Early Carboniferous ophiolite in central Qiangtang,northern Tibet: record of an oceanic back-arc system in the Palaeo-Tethys Ocean     

Tian-Yu Zhang  Cai Li  Chao-Ming Xie  Ming Wang  Yan-Wang Wu 《International Geology Review》2018,60(4):449-463

Zircon U–Pb dating of two samples of metagabbro from the Riwanchaka ophiolite yielded early Carboniferous ages of 354.4 ± 2.3 Ma and 356.7 ± 1.9 Ma. Their positive zircon εHf(t) values (+7.9 to +9.9) indicate that these rocks were derived from a relatively depleted mantle. The metagabbros can be considered as two types: R1 and R2. Both types are tholeiitic, with depletion of high-field-strength elements (HFSE) and enrichment of large-ion lithophile elements (LILE) similar to those of typical back-arc basin basalts (BABB), such as Mariana BABB and East Scotia Ridge BABB. Geochemical and isotopic characteristics indicate that the R1 metagabbro originated from a back-arc basin spreading ridge with addition of slab-derived fluids, whereas the R2 metagabbro was derived from a back-arc basin mantle source, with involvement of melts and fluids from subducted ocean crust. The Riwanchaka ophiolite exhibits both mid-ocean ridge basalts- and arc-like geochemical affinities, consistent with coeval ophiolites from central Qiangtang. Observations indicate that the Qiangtang ophiolites developed during the Late Devonian–early Carboniferous (D₃–C₁) in a back-arc spreading ridge above an intra-oceanic subduction zone. Based on our data and previous studies, we propose that an oceanic back-arc basin system existed in the Longmuco–Shuanghu–Lancang Palaeo-Tethys Ocean during the D₃–C₁ period.  相似文献   

The Carboniferous coal swamp floras of England: a window on an ancient tropical ecosystem     

Christopher J. Cleal 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(3):329-351

England has an exceptional range of Westphalian—Stephanian (late Bashkirian—Moscovian) fossil floras spanning some 10 million years. They represent vegetation growing in part of a swamp that covered large areas of tropical Euramerica and which was responsible for the removal of vast quantities of carbon from the atmosphere. This coincided with significant global climatic cooling—the Late Palaeozoic Ice Age. The cratonic Pennine Basin in central and northern England has some of the best preserved fossil floras of this age anywhere in the world, especially notable being those of the Barnsley Thick Seam in Yorkshire and Derbyshire, the Bensham Seam in Northumberland and Durham, and the Coseley Ten Foot Ironstone in the West Midlands. The floras in southern England are mostly not as well preserved but include the historically important Radstock flora of Somerset. The taxonomic diversity dynamics of the fossil floras of the Pennine Basin are rather different from those seen in South Wales, probably due to differences in landscape and habitat, which in turn probably reflect the different tectonic settings. However, evidence of a significant change from lycophyte- to fern-dominated vegetation in latest Westphalian times, recognisable across Euramerica, can be seen in the English floras.  相似文献   

徐淮地区早白垩世丰山和蔡山高镁埃达克质岩:拆沉下地壳熔融的熔体与地幔橄榄岩的反应     

霍腾飞  杨德彬  师江朋  杨浩田  许文良  王枫  卢毅 《岩石学报》2018,34(6):1669-1684

徐淮地区丰山花岗闪长斑岩和蔡山石英闪长玢岩的岩石地球化学、Sr-Nd-Hf同位素和石榴石的矿物化学研究对探讨华北克拉通东南缘早白垩世高镁埃达克质岩的岩石成因和构造演化具有重要意义。结果表明,丰山花岗闪长斑岩和蔡山石英闪长玢岩具高SiO_2(60.98%～67.88%)、富Al_2O_3(14.37%～15.04%)以及高的Na_2O/K_2O比值(1.58～2.24)和Mg~#值(57～66)的特征;富集LILE(Rb、Ba、Sr)和LREE,亏损HFSE(Nb、Ta、Ti)和HREE,具有Pb正异常和弱的Eu异常;结合高的Sr含量(579×10~(-6)～778×10~(-6))以及Sr/Y(33～69)和(La/Yb)_N比值(8.63～13.7),低的Y(10.5×10~(-6)～17.8×10~(-6))和Yb含量(0.74×10~(-6)～1.17×10~(-6)),暗示它们属于高镁埃达克质岩。丰山和蔡山埃达克质岩石的初始~(87)Sr/~(86)Sr比值介于0.7079～0.7086之间,ε_(Nd)(t)值变化于-10.77～-7.18之间,t_(DM2)=1504～1793Ma;岩浆锆石的ε_(Hf)(t)值为-14.2～-5.3,t_(DM2)=2101～2898Ma。徐淮地区早白垩世利国、班井、夹沟高镁埃达克质岩石中岩浆锆石的ε_(Hf)(t)值分别介于-13.4～-7.0、-13.4～-7.9和-15.9～-4.5之间,它们的t_(DM2)分别变化于2248～2825Ma、2331～2824Ma和2030～3048Ma之间。徐淮地区丰山和蔡山高镁埃达克质岩的Sr-Nd-Hf同位素组成和丰山花岗闪长斑岩中铁铝榴石残留晶的存在,结合它们高的Pb同位素组成和继承锆石U-Pb年代学暗示,丰山和蔡山高镁埃达克质岩浆主要起源于俯冲断离的扬子克拉通石榴辉石岩相下地壳物质熔融的熔体与地幔橄榄岩的反应,同时有拆沉的华北克拉通基底物质的参与,形成于华北克拉通东部岩石圈减薄的伸展构造背景。  相似文献   

福贡地区石炭系地层可能属于拉萨地块:来自碎屑锆石年龄谱的指示   总被引：1，自引：1，他引：0  

信迪  杨天南  薛传东  廖程  韩雪  梁明娟  李晓天 《岩石学报》2018,34(5):1335-1346

欧亚与印度大陆的碰撞使得印度大陆的北东角强烈挤入欧亚大陆内部,形成东构造结,其引发的强烈变形将南羌塘地块、北羌塘地块、拉萨地块、保山地块、兰坪-思茅地块以及地块之间的缝合带挤压于很窄的范围内,造成青藏高原腹地与其东南缘之间构造单元划分、对比困难。滇西福贡地区位于青藏高原东南缘,东构造结影响范围内,是三江造山带及青藏高原相关地块的汇聚处。福贡地区石炭系岩石组合表明其形成于较为稳定的浅海相沉积环境中,具有被动大陆边缘沉积特征。三件石炭系二云母石英片岩碎屑锆石表面年龄数据(194个有效分析点)形成五个主要年龄区间:2550~2400Ma、1800~1550Ma、1200~1080Ma、1000~850Ma和600~480Ma,峰值分别为~2502Ma、~1724Ma、~1120Ma、~886Ma和~512Ma。对比其他地块同时代地层的碎屑锆石年龄谱表明,福贡地区石炭系可能是拉萨地块的一部分,与腾冲地块一样,属于其南延部分。结合中新生代岩浆岩带的构造连续性,我们认为,印度板块东北角向北、北东方向的强烈挤压导致拉萨地块及冈底斯岩浆岩带弯曲、缩颈、并发生分离与旋转。  相似文献   

伊宁地块早石炭世球泡流纹岩的发现及地球化学特征   总被引：3，自引：2，他引：1  

李永军  王祚鹏  李新光  郭文杰  任朋飞  罗耀清  滕明耀  王冉 《岩石学报》2018,34(1):49-62

伊宁地块西段中部阿腾套山一带发现的球泡流纹岩,多数产于早石炭世海相无球泡流纹岩夹层中,整体呈层状、互层状,球泡通气孔顺层平行于流纹构造定向分布,推断是海相极浅水环境下的产物,因而具有精细指相意义。该球泡流纹岩与同期A型花岗岩类共生,属碱性岩类,SiO_2含量为69.18%~77.69%,K_2O大于4%,Al_2O_3为11.32%~16.24%,Ga/Al×10~4=2.1~7.8,∑REE=85.22×10~(-6)~348.8×10~(-6),(La/Yb)_N=6.7~14.8,相对富集轻稀土,δEu具明显负异常,稀土配分及大离子元素蛛网图均与A型花岗岩类地球化学特征总体相同。ε_(Hf)(t)均大于0(+12.5~+15.5),两阶段锆石Hf模式年龄大于锆石U-Pb年龄,表明源区可能为早古生代新生地壳物质部分熔融演化而来。A型球泡流纹岩与同期A型花岗岩类均形成于拉张构造环境,是区内早石炭世大哈拉军山组中迄今唯一发现的显示拉张构造环境的珍贵实例。为揭示和探讨伊宁地块总体处于汇聚大背景下的局部拉张环境,以及东窄西宽之楔形构造形态提供了可能。  相似文献   

大兴安岭北段新林战备村地区早白垩世火成岩年代学、地球化学及大地构造意义     

王文东  杨华本  刘涛  马治忠  郑吉林  胡军海  魏小勇 《世界地质》2018,37(1):21-36

大兴安岭北段新林战备村地区广泛发育晚中生代火山岩及花岗岩体。LA-ICP-MS锆石U-Pb测得战备村花岗岩体侵位年龄为121.0±0.8 Ma,其周围出露的白音高老组流纹岩喷发年龄为136.9±1.3 Ma,二者均形成于早白垩世。地球化学特征上,二者均属于过铝质高钾钙碱性-钾玄质系列,高SiO_2,富K_2O、Na_2O,低Mg、Ti,微量元素富集大离子亲石元素Rb、K、Ba及LREE,亏损高场强元素Nb、Ta、Ti及HREE,轻重稀土明显分异((La/Yb)_N=11~23.2),具中等-弱的Eu负异常(δEu=0.35~0.82),二者具同源岩浆演化的特征。结合前人研究结果,新林战备村地区早白垩世火成岩形成于后造山或碰撞后的构造环境,代表了伸展的大地构造背景,这些火成岩产出可能与蒙古—鄂霍茨克洋中侏罗世至早白垩世闭合造山后的伸展作用有关。  相似文献   

武夷山中段司前花岗岩体形成时代、地球化学特征及地质意义     

辛宇佳  李建华  董树文  张岳桥 《地球学报》2018,39(3):269-281

本文对武夷山中段的司前岩体进行了系统的岩石学、地球化学、年代学及Lu-Hf同位素研究。结果表明,司前岩体形成于(140±1)Ma,主要由黑云母二长花岗岩组成,富碱(K_2O+Na_2O=6.67 wt%~8.25 wt%),富钾(K_2O/Na_2O=1.16~2.41),A/CNK值介于1.01~1.25之间。岩体具有较高的∑REE(177.73×10~(-6)~427.88×10~(-6))、Zr+Nb+Y+Ce含量(262.6×10~(-6)~581.5×10~(-6))和Zr饱和温度(平均824°C),FeOt/MgO(3.06~3.93)和10 000×Ga/Al(2.64~3.28)比值均较高,属典型的铝质-过铝质A型花岗岩。岩体的锆石εHf(t)值均为负值(–18.6~–7.9),暗示其源于古老的地壳物质重熔。综合上述结果和区域背景推测,司前岩体的源岩为新元古代麻源群变质火山-碎屑岩,源岩可能经历早古生代和白垩纪两期熔融事件,地幔岩浆为花岗岩的形成提供了热源,但并未贡献物质,岩体的形成与古太平洋板块俯冲过程中因俯冲板片后撤诱发的弧后扩张作用有关。  相似文献   

Research Progress and Prospect on Reactivation of Ancient Landslides     

Yongshuang Zhang  Ruian Wu  Changbao Guo  Lichao Wang  Xin Yao  Zhihua Yang 《地球科学进展》2018,33(7):728-740

The ancient landslide has endured long-term slope evolution which results in its complicated material and special rock-soil properties. The risk of ancient landslide reactivation is substantially increasing due to the increase of intensified human engineering activities and the frequency of extreme weather events. Many ancient landslides have been reactivated all over the world and led to serious fatalities and severe damage to many important engineering facilities such as transportation and hydropower engineering projects. On the basis of the analysis of the research situation about the ancient landslides at home and abroad, the main research advances were summarized including the regional developing laws and recognizing of the ancient landslides, the mechanics properties of ancient landslide body and related sliding zone, reactivation mechanism of ancient landslides, reactivating process and modeling analysis of ancient landslides, early recognization of ancient landslide reactivation, etc. To meet the demands of disaster prevention and reduction, three key scientific issues were put forward to be solved: ①automaticaly establishing the methodology and identification criterions for recognition of ancient landslide; ②revealing the reactivation mechanism of ancient landslide based on a new strength theory; ③establishing the early rapid recognition method and predictive model for ancient landslide reactivation. Solving the above mentioned scientific theory and methodology will facilitate the planning and site selection of major projects as well as the disaster prevention and reduction in ancient landslide developing areas.  相似文献   

北羌塘地块上石炭统瓦垄山组的珊瑚化石     

张少文  李涌溪  武桂春  纪占胜 《地质论评》2021,67(Z1):67z1249-67z1250

  相似文献   

北秦岭西段早侏罗世A型花岗岩及其地质意义          下载免费PDF全文

宫相宽蔡宏明郭瑞清刘桂萍谭 俊木热地力·  买合苏提 《地质科学》2021,56(1):158-181

本文通过岩相学、岩石地球化学、锆石U?Pb定年和Lu?Hf同位素组成分析等方法,对出露于北秦岭西段宝鸡岩体王家山一带的黑云母花岗岩和其中的包体进行了研究。结果表明,该花岗岩形成时代为187±2 Ma,属于高钾钙碱性—钾玄岩系列岩石,富集Rb、Th、U等大离子亲石元素以及Nb、Zr和Hf等高场强元素,亏损Ba、Sr和Eu,具有高的全岩锆石饱和温度(825℃~838℃),显示A型花岗岩特征,形成于造山后的板内环境,可能为秦岭岩群副变质岩与安山质岩石部分熔融的产物。暗色包体显示塑性流变特征,具有岩浆结构,发育针状磷灰石和具有复杂成分环带的更长环斑结构长石,是幔源岩浆注入酸性岩浆发生混合作用的产物,形成时代为191±2 Ma,其锆石Hf同位素组成变化范围较大,εHf(t)值介于-11.26~-2.51,主要为富集地幔部分熔融产物。综合本文及前人已有研究结果,认为~190 Ma的早侏罗世早期秦岭地区早中生代碰撞造山过程已经结束,区域开始逐渐进入板内伸展构造演化阶段。  相似文献