Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane,south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting     

《Gondwana Research》2017

Post-collisional ultrapotassic magmatic rocks (15.2–18.8 Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo_87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo_77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5 wt%), Ni (145–346 ppm), Cr (289–610 ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (⁸⁷Sr/⁸⁶Sr)_i (0.7167–0.7274) and unradiogenic (¹⁴³Nd/¹⁴⁴Nd)_i (0.5118–0.5120), high (²⁰⁷Pb/²⁰⁴Pb)_i (15.740–15.816) and (²⁰⁸Pb/²⁰⁴Pb)_i (39.661–39.827) at a given (²⁰⁶Pb/²⁰⁴Pb)_i (18.363–18.790) with high δ¹⁸O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5 wt%) subgroup displays higher (¹⁴³Nd/¹⁴⁴Nd)_i, lower (⁸⁷Sr/⁸⁶Sr)_i and (²⁰⁶Pb/²⁰⁴Pb)_i ratios and lower δ¹⁸O values compared with the low-MgO (4.6–8.8 wt%) subgroup. High Ni (850–4862 ppm) contents of olivine phenocrysts and high whole-rock SiO₂, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO₂, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65 vol%) and orthopyroxene (30–50 vol%) with minor phlogopite (< 3 vol%) and quartz (< 2 vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75 vol%), orthopyroxene (20–35 vol%), clinopyroxene (< 3 vol%), phlogopite (< 2 vol%) and spinel (< 2 vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8 Ma in south Tibet.  相似文献   

Late Early Cretaceous magmatic rocks (118–113 Ma) in the middle segment of the Bangong–Nujiang suture zone,Tibetan Plateau: Evidence of lithospheric delamination     

《Gondwana Research》2017

Early Cretaceous magmatic rocks (including andesites, rhyolites, and granodiorites) occur in the Jiang Co and Zigetang Co areas of the middle segment of the Bangong–Nujiang suture zone, Tibet, and zircon UPb dating reveals that the magmatism took place between 118 and 113 Ma. The Zigetang Co andesites have geochemical features of the high-Mg adakitic andesites (HMAA) and are characterized by high K₂O (2.95–3.58 wt%), Th (12.5–15.0 ppm), MgO (2.50–3.31 wt%), and Mg# (58–59), and relatively juvenile ε_Hf(t) (+ 2.7 to + 6.4) isotopic compositions. These observations suggest that the andesites were derived from partial melting of the delaminated juvenile lower continental crust of the northern Lhasa terrane. The Zigetang Co granodiorites represent the melts generated by high-degree fractional crystallization of the HMAA magma. The andesites from the Jiang Co area are interpreted as deriving from partial melting of ancient heterogeneous subduction-modified lithospheric mantle. The Jiang Co rhyolites are probably products of crustal anatexis in a heterogeneous source. Taking into account previous data, we propose that these late Early Cretaceous magmatic rocks developed in a postcollisional tectonic setting and that they were related to the delamination of thickened lithosphere following the final Lhasa-Qiangtang amalgamation.  相似文献   

Singularity analysis of global zircon U-Pb age series and implication of continental crust evolution     

《Gondwana Research》2017

Recently compiled global databases of igneous and detrital zircon U-Pb ages have been integrated with other types of isotope data (e.g., neodymium, hafnium, and oxygen) to characterize the episodic growth of the continental crust and the development of supercontinents. However, to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long-standing question. Instead of analyzing amplitudes and periodicity of the age series, here, the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time-series records. The results indicate that zircon age distributions around peaks follow power law distribution, regardless of the bin-size used to measure the age distribution. Based on the commonly accepted mechanisms (phase transition, self-organized criticality and multiplicative cascade processes) for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to “avalanches” (superplumes, slab breakoff etc.) occurred during episodic convection of the mantle. The exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity. The values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature, with a mean duration of approximately 600–800 Myr. Both intensity of the zircon episodes and their duration for the interval from 3 Ga to 0.5 Ga depict a descending trend, which may signify mantle cooling.  相似文献   

Origin and tectonic setting of the giant Duolong Cu–Au deposit,South Qiangtang Terrane,Tibet: Evidence from geochronology and geochemistry of Early Cretaceous intrusive rocks     

《Ore Geology Reviews》2017

We constrain the origin and tectonic setting of the giant Duolong porphyry–epithermal Cu–Au deposit in the South Qiangtang Terrane of northern Tibet, based on new zircon U–Pb ages and Hf isotopic data, as well as whole-rock major and trace element data from poorly studied ore-associated intrusions in the Duolong area. The LA–ICP–MS zircon U–Pb dating indicates that the ore-associated rocks formed between 121 and 126 Ma. These ore-associated rocks are geochemically similar to low-K tholeiitic M-type granitoids and to mid- to high-K, calc-alkaline I-type granitoids. They have variable and predominantly positive zircon ε_Hf(t) values (− 1.4 to + 15.6) and variable crustal model ages (T^C_DM(Hf); 176–1122 Ma). Taking into account previous data and the regional geology of the study area, we propose that the ore-associated rocks originated from fractional crystallization of mantle-derived mafic melts and magma mixing of mantle-derived mafic and hybrid lower crust-derived felsic melts, and the hybrid lower crust included a mix of juvenile and older continental material. The Duolong porphyry–epithermal Cu–Au deposit formed within an ‘ensialic forearc’ of an active continental margin as a result of the northwards subduction of the Bangong–Nujiang Ocean crust beneath the South Qiangtang Terrane.  相似文献   

8.4 ka 以来纳木错湖芯介形类组合的环境变化意义   总被引：1，自引：1，他引：0  

谢曼平  朱立平  彭萍  甄晓林  汪勇  鞠建廷  A.CHWALB 《地理学报》2008,63(9):931-944

对青藏高原中南部纳木错长332cm的NMLC1孔湖芯研究发现,沉积物的介形类动物群计有6属15种。介形类属种生态特征和组合变化分析表明这些介形类对环境条件具有敏感性,其组合能够很好地反映过去环境变化的特征。结果表明,8．4ka以来具有三个不同的环境变化时期：早期在8400～6800aBP,湖泊由浅向深发展,环境具有相对冷湿的特征：中期在6800～2500aBP,湖泊深度逐渐加大,环境经历了暖湿-冷湿-冷干的变化．其中由冷湿向冷干的转化奠定了纳木错现今环境条件的基础：晚期在2500--9aBP,湖泊深度继续增加,这个时期的较早阶段,继续保持了前一时期的冷干特点,但湖水盐度可能开始增加．较晚阶段的冷干化加剧,陆面流水的活跃性大大降低。研究发现,纳木错NMLC1钻孔介形类黑色壳体的高峰值与介形类的最大生产量相一致,并且与沉积水动力条件增强相适应．指示了这些黑壳的产生与介形类的大量繁殖处于同一阶段．并且主要为异地搬运为主。钻孔中出现大量Candona幼虫壳体．其原因可能与沉积环境的水动力条件迅速改变有关。  相似文献   

The response of lake-glacier variations to climate change in Nam Co Catchment, central Tibetan Plateau, during 1970–2000     

WU Yanhong  ZHU Liping 《地理学报》2008,18(2):177-189

Based upon the 1970 aero-photo topographic map, and TM/ETM satellite images taken in 1991 and 2000, the authors artificially interpreted boundaries of lake and glaciers in Nam Co Catchment, and quantified lake-glacier area variations in different stages by “inte-grated method” with the support of GIS. Results show that from 1970 to 2000, lake area in-creased from 1942.34 km2 to 1979.79 km2 at a rate of 1.27 km2/a, while glacier area de-creased from 167.62 km2 to 141.88 km2 at a rate of 0.86 km2/a. The increasing rate of lake in 1991–2000 was 1.76 km2/a that was faster than 1.03 km2/a in 1970–1991, while in the same period of time, the shrinking rates of glaciers were 0.97 km2/a and 0.80 km2/a respectively. Important factors, relevant to lake and glacier response to the climate, such as air tempera-ture, precipitation, potential evapotranspiration and their values in warm and cold seasons, were discussed. The result suggests that temperature increasing is the main reason for the accelerated melting of glaciers. Lake expansion is mainly induced by the increase of the gla-cier melting water, increase of precipitation and obvious decrease of potential evapotranspi-ration. Precipitation, evaporation and their linkages with lake enlargement on regional scale need to be thoroughly studied under the background of global warming and glacier retreating.  相似文献   

Optically stimulated luminescence and radiocarbon dating of sediments from Lop Nur (Lop Nor), China     

《Quaternary Geochronology》2012

Lop Nur is a playa lake occupying the lowest part of the Tarim Basin, northwestern China, and is now a desolate and barren region. In the past decades, the ages of the lacustrine sediments from the lake were determined mainly by radiocarbon dating on bulk sediment. In this study, both optically stimulated luminescence (OSL) and radiocarbon methods were used to date the sediments from a pit in the central part of the lake. The OSL ages obtained for ten samples range from 0.5 to 9.4 ka, and are in stratigraphic order except for one sample. The ¹⁴C ages obtained for twenty-two bulk sediment samples range from 5.8 to 30.2 cal ka BP with erratic distribution. Based on the comparison of ¹⁴C with OSL ages and their age-depth models, we argue that the OSL ages are relatively reliable. The disequilibria in the U decay chain for some samples are deduced from the comparison of the NAA and TSAC results. We suggest that the OSL dating technique should preferably be applied to the playa sediments from Lop Nur, but the disequilibria in the U decay chain should be considered in evaluating dose rates. Additionally, radiocarbon reservoir effects in lakes in western China are reviewed.  相似文献   

Late pleistocene evolution of Lake Manas in western China with constraints of OSL ages of lacustrine sediments     

《Quaternary Geochronology》2012

Lake Manas, located in northwestern China, has been formed in a structure depression since early Quaternary. Optically stimulated luminescence (OSL) dating is applied to the lacustrine and aeolian deposits collected from the lake to construct a chronological frame for lake level fluctuations since the last inter-glacial period. The results yielded two stages of high lake stands (∼20 m above the present lake bed) occurred before 66 ka and 38–27 ka ago. No evidence for Holocene high lake level was found in the studying area.  相似文献   

持久性有机污染物在森林生态系统中的环境行为研究     

薛永刚  龚平  王小萍  姚檀栋 《地理科学进展》2013,32(2):278-287

持久性有机污染物(POPs)具有环境持久性和半挥发性, 可以在区域及全球范围内传输和分布。森林植被和林下土壤富含有机质, 森林生态系统因此成为POPs主要的储存库之一。植被叶片可快速吸附大气POPs, 并通过叶片凋落、雨水冲刷和干沉降等过程加强或加速大气POPs 向地面的沉降, 并使森林土壤成为POPs 的“汇”, 从而形成所谓的“森林过滤效应”, 进而影响POPs 在全球的分布。进入森林的POPs 在森林生态系统中将经历一系列的环境过程。本文简要介绍了森林过滤效应的特征和影响因素, 综述了叶片对大气POPs 的吸附、叶片凋落和干湿沉降、POPs 在土壤中的迁移和损失等3 个主要环境过程的研究进展, 报道了松针、树皮和苔藓作为被动采样器反映的森林POPs空间分布趋势。最后, 提出了森林POPs研究中亟待解决的科学问题, 并指出未来中国森林POPs研究的可能方向。  相似文献   

冰芯和气象记录揭示的青藏高原百年来典型冷暖时段气候变化特征     

德吉  姚檀栋  姚平  陈昱凝 《冰川冻土》2013,35(6):1382-1390

气候冷暖变化问题是全球科学家研究的一个聚焦点,但高海拔地区的气候变化过程尚不十分清楚,作为全球气候变化的敏感区的青藏高原更是如此. 以青藏高原北部的古里雅冰芯、唐古拉冰芯和南部的达索普冰芯、宁金岗桑冰芯δ¹⁸O记录作为温度代用指标,同时结合青藏高原西北缘的吉尔吉斯斯坦Naryn站长期气象记录和北半球同时期的气温变化进行比较,研究了过去100 a来青藏高原北部和南部的温度变化. 结果显示：青藏高原过去100 a来共出现1910年左右、1920年左右、1950年左右、1970年代4个冷期,各冷期之间对应出现4次暖期,并且变冷的程度越来越弱而变暖的程度越来越强. 其次,青藏高原气候的变冷变暖在不同地区和不同时段差异很大：从空间尺度上看,青藏高原北部变暖过程比南部更强烈;从时间尺度上看,1910年左右和1920年左右的两次变冷十分明显,但1950年左右和1970年代的两次变冷不明显. 另外,虽然有发生在1990年代早期的短暂降温过程,但与其说是一个冷事件,还不如说是一次变暖过程中的短暂停顿,随后表现为持续升温.  相似文献