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561.
The amalgamation of the southern Río de la Plata craton involves two possibly coeval Rhyacian sutures associated with the Transamazonian orogeny,rather than a single one as previously envisaged,i.e.the El Cortijo suture zone and the Salado suture.We circumscribe the Tandilia terrane to the region between these two sutures.The El Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the southern boundary of the Tandilia terrane,i.e.boundary between the Tandilia and Balcarce terranes.This extensive magnetic low,ca.300 km long,and ca.90 km wide,would be caused by demagnetization associated with shearing.At a more local scale,the trend of the El Cortijo suture zone often turns toward the EeW.At this scale,WNW trending tholeiitic dykes of Statherian age are seen to cut the Rhyacian El Cortijo suture zone.Spatially associated with the El Cortijo suture zone,there are small magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature related to those forming part of the El Cortijo Formation.We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean(w2650 Ma)crust occurred during Siderian times(2500e2300 Ma),causing the separation between the Balcarce,Tandilia and Buenos Aires terranes,and the development of narrow oceans at both north and south sides of the Tandilia terrane,accompanied by w2300e2200 Ma sedimentation over transitional econtinental to oceanice crust,and arc magmatism developed in the Tandilia terrane.The island arc represented by the El Cortijo Formation was also developed at this time.At late Rhyacian times,it occurred in both the closure of the narrow oceans developed previously,the entrapment of the El Cortijo island arc,as well as anatectic magmatism in the Balcarce terrane. 相似文献
562.
Evidence for lithospheric detachment in the central Andes from local earthquake tomography 总被引:4,自引:1,他引:3
Data from three temporary seismic networks were merged for tomographic inversion. Although the deployments did not coincide in time, spatial overlap was achieved by re-occupying existing sites. Travel times and t operators of about 1600 earthquakes were inverted for 3D models of νp, νp/νs and P-wave attenuation (Qp− 1). All three attributes provide a consistent image of the entire subduction zone on a lithospheric scale. The tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the Western Cordillera and most of the Puna plateau, indicative of weak rheology and mostly asthenospheric mantle. In contrast, forearc and eastern foreland are characterized by high Qp values, corresponding to cold temperatures in accordance with thermal models. In the backarc, between 23°S and 24°S, a high velocity, high Qp structure beneath the Eastern Cordillera and eastern Puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process. South of this structure, the mantle is characterized by low velocities, high νp/νs ratios, and low Qp values. Here it is believed that lithosphere originally underlying Andean crust has already been removed. This is supported by new estimates of crustal thickness and volcanic activity. 相似文献
563.
During its storage or ascent, basaltic magma inevitably interacts with the surrounding rocks. In this study, schist xenoliths incorporated within ascending basalt are examined. Heating of the xenoliths combined with decompression effect of rapid magma uprise led to dehydration melting of hydrous minerals producing hercynitic spinel, melt, sillimanite and FeTi oxides. The melt is rhyolitic, strongly peraluminous (1.77 < A/CNK < 2.35) and corundum normative. It may contain up to 8 wt%FeOt. It occurs between the foliation planes and in the intragranular environment. Dehydration melting of micas in the schist is probably related to combined effects of heating by basaltic magma and decompression due to the rapid rise. Melting of xenoliths was a progressive process at low pressure. To cite this article: H. Bayhan et al., C. R. Geoscience 338 (2006). 相似文献
564.
565.
Until now, no satisfactory geodynamic model has been delivered concerning the three main West African orogens: Panafrican 1 (Bassaride belt), Panafrican 2 (Rokelide belt) and Hercynian (Mauritanide belt). However, since the last synthetic paper (Villeneuve, 2008), new geological, geophysical and geochronological data, from the Moroccan Sahara to Sierra Leone, allow us to propose a new geodynamical model. It includes the two Panafrican events in a single model very similar to the present western Pacific margin. An old “West African Neoproterozoic ocean” (WANO) was limited by a set of island arcs separated from the West African craton by a series of “back arc basins”. The closure of this first round of back arc basins around 650 Ma led to the Bassaride belt (Panafrican 1). Then the WANO was subducting underneath the island arcs (between 650 and 550 Ma) meanwhile a new generation of “back arc basins” opened to the east between the arcs and the craton margin. The closure of the WANO and associated island arcs and back arc basins (550 to 500 Ma) led to the Rokelide belt (Panafrican 2). The Hercynian structures involving a Palaeozoic cover (made with continental material) associated to a “greeenschist facies” metamorphism is ascribed to an intracontinental belt. 相似文献
566.
Xian-Hua Li Weng-Guo Long Qiu-Li Li Yu Liu Yong-Fei Zheng Yue-Heng Yang Kevin R. Chamberlain De-Fang Wan Cun-Hua Guo Xuan-Ce Wang Hua Tao 《Geostandards and Geoanalytical Research》2010,34(2):117-134
We introduce a potential new working reference material – natural zircon megacrysts from an Early Pliocene alkaline basalt (from Penglai, northern Hainan Island, southern China) – for the microbeam determination of O and Hf isotopes, and U–Pb age dating. The Penglai zircon megacrysts were found to be fairly homogeneous in Hf and O isotopes based on large numbers of measurements by LA-multiple collector (MC)-ICP-MS and SIMS, respectively. Precise determinations of O isotopes by isotope ratio mass spectrometry (IRMS) and Hf isotopes by solution MC-ICP-MS were in good agreement with the statistical mean of microbeam measurements. The mean δ18O value of 5.31 ± 0.10‰ (2s) by IRMS and the mean 176Hf/177Hf value of 0.282906 ± 0.0000010 (2s) by solution MC-ICP-MS are the best reference values for the Penglai zircons. SIMS and isotope dilution-TIMS measurements yielded consistent 206Pb/238U ages within analytical uncertainties, and the preferred 206Pb/238U age was found to be 4.4 ± 0.1 Ma (95% confidence interval). The young age and variably high common Pb content make the Penglai zircons unsuitable as a primary U–Pb age reference material for calibration of unknown samples by microbeam analysis; however, they can be used as a secondary working reference material for quality control of U–Pb age determination for young (particularly < 10 Ma) zircon samples. 相似文献
567.
Stephen F. Crowley 《Geostandards and Geoanalytical Research》2010,34(2):193-206
International carbon and oxygen isotope calibration material NBS 19 and reference materials NBS 18, International Atomic Energy Agency (IAEA)-CO-1 and IAEA-CO-8 are prepared from naturally occurring rock specimens of marble and carbonatite. Mineralogical and chemical analysis showed that only NBS 19 and IAEA-CO-1 represent essentially pure samples of calcite containing < and minimal (< 1%) quantities of quartz. In contrast, both NBS 18 and IAEA-CO-8, although primarily composed of calcite, are contaminated by a range of additional phases. NBS 18 was estimated to contain 1% Fe-dolomite and trace (< 1%) quantities of apatite and quartz. IAEA-CO-8 was estimated to contain at least 4% non-carbonate material (including apatite, barite, biotite and magnetite). NBS 18 and IAEA-CO-8 are both derived from samples of carbonatite and the calcite component of each material is characterised by appreciable substitution of Mg + Mn + Sr ± Fe ± Ba (Σ ≈ 14000–15000 μg g-1) for Ca. The observations reported in this study complement data in the literature detailing significant grain-scale isotopic heterogeneity in NBS 18 and IAEA-CO-8. Both data sets highlight the need for careful characterisation of calibration materials prior to distribution. 相似文献
568.
Paul F. Schofield Andrew D. Smith J. Frederick W. Mosselmans Hendrik Ohldag Andreas Scholl Simone Raoux Gordon Cressey Barbara A. Cressey Paul D. Quinn Caroline A. Kirk Simon C. Hogg 《Geostandards and Geoanalytical Research》2010,34(2):145-159
This work describes the application of microfocus X-ray absorption spectroscopy (XAS) and X-ray photo-emission electron microscopy (XPEEM) to the study of the complex mineralogical intergrowths within the Santa Catharina meteorite. The Santa Catharina meteorite of this study (BM52283 from the meteorite collection of the Natural History Museum, London, UK) primarily comprises a taenite bulk host phase (Fe:Ni ratio = 70.9 ± 0.8%:29.1 ± 0.8%) with a set of oxide-bearing cloudy zone textured regions (Fe:Ni:O ratio = 40.4 ± 0.3%:49.0 ± 0.7%:10.6 ± 0.8% at the core and Fe:Ni:O ratio = 34.4 ± 1.5%:42.7 ± 0.6%:22.9 ± 1.8% towards the rims) and numerous schreibersite (Fe:Ni:P ratio = 38.6 ± 1.6%:38.4 ± 0.9%:23.0 ± 0.5%) inclusions. Between the schreibersite and the taenite are rims up to 50 μm across of Ni-rich kamacite (Fe:Ni ratio = 93.4 ± 0.4%:6.6 ± 0.5%). No chemical zoning or spatial variations in the Fe and Ni speciation was observed within either the schreibersite or the kamacite phases. The oxide-bearing cloudy zone textured regions mostly comprise metallic Fe–Ni alloy, predominantly tetrataenite. Within the oxide phases, the Fe is predominantly, but not entirely, tetrahedrally co-ordinated Fe3+ and the Ni is octahedrally co-ordinated Ni2+. Structural analysis supports the suggestion that non-stoichiometric Fe2NiO4 trevorite is the oxide phase. The trevorite:tetrataenite ratio increases at the edges of the oxide-bearing cloudy zone textured regions indicating increased oxidation at the edges of these zones. The spatial resolution of the XPEEM achieved was between 110 and 150 nm, which precluded the study of either the previously reported ∼ 10 nm precipitates of tetrataenite within the bulk taenite or any antitaenite. 相似文献
569.
John Eliades Xiaolei Zhao William E. Kieser Albert E. Litherland 《Geostandards and Geoanalytical Research》2010,34(2):107-115
Preliminary tests of a prototype radio-frequency quadrupole (RFQ) collision cell system, known as an isobar separator for anions (ISA), for the removal of isobaric interferences for accelerator mass spectrometry (AMS) and for studies of anion–gas interactions are reported. The ISA decelerated a mass-analysed beam of anions from an energy (∼ 20 keV) typically generated by an AMS ion source to < 10 eV. RFQs and electrostatic lenses then guided the ions through the collision cell where ion-gas collisions reduced both the energy and energy spread of the ion beam (cooled the ions) and ion-gas reactions attenuated most of the unwanted isobars. The anions were then re-accelerated to their original energy for injection into the rest of the AMS system. With the ISA installed on a full 3 MV AMS system, attenuations of 32S-, 12C3- and 39K- by six, seven and greater than ten orders of magnitude, respectively were achieved using 0.7–1 Pa NO2 gas in the collision cell, while maintaining approximately 10–30% of the chlorine anion transmission. A further measurement of a 36Cl/Cl = 4.1 × 10-11 RM is also described. The results suggest that the 36Cl/Cl lower detection limit of the current system was 10-14–10-15 for samples that could be prepared with S/Cl ratios below 10 μg g-1. 相似文献
570.
B. Batjargal B. Davaasuren D. Erdenetsetseg 《Geostandards and Geoanalytical Research》2010,34(3):231-236
This paper briefly outlines the production and certification of reference materials at the Central Geological Laboratory (CGL) of Mongolia. The marketing of CRMs produced in Mongolia, as well as problems encountered in internationally recognised certification attempts and some proposed solutions, are discussed. The basic elements of the CGL’s strategy for the development of the CRM sector are to produce high quality CRMs according to the requirements of internationally recognised norms, to consider the market needs, to certify the RMs at the international level and to widely advertise them to the geochemical community. The CGL has already established the basis for the further development of this sector by, for instance, accreditation under ISO/IEC 17025, by modernising its preparation technology, by cooperation with international organisations in the field of CRMs and by permanent participation in the International Association of Geoanalysts’ GeoPT™ proficiency testing programme. 相似文献