driven system that started near the end of the Archaean.     

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·4–4km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an ‘onion-skin’ structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km³, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure     

鄂东南地区鄂城岩体的时代、成因及其对成矿作用的指示

鄂城岩体位于鄂东南地区的最北部,是鄂东南地区的六大岩体之一.在该岩体的南缘接触带上产出了长江中下游地区最大的矽卡岩型铁矿床——程潮铁矿床.众多研究表明,程潮铁矿化与鄂城杂岩体的岩浆演化密切相关,然而目前对于成矿作用究竟是与花岗质岩还是闪长质岩有关仍存在争议.通过对鄂城杂岩体开展系统的锆石U-Pb年代学、元素地球化学和Sr-Nd-Hf同位素研究,结果表明该岩体主要由花岗岩、石英二长岩、花岗斑岩以及小面积的闪长岩组成,最早侵位于140±1 Ma（中粒闪长岩）,之后依次侵位形成了细粒闪长岩（132±2 Ma）、花岗斑岩（130±2 Ma）、花岗岩（中细粒花岗岩129±2 Ma,中粒花岗岩129±1 Ma）和石英二长岩（129±1 Ma）.根据全岩地球化学特征,鄂城杂岩体的岩石组成大致可以分为两组:（1）花岗岩类,包括花岗岩、花岗斑岩和角闪石英二长岩,钾质,具有高SiO₂,低TiO₂、FeO_t、MnO、MgO含量等特征;（2）闪长岩类,包括中、细粒闪长岩,钠质,具有低SiO₂,高TiO₂、FeO_t、MnO、MgO含量等特征.这些岩石均富集轻稀土元素（LREE）和大离子亲石元素（LILE,如Rb、Th等）,亏损高场强元素（HFSE,如Nb、P、Ti）等,且花岗岩类具明显的负Eu异常,而闪长岩类则无此特征.在同位素组成方面,鄂城花岗岩类具有较负的全岩ε_Nd（t）值（-11.7~-10.1）和锆石ε_Hf（t）值（-22.91~-9.83）,闪长岩类则具有稍高的全岩ε_Nd（t）值（-7.6）和锆石ε_Hf（t）值（-12.04~-4.69）.元素和同位素地球化学特征共同表明,鄂城花岗岩类属于高分异Ⅰ型花岗岩,且主要来源于古元古代基底物质的部分熔融作用,源区可能有少量幔源物质的加入;闪长岩类主要来源于富集岩石圈地幔,且经历了一定的分离结晶作用.年代学结果显示,鄂城花岗岩类和细粒闪长岩的侵位时间均与程潮铁矿床的主成矿期吻合.结合野外接触关系以及前人的研究,程潮铁矿化可能与上述两类岩石均密切相关.从整个鄂东南地区的成矿作用来看,随着岩浆源区壳源物质贡献的增大以及岩浆分异程度的增加,岩浆作用与铁矿化的关系也更加密切.      

Petrology and Geochemistry of the Huangshan Granitic Intrusion in Anhui Province, Southeast China: Implications for Petrogenesis and Geodynamics

<正>The Huangshan granitic intrusion in Anhui province,SE China,is tectonically located at the southeastern boundary of the Yangtze Block.Based on the contact relation and the petrography,the Huangshan granitic intrusion can be divided into four stages,from early to late,medium-grained monzogranite,coarse-grained porphyric granite,fine-medium grained porphyric granite,and finecoarse grained granite.All rocks from the Huangshan granitic intrusion display similar petrological and geochemical characteristics,i.e.relatively high SiO_2(75%) and alkali(7.85%-8.59%),low CaO (1%),high Fe-number(FeO_T/(MgO+FeO_T) = 0.93-0.97) and A/CNK(atomic Al/(Ca+Na+K))=1.04- 1.19.They are also enriched in rare earth elements(REE,except for Eu,with a total REE contents ranging from 116 ppm to 421 ppm),high strength field elements such as Zr,Hf,Nb,but depleted in Ba,Sr and Ni.The 10 000×Ga/Al ratios are higher than 2.6,which are consistent with the A-type granitoids.Based on the classification diagrams proposed by Eby,the Huangshan granite can be classified into the A2 group,which is usually believed to be formed under an extensional tectonic setting.Their Nd isotopic compositions suggest that the primary magmas of the Huangshan granite are predominantly derived from the Proterozoic andesitic rocks in the region,and this conclusion is also supported by REE modeling.The systemic investigations on the geochemistry of the Huangshan granitic intrusion can provide significant implications for the understanding of the petrogenesis and the geodynamic regime of southeastern China during the Late-Mesozoic.     

Lead isotope distribution in podzolic soil profiles on different types of bedrock in a formerly glaciated terrain (Oslo,Norway)

Lead has been exploited by man over thousands of years for a variety of metallurgical, medicinal, and industrial purposes. The cumulative output of Pb from mining is estimated to be 260 million metric tonnes and 85% of this has occurred over the last two centuries. Global annual production of Pb from mining was about 3 million tonnes at the turn of the millenium. Terrestrial ecosystems all over Norway have been contaminated moderately to strongly by Pb and other trace elements from atmospheric deposition. With the aim of developing a method for mapping the accumulated content of anthropogenic Pb and how deep in the soil profile the atmospherically deposited Pb has penetrated, the concentration of Pb and the ²⁰⁶Pb/²⁰⁷Pb ratio has been studied in podzolic forest soils at four locations with different lithology, i.e. age and type of bedrock, in the Oslo area. The concentrations of Pb in the soil profiles are 6.6–38.1 mg/kg (median 10.3). The ²⁰⁶Pb/²⁰⁷Pb ratio ranges between 1.168 and 1.314 (median 1.267) over the entire profile. In the upper 5 cm the range is 1.168–1.191, similar to ratios determined in recent atmospheric deposition. Applying three different methods, the amount of anthropogenically deposited Pb is estimated at 1–6 t/km².