Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley,South Tianshan,NW China: phase equilibria and P–T path     

Z. L. TIAN  C. J. WEI 《Journal of Metamorphic Geology》2013,31(3):281-300

Eclogites from the Kebuerte Valley, Chinese South Tianshan, consist of garnet, omphacite, phengite, paragonite, glaucophane, hornblendic amphibole, epidote, quartz and accessory rutile, titanite, apatite and carbonate minerals with occasional presence of coesite or quartz pseudomorphs after coesite. The eclogites are grouped into two: type I contains porphyroblastic garnet, epidote, paragonite and glaucophane in a matrix dominated by omphacite where the proportion of omphacite and garnet is >50 vol.%; and type II contains porphyroblastic epidote in a matrix consisting mainly of fine‐grained garnet, omphacite and glaucophane where the proportion of omphacite and garnet is <50 vol.%. Garnet in both types of eclogites mostly exhibits core–rim zoning with increasing grossular (X_gr) and pyrope (X_py) contents, but a few porphyroblastic garnet grains in type I eclogite shows core–mantle zoning with increasing X_py and a slight decrease in X_gr, and mantle–rim zoning with increases in both X_gr and X_py. Garnet rims in type I eclogite have higher X_py than in type II. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for three representative samples suggest that the eclogites have experienced four stages of metamorphism: stage I is the pre‐peak temperature prograde heating to the pressure peak (P_max) which was recognized by the garnet core–mantle zoning with increasing X_py and decreasing X_gr. The P–T conditions at P_max constrained from garnet mantle or core compositions with minimum X_gr content are 29–30 kbar at 526–540 °C for type I and 28.2 kbar at 518 °C for type II, suggesting an apparent thermal gradient of ～5.5 °C km^?1. Stage II is the post‐P_max decompression and heating to the temperature peak (T_max), which was modelled from the garnet zoning with increasing X_gr and X_py contents. The P–T conditions at T_max, defined using the garnet rim compositions with maximum X_py content and the Si content in phengite, are 24–27 kbar at 590 °C for type I and 22 kbar at 540 °C for type II. Stage III is the post‐T_max isothermal decompression characterized by the decomposition of lawsonite, which may have resulted in the release of a large amount of fluid bound in the rocks, leading to the formation of epidote, paragonite and glaucophane porphyroblasts. Stage IV is the late retrograde evolution characterized by the overprint of hornblendic amphibole in eclogite and the occurrence of epidote–amphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The P–T estimates obtained from conventional garnet–clinopyroxene–phengite thermobarometry for the Tianshan eclogites are roughly consistent with the P–T conditions of stage II at T_max, but with large uncertainties in temperature. On the basis of these metamorphic stages or P–T paths, we reinterpreted that the recently reported zircon U–Pb ages for eclogite may date the T_max stage or the later decompression stage, and the widely distributed (rutile‐bearing) quartz veins in the eclogite terrane may have originated from the lawsonite decomposition during the decompression stage rather than from the transition from blueschist to eclogite as previously proposed.  相似文献   

新疆北天山巴音沟蛇绿岩的地质特征   总被引：3，自引：1，他引：3  

邬继易  刘成德 《岩石学报》1989,5(2):76-87

巴音沟蛇绿岩虽受强热构造作用肢解,但仍保存有较完整的蛇纹石化超基性岩、状层辉长岩、基性熔岩(下部块状、上部枕状)和放射虫硅质岩的层序组合。岩石化学、地球化学、放射虫等古生物资料表明,它代表一个中石炭世陆缘海盆迅速扩张形成的洋壳和上地幔的残片。其侵位发生在中石炭世未海盆的封闭期间。  相似文献   

Origin of calcium in aerosols over the western North Pacific     

T. Suzuki  S. Tsunogai 《Journal of Atmospheric Chemistry》1988,6(4):363-374

Weekly aerosol samples were collected from March 1981 to June 1983 at the six stations in the western North Pacific region and analyzed for Ca and Na. By coupling data with those previously reported for Al (Tsunogai et al., 1985), the following results and conclusion have been obtained. There was a positive correlation between the atmospheric concentration of Al and the concentration of nonsea salt Ca (nssCa). The nssCa/Al ratios from the six stations, however, considerably varied (from 0.84±0.36 to 3.00±1.91), and the ratios were usally larger than those of the crustal average or of usual soil in Japan. The Ca/Al ratios of Asian desert soil and loess vary from 0.52 to 1.29, which are similar to the nssCa/Al ratios of aerosols in the surface air over the western North Pacific region except at Onna, Okinawa. The exception may be due to a local effect of coral. These results suggest that a large part of nonsea salt Ca in the surface air over the western North Pacific is derived from arid regions in Asia and that the nssCa/Al ratio in aerosol varies with that of the source material.  相似文献   

周口店新发现距今11～1.9万年的洞穴堆积层的成因及年代学研究     

田明中  曹伯勋 《现代地质》1993,7(4):421-427

本文对周口店新发现距今11～1.9万年的东岭子洞穴堆积物进行了成因研究,认为该套堆积物并非全属岩溶堆积物,其中也有风成黄土沉积。应用古地磁、热发光及铀系法等年代学方法测定,认为这套堆积为距今11～1.9万年间沉积,弥补了周口店地区近5万年的晚更新世洞穴地层空缺,是一个含有丰富地质事件的晚更新世洞穴地层剖面。  相似文献   

天山南北麓山前拗陷的第四纪构造变动     

王永兴  王树基 《干旱区地理》1994,(4)

本文在大量实际调查资料的基础上，详细论述了天山南北麓库车山前拗陷和乌鲁木齐山前拗陷在第四纪时期主要的构造变动及形成的最新构造形迹，并利用石油地质研究的最近成果对山前拗陷中最新构造的形成、演化以及褶皱构造与基底断裂活动的关系进行了讨论。  相似文献   

天山高山冰缘环境的融冻泥流作用   总被引：2，自引：0，他引：2  

刘耕年  熊黑钢 《地理学报》1994,49(4):363-370

根据实地观测结果分析，本文得出：天山冰缘环境发育的融冻泥流分布在海拔２５００─３６００ｍ间的阴坡。融冻泥流发育要求地下含水丰富，粒度以细粉砂为主，地表坡度１０─３１°。融冻泥流的形态多数为长宽比小于１的叶状，少量为舌状。融冻泥流表面平均运动速度为１１．１４ｃｍ／ａ，舌前缘前进的平均速度的１．８６ｃｍ／ａ，中轴部位的运动速度比两侧大。  相似文献   

A geomagnetic event recorded in loess deposits of the Tönchesberg (Germany): identification of the Blake magnetic polarity episode     

Jan Reinders  Ulrich Hambach 《Geophysical Journal International》1995,122(2):407-418

  相似文献   

Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China   总被引：5，自引：0，他引：5  

Christopher P. Hunt  Subir K. Banerjee  Jiamao Han  Peter A. Solheid  Eric Oches  Weiwei Sun  Tungsheng Liu 《Geophysical Journal International》1995,123(1):232-244

Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining-one of the few palaeoclimate sites in the currently arid western Loess Plateau of China-illustrate the importance of making many types of rock-magnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction.
The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. the resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. the data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites.
In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.  相似文献   

天山第四纪构造地貌演化：活动性陆内古板缘的一种造山模式     

王永兴 《干旱区地理》1995,18(1):27-35

本文在对天山构造地貌形成、演化及构造地貌特征分析的基础上，对天山作为陆内古板缘山的构造、边界及动力条件进行了研究，进而提出了活动性陆内古板缘山地的造山旋回模式。  相似文献   

我国天山山区公路山地灾害及其防治     

朱诚 《山地学报》1993,(2)

天山山区公路山地灾害主要出现于海拔>3000米的现代冰缘区,遭山地灾害威胁较严重的是独山子—库车公路和乌鲁木齐—库尔勒公路。较突出的山地灾害是雪崩、风吹雪、自然积雪、泥石流与石冰川,其次是倒石堆、热融滑塌和热融掏蚀。该区公路山地灾害防治应采取预防为主、防治结合的原则,此外应加强灾害预报。  相似文献