A Preliminary Study on Paleomagnetism and Rock Magnetism of Eclogite from the Maobei Area

A preliminary study of paleomagnetism and rock magnetism has been done on 55 eclogite samples collected from the Chinese Continental Scientific Drilling (CCSD) site at the Maobei (毛北) area, Donghai (东海) County, Jiangsu (江苏) Province. Also the isothermal remanence, hysteresis loop, magnetic fabric, thermal susceptibility were measured, and analyses were made by electron-probe and scanning electric microscope on some samples synchronously. The result indicates that there are two groups of stable remanence, the normal and reversed magnetization. The remanence orientations are: D=94.3o, I=-29.1o and D=273.7o, I=15.4o, respectively. The magnetization intensity and the density of the samples which carry the normal magnetization are very different from those bearing reversed magnetization. The magnetic anisotropy is weak, and the minimum axis is hardly determined. The isothermal remanence and the hysteresis loop show that the magnetic carriers of the eclogite are likely SD (single domain) and PSD (pseudo-single domain) magnetite. According to the magnetic property, the cause of formation of magnetic carriers, the mechanism of the remanence, and the significance for the tectonics are discussed.     

Rheological Strength of UHP Eclogite from Dabieshan: Evidences from High p-T Experiments

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Oxygen, carbon, and strontium isotope geochemistry of diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav Massif, Kazakhstan

Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with δ¹⁸O_VSMOW values as low as +9‰, δ¹³C_VPDB=−9‰, and ⁸⁷Sr/⁸⁶Sr as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of ¹³C/¹²C fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals.     

Early Cooling History of Eciogites from the Dabie-Sulu Orogen： Constraints from Diffusion Kinetics of Garnet

Abstract For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie‐Sulu orogen. Various zonings of cation diffusion were preserved in the garnets, enabling the acquirement of average cooling rates for the high‐to ultrahigh‐pressure rocks without using geochronological approaches. The coesite‐bearing hot eclogites yield fast cooling rates of about 20 to 30°C/Ma subsequent to peak metamorphic temperatures, whereas the cold eclogite gives a relatively slow cooling rate of 8°C/Ma at its initial exhumation. A very slow cooling rate of <0.3°C/Ma is obtained for the granulite at Huangtuling, suggesting that the granulite may not be involved in the continental deep subduction.     

Petrogenesis and mantle source characteristics of the late Cenozoic Baekdusan (Changbaishan) basalts,North China Craton

Late Cenozoic intraplate basaltic rocks in northeastern China have been interpreted as being derived from a mantle source composed of DMM and EM1 components. To constrain the origin of the enriched mantle component, we have now determined the geochemical compositions of basaltic rocks from the active Baekdusan volcano on the border of China and North Korea. The samples show LREE-enriched patterns, with positive Eu and negative Ce anomalies. On a trace element distribution diagram, they show typical oceanic island basalt (OIB)-like LILE enrichments without significant Nb or Ta depletions. However, compared with OIB, they show enrichments in Ba, Rb, K, Pb, Sr, and P. The Nb/U ratios are generally within the range of OIB, but the Ce/Pb ratios are lower than those of OIB. Olivine phenocrysts are characterized by low Ca and high Ni contents. The radiogenic isotopic characteristics (⁸⁷Sr/⁸⁶Sr = 0.70449 to 0.70554; ε_Nd = −2.0 to +1.8; ε_Hf = −1.7 to +6.1; ²⁰⁶Pb/²⁰⁴Pb = 17.26 to 18.12) suggest derivation from an EM1-like source together with an Indian MORB-like depleted mantle. The Mg isotopic compositions (δ²⁶Mg = −0.39 ± 0.17‰) are generally lower than the average upper mantle, indicating carbonates in the source. The ⁸⁷Sr/⁸⁶Sr ratios decrease with decreasing δ²⁶Mg values whereas the ¹⁴³Nd/¹⁴⁴Nd and (Nb/La)_N ratios increase. These observations suggest the mantle source of the Baekdusan basalts contained at least two components that resided in the mantle transition zone (MTZ): (1) recycled subducted ancient (∼2.2–1.6 Ga) terrigenous silicate sediments, possessing EM1-like Sr–Nd–Pb–Hf isotopic signatures and relatively high values of δ²⁶Mg; and (2) carbonated eclogites with relatively MORB-like radiogenic isotopic compositions and low values of δ²⁶Mg. These components might have acted as metasomatizing agents in refertilizing the asthenosphere, eventually influencing the composition of the MTZ-derived plume that produced the Baekdusan volcanism.     

苏—鲁超高压变质岩带从上地幔返回地表的证据

印支期苏—鲁超高压榴辉岩带是初始欧亚板块东部大陆边缘在与古太平洋板块强烈挤压作用过程中,由黄海—苏东北地体A—型俯冲至上地幔形成的。其回返地表主要是在中—新生代由弧后扩张作用引起的地壳拉伸、增长、减薄过程中实现的。中—晚白垩世非造山的碱性花岗岩带与榴辉岩带的重叠、晚白垩世红色盆地、第三纪巨厚沉积岩系和新生代大陆裂谷带玄武岩等的形成和发育等种种地质证据表明俯冲至上地幔中的榴辉岩带后来被向上拉伸至地表的。     

An example of high-pressure low-temperature metamorphic rocks from an island-arc: the Paikon Series (Innermost Hellenides, Greece)

Abstract The Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn- to post-kinematic with respect to a pre-Tithonian tectonic phase and range from the lawsonite-chlorite-albite facies through transitional Na-amphibole-greenschist facies to the chlorite sub-zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thrusting, is suspected.     

西藏松多榴辉岩的矿物原位微量元素地球化学研究

对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。     

Multistage metamorphism and deformation in high‐pressure metabasites of the northern Adula Nappe Complex (Central Alps,Switzerland)

Metabasites from the northern Adula Nappe Complex (ANC) display a complex microstructural evolution recording episodes of deformation and metamorphic re‐equilibration that were obliterated in the surrounding phengite‐bearing schists. Pre‐D1 and D1 deformation episodes are preserved as mineral inclusions within garnet cores of some amphibole‐bearing eclogites and record high‐temperature greenschist‐/amphibolite‐facies conditions. D2 produced an eclogite‐facies foliation which developed at 580 ± 70°C and 19 ± 3 kbar. D3 was a composite deformation episode which can be divided into three sub‐episodes D3m, D3a and D3b which occurred as the metamorphism evolved from post‐eclogitic high‐pressure and low‐temperature conditions through to amphibolite‐facies conditions at 590 ± 30°C and 11.7 ± 1.3 kbar. The D3 deformation episode was responsible for the development of the S3 regional‐scale foliation in the surrounding schists, whilst D4 caused the development of an S4 greenschist foliation. The composite nature of the D3 episode indicates that rocks of the northern ANC experienced a protracted post‐eclogitic structural reworking and that the current structure of this part of the Alps is a late‐Alpine feature. Copyright © 2010 John Wiley & Sons, Ltd.     

高压基性麻粒岩与榴辉岩的抬升转换作用--信阳超镁铁质火山角砾岩管的捕虏体研究

通过对华北地块与秦岭-大别造山带结合部位豫南信阳北超基性火山角砾岩管中基性麻粒岩、榴辉岩和石榴石辉石岩捕虏体的岩相学、矿物化学成分的初步研究和对比,发现这些基性岩石捕虏体中大多存在着因抬升降压而发生的转换反应作用.这些转换反应可能与华北地块和秦岭-大别造山带相互作用过程中的抬升过程有关.