U-Pb and Rb-Sr dating of a polymetamorphic nappe terrain: The Caledonian Jotun nappe,southern Norway

The Caledonian Jotun nappe in the Tyin area of southern Norway has been investigated using U-Pb analysis of zircon and sphene and Rb-Sr measurements of minerals and whole rocks, with special reference to the reaction of the isotope systems to various kinds of metamorphic event. The nappe consists of Precambrian basement rocks and their cover, inversely thrust over the Baltic shield and its parautochthonous, presumably lower Paleozoic sediments during the Caledonian orogeny. While the nappe basement retained its pre-Caledonian structures, the nappe cover was penetratively deformed and metamorphosed to lower greenschist facies conditions.U-Pb analyses of zircon and sphene ofnappe basement rocks point to the crystallization of a syenitic to monzonitic magma at 1694± 20m.y., the intrusion of gabbros into the syenites and monzonites at 1252_?25⁺²⁸ m.y., and the metamorphism (upper greenschist-lower amphibolite facies) and deformation (gneissification and mylonitization) of the whole complex at 909± 16m.y. Although this latest event caused strong lead loss in all zircon populations, it had no influence on the Rb-Sr system on a whole rock scale. The Caledonian movements did not disturb the U-Pb system of zircon and sphene but strongly influenced the Rb-Sr system in certain minerals and zircon and/or its inclusions (K-feldspar and brown biotite partially, green biotite completely reset).In contrast to the nappe basement, zircons from thecover rocks show pronounced lead loss during the Caledonian metamorphism/deformation — U-Pb analysis give discordia lower intercept ages of 415± 21 m.y. and 395± 6m.y. Again, however, the Rb-Sr whole-rock system has not been reset in Caledonian time. Minerals from the same rocks which provided the zircons give Rb-Sr isochron ages of 390± 11m.y. and 386 ± 20 m.y.Comparison of U-Pb and Rb-Sr results from the nappe basement demonstrates that only the analysis of U-Pb systematics in zircon and especially sphene resolved the 900-m.y. event. Rb-Sr mineral data alone yield ambiguous results.The results from the nappe cover confirm that U-Pb analyses from penetratively deformed sediments are very useful in dating a metamorphic/tectonic event in lower greenschist facies conditions.From the measured cell dimensions of the zircon populations it is concluded that lead can be completely retained in zircon during recrystallization.     

Neotectonics of Hispaniola: plate motion, sedimentation, and seismicity at a restraining bend

A series of different plutons from the Lhasa-Xigaze segment of the Gangdese (Transhimalaya) belt has been studied by high-resolution UPb analyses of zircon (using zircon fractions of 5–100 grains each, selected upon specific grain characteristics). For two diorites, located east of Xigaze (Dazhuka), the zircons yield concordant ages of 93.4 ± 1.0 and 94.2 ± 1.0 m.y., respectively. Also concordant ages of 41.1 ± 0.4 and 41.7 ± 0.4 m.y. have been obtained for two granodiorites, collected southwest of Lhasa (Qushui). The precision on the ages of two granites from the Xigaze and Lhasa area, is limited by two factors: the presence of inherited radiogenic lead and the occurrence of subsequent lead loss. However, some concordant zircons, detected in both granites, define approximate ages of about 67 and 53 m.y., respectively. The inherited lead components show that melting of Precambrian material was involved in magma genesis.The UPb ages substantiate a magmatic activity lasting from mid-Cretceous (Cenomanian) to Eocene (Lutetian) time. Such a period of plutonism at the southern margin of Eurasia, as well as the occurrence of magma generation from continental crust, suggest that the Gangdese range results from the subduction of Tethys oceanic lithosphere (Indian plate) underneath Eurasia (Eurasian plate). If this model is true, the collision of India with Eurasia (along the Lhasa-Xigaze sector) postdates the emplacement of the 41 m.y. old Gangdese granodiorites, i.e. the collision occurred after Lutetian time.     

Etudes chimiques et isotopiques de l'uranium,du plomb et de plusieurs produits de fission dans un echantillon de minerai du reacteur naturel d'Oklo

Isotopic dilution technique and mass spectrometry are used in order to measure isotopic compositions and concentrations of some particular elements on the Oklo sample KN50-3548, which are interesting for describing the nuclear reaction. These analyses give coherent results leading to the determination of the following parameters: neutron fluence, spectrum index, total number of fissions relative to uranium, and percentage of²³⁸U and²³⁹Pu fissions. From the percentage of plutonium fissions, the duration of the nuclear reaction is estimated to be 800,000 years. The age of the nuclear reaction is measured from the lead analysis and from the number of uranium fissions; these two determinations give respectivelyd = 1730 ± 10m.y. andd = 2000 ± 100m.y. The second method, based on the assumption that uranium and rare earths have not migrated, could mean that the nuclear reaction might be older than the Francevillian. It is also shown that no contamination by natural elements has occurred for lead, rare earths, palladium and rare gases, after the reaction.     

Further U-Pb dates on zircons from the early Precambrian rocks of the Godthaabsfjord area,West Greenland

U-Pb analyses of zircons from the Godthaabsfjord area in West Greenland yield dates of 2530 ± 30 m.y. for the Qo?rqut granite, 2820 ± 50 m.y. for the Nu?k gneiss, 2600–2900 m.y. for Malene-type supracrustal rocks, and >3800 m.y. for Isua supracrustal rocks. The zircon data are in accord with the stratigraphic sequence except for the Malene supracrustal zircons, which appear to have been variably affected by later metamorphism that did not reset the Nu?k or Amîtsoq zircons in the same area.     

The Palung granite (Himalaya); high-resolution UPb systematics in zircon and monazite

UPb analyses of fractions of zircon and monazite (3–8 grains each) and of single zircon grains resolve a lower Ordovician age of 470 ±4m.y. for the Palung granite which occurs in the High Himalayan nappes south of Kathmandu. Its thrusting during the Alpine orogeny under lower greenschist facies conditions did not affect the UPb systems in zircon and monazite. The granite crystallized from a magma which was mainly generated by anatexis of Precambrian continental crust. The magma was heterogeneous with respect to primary ages and/or metamorphic histories of the magma source rocks. This indicates either a derivation from (meta-) sediments or an intense mixing of different crustally derived magmas. The genesis of the Palung granite is possibly related to an orogeny which affected the Indian shield in lower Palaeozoic times. The detected inherited radiogenic lead in the Palung zircons occurs in perfectly homogeneous, transparent crystals; i.e. this radiogenic (“excess”) lead is not related to the presence of old, microscopically visible, overgrown zircon cores. The minimum ages of the inherited lead components range from about 800 to 1700 m.y.     

Deformation mechanism maps for polycrystalline galena

A deformation mechanism map, depicting the fields of stress and temperature in which modes of plastic flow are dominant (i.e. provide the fastest strain rate) has been calculated for polycrystalline, stoichiometric galena for each of two grain sizes; namely, 10 and 10³ μm. The deformation mechanisms considered were dislocation glide, dislocation creep (i.e. creep involving dislocation climb), Nabarro-Herring creep and Coble creep.During folding and related tectonic deformation in the earth's crust steady-state flow of galena may occur by either dislocation or diffusion creep at very low differential stresses (typically ranging from 100 bars down to 10^?2 bar, or less). The dislocation creep field will be enlarged at the expense of that for diffusion creep, however, if (a) the stress dependence of strain rate for dislocation creep decreases at low stresses and if the grain size is greater than 10 μm, or (b) diffusion creep rates decline at high strains due to the presence of second-phase particles in the grain boundaries. It is probable that dislocation glide will be the dominant deformation mechanism in galena only at low temperatures and fast strain rates and it is unlikely to be the dominant mechanism during folding. Kinking may have some potential as an indicator of these temperatures and strain rates.     

Lead isotopic study of aplites from the Precambrian basement rocks near Ibadan,Southwestern Nigeria

Pb isotopic compositions for three total-rock samples of aplite and their constituent K-feldspars from the Nigerian basement assemblage near Ibadan show lead homogenization during the Pan-African thermo-tectonic event. A secondary isochron formed by the K-feldspars data points is used to calculate a primary age of about 2750 m.y. for the aplites. The aplites do not register any Pb isotopic effect from the intrusion of granite gneiss in the area at 2330 m.y.     

Mass spectrometric study of some selected galenas from the Eastern Desert of Egypt

Summary Thermal emission mass spectrometric technique is applied to the determination of the lead isotopic composition of some selected galenas from the Eastern Desert of Egypt. Recent methods of calculations for galena dating are used in determining the geochronological significance of the data. A correlation is made between the data obtained and the modes of occurrences of the deposits to enter any processes that cause their isotopic variations.     

Uranium-lead isotope systematics in a regionally metamorphosed tonalite from the eastern Alps

U-Pb isotopic analyses were made on sphene, three epidote fractions, apatite, K-feldspar and plagioclase from a 314-m.y.-old tonalite member of the Zentralgneis plutonic suite in the southeast Tauern Window. The tonalite reached temperatures in excess of 550°C during the Tertiary Alpine metamorphism.Apatite, fine-grained clinozoisite and feldspars equilibrated during the metamorphism, and the apatite yields an age of 22 m.y. which is 6 m.y. older than the Rb-Sr age of coexisting biotite.Sphene and coarse-grained iron-rich epidote did not reach equilibrium during Alpine metamorphism and the sphene data indicate crystallisation before 215 m.y. These minerals contain a large proportion of the uranium and thorium in the rock and the data thus imply that the present distribution of heat-producing elements in the tonalite was established long before the Alpine metamorphism.The very high closure temperatures for sphene and epidote implied by the data suggest they may be of value in dating metamorphism.     

U(Th)Pb systematics and ages of Himalayan leucogranites, South Tibet

The age and origin of five leucogranites from the High and Tethys Himalaya, and two country-rock gneisses were investigated by UPb dating of zircon fractions and single grains, and fractions of monazite. Additionally, ThU concentrations in whole rock powders and isotopic compositions of Pb in leached K-feldspars were determined. Monazites yield ages of 16.8 ± 0.6 m.y. for the Nialam migmatite-granite, 15.1 ± 0.5 m.y. for the Lhagoi Kangri granite, 14.3 ± 0.6 m.y. for a granite from Mt. Everest, and 9.8 ± 0.7 m.y. and 9.2 ± 0.9 m.y. for two varieties of the Maja granite. These data, together with monazite ages of 21.9 ± 0.2 and 24.0 ± 0.4 m.y., determined earlier on the Makalu granite [1], substantiate a period of intracontinental granite emplacements from 24 to 9 m.y. ago, i.e. from uppermost Oligocene to late Miocene times. Such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of India with Eurasia. Furthermore, the individual ages, together with structural relationships between granites and country rocks suggest that granite formation and tectono-metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m.y. Inherited lead components, present in all granite zircons point to large proportions of Precambrian material in the magma source regions, up to 2200 m.y. old.ThU systematics between monazite and country rocks indicate that U has been leached from most of the granites after crystallisation of monazite.Zircon dating of the Kangmar granite gneiss, which occurs in a window through the Tethys Himalayan sediments, shows that this pluton, transformed to a gneiss during the Alpine orogeny, crystallised in lowermost Palaeozoic times 562 ± 4 m.y. ago.     

Heat generation versus depth of crystallization for Norwegian monzonitic rocks

Thorium, U and K analyses by γ-ray spectrometry of monzonitic intrusives from Lofoten-Vesterålen (ca. 1800 m.y.), Bjerkreim-Sogndal (ca. 1000 m.y.) and the Oslo region (ca. 275 m.y.) are used to calculate values of radioactive heat generation. A strong negative correlation between heat generation and depth of crystallization is demonstrated, indicating that the chemical zoning of the continental crust with respect to Th and U is of a rather regional and regular character, and was probably firmly established at least 1800 m.y. ago. The K contents, in contrast to Th and U, show an increase with depths of crystallization for these rocks. This may be explained by considerations of the pressure dependence of the partition coefficients for these elements between minerals and magma, and the importance of a fluid phase as a transport medium for the incompatible elements Th and U. A positive correlation between K content and age of intrusion is discussed on a tentative basis.     

Discovery of Precambrian rocks in New Zealand: Age relations of the Greenland Group and Constant Gneiss,West Coast,South Island

Rb-Sr whole-rock analyses yield a Cambro-Ordovician (495 ± 11 m.y.) sedimentation age for the supposed Precambrian Greenland Group and a late Precambrian age, 680 ± 21 m.y., for parts of the Constant Gneiss, the first confirmation of Precambrian rocks in New Zealand. A Precambrian age for the Greenland Group is thus unlikely and the large area of Upper Cambrian-Lower Ordovician rocks now established can be considered as a lateral equivalent of the fossiliferous Lower Palaeozoic succession of northwest Nelson to the east. The Greenland Group, especially in the Paparoa Range has been affected subsequently by a thermal metamorphic overprint about 360 m.y. ago during the Tuhuan Orogeny. Although the Constant Gneiss must form the local basement to the Greenland Group in north Westland, the former does not appear to be the source of the sediments and the true provenance must lie elsewhere.     

North American Precambrian history recorded in a single sample: high-resolution UPb systematics of the Potsdam sandstone detrital zircons,New York State

Zircons separated from the Cambrian Potsdam sandstone of New York yield four distinct populations which can be defined by a number of analytical techniques. U-Pb isotopic analyses of small samples and monozircons of each population reveal a fine chronology not apparent in milligram-sized sample analysis, and define source area ages of 1180, 1320, 2100 and 2700 m.y. for the Cambrian detrital suite. These ages correspond to well-defined sources in the Superior and Grenville Provinces of the Canadian Shield (2700 and 2100 m.y.) and the well established Grenville age rocks of the Adirondack Mountains (1180 m.y.). The 1320-m.y. age appears to be derived from the Adirondacks, and suggests the existence of pre-Grenville basement in that massif. Our techniques allow the interpretation of the Precambrian history of a large portion of eastern North America from a single sample, and thus should be valuable in the definition of source areas in paleogeographic reconstruction, and in studies of continental crustal evolution.     

MULTIPARAMETER GEOPHYSICAL LOGGING AT THE YAVA LEAD DEPOSIT: A STATISTICAL APPROACH1

The present study describes multiparameter geophysical logging carried out at the Yava sandstone lead deposit, Nova Scotia, Canada. Statistical analysis of the multiparameter data set shows that the spectral gamma-gamma ratio log (SGG ratio) is the most useful technique for characterizing the disseminated sulphide mineralization. Principal component analysis (PCA) indicates that the apparent chargeability (IP parameter) responds to the presence of clay minerals in the sandstone in addition to disseminated sulphides, so that the induced polarization method (IP) does not accurately delineate the disseminated galena content as was originally assumed in the preliminary log interpretation. PCA has also confirmed that the SGG ratio and density are related to lead content and that lithological variations can be delineated with natural radioactivity and resistivity. The zinc content of the deposit was poorly characterized by geophysical logs. Sphalerite occurrences seem to be localized as narrow bands (< 1 cm) which were not geophysically detectable.     

Elucidating geochemical response of shallow heterogeneous aquifers to CO2 leakage using high-performance computing: Implications for monitoring of CO2 sequestration

Predicting and quantifying impacts of potential carbon dioxide (CO₂) leakage into shallow aquifers that overlie geologic CO₂ storage formations is an important part of developing reliable carbon storage techniques. Leakage of CO₂ through fractures, faults or faulty wellbores can reduce groundwater pH, inducing geochemical reactions that release solutes into the groundwater and pose a risk of degrading groundwater quality. In order to help quantify this risk, predictions of metal concentrations are needed during geologic storage of CO₂. Here, we present regional-scale reactive transport simulations, at relatively fine-scale, of CO₂ leakage into shallow aquifers run on the PFLOTRAN platform using high-performance computing. Multiple realizations of heterogeneous permeability distributions were generated using standard geostatistical methods. Increased statistical anisotropy of the permeability field resulted in more lateral and vertical spreading of the plume of impacted water, leading to increased Pb²⁺ (lead) concentrations and lower pH at a well down gradient of the CO₂ leak. Pb²⁺ concentrations were higher in simulations where calcite was the source of Pb²⁺ compared to galena. The low solubility of galena effectively buffered the Pb²⁺ concentrations as galena reached saturation under reducing conditions along the flow path. In all cases, Pb²⁺ concentrations remained below the maximum contaminant level set by the EPA. Results from this study, compared to natural variability observed in aquifers, suggest that bicarbonate (HCO₃⁻) concentrations may be a better geochemical indicator of a CO₂ leak under the conditions simulated here.     

Perspectives on the ethiopian volcanic province

Published major-element analyses of Ethiopian volcanic rocks have been subjected to a systematic discriminant analysis. The plateau regions can be subdivided according to the proportions of alkaline and tholeiitic basalts. In northern Ethiopia, these subprovinces show increasing basalt alkalinity with time. The most voluminous basalts have lowest magnesium values, independent of the degree of alkalinity. Rift and Afar basalt chemistry falls within the spectrum observed for the plateau basalts, with no perceptible difference resulting from lithospheric attenuation beneath Afar. However, silicic volcanics of the Rift-Afar floor differ in bulk terms from those of the plateau margins in showing a stronger bias towards peralkalinity, and having higher Na/K values. Two particularly voluminous volcanic episodes have occurred in Ethiopia, dated at ?30–19 m.y. and 4.5–0 m.y. and which link well with one model for seafloor spreading in the Red Sea and Gulf of Aden. Evidence for a mantle hotspot under Ethiopia remains ambiguous.     

Episodic accretion and plutonism in southwestern Alaska

Paleontologic and radiometric dating of the accretionary prism and magmatic arc of southwestern Alaska reveal an history of episodic accretion and plutonism. Possible accretion events in the Triassic (220-195 m.y.) and Early Jurassic (184-176 m.y.) were followed by Middle Cretaceous (108-83 m.y.), earliest Paleogene (65-60 m.y.), Middle Paleogene (50-40 m.y.), and Neogene (25-0 m.y.) accretion episodes. Plutonic events, which alternate with the accretion events, occurred in the Early Jurassic (193-184 m.y.), Middle/Late Jurassic (176-145 m.y.), Late Cretaceous/Early Paleogene (83-50 m.y.), and Late Paleogene (38-26 m.y.). Episodicity of accretion events is an apparent cause of incomplete stratigraphic records in the accretionary prism and forearc basin.     

The 15 m.y. geomagnetic reversal periodicity: a quantitative test

We have analyzed the three most commonly used geomagnetic polarity time scales that cover the past 100 m.y.. In the three cases, different spectral analyses show a 13–16 m.y. periodicity in the rate of reversal occurrence. To test whether this periodicity is real or simply arises from a random generator we have compared these polarity time scales with a large number of synthetic sequences produced by a random process, characterized by a linear time variation of its mean activity. Geomagnetic and generated sequences were regularly sampled by using sliding windows, anf then the Fourier spectra of the obtained frequency signals were compared. This test shows that the detected periodicity is presumably not a simple statistical fluctuation of an aperiodic generator, and consequently that a long-term periodicity in the geodynamo must be seriously considered.     

Source of lead in Central American and Caribbean mineralization,II. Lead isotope provinces

In an earlier study of Mesozoic and Cenozoic mineralization in Central America and the Caribbean region, we found that lead isotopic compositions of deposits in northern Central America, which is underlain by a pre-Mesozoic craton, ranged to higher²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb compositions than did deposits from elsewhere in the region, where the basement is Mesozoic oceanic material. Using 16 analyses for 12 new deposits, as well as new analyses for 11 of the samples studied previously, we have found that lead isotopic compositions correlate closely with crustal type but show little or no correlation with depth to the M-discontinuity. The deposits are divisible into three main groups including (in order of increasing²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratio): (1) deposits in southern Central America and all deposits in the Greater Antilles except Cuba; (2) all deposits in northern Central America; and (3) the Cuban deposits. Southern Central American and Caribbean lead is higher in²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb than most mid-ocean ridge basalts but could have been derived directly or indirectly from undepleted mantle. Northern Central America can be divided into the Maya block, which belongs to the Americas plate, and the Chortis block, which belongs to the Caribbean plate. Maya block deposits fall along a linear array whereas those of the Chortis block (except the Monte Cristo deposit) form a cluster. These results suggest that the Maya block is underlain by crust or mantle with a large range of U/Pb and Th/U ratios, whereas the Chortis block basement is more homogeneous. Two-stage model calculations indicate an age of about 2280±310 m.y. for the Maya block basement, although no such rocks are known in the region. Comparison of the Chortis block data to our recently published lead isotopic analyses of Mexican deposits shows considerable similarities suggesting that the Chortis block could have been derived from Mexico.