大兴安岭北段上侏罗统哈日陶勒盖玄武岩的厘定及其地质意义

大兴安岭北段西部新巴尔虎右旗地区新厘定出非正式填图单元——上侏罗统哈日陶勒盖玄武岩,其岩石组合为灰黑色、深绿灰色、紫褐色玄武岩以及气孔杏仁状玄武岩、碱性橄榄玄武岩、玄武粗安岩等。锆石SHRIMP U Pb定年法测得其年龄数据为(1467±31) Ma,时代为晚侏罗世。岩石地球化学分析显示,哈日陶勒盖玄武岩属高钾钙碱性系列,岩石中Sr含量低,富集大离子亲石元素Rb、K、Ba,亏损高场强元素Nb、Ti,稀土元素含量总体较高,轻重稀土分馏明显,略具有铕负异常。上侏罗统哈日陶勒盖玄武岩的厘定说明,大兴安岭地区存在晚侏罗世基性火山岩浆活动事件。通过区域地质调查和地层对比,上侏罗统哈日陶勒盖玄武岩与上侏罗统伊列克得组原始定义相符,二者实则为同物异名,建议恢复上侏罗统伊列克得组。这一认识对大兴安岭中生代构造岩浆活动的研究及中生代火山岩地层划分对比具有重要地质意义。     

Petrological and Os Isotopic Characteristics of Zedong Peridotites in the Eastern Yarlung–Zangbo Suture in Tibet

The Zedong ophiolites in the eastern Yarlung–Zangbo suture zone of Tibet represent a mantle slice of more than 45 km~2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine(Fo89.7–91.2), orthopyroxene(En_(88–92)), clinopyroxene(En_(45–49) Wo_(47–51) Fs_(2–4)) and spinel [Mg~#=100×Mg/(Mg+Fe)]=49.1–70.7; Cr~#=(100×Cr/(Cr+Al)=18.8–76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%–40%, indicating that the Zedong mantle peridotites may experience a multi–stage process. The peridotites are characterized by depleted major element compositions and low REE content(0.08–0.62 ppm). Their "spoon–shaped" primitive–mantle normalized REE patterns with(La/Sm)_N being 0.50–6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites(18.19–50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54–0.60 and Pt/Pd being 1.09–1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with ~(187)Os/~(188)Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge(MOR) and were later modified in supra–subduction zone(SSZ) environment.     

藏北羌塘南部埃迪卡拉系达布热组的建立及其地质意义

羌塘位于青藏高原腹地,构造上处于冈瓦纳大陆北缘。因其特殊的构造位置,羌塘地体的起源及构造演化对于探讨青藏高原的早期形成演化、冈瓦纳大陆裂解,以及特提斯洋演化等关键科学问题至关重要。最近,在羌塘南部达布热地区发现一套碎屑岩夹玄武岩的岩石组合,碎屑岩具有低成分成熟度的特点,虽然岩石发生了低绿片岩相变质,但仍然保留了原岩类复理石沉积的特点。根据碎屑锆石定年结果,该套地层中碎屑锆石的最年轻年龄为550Ma左右。此外,该套地层中玄武岩夹层的测年结果表明,该套地层形成于埃迪卡拉纪(约550Ma)。结合地层剖面及区域地层对比,建立了埃迪卡拉纪达布热组。达布热组是羌塘地区首次发现的埃迪卡拉纪地层,该组地层的建立为探讨冈瓦纳大陆北缘构造演化提供了重要线索。     

The terrestrial fauna of the Late Triassic Pant-y-ffynnon Quarry fissures,South Wales,UK and a new species of Clevosaurus (Lepidosauria: Rhynchocephalia)

Pant-y-ffynnon Quarry in South Wales yielded a rich cache of fossils in the early 1950s, including articulated specimens of new species (the small sauropodomorph dinosaur Pantydraco caducus and the crocodylomorph Terrestrisuchus gracilis), but no substantial study of the wider fauna of the Pant-y-ffynnon fissure systems has been published. Here, our overview of existing specimens, a few described but mostly undescribed, as well as freshly processed material, provides a comprehensive picture of the Pant-y-ffynnon palaeo-island of the Late Triassic. This was an island with a relatively impoverished fauna dominated by small clevosaurs (rhynchocephalians), including a new species, Clevosaurus cambrica, described here from a partially articulated specimen and isolated bones. The new species has a dental morphology that is intermediate between the Late Triassic Clevosaurus hudsoni, from Cromhall Quarry to the east, and the younger C. convallis from Pant Quarry to the west, suggesting adaptive radiation of clevosaurs in the palaeo-archipelago. The larger reptiles on the palaeo-island do not exceed 1.5?m in length, including a small carnivorous crocodylomorph, Terrestrisuchus, and a possible example of insular dwarfism in the basal dinosaur Pantydraco.     

华夏陆块钠质高镁玄武岩地球化学特征及地质意义

赣州盆地位于钦杭(钦州—杭州)结合带南侧华夏陆块西部,由晚白垩世早期红色碎屑沉积岩系组成,其底部夹有一套玄武岩,全岩Si O_2含量为46.50%~48.13%,具有钠质高镁特征(Mg O=15.47%~18.24%,Mg~#=74~77,K_2O/Na_2O=0.22~0.39)。稀土元素配分型式表现出轻稀土富集右倾,没有明显的Eu异常的特点,表明岩浆没有明显的斜长石分离结晶作用;微量元素上富集大离子亲石元素,高场强元素特别是Ta、Nb、Ti亏损不明显,Cr、Ni的含量分别为235×10~(-6)~617×10~(-6)、157×10~(-6)~493×10~(-6),与原生岩浆的Cr、Ni含量接近,Ba/Nb值(13.51~23.18)和La/Nb值(0.82~1.03)表明源区有富集地幔组分;Sr—Nd—Pb—O同位素显示出源区性质具有EMⅡ型富集地幔特征。岩石学和地球化学特征表明,玄武岩具有原生岩浆性质,是富集地幔部分熔融的产物。同时为中生代华夏陆块EMⅡ型岩石圈地幔提供了准确的证据。由赣州盆地晚白垩世构造背景和构造环境判别图解可知,本区玄武岩形成于板内拉张环境,可能与古太平洋俯冲条件的转变有关。     

塔里木溢流玄武岩火山通道的三维结构及其热成因气体释放

塔里木上奥陶统-志留系沉积地层中广泛发育早二叠世溢流玄武岩的火山通道相岩床-岩墙网络。三维地震数据解释结果显示,这些火山通道以平行围岩地层的岩床和斜切围岩地层的"碟状岩床"为主要特征。在玄武岩喷发过程中,火山通道岩浆的热量可以导致沉积围岩发生热接触变质并将沉积围岩中的有机质转化为"热成因气体"。在塔北英买2井区火山通道烘烤沉积围岩模型基础上,利用有限元热模拟方法确定了该区早二叠世玄武岩喷发时火山通道热烘烤影响范围随时间的变化。基于沉积围岩有机质丰度估算,该区热烘烤成因甲烷释放量可达11.3Gt(即113亿吨)。如果整个塔里木溢流玄武岩省具有与英买2地区相同的释放强度,则塔里木溢流玄武岩省活动期间释放的甲烷总量可达7062.5Gt,必然导致非常显著的环境效应。同时,玄武岩火山通道岩浆引起的热接触变质作用对已存在的油藏具有明显的破坏作用,塔里木盆地古生界总量约8~10Gt的油藏破坏和大量沥青的形成可能与此有关。     

中国东部四明山古夷平面遗迹特征与演化

古夷平面研究在地貌演化和新构造运动研究中占有极其重要的地位,但由于遭受构造运动和剥蚀作用的影响,很多古夷平面遗迹已无从清晰辨识。浙江四明山顶古近纪夷平古风化壳面的发现,为研究浙江东部甚至华东地区的新生代地貌演化提供了重要地质信息,同时为始新世和中新世时的古气候、古环境研究提供了重要物质证据。通过详细的野外地质调查,系统采集了该区5个点位的20个古风化壳样品和7个点位的15个现代风化壳样品,开展黏土矿物分析和玄武岩K-Ar年龄测试。研究确认四明山地区发育有2期古夷平面遗迹,即古近纪(E)和新近纪(N)古夷平面遗迹:第一期夷平面发育时间为65. 0～32. 2 Ma,解体和古夷平面遗迹保存时间为32. 2～21. 7 Ma;第二期夷平面发育时间为21. 7～10. 5 Ma,解体和古夷平面遗迹保存时间为10. 5～3. 0 Ma。研究区2期火山活动喷发所形成的玄武岩盖层有效保护了该区的古风化壳,从而使四明山古夷平面遗迹具有空间展布完整、剖面信息清晰等特征,呈现出由古夷平面(古风化壳)与玄武岩堆积面(现代风化壳)组成的"双风化壳"现象,为观察古夷平面遗迹特征提供了最佳场所。     

Integration of zircon and apatite U–Pb geochronology and geochemical mapping of the Wude basalts(Emeishan large igneous province):A tool for a better understanding of the tectonothermal and geodynamic evolution of the Emeishan LIP

Radiogenic isotopic dating and Lu–Hf isotopic composition using laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)of the Wude basalt in Yunnan province from the Emeishan large igneous province(ELIP)yielded timing of formation and post-eruption tectonothermal event.Holistic lithogeochemistry and elements mapping of basaltic rocks were further reevaluated to provide insights into crustal contamination and formation of the ELIP.A zircon U–Pb age of 251.3±2.0 Ma of the Wude basalt recorded the youngest volcanic eruption event and was consistent with the age span of 251-263 Ma for the emplacement of the ELIP.Such zircons hadε_Hf(t)values ranging from7.3 to+2.2,identical to those of magmatic zircons from the intrusive rocks of the ELIP,suggesting that crust-mantle interaction occurred during magmatic emplacement,or crust-mantle mixing existed in the deep source region prior to deep melting.The apatite U–Pb age at 53.6±3.4 Ma recorded an early Eocene magmatic superimposition of a regional tectonothermal event,corresponding to the Indian–Eurasian plate collision.Negative Nb,Ta,Ti and P anomalies of the Emeishan basalt may reflect crustal contamination.The uneven Nb/La and Th/Ta values distribution throughout the ELIP supported a mantle plume model origin.Therefore,the ELIP was formed as a result of a mantle plume which was later superimposed by a regional tectonothermal event attributed to the Indian–Eurasian plate collision during early Eocene.     

Delineation of the 85°E ridge and its structure in the Mahanadi Offshore Basin,Eastern Continental Margin of India (ECMI), from seismic reflection imaging

The passive Eastern Continental Margin of India (ECMI) evolved during the break up of India and East Antarctica in the Early Cretaceous. The 85°E ridge is a prominent linear aseismic feature extending from the Afanasy Nikitin Seamounts northward to the Mahanadi basin along the ECMI. Earlier workers have interpreted the ridge to be a prominent hot spot trail. In the absence of conclusive data, the extension of the ridge towards its northern extremity below the thick Bengal Fan sediments was a matter of postulation. In the present study, interpretation of high resolution 2-D reflection data from the Mahanadi Offshore Basin, located in the northern part of the ridge, unequivocally indicates continuation of the ridge across the continent–ocean boundary into the slope and shelf tracts of the ECMI. Its morphology and internal architecture suggest a volcanic plume related origin that can be correlated with the activity of the Kerguelen hot spot in the nascent Indian Ocean. In the continental region, the plume related volcanic activity appears to have obliterated all seismic features typical of continental crust. The deeper oceanic crust, over which the hot spot plume erupted, shows the presence of linear NS aligned basement highs, corresponding with the ridge, underlain by a depressed Moho discontinuity. In the deep oceanic basin, the ridge influences the sediment dispersal pattern from the Early Cretaceous (?)/early part of Late Cretaceous times till the end of Oligocene, which is an important aspect for understanding the hydrocarbon potential of the basin.     

The age,extent and geomorphological significance of the Sassafras basalt,south‐eastern New South Wales

Basalt at Sassafras was erupted in the Middle Eocene. The K‐Ar ages average 45.3 ± 4.9 Ma on whole rock and 48.4 ± 1.9 Ma on plagioclase. The basalt is not limited to a plateau capping, but extends 150 m down into adjacent valleys. Comparison with nearby Eocene basalts shows that there was in excess of 250 m of local relief in the central Shoalhaven valley by the Early Tertiary. The basalts were extruded at high elevation, and denudation of the coastal margin of the upland was already well advanced. Post‐basaltic denudation has been very slow, and the Early Tertiary landscape is well preserved.