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991.
鲁西孟家屯岩组中发现红柱石和锌尖晶石 总被引:1,自引:7,他引:1
侯增谦、高永丰、黄卫于2001年发表的《西藏高原雅鲁藏布江北岸蛇绿岩带的发现及其地质意义》一文中,把所发现的蛇绿岩带描述为“谢通门蛇绿岩带东起大竹卡区,西至谢通门以西,长数百千米,宽3~5km,由一系列自北而南逆冲推覆的构造岩片构成,其中,基性玄武熔岩岩片、巨厚辉长岩岩片、超镁铁质辉石岩-纯橄岩岩片,依次由雅江近岸至冈底斯弧内部呈叠瓦状排布。变质橄榄岩出露零星,少许孤立的岩体出露于雅江近岸,发育强烈碳酸盐化和蛇纹石化;大量散布的岩群出露于堆积岩岩片的北侧,多在冈底斯弧花岗岩基和弧火山-沉积岩系中呈… 相似文献
992.
吉林省中部红帘石硅质岩的特征及意义 总被引:10,自引:2,他引:10
吉林省中部磐石县境内烟筒山地区的红帘石硅质岩存在于古生代地层之中。地球化学研究发现,该硅质岩具有较低的SiO2(80%左右)和较高的Al2O3(10%左右)。微量元素中V的含量也较低,稀土元素中不出现明显的Ce负异常,表明其形成于与陆源物质关系密切的大陆边缘环境。岩石的强烈变形和无层序特点暗示该区的古生代地层可能大多为构造混杂岩,不具备传统的地层划分和对比意义。矿物温压计算结果显示,该硅质岩经历了约430℃、大于6.OGPa的中压相系变质作用。不存在其他高压变质作用矿物,可能反映了本区为兴蒙造山带中古亚洲洋最后闭合的地点所在,并推测其最后闭合的时间应在古生代末一中生代初期。 相似文献
993.
滇中妥甸组中首次发现昆虫化石 总被引:3,自引:1,他引:3
首次在云南滇中地区晚侏罗世妥甸组滥泥阱剖面和级山坡一带发现昆虫化石2新种:Mesobluttula tuodianensis sp.nov.和Neorthophlebia yunnanensis sp.nov.,未定种3种。其中长翅目新直脉蝎蛉新直脉蝎蛉属(Neorthophlebia)在国内为首次发现。 相似文献
994.
995.
Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya 总被引:15,自引:0,他引:15
C. G. Daniel L. S. Hollister R. R. Parrish D. Grujic 《Journal of Metamorphic Geology》2003,21(4):317-334
Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year?1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion. 相似文献
996.
Timing of Himalayan ultrahigh-pressure metamorphism: sinking rate and subduction angle of the Indian continental crust beneath Asia 总被引:28,自引:0,他引:28
Y. Kaneko I. Katayama H. Yamamoto K. Misawa M. Ishikawa H. U. Rehman A. B. Kausar K. Shiraishi 《Journal of Metamorphic Geology》2003,21(6):589-599
Coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from Higher Himalayan Crystalline rocks in the upper Kaghan Valley, north‐west Himalaya. The metamorphic peak conditions of the coesite‐bearing eclogites are estimated to be 27–32 kbar and 700–770 °C, using garnet–pyroxene–phengite geobarometry and garnet–pyroxene geothermometry, respectively. Cathodoluminescence (CL) and backscattered electron (BSE) imaging distinguished three different domains in zircon: inner detrital core, widely spaced euhedral oscillatory zones, and thin, broadly zoned outermost rims. Each zircon domain contains a characteristic suite of micrometre‐sized mineral inclusions which were identified by in situ laser Raman microspectroscopy. Core and mantle domains contain quartz, apatite, plagioclase, muscovite and rutile. In contrast, the rim domains contain coesite and minor muscovite. Quartz inclusions were identified in all coesite‐bearing zircon grains, but not coexisting with coesite in the same growth domain (rim domain). 206Pb/238U zircon ages reveal that the quartz‐bearing mantle domains and the coesite‐bearing rim were formed at c. 50 Ma and 46.2 ± 0.7 Ma, respectively. These facts demonstrate that the continental materials were buried to 100 km within 7–9 Myr after initiation of the India–Asia collision (palaeomagnetic data from the Indian oceanic floor supports an initial India‐Asia contact at 55–53 Ma). Combination of the sinking rate of 1.1–1.4 cm year?1 with Indian plate velocity of 4.5 cm year?1 suggests that the Indian continent subducted to about 100 km depth at an average subduction angle of 14–19°. 相似文献
997.
Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model. 相似文献
998.
B.P. Singh 《Earth and Planetary Science Letters》2003,216(4):717-724
The evolution of the Himalayan foreland is the result of continent-continent collision and related large-scale tectonics in the region. The initial foredeep basin sequences are exposed in limited areas of the western Himalaya, which makes these areas very significant in unraveling the earliest evolution of the foreland system. The basal interval of the Himalayan foredeep is exposed in the Jammu area (India), which preserves silicified breccia formed by the erosion of hanging walls of shallow faults. Two Paleocene sections are analyzed that suggest the existence of growth faults which developed in response to the India-Asia collision in the Late Paleocene (∼57.9-54.7 Ma). The pebble-size clasts and their derivation entirely from the basement demonstrate rapid sedimentation in response to rapid subsidence at the onset of basin evolution. The angular unconformity showing effects of erosion associated with a thin soil horizon may be due to a forebulge at the site of the unconformity. The reworked bauxite above this soil horizon demonstrates erosion of another forebulge from the cratonward side. 相似文献
999.
1000.
Houssa Ouali Bernard Briand Jean-Luc Bouchardon Paul Capiez 《Comptes Rendus Geoscience》2003,335(5):425-433
In southeastern Central Morocco, the Bou-Acila volcanic complex is considered of Cambrian age. In spite of low-grade metamorphic effect, initial volcanic texture and mineralogy can be recognized and volcanic rocks are dominated by dolerites and porphyric dolerites. The initial mineralogy is composed of plagioclases, pyroxenes and dark minerals. A secondary mineral assemblage is composed of albite, epidote, chlorite and calcite. According to their immobile elements compositions, the southeastern central Morocco metavolcanites are of within-plate continental tholeiites. This volcanism and those recognized in many other areas in Morocco confirm a Cambrian extensive episode within the Gondwana supercontinent. To cite this article: H. Ouali et al., C. R. Geoscience 335 (2003).To cite this article: H. Ouali et al., C. R. Geoscience 335 (2003). 相似文献