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1.
P. Černý R. Chapman J. Staně M. Nová H. Baadsgaard M. Rieder M. Kavalová L. Ottolini 《Mineralogy and Petrology》1995,55(1-3):177-201
Summary The chemistry, structural parameters, polytypism, optical properties and Rb-Sr isotopes were examined in 11 to 60 samples of biotite, muscovite and lepidolite from the pegmatites at Roná (the type locality of lepidolite; 323 ± 4Ma) and Dobrá Voda (306 ± 9Ma) in western Moravia. At both localities, early endocontact biotite is followed inwards by muscovite and lepidolite, which is concentrated in and around the core. At Roná, a 1M lepidolite follows after 2M1 muscovite but all later generations of lepidolite are 2M2, close to Tri50 Ply50 and in part associated with muscovite 2M1. At Dobrá Voda, all lepidolite types are 1M and free of muscovite, and the late varieties approximate Tri30 Ply70. At both localities, a trend of increasing HF is indicated during the progress of mica crystallization, culminating in precipitation of topaz. Polytypism of lepidolite is not correlatable with any compositional or growth feature, or their combination. Throughout the mica crystallization, Rb/Cs decreases but K/Rb becomes reversed after an initial decrease. Boron is partitioned preferentially into muscovite (up to 1.10 wt.% B2O3) but Be, Zn, Mn and Sc are enhanced in lepidolite. A slight increase in Fe, Ba and Cl in the last generation of lepidolite might be possibly due to mixing of residual pegmatite fluids with metamorphic pore solutions.
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Geochemische und strukturelle Entwicklung der Glimmer in den Pegmatiten von Roná und Dobrá Voda, Tschechische Republik
Zusammenfassung In 11 bis 60 Proben von Biotit, Muskovit und Lepidolith aus den Pegmatiten von Roná (Typlokalität des Lepidoliths; 323 ± 9 Ma) in Westmähren wurden Chemie, Struktur-parameter, Polytypie, optische Eigenschaften und Rb-Sr-Isotopie untersucht. An beiden Lokalitäten wird früher Biotit an Endokontakten nach Innen von Muskovit und Lepidolith gefolgt, letzterer ist in und um den Kern konzentriert. In Roná folgt 1M-Lepidolith auf 2M1-Muskovit, aber alle späteren Lepidolithgenerationen sind 2M2, nahe Tri50Ply50 und zum Teil mit 2M1-Muskovit vergesellschaftet. In Dobrá Voda sind alle Lepidolithe vom Typ 1M und frei von Muskovit, die späten Varietäten kommen Tri50Ply50 nahe. An beiden Lokalitäten ist während des Fortschreitens der Glimmerkristallisation eine Tendenz von steigendem HF angezeigt, die in der Ausfällung von Topas ihren Höhepunkt findet. Die Polytypie des Lepidoliths kann nicht mit irgendeiner Eigenheit der Zusammensetzung oder des Wachstums korreliert worden, auch nicht mit einer Kombination von diesen. Während der ganzen Glimmerkristallisation nimmt Rb/Cs ab, aber die Tendenz von K/Rb ändert sich nach anfänglichem Abfall. Das Bor verteilt sich bevorzugt auf den Muskovit (bis zu 1.10 Gew. -% B2O3), aber die Be-, Zn-, Mn- und Sc-Gehalte sind im Lepidolith erhöht. Ein leichter Ansteig von Fe, Ba und Cl in der letzten Lepidolithgeneration könnte vielleicht durch eine Mischung von pegmatitischen Restlösungen mit metamorphen Porenlösungen verursacht sein.
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2.
G.S. Odin V. Barbin A.J. Hurford H. Baadsgaard B. Galbrun P.-Y. Gillot 《Earth and Planetary Science Letters》1991,106(1-4)
A comparison between conventional KAr (biotite) ages and fission track (zircon and apatite) and UPb (zircon) ages obtained from stratigraphically well-constrained Priabonian (Late Eocene) volcano-sedimentary deposits of northern Italy is presented. Two sections at Priabona (one level) and Possagno (two levels) were dated. The application of fission track dating appears fruitful for obtaining reasonably precise (±4 to 5% 2σ errors) ages useful for time-scale calibration. The concordancy of apatite and zircon fission track ages, and the reproducibility of results provide the time of volcanic eruption and deposition. The UPb analysis of the zircons has not been unsuccessful, but discordancy does not permit accurate dating. Significant dates obtained from Possagno are: KAr method, 35.0 ± 0.5 Ma (duplicate analysis on K-rich biotite from the same level); fission track dating method, 35.8 ± 1.4 Ma (weighted mean age on 2 apatite and 3 zircon separates from the same level); UPb method, 36.7 ± 1.0 Ma (maximum age of discordant zircons from the same level). The comparison between the present results and recent multi-method and multi-laboratory results obtained from time equivalent Priabonian (Late Eocene) biotite-rich layers from the Apennines shows perfect agreement and supports the location of a Priabonian stage between about 37.5 Ma and about 33.7 (±0.5) Ma; the alternative ages preferred by the Decade of North American Geology convention should be abandoned and a large portion of this scale revised accordingly. 相似文献
3.
U-Th-Pb, Pb-Pb, Rb-Sr and K-Ar radiometric relationships in the minerals from six selected Amîtsoq gneiss samples reveal a complicated history of variable mineral response to polymetamorphism.K-Ar dates on biotite range from 2170 to 3220 m.y. (excess argon present), on hornblende from 2340 to 2510, and on a single muscovite at 1670 m.y.Rb-Sr whole rock results give an apparent isochron of at least 4065 m.y., but this result is likely fortuitous from a small sample selection since Pb-Pb whole rock analyses give ~ 3600 m.y. and the zircons in these rocks yield a concordia-discordia intersection at 3600 m.y. Rb-Sr mineral analyses generally give a confusing and variable pattern of isotopic relationships; but hornblende, K-feldspar, apatite, allanite and sphene appear to have last responded to metamorphism at 2200–2600 m.y. Rb-Sr in biotite, epidote and, in part, plagioclase have been affected by an event at ~ 1550 m.y.U-Th-Pb data from sphene, apatite and allanite give almost concordant dates at 2500–2600 m.y. vs plots yield two separate lines for apatite (slope age 2435 m.y.) and for sphene + allanite (slope age 2530 m.y.), indicating apatite to have a different (less-radiogenic) ‘initial’ Pb than that for sphene and allanite. A similar pattern is found for the vs plot for sphene and apatite. The Pb-isotopic composition of the feldspars is very homogeneous and the least-radiogenic of all components, pointing towards a homogeneous parent material for the now lithologically diverse Amîtsoq gneisses. Using a0 = 9.307, b0 = 10.294, C0 = 29.476, t0 = 4.56 b.y., ω = 6.9 and ; the feldspars give a model Pb age of 3500–3600 m.y. by either U-derived or Th-derived Pb. The segregation of the present Amîtsoq gneisses from the homogeneous parent material was apparently accompanied by a U and Th loss with preservation or enrichment of Pb at ~ 3600 m.y. ago. No consistent treatment of the present U-Th-Pb data will produce viable data indicating an age > 3600 m.y. for the parent materials of the Amîtsoq gneiss.Petrographie observations generally concur with radiometric results and permit the postulation of the reaction: Hbl + K-feld→ biotite + epidote + sodic plag, to account for some of the effects of the latest metamorphism.The total internal radiometric evidence indicates three major metamorphic events affected the Amîtsoq gneisses close to 3600, 2500 and 1550 m.y. 相似文献
4.
D.W. Davis J. Gray G.l. Gumming H. Baadsgaard 《Geochimica et cosmochimica acta》1977,41(12):1745-1749
The decay constant 87Rb has been redetermined by measuring the amount of radiogenic 87Sr produced over a period of 19 years, in 20 g samples of purified RbClO4, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO3)2. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of . [.] 相似文献
5.
Impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth. Age-dating of impact events is critical in correlating cause and effects for these catastrophic processes. The Falconbridge and Drury Township (Twp) intrusions were emplaced at the contact between Neoarchaean basement and Palaeoproterozoic volcanosedimentary rocks, and also lie at the southeast and southwest edges of the Sudbury Igneous Complex (SIC), within its thermal contact aureole. The Falconbridge Twp intrusion is dated at 2441 ± 3 Ma by U-Pb zircon, with evidence of Archaean inheritance from its host granitoids. Granitoids from the southernmost Abitibi Subprovince are dated here between 2670 ± 11 Ma for an undeformed Algoman granite, and 2696 ± 18 Ma for a foliated granitoid, consistent with existing data from the Abitibi Greenstone Belt and from the Wawa Subprovince. Major and trace element geochemical evidence, common-Pb isotopic compositions, and εNd2440 values between 0 and −1 are all consistent with a Palaeoproterozoic origin for the Falconbridge Twp intrusion, and support inclusion in the East Bull Lake-type suite of leucogabbroic plutons and sills.In contrast, the Drury Twp intrusion gives a U-Pb zircon age of 1859 ± 13 Ma, coincident with the date of SIC-emplacement. While the major and trace element compositions are comparable to the Falconbridge data, the Drury displays significant heterogeneity in εNd2440, with values ranging from +3.7 to −0.1, and contains more radiogenic Pb isotopic compositions. Field, geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the SIC itself, and indicates that the Drury too is a Palaeoproterozoic intrusion. This requires that apparently unshocked, undeformed magmatic-looking zircon has been grown or reset in a postmagmatic setting. This has significant implications for the identification of mantle-derived magmas and crustal remelts associated with large impact craters. A resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen-complexing is proposed, inducing rapid, postshock lead loss and subsequent annealing. 相似文献
6.
H. Baadsgaard 《Earth and Planetary Science Letters》1976,33(2):261-267
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. 相似文献
7.
A continuous velocity field for Norway 总被引：1，自引：0，他引：1
Halfdan Pascal Kierulf Mohammed Ouassou Matthew James Ross Simpson Olav Vestøl 《Journal of Geodesy》2013,87(4):337-349
In Norway, as in the rest of Fennoscandia, the process of Glacial Isostatic Adjustment causes ongoing crustal deformation. The vertical and horizontal movements of the Earth can be measured to a high degree of precision using GNSS. The Norwegian GNSS network has gradually been established since the early 1990s and today contains approximately 140 stations. The stations are established both for navigation purposes and for studies of geophysical processes. Only a few of these stations have been analyzed previously. We present new velocity estimates for the Norwegian GNSS network using the processing package GAMIT. We examine the relation between time-series length and precision. With approximately 3.5 years of data, we are able to reproduce the secular vertical rate with a precision of 0.5 mm/year. To establish a continuous crustal velocity field in areas where we have no GNSS receivers or the observation period is too short to obtain reliable results, either interpolation or modeling is required. We experiment with both approaches in this analysis by using (i) a statistical interpolation method called Kriging and (ii) a GIA forward model. In addition, we examine how our vertical velocity field solution is affected by the inclusion of data from repeated leveling. Results from our geophysical model give better estimates on the edge of the network, but inside the network the statistical interpolation method performs better. In general, we find that if we have less than 3.5 years of data for a GNSS station, the interpolated value is better than the velocity estimate based on a single time-series. 相似文献
8.
Estimates of twenty-first century sea-level changes for Norway 总被引：1，自引：0，他引：1
In this work we establish a framework for estimating future regional sea-level changes for Norway. Following recently published works, we consider how different physical processes drive non-uniform sea-level changes by accounting for spatial variations in (1) ocean density and circulation (2) ice and ocean mass changes and associated gravitational effects on sea level and (3) vertical land motion arising from past surface loading change and associated gravitational effects on sea level. An important component of past and present sea-level change in Norway is glacial isostatic adjustment. Central to our study, therefore, is a reassessment of vertical land motion using a far larger set of new observations from a permanent GNSS network. Our twenty-first century sea-level estimates are split into two parts. Firstly, we show regional projections largely based on findings from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) and dependent on the emission scenarios A2, A1B and B1. These indicate that twenty-first century relative sea-level changes in Norway will vary between ?0.2 to 0.3 m (1-sigma ± 0.13 m). Secondly, we explore a high-end scenario, in which a global atmospheric temperature rise of up to 6 °C and emerging collapse for some areas of the Antarctic ice sheets are assumed. Using this approach twenty-first century relative sea-level changes in Norway are found to vary between 0.25 and 0.85 m (min/max ± 0.45 m). We attach no likelihood to any of our projections owing to the lack of understanding of some of the processes that cause sea-level change. 相似文献
9.
U-Pb analyses were made on zircons separated from different lithologies within the Nûk gneisses taken from Godthåb (Nuuk) townsite and from the south-west shore of Bjørneøen. Size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the Nûk gneisses of 2890–3065 Myr. This spread of ages is not a result of multiple intrusion, but more likely results from different degrees of ‘diffusion’ and metamorphic episodic lead loss by the zircons. Since U-Pb and Th-Pb discordance are similar, loss of radiogenic lead (and not U or Th-loss/gain) by radiation-damage diffusion and through metamorphism is the main factor causing discordance of the U-Pb dates. The results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed. This interpretation is supported by Rb-Sr whole rock analyses of the same rocks, in which the Nûk townsite rocks scatter about a reference isochron of 2840 Myr, while the Bjørneøen samples give an isochron (MSWD = 0.55) of 3076 ± 27 Myr and the initial ratio at 0.70205 ± 0.00009. The zircon results do not rule out the possibility that more than one metamorphic episode affected the original Nûk gneiss parents prior to 2520 Myr (Qôrqut granite intrusion). A date of 2660 Myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south-western Bjørneøen and Godthåb town. 相似文献
10.
Peter A. Nielsen C. Willem Langenberg H. Baadsgaard John D. Godfrey 《Precambrian Research》1981,16(3):171-193
A complex of Precambrian polymetamorphic gneisses and granitoids of the Churchill structural province, northeastern Alberta, Canada has been examined structurally, petrographically, chemically and geochronologically. An Archean basement gneiss complex is indicated by Rb-Sr dating of pegmatites which cut both gneisses and granitoids (2470 ± 26 Ma with an initial 87Sr/86Sr ratio of 0.7030 ± 0.0008). A high-pressure granulite facies (M1) mineral assemblage and older structures (D1) are assigned to the Archean. A moderate-pressure granulite facies (M2.1), a low-pressure amphibolite facies (M2.2), a greenschist facies (M2.3), and younger structures (D2) are of Aphebian age. Formation of granitoids by anatexis of the pre-existing Archean basement complex during M2.1 is indicated by their Aphebian ages (ca. 1900 Ma) and high initial 87Sr/86Sr ratios (0.7100 ± 0.0018). The path of retrograde metamorphism is linked with relatively slow rates of uplift and cooling. Late Aphebian sediments attained low-grade greenschist facies metamorphism only and are younger than the other metamorphic rocks. The tectonic evolution of this Precambrian mobile belt during the Aphebian contrasts with the stable Archean cratonic block in the Slave province to the north. 相似文献