班公湖-怒江结合带东段早石炭世洋壳残片及其古特提斯意义

班公湖-怒江结合带东段是破译班公湖-怒江特提斯洋早期演化的重要窗口。在对八宿县怒江大桥-邦达地区嘉玉桥岩群详细野外观察基础上,对赋存其中的玄武岩岩块及其围岩进行了锆石U-Pb定年,并对玄武岩进行了全岩地球化学研究。嘉玉桥岩群具有构造混杂岩的特征,基质主要为板岩、千枚状板岩、薄层状结晶灰岩,岩块主要为玄武岩、结晶灰岩、大理岩。玄武岩岩块锆石的外部形态、内部结构及均一的年龄结果显示其为基性岩浆结晶锆石。基性锆石²⁰⁶Pb/²³⁸U年龄的加权平均值为338±2Ma,代表玄武岩形成时代。玄武岩的围岩中碎屑锆石U-Pb年龄范围为3157~500Ma,最年轻一组年龄为513~500Ma,限定其原始沉积时代不早于该年龄范围。玄武岩轻稀土元素亏损,高场强元素Nb、Ta、Zr、Hf无异常,ε_Nd（t）值为+7.4~+8.5,这些地球化学特征与N-MORB型玄武岩一致。受海水蚀变影响,Rb、Ba、U、Sr元素和⁸⁷Sr/⁸⁶Sr（i）同位素有不同程度的富集。玄武岩岩块是由亏损地幔中尖晶石相二辉橄榄岩部分熔融形成,代表洋壳残片。八宿县怒江大桥-邦达地区嘉玉桥岩群是一套俯冲增生杂岩,其基质岩石和岩块主要形成于早石炭世,构造就位时间有待进一步限定。班公湖-怒江结合带存在较多前侏罗纪的化石和同位素年龄信息,本研究揭示班公湖-怒江洋在338±2Ma已经具有成熟的洋壳,指示班公湖-怒江特提斯洋打开时间应早于早石炭世（338±2Ma）。

     

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge,SW Japan

To understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.     

Geochemical contrast between the Sanbagawa psammitic schists (Oboke unit) and the Cretaceous Shimanto sandstones in Shikoku, Southwest Japan and its geologic significance

Abstract A systematic geochemical study of sandstones from the Cretaceous Shimanto Supergroup and psammitic schists from the Oboke unit in Shikoku has been carried out in order to clarify the depositional age of the protoliths of the Oboke psammitic schists. The geochemical data, together with chronological and geologic data, led to the following conclusions. (i) It is inferred that Oboke psammitic schists are metamorphically equivalent to sandstones in the Hiwasa Formation of the Shimanto accretionary complex, deposited in a trench area during the Campanian, in eastern Shikoku. (ii) The protolith attained to maximum metamorphic conditions within 20 million years after the deposition. (iii) The accumulation of a large amount of coarse-grained clastic sediments in the trench area induced offscraping and underplating of the sediments in the subduction zone, forming the Hiwasa Formation and Oboke unit, respectively.     

台湾增生楔的构造单元划分及其变形特征

台湾增生楔位于欧亚板块、菲律宾海微板块和南海的结合部位,是现代弧陆碰撞研究的理想场所。通过对南海973航次在该区域的多道地震剖面的解释,对该增生楔进行了构造单元的划分,并分析了变形特征。认为台湾增生楔是由3个部分,即弧陆碰撞产生的增生部分、洋内俯冲产生的增生部分和增生楔后端在恒春海脊和北吕宋海槽之间的构造楔组成,研究区的高屏斜坡、恒春海脊和北吕宋海槽西端变形带分别是3个部分的反映。自中中新世以来,南海洋壳开始沿着马尼拉海沟向菲律宾海微板块俯冲,形成增生楔中洋内俯冲增生部分;与此同时菲律宾海微板块开始向NW方向移动,前缘的吕宋岛弧自6．5Ma B．P以来与亚洲陆缘斜向碰撞,形成增生楔中弧陆碰撞增生部分。碰撞首先发生在台湾岛的北部,由于弧陆强烈的挤压作用,增生楔后端部分向北吕宋海槽倒冲楔人,使得上部的北吕宋海槽的沉积发生隆升变形。滨海的各个地貌单元可以和台湾陆上的地貌单元相联系,它们具有相似的地质特征,但是由于陆上部分增生历史久,不仅抬升为陆,而且地层的年代也更老。     

Structure of the Middle Urals,east of the main Uralian Fault

In the Middle Urals, volcanic-arc and back-arc basin rocks of Ordovician to Devonian age occur in the Tagil Synform. These outboard terranes were thrust westwards in the late Carboniferous onto continental margin associations of late Proterozoic and Palaeozoic age, now exposed in the Central Uralian Uplift. The Main Uralian Fault coincides approximately with the suture separating the outboard terranes from the East European Platform margin. New fieldwork in the hinterland of the Middle Urals in the area east of the Tagil Synform has found structural evidence favouring E-directed thrusting of accreted terranes and eugeoclinal allochthons in the late Palaeozoic. The upper tectonic units are composed of ophiolite mélange and volcano-sedimentary rocks of Ordovician to Devonian age; they are thrust onto high-grade gneisses, some of possible microcontinental affinities, extensively intruded by mid-Palaeozoic granitic plutons. The nappes in the hinterland are refolded by major upright antiforms and synforms that fold the entire tectonostratigraphy. After thrust assembly, all tectonic units east of the Main Uralian Fault were intruded by late Carboniferous to early Permian granites. Reflection seismic profiles (recorded to 8 s TWT), recently reprocessed at Cornell University, image the major fold structures and demonstrate that they are restricted to the upper crust, being underlain by an extensive zone of flat-lying middle crustal reflectivity. At 10–15 km depth the latter appears to truncate all structures, including the late- to post-tectonic granitoids and extensional faults, east of the Main Uralian Fault. Previous studies (potential-field, refraction- and wide-angle-reflection seismics) have identified an anomalously deep crust under the Tagil Synform and have concluded that the root zone of the orogen is located beneath this belt. The new evidence presented here supports this interpretation, with back-thrusting of the oceanic rocks eastwards over Palaeozoic accreted terranes. © 1998 John Wiley & Sons, Ltd.     

琼中高级变质杂岩中单颗粒锆石Pb-Pb年龄及其地质意义

琼中地区存在由片麻岩、混合岩、麻粒岩和紫苏花岗岩组成的高级变质杂岩。对其中片麻岩和麻粒岩开展了单颗粒锆石Pb-Pb年代学研究,分别获得1483±13Ma和499±24Ma的年龄数据。结合现有地质资料, 认为琼中地区先后经历了新太古代表壳岩系形成、中元古代变质—岩浆再造和加里期构造热事件改造等3个主要演化阶段。     

南海东北部陆坡与恒春海脊天然气水合物分布的地震反射特征对比

对“探宝号”调查船2001年8月在南海东北部陆坡及台湾南部恒春海脊海域采集的多道地震剖面资料进行了地震反射波数据分析、解释和研究,并对南海北部陆坡、陆隆及其东侧俯冲带等区域天然气水合物矿藏的成藏规律及分布特征作了初步的分析与探讨,结果表明:(1)南海东北部陆坡和台湾南部恒春海脊海域地震剖面上均显示有BSR,但两区域构造成因、形式和相关地质环境的不同造成了天然气水合物的成因及过程不同。(2)南海东北部陆坡区域的天然气水合物形成与该区广泛发育的断裂带、滑塌构造体及其所形成的压力场屏蔽环境有关,而台湾南部恒春海脊海域天然气水合物的形成则与马尼拉海沟俯冲带相关的逆冲推覆构造、增生楔及其所对应的海底流体疏导体系有关。(3)南海陆缘区域广泛发育有各种断裂带、滑塌构造体、泥底辟、俯冲带、增生楔等,且温压环境合适,是天然气水合物矿藏极有可能广泛分布的区域。     

The origin of strata within the inner accretionary prism of Nankai Trough: Evidence from clay mineral assemblages along the NanTroSEIZE transect

One of the more prominent architectural elements of the Nankai subduction margin, offshore southwest Japan, is an out‐of‐sequence thrust fault (megasplay) that separates the inner accretionary prism from the outer prism. The inner prism (hanging wall of the megasplay) is dominated by mudstone, which is enigmatic when the sedimentary facies is compared to coeval deposits in the Shikoku Basin (i.e. inputs from the subducting Philippine Sea plate) and to coarser‐grained turbidite sequences from the Quaternary trench wedge. Clay mineral assemblages amplify the mismatches of sedimentary facies. Mudstones from the inner prism are uniformly depleted in smectite, with average bulk values of 23–24 wt%, whereas the Shikoku Basin deposits show progressive decreases in proportions of smectite over time, from averages of 46–48 wt% at 10 Ma to 17–21 wt% at 1 Ma. Plate‐boundary reconstructions for the Philippine Sea region provide one solution to the conundrum. Between 15 Ma and 10 Ma, the Pacific plate subducted near the NanTroSEIZE transect, and a trench‐trench‐trench triple junction migrated to the northeast. Accretion during that period involved sediments that had been deposited on the Pacific plate. Motion of the Philippine Sea plate changed from 10 Ma to 6 Ma, resulting in sinistral slip along the proto‐Nankai Trough. Sediments accreted during that period probably had been deposited near the triple junction, with a hybrid detrital provenance. Renewed subduction of the Philippine Sea plate at 6 Ma led to reorganization of watersheds near the Izu–Honshu collision zone and gradual incision of large submarine canyons on both sides of the colliding Izu arc. Accreted Pliocene mudstones share more of an affinity to the triple junction paleoenvironment than they do to Shikoku Basin. These differences between subducting Shikoku Basin strata and accreted Pacific plate sediments have important implications for interpretations of frictional properties, structural architecture, and diagenetic fluid production.     

Palaeokarst development in a lower Frasnian (Devonian) platform succession,Canning Basin,northwestern Australia

Recognition of palaeokarst in the oldest exposed Devonian (Givetian ‐ lower Frasnian) platform successions of the Canning Basin reef complexes has eluded investigators for over forty years. The first evidence for palaeokarst, developed on microbial mud‐mounds in a single stratigraphic horizon, is documented and records an episode of exposure during early carbonate platform development. Surface palaeokarst features are scalloped surfaces, solution pits and a pipe, underlain by fenestral limestone with sediment‐filled fossil moulds and vugs. The platform succession has variably developed metre‐scale cycles which are composed predominantly of shallowing‐upward subtidal facies, with some cycles having fenestral peloidal mudstone caps. Changes in facies type and stratigraphic arrangement up the succession define two deepening‐upward units (～70 and 180 m thick), with the palaeokarst surface representing emergence following rapid shallowing at the top of the lower unit. The stratigraphic position of the palaeokarst between these two units suggests it may represent a sequence boundary. This may have been caused by a low‐magnitude eustatic fall or footwall‐uplift event superimposed on a rapidly subsiding basin margin.     

含金磷酸盐矿物的发现及其地质意义 ——以山东牟平腊子沟金矿为例

山东牟平腊子沟金矿位于华北板块苏鲁造山带胶南-威海隆起区,牟平-乳山金矿成矿带中段,区内岩浆活动频繁,构造活动强烈,成矿地质条件优越。本文通过镜下显微结构观察、电子探针测试分析对腊子沟金矿载金矿物的组成、成分及金的赋存状态进行了研究,首次在该矿区发现含金独居石和含金磷灰石,金含量达到0.283%～0.456%,推测金可能通过类质同象形式替代磷灰石中的Ca2+或独居石中的稀土元素(Ce2+、 La2+)以晶格金的形式存在。前人研究认为成矿流体中金主要以Au[HS]2－和AuCl2－络合物形式进行迁移,本次含金磷酸盐矿物的发现表明：可能部分金以磷酸根络合物形式进行迁移。