Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen,southern Central Asian Orogenic Belt:Constraints from P-T path and zircon U-Pb dating     

《地学前缘(英文版)》2020,11(4):1189-1201

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen,which connects the Tianshan Orogen to the west and the Mongolia-Xing'anling Orogen to the east.The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen.According to the microstructures,mineral relationships,and geothermobarometry,we identified four stages of mineral assemblages from the garnet amphibolite sample:(1) a pre-peak stage,which is recorded by the cores of garnet together with core-inclusions of plagioclase(Pl_1);(2) a peak stage,which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase(Pl_2)+amphibole(Amp_1)+Ilmenite(Ilm_1)+biotite(Bt_1),developed at temperature-pressure(P-T) conditions of 818.9-836.5℃ and7.3-9.2 kbar;(3) a retrograde stage,which is recorded by garnet rims + plagioclase(Pl_3)+amphibole(Amp_2)+orthopyroxene(Opx_1)+biotite(Bt_2)+Ilmenite(Ilm_2),developed at P-T conditions of 796.1-836.9℃ and5.6-7.5 kbar;(4) a symplectitic stage,which is recorded by plagioclase(Pl_4)+orthopyroxene(Opx_2)+amphibole(Amp_3)+biotite(Bt_3) symplectites,developed at P-T conditions of 732 ±59.6℃ and 6.1 ±0.6 kbar.Moreover,the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ±4.7 Ma for the protolith and 281.4±8.5 Ma for the peak metamorphic age.Therefore,the mineral assemblage,P-T conditions,and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t(Pressure-Temperature-time) path,indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian,then the southern Beishan area underwent a process of thinning of the continental crust.  相似文献   

福建北部近岸东台山岛火山岩形成时代、地球化学与古太平洋板块俯冲的响应          下载免费PDF全文

王飞飞  张勇  韩宗珠  宋维宇  吴浩  宁泽 《中国地质》2020,47(5):1426-1437

福建北部沿岸岛屿岩石组合以晚中生代火成岩为主，研究认为是古太平洋俯冲消减的产物，对反演洋盆构造演化具有重要的指示意义。本次对其中的福建北部海域东台山岛上广泛发育的酸性火山岩进行了锆石U-Pb定年以及全岩主微量地球化学分析工作。2件年代学样品分别获得了92 Ma和86 Ma的锆石U-Pb年龄，确定东台山岛火山岩形成于晚白垩世。全岩地球化学特征指示火山岩样品以酸性钙碱性岩石为主，整体富集Rb、Ba等元素，亏损Nb、Ta、Sr、Eu等元素，显示弧型岩浆岩的地球化学组成。研究认为东台山岛火山岩起源于古老下地壳变沉积岩熔融，并在浅层岩浆房内经历了不同程度的结晶分异过程。结合区域上晚中生代岩浆作用由陆向海的时空迁移特征，福建北部沿岸岛屿火山岩形成的深部动力学机制应该与古太平洋俯冲过程中的板片回转过程相关。  相似文献   

大兴安岭南部二叠系—三叠系界线及沉积环境演化     

陈树旺  郑月娟  张健  张海华  张德军  苏飞  黄欣  公繁浩  甄甄 《中国地质》2020,47(3):821-838

北方陆相二叠系—三叠系界线研究是界线地层研究的重要问题之一,对采自内蒙古巴林右旗幸福之路组二段建组剖面的安山质岩屑晶屑凝灰岩开展锆石LA-ICP-MS U-Pb测年,结果为(255.5±1.1)Ma,火山岩上部多个层位粉砂质泥岩中发现孢粉化石,计17属20种,时代为晚二叠世—早三叠世,推断幸福之路组的时代为晚二叠世—早三叠世,大兴安岭地区二叠系—三叠系界线在幸福之路组内部,二者为整合接触。幸福之路组以红层做为划分标志,岩性为正常沉积碎屑岩夹火山岩,见大量泥灰岩夹层、钙质结核及干裂现象,反映了炎热、干燥及强氧化条件的气候环境,大兴安岭地区晚二叠世晚期(幸福之路组二段下部)就已出现反映干旱气候的红层沉积,与相邻的华北及新疆准噶尔盆地相似。  相似文献   

新疆别也萨麻斯矿区钽锰矿的矿物学特征及其TIMS U-Pb定年     

何晗晗  艾尔肯&#;吐尔孙  王登红  王瑞江  陈振宇 《岩矿测试》2020,39(4):609-619

钽矿是我国的紧缺资源,近年来对别也萨麻斯地区钽矿取得了找矿新进展,包括新矿点的发现以及花岗伟晶岩型稀有金属资源的找矿突破。区内伟晶岩脉广泛发育,为探究含矿脉体的成矿时代、查明区内典型铌钽矿物的矿物学特征,本文以L18号伟晶岩脉中的钽锰矿为研究对象,对其物理性质、化学成分、地质年代等进行了分析。应用电子探针测试钽锰矿的化学组成,热电离质谱法(TIMS)测定其U-Pb年龄,确定含矿脉体的形成年代。结果表明,研究区钽锰矿中Ta_2O_5含量为51.58%～74.80%,均值68.49%,Nb_2O_5含量为6.15%～27.63%;部分主量元素分布不均,未表现出规律的分带性,但矿物颗粒中心部位的CaO含量较边部低,横剖面上SiO_2含量相对稳定,TiO_2与WO_3显示不规律波动。这种特征表明钽锰矿并非单纯由结晶分异作用形成,而是可能受到了后期交代作用的影响。钽锰矿的U-Pb年龄为160Ma,说明钽锰矿化发生于晚侏罗世早期,与围岩海西期二云母花岗岩相差甚远,后者并非L18号脉体的成矿母岩。  相似文献   

云南哀牢山-红河剪切带新生代富碱侵入岩中榍石年代学和地球化学特征     

马倩  杨岳衡  赵志丹  佟鑫  武精凯  苗壮  李晨  雷杭山 《岩石学报》2020,36(9):2751-2764

榍石作为副矿物在哀牢山-红河剪切带新生代富碱侵入岩中广泛存在。原位获取榍石矿物内部微量元素、U-Pb年龄和Sm-Nd同位素的空间变化对获取矿物和岩石的成因演化信息具有重要意义。本文使用四级杆/多接收电感耦合等离子体质谱(Quadrupole/Multi-Collector Inductively Coupled Plasma Mass Spectrometry,Q/MC-ICP-MS)与激光剥蚀系统(Laser Ablation,LA)联用,对哀牢山-红河剪切带5个富碱侵入岩体(桃花岩体、宁蒗-永胜岩体、哈播岩体、铜厂岩体和十里村岩体)中榍石开展了微区原位微量元素、U-Pb年代学和Sm-Nd同位素研究。微量元素分析结果表明,三江富碱侵入岩中榍石为岩浆成因,亏损Rb、Ba、Pb、Sr等大离子亲石元素,富集Th、U、Nb、Ta、Zr、Hf等高场强元素。榍石的稀土配分图均表现为明显右倾,不具有或具有弱的Eu负异常。与云南北衙、马厂箐矽卡岩矿床中的热液榍石相比,本文榍石在稀土元素组成上,具有较高的稀土元素总量、较高的Th/U、LREE/HREE和Ce/Ce*比值,具有较低的Eu/Eu*、Nb/Ta、Zr/Hf比值。微区原位LA-Q-ICP-MS U-Pb定年结果表明,研究区富碱岩体中榍石结晶年龄在32.5～37.9Ma之间,代表了岩体形成时代,与三江地区哀牢山-红河剪切带及其附近新生代富碱岩浆活动高峰期(～35Ma)一致,属于青藏高原晚碰撞期岩浆作用的产物。榍石微区原位LA-MCICP-MS Sm-Nd同位素分析结果显示,榍石颗粒的Nd同位素组成均一,表明榍石结晶过程中寄主岩浆的Nd同位素组成没有发生明显变化。各个富碱岩体之间的榍石Nd同位素组成变化范围在-6.8～-2.1之间,与全岩的同位素特征一致,表明榍石的原位微区Sm-Nd同位素可以作为富碱侵入岩研究中有效的示踪手段之一。  相似文献   

High‐T and low‐P metamorphism in the Xilingol Complex of central Inner Mongolia,China: An indicator of extension in a previous orogeny          下载免费PDF全文

Jin‐Rui Zhang  Chun‐Jing Wei  H. Chu 《Journal of Metamorphic Geology》2018,36(4):393-417

The Xilingol Complex comprises biotite gneisses and amphibolite interlayers with extensive migmatization. Four representative samples were documented and found to record either two or three metamorphic stages. Phase modelling using thermocalc suggests that the observed assemblages represent the final stages that underwent cooling from temperature peaks, and are consistent with a fluid‐absent solidus in P–T pseudosections. Their P–T conditions are further constrained to be 5–6 kbar/680–725°C and 4–5 kbar/650–680°C for two garnet‐bearing gneiss samples, 4–5 kbar/660–730°C for a cordierite‐bearing gneiss sample, and 4–5 kbar/680–710°C for an amphibolite sample based on mineral composition isopleths, involving measured Mg content in biotite, anorthite in plagioclase, grossular and pyrope in garnet and Ti content in amphibole. The peak temperature conditions recovered are 760–790°C or >760°C at 5–6 kbar based on the composition isopleths of plagioclase, biotite, garnet and especially the comparison of melt contents between the calculated and observed. A pre‐peak heating process with slight decompression can be suggested for some samples on the basis of the core–rim increase in the plagioclase anorthite, and the stability of ilmenite. Zircon U–Pb dating using the LA‐ICP‐MS method provides systemic constraints on the metamorphic ages of the Xilingol Complex to be 348–305 Ma, interpreted to represent the post‐peak cooling stages. Moreover, metagabbroic dykes that intruded into the Xilingol Complex yield 317 ± 3 Ma from magmatic zircon, and are considered to have played a significant role for heat advection triggering the high‐T and low‐P metamorphism. Thus, the clockwise P–T paths involving pre‐peak heating, peak and post‐peak cooling recovered for the Xilingol Complex are consistent with an extensional setting in the Carboniferous that developed on a previous orogen in response to addition of mantle‐derived materials probably together with upwelling of the asthenospheric mantle.  相似文献   

Dating of paleolandslides in western Finnish Lapland          下载免费PDF全文

Antti E.K. Ojala  Mira Markovaara‐Koivisto  Maarit Middleton  Timo Ruskeeniemi  Jussi Mattila  Raimo Sutinen 《地球表面变化过程与地形》2018,43(11):2449-2462

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Deglaciation history at the Alpine-Mediterranean transition (Argentera-Mercantour,SW Alps) from 10Be dating of moraines and glacially polished bedrock     

Yann Rolland  Romain Darnault  Régis Braucher  Didier Bourlès  Carole Petit  Stéphane Bouissou  ASTER Team 《地球表面变化过程与地形》2020,45(2):393-410

Estimating the extent and age of the last glacial maxima as well as the chronology of glacial recessions in various environmental contexts is key to source-to-sink studies and paleoclimate reconstructions. The Argentera-Mercantour massif is located at the transition between the Alps and the Mediterranean Sea, therefore, its deglaciation chronology can be compared to the sediment budget of the Var River basin. Based on 13 new cosmic-ray exposure (CRE) beryllium-10 (¹⁰Be) datings performed on moraines and polished crystalline bedrocks and 22 reassessed ¹⁰Be CRE ages from similar altitude nearby steep basement surfaces, and from a lake sediment core, we can constrain the deglaciation chronology of the Argentera-Mercantour massif. These data allow for the first time to fully reconstruct the deglaciation history at the scale of the entire massif in agreement with a major glacier recession at c. 15 ka, at the onset of Bølling transition between the Oldest and Older Dryas. Main deglaciation of the upper slopes [2700–2800 m above sea level (a.s.l.)] occurred after the Last Glacial Maximum (LGM) at 20.8–18.6 ka, followed by the main deglaciation of the lower slopes (2300 m a.s.l.) at 15.3–14.2 ka. Finally, the flat polished surfaces above 2600 m a.s.l. and the zones confined within narrow lateral valleys were likely affected by progressive ice melting of remaining debris covered glaciers and moraine erosion following the Younger Dryas re-advance stage between 12 and 8–9 ka. At lower elevations, the Vens Lake located at 2300 m a.s.l., allows evidence of the onset of lake sedimentation at c. 14 ka and a transition towards a vegetated environment that mainly occurred before 8 ka. Moraine final stabilization at 5 ka might reflect denudation acceleration during the Holocene humid phase. This contribution reveals a glacier–climate relationship more sensitive to warming phases in the southern Alps highlighted by a major decrease of glaciers after c. 15 ka. This major deglaciation is correlated with a 2.5-fold decrease of sediment discharge of rivers into the Mediterranean Sea. © 2019 John Wiley & Sons, Ltd.  相似文献   

Climatic and structural controls on Late-glacial and Holocene rockfall occurrence in high-elevated rock walls of the Mont Blanc massif (Western Alps)     

Xavi Gallach  Julien Carcaillet  Ludovic Ravanel  Philip Deline  Christophe Ogier  Magali Rossi  Emmanuel Malet  David Garcia-Sellés 《地球表面变化过程与地形》2020,45(13):3071-3091

In the Mont Blanc massif (European Western Alps), rockfalls are one of the main natural hazards for alpinists and infrastructure. Rockfall activity after the Little Ice Age is well documented. An increase in frequency during the last three decades is related to permafrost degradation caused by rising air temperatures. In order to understand whether climate exerts a long-term control on rockfall occurrence, a selection of paleo-rockfall scars was dated in the Glacier du Géant basin [>3200 m above sea level (a.s.l.)] using terrestrial cosmogenic nuclides. Rockfall occurrence was compared to different climatic and glacial proxies. This study presents 55 new samples (including replicates) and 25 previously-published ages from nine sampling sites. In total, 62 dated rockfall events display ages ranging from 0.03 ± 0.02 ka to 88.40 ± 7.60 ka. Holocene ages and their uncertainties were used to perform a Kernel density function into a continuous dataset displaying rockfall probability per 100 years. Results highlight four Holocene periods of enhanced rockfall occurrence: (i) c. 7–5.7 ka, related to the Holocene Warm Periods; (ii) c. 4.5–4 ka, related to the Sub-boreal Warm Period; (iii) c. 2.3–1.6 ka, related to the Roman Warm Period; and (iv) c. 0.9–0.3 ka, related to the Medieval Warm Period and beginning of the Little Ice Age. Laser and photogrammetric three-dimensional (3D) models of the rock walls were produced to reconstruct the detached volumes from the best-preserved rockfall scars (≤0.91 ± 0.12 ka). A structural study was carried out at the scale of the Glacier du Géant basin using aerial photographs, and at the scale of four selected rock walls using the 3D models. Two main vertical and one horizontal fracture sets were identified. They correspond respectively to alpine shear zones and veins opened-up during long-term exhumation of the Mont Blanc massif. Our study confirms that climate primarily controls rockfall occurrence, and that structural settings, coincident at both the massif and the rock wall scales, control the rock-wall shapes as well as the geometry and volume of the rockfall events. © 2020 John Wiley & Sons, Ltd.  相似文献   

Late Quaternary climate and environmental change derived from glacial deposits in the Parlung Zangbo Valley,southeastern Tibetan Plateau     

Renrong Chen  Yingkui Li  Zhongping Lai  Xianjiao Ou  Xiaoyu Lu 《自然地理学》2020,41(2):99-125

ABSTRACT

This study investigated the late Quaternary climate and environmental characteristics of two tributary valleys (Xingmu and Depu Valleys) in the Parlung Zangbo Valley, southeastern Tibetan Plateau. Optically stimulated luminescence (OSL) samples collected from moraines at the mouth of Xingmu Valley produce a wide age range from 13.9 ka to 76 ka. The ages measured from the lenticular sand are consistent with the relative geomorphic sequence of the landforms. Lenticular sand layers below the moraine were dated to 37.9 ka and 44.7 ka, indicating that fluvial processes were likely dominant in the valley during Marine Isotope Stage (MIS) 3. The outer moraine ridges at the valley mouth were formed during 13.9 ka and 26.5 ka, corresponding to MIS2. At Depu Valley, OSL samples from two sets of lateral and terminal moraines close to the modern glacier, provide ages from 1.4 ka to 29.2 ka. The paleosol layer widely developed during 2.6 cal ka BP and 8.7 cal ka BP in the study area, reflecting a relatively warm condition during the mid-Holocene.  相似文献