Tripartite climate reversal in Central Europe 5600-5300 years ago

The history of variations in water level of Lake Constance, as reconstructed from sediment and pollen analysis of a sediment sequence from the archaeological site of Arbon-Bleiche 3, shows an abrupt rise in lake level dendrochronologically dated to 5375 yr ago (5320 yr relative to AD 1950). This event, paralleled by the destruction of the Neolithic village by fire, provoked the abandonment of this prehistoric lake-shore location established in the former shallow bay of Arbon-Bleiche, and was the last of a series of three episodes of successively higher lake level, the first occurring at 5600-5500 cal yr B.P. The dendrochronologically dated rise event was synchronous with an abrupt increase in atmospheric ¹⁴C. This supports the hypothesis of an abrupt climate change forced by varying solar activity. Moreover, the three successive episodes of higher lake level between 5600 and 5300 cal yr B.P. at Arbon-Bleiche 3 coincided with climatic cooling and/or changes in moisture conditions in various regions of both hemispheres. This period corresponds to the mid-Holocene climate transition (onset of the Neoglaciation) and suggests inter-hemispheric linkages for the climate variations recorded at Arbon-Bleiche 3. This mid-Holocene climate reversal may have resulted from complex interactions between changes in orbital forcing, ocean circulation and solar activity. Finally, despite different seasonal hydrological regimes, the similarities between lake-level records from Lake Constance and from Jurassian lakes over the mid-Holocene period point to time scale as a crucial factor in considering the possible impact of climate change on environments.     

Age and origin of magmatism along the Cenozoic Red River shear belt, China

To decipher the geodynamic significance of Cenozoic magmatism along the Red River shear belt, geochemical analyses, U-Pb and Rb-Sr dating, and Pb-Sr-Nd isotope tracing were undertaken. Zircon, monazite, titanite, and a Ti-U-oxide from foliated granitoid intrusions in the shear belt gneisses yield U-Pb emplacement ages of 33.1?±?0.2 (2σ), 31.9?±?0.3, 25.8?±?0.2 and 24.7?±?0.2?Ma, and an age of 35.0?±?0.3?Ma was obtained for the roughly 100?km long, adjacent Jinping (Phan Si Pang) alkali granite. Together with our previous data the new ages suggest that magmatism and left-lateral strike-slip movements occurred coevally during latest Eocene–Oligocene times from 33 to 22?Ma. The Rb-Sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500?°C occurred at 52.6?±?1.1?Ma, pre-dating the onset of magmatism, whereas further cooling to 300?°C took place at 28.9?±?0.6. This shows that unroofing in the north took place almost 9?million years earlier than in the central gneiss segments of the shear zone. Geochemical data substantiate two types of magmas: (1) amphibole-bearing intrusions of alkaline trend which are derived from sources with I_sr: 0.7065–0.7089 and _i ^Nd: ?3.7 to ?6.6; (2) leucogranitic layers and bodies having I_sr: 0.7084–0.7354 and _i ^Nd: ?3.3 to ?13.4. The former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks, whereas leucogranites are restricted to the shear belt gneisses. Source signatures of the alkaline intrusions lie adjacent to the those of OIB, plotting at the lower end of the Mantle Array. Contamination of these melts by continental material seems to be very limited. On the other hand, the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures, requiring melting of crust different from the actually exposed gneisses. Magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains, most likely present in the lithosphere underneath the region. Since lithospheric thickening or subduction can be ruled out to produce both types of magmas, the presence of an important thermal anomaly is required, which is coevally active with left-lateral strike-slip shear. Adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas, which successively migrate through the crust to induce anatectic melting at 20–15?km crustal depth. Alkaline magmas largely dominate the volume of magmatism along the belt, being continuously present in the shear zone for millions of years. Such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the Red River belt, possibly also playing a rôle for where and when zones of plate-scale lateral extrusion develop.     

Determination of Mislocation Vectors to Evaluate bias at GSETT-3 primary stations

Using the database provided by the Reviewed Event Bulletins (REBs) for the first 2.5 years of the Group of Scientific Experts Technical Test-3 (GSETT-3) experiment, we compiled mislocation vectors for both arrays and selected three-component stations of the primary network from the published slowness and azimuth information gained through f-k- and polarization analysis. Imposing constraints such as a minimum signal-to-noise ratio (SNR) and number of defining phases, we aim at eliminating location bias as the hypocentral parameters are taken from the REBs. Results from 14 arrays with apertures from about 1 km to more than 20 km are presented as well as from 18 three-component stations, which indicate that the mislocation vectors in many cases can improve location accuracy considerably. If these mislocation vectors are compiled to provide coverage of a sufficient portion of the slowness domain these empirical corrections can easily be applied prior to location processing. In the context of the Comprehensive Nuclear Test-Ban Treaty (CTBT), these mislocation patterns could be essential for providing accurate event location of suspicious low-magnitude events, as these location parameters will be used to pinpoint the area where to conduct an on-site inspection.     

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.     

Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite–granite–leucogranite associations: constraints from Sr–Nd–O isotopes from the Bandombaai Complex, Namibia

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (_{Nd (540 Ma)}=−6.3 to −19.8; δ¹⁸O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (⁸⁷Sr/⁸⁶Sr_initial=0.70794–0.70982). Two leucogranites and one aplite have higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70828–0.71559), but similar initial _Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K₂O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.     

Electrodynamic Disaggregation: Does it Affect Apatite Fission-Track and (U-Th)/He Analyses?

Apatite fission-track and (U-Th)/He analyses require the liberation of intact idiomorphic apatite grains from rock samples. While routinely being carried out by mechanical methods, electrodynamic disaggregation (ED) offers an alternative approach. The high-voltage discharges produced during the ED process create localised temperature peaks (10000 K) along a narrow plasma channel. In apatite, such high temperatures could potentially reduce the length of fission tracks, which start to anneal at temperatures > 60 °C, and could also enhance He diffusion, which becomes significant at 30–40 °C over geological time scales. A comparison of fission-track analyses and (U-Th)/He ages of apatites prepared both by mechanical (jaw crusher, disk mill) and ED processing provides a way of determining whether heating during the latter method has any significant effect. Apatites from three samples of different geological settings (an orthogneiss from Madagascar, the Fish Canyon Tuff, and a muscovite-gneiss from Greece) yielded statistically identical track length distributions compared to samples prepared mechanically. Additionally, (U-Th)/He ages of apatites from a leucogranite from Morocco prepared by both methods were indistinguishable. These first results indicated that during electrodynamic disaggregation apatite crystals were not heated enough to partially anneal the fission tracks or induce significant diffusive loss of He.     

Age and evolution of scheelite-hosting rocks in the Felbertal deposit (Eastern Alps): U-Pb geochronology of zircon and titanite

U-Pb isotope analyses of zircon and titanite extracted from different rocks of the Felbertal scheelite deposit yield the following information: (1) An age of 593±22 Ma (2) is obtained for zircon crystallization in the scheelite-bearing matrix of an eruption breccia in the western ore field. (2) Discordant zircons from an elongated, up to 8 m thick scheelite-rich quartzite body in the eastern ore field give an upper intercept age of 544±5 Ma. This quartzite contains a laminated, fine-grained scheelite mineralization. (3) Zircons from a small granitoid intrusion of the western ore field reveal an age of 336±16 Ma, and concordant titanites document an age of 282±2 Ma for Variscan amphibolite facies metamorphism. Both events, granitoid intrusion and later metamorphism caused ore re-mobilization, including the formation of yellowish fluorescent (molybdo-) scheelite porphyroblasts. (4) For a narrow lamprop-1hyric dike in the western ore field, a concordant titanite age of 283±7 Ma is obtained. This age is identical with the titanites from the amphibolite facies metamorphic intrusion. Tiny scheelite grains were tapped by the dike from pre-existing scheelite mineralizations in the truncated host rocks. (5) Alpine metamorphism at 31±4 Ma did not exceed lowermost amphibolite facies conditions, and it caused scheelite re-mobilization on a minor scale only, producing bluish fluorescent porphyroblasts in quartz veinlets and veins, as well as bluish fluorescent scheelite rims around older scheelite grains. Moreover, crosscutting Alpine fissure fillings show bluish fluorescent, inclusion-free scheelite. (6) The preservation of Variscan titanites, the absence of Alpine titanite growth, and the large degree of Variscan scheelite re-mobilization demonstrate that amphibolite facies metamorphism in the Felbertal area has a Variscan age. This result clearly documents Variscan tectono-metamorphism to be the dominant event, instead of the hitherto surmised Alpine metamorphism. This multi-stage evolution of the Felbertal ore bodies corroborates the view that tungsten deposits are conditioned by several succeeding thermal events, leading to a series of stages that ultimately produce high-grade scheelite concentrations. These high-grade ores predominately occur along shear zones of different age, accompanied by the formation of large volumes of low-grade scheelite mineralizations along host rock foliations and quartz veinlets and veins.     

Earthquakes in Switzerland and surrounding regions during 2008

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2008. During this period, 451 earthquakes and 75 quarry blasts were detected and located in the region under consideration. The three strongest events occurred in the Valais, near Lac des Toules (M_L 3.6), and in Graubünden, near Ilanz (M_L 3.7) and Paspels (M_L 4.0). Although felt by the population, they were not reported to have caused any damage. However, with a total of only 15 events with M_L ≥ 2.5, the seismic activity in the year 2008 was far below the average over the previous 33 years.