Archean granulite gneisses from eastern Hebei Province,China: rare earth geochemistry and tectonic implications   总被引：22，自引：0，他引：22  

Bor-ming Jahn  Zong-qing Zhang 《Contributions to Mineralogy and Petrology》1984,85(3):224-243

The granulite gneisses and their retrograded products of the Qianxi Group from eastern Hebei Province, China, have been investigated for their isotope and trace element geochemistry. A consistent age of about 2.5 AE has been obtained by the Rb-Sr and Sm-Nd whole-rock isochron methods, in agreement with the zircon U-Pb data (Pidgeon 1980; D.Y. Liu, unpubl.). Geochemical arguments from initial isotopic ratios (I_Sr and I_Nd) and elemental distribution patterns have led us to conclude that this age of about 2.5 AE represents the time of granulite facies metamorphism, which must have followed closely the primary emplacement of their protoliths. Previous claims for early Archean ages (>3.5 AE) of these granulites are not substantiated. The mineral isotope systematics register an important thermal event at about 1.7 AE, roughly corresponding to the time of the widespread Luliang Orogeny (Ma and Wu 1981) or Chungtiao Movement (Huang 1978).The granulites of the Qianxi Group have diverse compositions ranging from ultrabasic through basic-intermediate to acid. Discriminant function calculations suggest that most analyzed samples have igneous parentage. Only a few show characteristics of metasedimentary rocks. The igneous protoliths apparently belong to two series — tholeiitic and calc-alkaline, with the latter dominating in abundance. The majority of the acid granulites have compositions corresponding to tonalite-granodiorite.Except for ultrabasic and metasedimentary rocks, all REE patterns are significantly fractionated with LREE enrichment. The degree of fractionation, as measured by the (La/Yb)_N ratios, is most important in the acid granulites. These rocks often show positive Eu anomalies and HREE depletions that are typical of Archean TTG rocks (tonalitetrondhjemite-granodiorite).The existence of komatiites has been previously reported in this region. Although a few rocks have a major element chemistry similar to that for peridotitic komatiites, the lack of associated members in a komatiitic series and the scarcity of REE data have not confirmed the true komatiite occurrence in this region.Many Qianxi granulites are highly depleted in Rb relative to K and Sr. This preferential Rb depletion during granulite facies metamorphism has led to very high K/Rb and very low Rb/Sr ratios. The most comparable case is found in Lewisian granulites.Although the fractionated REE patterns of the basic granulites somewhat resemble those of continental flood basalts, the highly different abundances in other incompatible elements (Ti, Zr, and Ba) readily distinguish them from each other. Nevertheless, the LREE enriched patterns of the basic granulites may suggest an origin of their protoliths by partial melting of LREE-enriched mantle sources. On the other hand, the REE patterns of acid granulites suggest that their protoliths could be derived by partial melting of quartz eclogite, amphibolite or basic granulite.The close time relationship for a series of geologic events, namely, from initial melting of mantle peridotites, through fractional crystallisation of basaltic magmas, to granulite facies metamorphism, seems to occur in many granulite terrains. This relationship, together with the juxtaposition of lithologies of different origins and the exceptionally high pressure conditions (>10 Kb) can be best explained by crustal underplating combined with intracrustal thin-skinned thrusting and stacking of crustal slices. The andesitic or island arc model for the formation of the lower continental crust is not in good agreement with the present geochemical data.  相似文献   

NdSr isotope and REE geochemistry of alkali basalts from the Massif Central,France     

Catherine Chauvel  Bor-Ming Jahn 《Geochimica et cosmochimica acta》1984,48(1):93-110

Nd and Sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the Massif Central, in France. Samples show a typical enrichment in incompatible elements. In particular, the REE patterns exhibit a strong fractionation characterized by a ($\text{La}\text{Yb}\text{)}{}_{\mathrm{<mtext>N</mtext>}}$ ratio of about 20. The Yb_N content is about 10 times chondrite. The $\text{143}\text{Nd}\text{144}\text{Nd}$ ratios exhibit a range from 0.512775 to 0.512989, values quite comparable to those from oceanic island basalts. The $\text{87}\text{Sr}\text{86}\text{Sr}$ ratios vary between 0.70338 and 0.70458 and are anti-correlated with the Nd isotopic ratio.The isotopic and the trace element (in particular REE) data have been used in order to quantitatively model the genesis of the alkali basalts. Among the several types of models tested here, the most likely one appears to be the model of mantle metasomatism. A semi-quantitative approach shows that the source of alkali basalts from the Massif Central was metasomatized prior to melting. In such a model, the basalts could be produced by rather high degrees of partial melting (such as 10 or 15%) of the metasomatically enriched mantle.  相似文献   

Geomorphological modelling and nature protection in arctic and subarctic environments     

Alfred Jahn 《Geoforum》1976,7(2):121-137

Geomorphological processes of the cold climate zone can be divided into natural and accelerated. In the former group the following processes are being discussed: frost weathering, mass wasting, solifluction, landslides, slopewash and linear erosion and wind action. Processes of accelerated action are due to natural and artificial stimuli. To the former belong for instance heavy rains which sometimes occur in polar zones (as in Spitsbergen in July 1972). More frequent however, are, artificial causes which are connected with man's activities.The action of geomorphological processes is indicated by the denudational balance which can be either positive or negative. The author has determined this balance for some regions of the Arctic and Subarctic, especially for Siberia. The balance is positive (degradation) in the zone of polar desert and in the oceanic part of the Arctic, and also in high mountains of this zone. Negative denudation balance prevails in the continental section of the Subarctic. The areas of positive denudational balance are threatened with nature destruction and therefore particularly deserve to be covered by a nature protection scheme.  相似文献   

On the Age of Rhodesian Greenstone Belts     

Bor-Ming Jahn  Kent C. Condie 《Contributions to Mineralogy and Petrology》1976,55(3):317-330

Deformation experiments have been carried out on a microgranodiorite under undrained conditions at a pressure of 0.448 GPa and temperatures up to 720 °C. The granodiorite contained a small amount of chlorite and amphibole minerals which decomposed at elevated temperatures, giving rise to a pore pressure. Evidence about the decomposition reaction was obtained from differential thermal analysis and optical microscopy. In some experiments small amounts of free water were enclosed with the sealed rock sample. The strength (at fracture or 2 per cent strain) fell to very low values (0.02 GPa) at 700 °C, and at temperatures of 670 °C and above there was evidence of partial melting which led to ductile behaviour. At somewhat lower temperatures the rock was relatively weak but brittle.  相似文献   

Zircon U–Pb dating of eclogite from the Qiangtang terrane,north-central Tibet: a case of metamorphic zircon with magmatic geochemical features     

Zhai  Qing-guo  Jahn  Bor-ming  Li  Xian-hua  Zhang  Ru-yuan  Li  Qiu-li  Yang  Ya-nan  Wang  Jun  Liu  Tong  Hu  Pei-yuan  Tang  Suo-han 《International Journal of Earth Sciences》2017,106(4):1239-1255

International Journal of Earth Sciences - Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin...  相似文献   

Desalinization of running waters: II. Benthic diatom communities: A comparative field study on responses to decreasing salinities     

Svenja Busse  Regine Jahn  Claus-Jürgen Schulz   《Limnologica》1999,29(4):465-474

Desalinization effects on benthic diatom communities were investigated in two adjacent streams in Northern Thuringia, the creek Urbach and the river Helbe. Salt-saturated stockpile effluents had been introduced into the Urbach until 1996, resulting in fluctuating chloride concentrations with maximum values up to 25.4 g · 1⁻¹ (Helbe: max. 3.7 g · 1⁻¹).

The diatom vegetation and hydrographical and chemical data of an Urbach site were compared to those of a sampling site further downstream. Monthly sampling in 1996 started at the end of salt influx in January. Reference samples from the years 1995, 1986 and 1963 were evaluated along with the 1996 samples.

After termination of the introduction of salt-loaded effluents, relative abundances of brackish water species greatly decreased within three months. The Halobion Index (Ziemann 1982), reflecting osmotic and ion composition effects on diatoms, decreased until May 1996, then stabilized. In summer 1996, several diatoms had a mass development, showing that salinization impacts had previously overruled effects of other environmental variables.  相似文献   

Magnetic starspots     

K. Jahn  K. St&#x;pie&#x; 《Astronomische Nachrichten》1984,305(5):250-250

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Paleozoic tectonics of the southern Chinese Tianshan: Insights from structural,chronological and geochemical studies of the Heiyingshan ophiolitic mélange (NW China)     

Bo Wang  Liangshu Shu  Michel Faure  Bor-ming Jahn  Dominique Cluzel  Jacques Charvet  Sun-lin Chung  Sébastien Meffre 《Tectonophysics》2011,497(1-4):85-104

In the southern Chinese Tianshan, the southernmost part of the Central Asian Orogenic Belt (CAOB), widespread ophiolitic mélanges form distinct tectonic units that are crucial for understanding the formation of the CAOB. However, the timing of tectonic events and the subduction polarity are still in controversy. In order to better understand these geological problems, a comprehensive study was conducted on the Heiyingshan ophiolitic mélange in the SW Chinese Tianshan. Detailed structural analysis reveals that the ophiolitic mélange is tectonically underlain by sheared and weakly metamorphosed pre-Middle Devonian rocks, and unconformably overlain by non-metamorphic and undeformed lower Carboniferous (Serpukhovian) to Permian strata. The igneous assemblage of the mélange comprises OIB-like alkali basalt and andesite, N-MORB-like tholeiitic basalt, sheeted diabase dikes, cumulate gabbro and peridotite. Mafic rocks display supra-subduction signatures, and some bear evidence of contamination with the continental crust, suggesting a continental marginal (back-arc) basin setting. Zircons of a gabbro were dated at 392 ± 5 Ma by the U–Pb LA-ICP-MS method. Famennian–Visean radiolarian microfossils were found in the siliceous matrix of the ophiolitic mélange. Mylonitic phyllite which displays northward-directed kinematic evidence yielded muscovite ⁴⁰Ar/³⁹Ar plateau ages of 359 ± 2 Ma and 356 ± 2 Ma.These new data, combined with previously published results, suggest that the mafic protoliths originally formed in a back-arc basin in the Chinese southern Tianshan during the late Silurian to Middle Devonian and were subsequently incorporated into the ophiolitic mélange and thrust northward during the Late Devonian to early Carboniferous. Opening of the back-arc basin was probably induced by south-dipping subduction of the Paleo-Tianshan Ocean in the early Paleozoic, and the Central Tianshan block was rifted away from the Tarim block. Closure of the back-arc basin in the early Carboniferous formed the South Tianshan Suture Zone and re-amalgamated the two blocks.  相似文献   

Mesozoic age of the Gilyui Metamorphic Complex in the junction zone of the Selenga–Stanovoi and Dzhugdzhur–Stanovoi superterranes,Central Asian fold belt     

S. D. Velikoslavinskii  A. B. Kotov  V. P. Kovach  A. M. Larin  A. A. Sorokin  A. P. Sorokin  E. V. Tolmacheva  E. B. Salnikova  K. L. Wang  B. M. Jahn  S. L. Cung 《Doklady Earth Sciences》2016,468(2):561-565

The Gilyui Complex includes sedimentary and volcanic rocks metamorphosed to amphibolite and epidote–amphibolite facies, which constitute blocks confined to the main structural sutures of the Dzhugdzhur–Stanovoi superterrane in the Central Asian fold belt. In recent stratigraphic scales, they are considered as being Neoarchean in age with Nd model age values of 1.5–3.0 Ga. The youngest detrital zircons from metamorphosed mudstone of the Gilyui Complex yield a date of 285 ± 4 Ma, which determines the lower age limit for the formation of its protolith. The age of crystallization of rhyolites from the Gilyui Complex is determined to be 231 ± 4 Ma. If the rhyolites form volcanic flow units or sills, the Gilyui Complex is approximately 230 Ma or 231 ± 4 to 285 ± 4 Ma old, respectively.  相似文献   

Phase transitions in the system CaCO<Subscript>3</Subscript> at high P and T determined by in situ vibrational spectroscopy in diamond anvil cells and first-principles simulations     

Monika Koch-Müller  Sandro Jahn  Natalie Birkholz  Eglof Ritter  Ulrich Schade 《Physics and Chemistry of Minerals》2016,43(8):545-561

The stability of the high-pressure CaCO₃ calcite (cc)-related polymorphs was studied in experiments that were performed in conventional diamond anvil cells (DAC) at room temperature as a function of pressure up to 30 GPa as well as in internally heated diamond anvil cells (DAC-HT) at pressures and temperatures up to 20 GPa and 800 K. To probe structural changes, we used Raman and FTIR spectroscopy. For the latter, we applied conventional and synchrotron mid-infrared as well as synchrotron far-infrared radiation. Within the cc-III stability field (2.2–15 GPa at room temperature, e.g., Catalli and Williams in Phys Chem Miner 32(5–6):412–417, 2005), we observed in the Raman spectra consistently three different spectral patterns: Two patterns at pressures below and above 3.3 GPa were already described in Pippinger et al. (Phys Chem Miner 42(1):29–43, 2015) and assigned to the phase transition of cc-IIIb to cc-III at 3.3 GPa. In addition, we observed a clear change between 5 and 6 GPa that is independent of the starting material and the pressure path and time path of the experiments. This apparent change in the spectral pattern is only visible in the low-frequency range of the Raman spectra—not in the infrared spectra. Complementary electronic structure calculations confirm the existence of three distinct stability regions of cc-III-type phases at pressures up to about 15 GPa. By combining experimental and simulation data, we interpret the transition at 5–6 GPa as a re-appearance of the cc-IIIb phase. In all types of experiments, we confirmed the transition from cc-IIIb to cc-VI at about 15 GPa at room temperature. We found that temperature stabilizes cc-VI to lower pressure. The reaction cc-IIIb to cc-VI has a negative slope of ?7.0 × 10^?3 GPa K^?1. Finally, we discuss the possibility of the dense cc-VI phase being more stable than aragonite at certain pressure and temperature conditions relevant to the Earth’s mantle.  相似文献