东昆仑夏日哈木铜镍矿镁铁质超镁铁质岩体岩相学、锆石U-Pb年代学、地球化学及其构造意义

文章报道了东昆仑夏日哈木铜镍矿成矿岩体的岩相学、锆石U-Pb年代学、全岩地球化学以及锆石Hf同位素资料,以确定该岩体的形成时代、岩石成因及其形成的构造环境。夏日哈木Ⅰ号镁铁质超镁铁质岩体位于昆中基底隆起花岗岩带中段,北侧靠近昆北断裂。岩体走向NEE,剖面呈平缓的“岩盆状”,地表出露面积约0.7 km2。该杂岩体主要由辉长苏长岩、斜方辉石岩、橄榄辉石岩、斜长二辉橄榄岩和方辉橄榄岩组成,橄榄岩相和辉石岩相是主要的Cu、Ni赋矿岩相。镁铁质超镁铁质岩体主量元素具有低硅(w(SiO₂)=36.68%~52.58%)、低钛(w(TiO2)=0.13%~0.47%)、高镁(w(MgO)=10.91%~35.81%)、贫碱(w(K₂O+Na₂O)=0.26%~1.95%)的特征,属亚碱性系列岩石,m/f为3.88~6.29,属铁质超基性岩类(m/f=2~6.5)。岩石稀土元素球粒陨石标准化配分模式为轻稀土富集型,(La/Yb)N=1.44~2.98,Eu异常不明显,相似的稀土元素配分模式说明岩体的同源性。岩石富集大离子亲石元素(Rb、Th、U、K),相对亏损高场强元素(Nb、Ta)。岩体中存在新元古代花岗岩的捕虏体以及La/Yb、Ce/Yb、Th/Yb、Nb/La、La/Sm比值显示岩体经历了有限的地壳混染。辉长苏长岩锆石的LA-MC ICPMS、U-Pb年代学研究表明,岩体形成年龄为(423±1) Ma,MSWD=0.14,属晚志留世。锆石的176Hf/177Hf比值为0.282 628~0.282 833,相应的εHf(t)均为正值(4.0~10.9),Lu-Hf的单阶段模式年龄(tDM1)为610~875 Ma,平均值为788 Ma,大于锆石U-Pb年龄。研究认为,岩体的岩浆源区主要为亏损地幔,可能有早期流体交代的富集岩石圈地幔组分的加入和地壳物质的混染。结合区域构造演化,文章认为岩体形成于碰撞后伸展的构造环境,可能与俯冲板片的断离作用有关。岩浆演化过程中橄榄石和斜方辉石的分离结晶作用和地壳中硫的加入可能是促使岩浆体系达到硫饱和的主要机制。     

Microfossil analysis of sediments representing the 1964 earthquake, exposed at Girdwood Flats, Alaska, USA

Diatom, pollen, foraminifera and thecamoebian assemblages from an outcrop of peat and silt at Girdwood Flats, in the upper Turnagain Arm of the Cook Inlet, Alaska, record four phases of relative land and sea-level changes. The first phase is the development of freshwater swamp above high marsh sediments during relative land uplift, caused by strain accumulation along the locked portion of the Alaska-Aleutian subduction zone. In second phase, the top 2 cm of the peat, all microfossil groups record pre-seismic relative sea-level rise (relative land subsidence). The third phase is rapid land subsidence, 1.7 m, during the earthquake of March 1964 that initiated intertidal silt accumulation above the peat. The final phase is the colonisation of mudflat by salt marsh communities during post-seismic land uplift. The microfossil data compare favourably with sequences from Washington, Oregon and British Columbia that record late Holocene submergence events caused by earthquakes. The comparable changes in microfossil assemblages record the different phases of relative land and sea-level changes and the magnitude of land subsidence caused by each earthquake (expressed relative to the tidal range at the site). These results raise the question whether preseismic sea-level rise represents any kind of warning of large earthquakes.     

Melting-induced fluid flow during exhumation of gneisses of the Sulu ultrahigh-pressure terrane

Hydrothermally altered rocks are products of fluid–rock interactions, and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks. Thus, quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure. In order to decipher the fluid activity in the Sulu ultrahigh-pressure (UHP) terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. In one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies overprinting of the eclogite lens decreases from the rim to the core of the lens, with fresh eclogite preserved in the core. The foliated biotite gneiss contains felsic veins and residual phengites. Zircon rims from the gneiss are characterized by melt-related signatures with steep HREE patterns, high Hf contents and negative Eu anomalies, and a pool of weighted average ²⁰⁶Pb/²³⁸U analyses reveal an age of 219 ± 3 Ma (2σ), which is younger than the UHP metamorphic age (236 ± 2 Ma, 2σ) recorded by zircons from the eclogite lens. This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting due to phengite dehydration during the “hot” exhumation stage.The formation age of the quartz vein (219 ± 2 Ma, 2σ) defined by zircon rims agrees well with the partial melting time (219 ± 3 Ma, 2σ) of the host gneiss. The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217 ± 5 Ma (2σ), overlap with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced a significant fluid flow during the exhumation stage, and thus contributed significantly to the extensive retrogression of eclogites in the Sulu UHP terrane.     

裂缝发育区平面分布的分形评价

裂缝是一种具有强烈非均质特征的油气储集 体,其在三维空间内的分布具有非 线性特征。运用相似维数统计法对裂缝发育带在二维平面内分布的分形特征进行研究,并用 该定量参数进行层间裂缝发育程度的定量对比。理论上证实,相似维数这一分形参数可以定 量表征裂缝发育带对二维空间的充填能力,实际统计结果表明,裂缝发育带的统计相似维数 高低与层间总体产能的对比关系是完全吻合的。     

In situ U–Pb dating and trace element analysis of zircons in thin sections of eclogite: Refining constraints on the ultra high-pressure metamorphism of the Sulu terrane,China

In situ U–Pb dating and trace element analysis of zircons, combined with a textural relationship investigation in thin section, is a powerful tool to constrain the ultra high-pressure stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the main hole of the Chinese Continental Scientific Drill project in the Sulu UHP terrane. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average ²⁰⁶Pb/²³⁸U age of these zircon domains (230 ± 4 Ma, 2σ) agrees well with the published age of coesite-bearing zircon separates (230 ± 1 Ma, 2σ), suggesting that the peak UHP metamorphism in the Sulu terrane may have occurred at ~ 230 Ma.Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average ²⁰⁶Pb/²³⁸U age of 209 ± 4 Ma (2σ). These retrograde zircon domains have similar REE compositions to the ~ 230 Ma UHP zircon domains. These observations imply that the ~ 209 Ma zircon domains could have formed by fluid activity-associated alterations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REEs in these domains.     

软土地基中单桩施工引起的超孔隙水压力

通过对桩基施工过程中实测资料的分析，探讨了沉桩时单桩周围土中产生的超孔隙水压力的大小、分布及影响范围，并与理论解进行了对比。还对桩-土界面处的超孔隙水压力进行了讨论。     

Tidal marsh stratigraphy, sea-level change and large earthquakes, i: a 5000 year record in washington, U.S.A.

Many of the estuaries of the Pacific Northwest of the U.S.A. and Canada contain stratigraphic sequences typified by alternating peat-mud couplets. Recent studies in this region interpret such couplets as the product of repeated large (magnitude S or 9) earthquakes on the Cascadia subduction zone. The resultant pattern of land-level movements is described by a model, the ‘earthquake deformation cycle’, of coseismic land subsidence followed by land uplift during interseismic strain accumulation. However, peat-mud couplets similar to those recorded in the Pacific Northwest are found on other less tectonically active temperate-latitude coasts, such as northwest Europe and the Atlantic coast of the U.S.A., where they have been interpreted as the product of non-seismic coastal processes. In this paper we apply the methods and scientific framework common to sea-level investigations in northwest Europe to a sequence of peat-mud couplets recorded in the lower Johns River, an estuary in southern Washington, to provide a test of the ‘earthquake deformation cycle’.Stratigraphic investigations of the intertidal sediments along the lower Johns River, using lithological, pollen, diatom and foraminiferal data, show evidence for eight coastal submergence events during the last 5000 years. To evaluate the ‘earthquake deformation cycle’ we assess the lateral extent of peat-mud couplets, the synchroneity of submergence, the presence of tsunami deposits accompanying submergence, and the suddenness and amount of submergence. Each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the ‘earthquake deformation cycle’, demonstrating the continued intermittent seismic activity of the Cascadia subduction zone throughout the mid and late-Holocene.Quantitative analyses of contemporary and fossil biostratigraphic data, using TWINSPAN and Detrended Correspondence Analysis, enable us to estimate the magnitude of submergence accompanying each peat-mud couplet. One event was accompanied by submergence of about 1.5 m or more, four events by intermediate submergence of about 1±0.5 m, and a further three events by submergence of <0.5 m. There is evidence for non-seismic relative sea-level rise prior to two of the eight submergence events, but for at least the last 3500 years the magnitude of relative sea-level rise has been less than the combined influence of sediment accretion following submergence and interseismic land uplift.     

西准噶尔志留纪-泥盆纪弧盆格局的确定及其区域构造演化意义

新疆西准噶尔克拉玛依-额敏一带广泛出露古生代的物质建造,记录着古亚洲洋演化的重要信息.古生代期间特别是晚古生代期间经历了复杂的洋陆转换过程,残留了系列近东西向及北东向蛇绿构造混杂岩系以及古大陆边缘的增生体系,造就了研究区复杂的岩石地层系统.目前对西准噶尔志留系、泥盆系构造古地理属性及志留纪-泥盆纪构造格局的刻画并不够精细.通过详细地野外地质调查及室内数据综合分析,结合前人的研究资料,对西准噶尔克拉玛依-额敏一带志留系及泥盆系代表性的岩石地层的构造古地理属性进行了界定,并在此基础上,对研究区志留纪-泥盆纪的弧盆格局进行探讨.综合分析认为志留纪西准噶尔克拉玛依-额敏一带古亚洲洋开始强烈的俯冲消减,泥盆纪古亚洲洋进入全面俯冲拼贴的演化阶段,形成多块体拼贴增生的多岛弧盆格局.泥盆纪后古亚洲洋在研究区开始进入残余洋盆的演化阶段.      

西准噶尔古生代地层区划及古地理演化

根据大地构造环境与沉积组合(建造)类型,地层序列与地层接触关系,古地理格局与古环境条件,古生物类型与生物古地理区系,地层类型与地层的变形、变质和变位特征,地层区划的边界类型与识别标志,地层区划可以区分为综合和断代地层区划2类,都可以分为4级:地层大区(stratomegaregion)、地层区(stratoregion)、地层分区(stratosubregion)和地层小区(stratomicroregion).基于近年来取得的大量新资料、新认识和上述地层区划6方面的判据,西准噶尔地区古生代地层区划自北向南划分为萨吾尔山地层小区、沙尔布尔提山地层小区、玛依力山地层小区和克拉玛依地层小区.在构造古地理上,西准噶尔地区古生代表现为多岛洋和软碰撞的特点,志留纪后期至早石炭世是多岛洋和软碰撞的鼎盛时期,也是西准噶尔地区古生代地层区划的重要形成时期;晚石炭世至二叠纪,西准噶尔地区主体脱离海洋环境,进入陆内造山阶段,西准噶尔地区古生代地层的分区性逐渐消失.在生物古地理上,早古生代西准噶尔地区属于介于太平洋生物大区与大西洋生物大区之间的混生生物大区,不同于东北部西伯利亚板块南部由Tuvaella(图瓦贝)动物群所代表的生物区系;从志留纪至泥盆纪,西准噶尔地区的生物组合面貌明显属于热带-亚热带的古特提斯生物大区;晚石炭世-二叠纪西准噶尔地区陆相地层中的植物群面貌显示出明显的北温带安加拉植物群的特点.在沉积古地理上,西准噶尔地区古生代的作用相包括正常沉积与事件沉积,特别是反映活动构造环境的内力事件沉积特别发育,如火山爆发相、火山溢流相和震积岩相;环境相包括古陆、河流相、滨-浅海相和半深海-深海相.      

新疆额敏东部志留纪化石的发现及其地质意义

在额敏东部原划为晚泥盆世塔尔巴哈台组的火山碎屑岩夹碳酸盐岩地层中发现了丰富的志留纪文洛克世动物化石, 包括珊瑚Halysites hoboksarensis、H. sp.、Mesofavosites sp., 腕足动物Atrypa sp.、Eospirifer radiatus, 三叶虫Encrinuroides sp.、Encrinurus sp.1、E. sp.2、Cheirurus sp.、Sthenarocalymene sp., 短剑类Lepidocoleus sp.和层孔虫等, 通过区域岩性组合对比, 将该套地层改划为文洛克统沙尔布尔组.珊瑚和层孔虫等对环境要求较高的生物化石的发现, 同时结合地层中风暴沉积的粒序层理、包卷层理及火焰构造等相标志, 说明了额敏东部在文洛克世为火山碎屑质的开阔台地相沉积环境, 处于热带、亚热带气候区.