南海北部深水区新生代岩浆岩分布规律及其与海底地质灾害的相关性

前人对南海北部新生代陆缘盆地结构、构造样式和期次等做了大量的研究工作,但对于具体体现内动力过程的岩浆活动关注较少,尤其是北部深水区的研究鲜有涉及。本文基于二维地震剖面的解译分析,将南海北部新生代岩浆岩划分为3个集中分布区:琼东南-西沙区、神狐区和东沙区。各区域岩浆岩的产出状态有所差异,其中琼东南-西沙区岩浆岩的平面展布规律性极强,主要受右行右阶的走滑断裂控制;神狐区岩浆岩位于NW向与NE向断裂的交汇处,侵位空间受先存断裂制约;东沙区岩浆岩的产生与东沙运动期间区域伸展过程有直接关系。作为内动力的表征,岩浆作用对于海底地形地貌具有重要的改造作用,与活动断裂、海底滑坡、浅层气等海底地质灾害也具有成因上的关联。     

Helen's Reef: a microgabbroic intrusion in the Rockall intrusive centre,Rockall Bank

Rockall Island is composed of 52 ± 9 Ma (m.y.) aegirine granite and forms part of a nearly planated intrusive complex. The nearby Helen's Reef lies within the complex but is composed of Cretaceous (81 ± 3 Ma) microgabbros. The petrography and chemistry of the microgabbros differ from Mid-Ocean Ridge tholeiites but are similar to the Tertiary gabbroic rocks of northwestern Scotland. The relationship of the Cretaceous igneous activity to the evolution of the North Atlantic Ocean is discussed briefly.     

Basalts and serpentinite from the Puerto Rico Trench, 1. Petrology

Typical abyssal tholeiites occur on both north and south walls of the Puerto Rico Trench. Probably they were erupted in the Mid-Atlantic Ridge in Cretaceous time and afterwards were moved to the present position. In addition, doleritic basalts containing phenocrysts of pigeonite rimmed by augite were found from the north wall, and these rocks presumably represent a new type of abyssal tholeiites characterized by $\text{CaO}\text{Na}{}_2\text{O}$ ratios lower than those of ordinary abyssal tholeiites.A new analysis of serpentinite from the north wall is also recorded here.     

North-east Greenland Shelf north of 79°N: results of a reflection seismic experiment in sea ice

The first multichannel seismic reflection measurements of the north-east Greenland shelf north of 79°N were made as part of the R.V. Polarstern cruise ARK V/1988. This area is perennially covered by sea ice. The sparse seismic data show a very deep rift basin filled with up to 10 km of high velocity rocks interpreted as Carboniferous and Permian sediments. Conspicuous structures interpreted as marginal reefs are associated with the western flank of the rift basin.     

南海北部陆缘张裂--岩石圈拆沉的地壳响应

南海北部陆缘在中生代晚期曾形成宏伟的华夏陆缘造山带。火成岩岩石学、岩相古地理学和地球物理学证据显示，该造山带不仅具有巨厚(50～60 km)的陆壳，而且还有巨厚(160～180 km)的岩石圈根，在地势上曾出现过高3 500～4 000 m 的华夏山系。陆缘裂陷盆地的形成发育历史、地壳-岩石圈深部结构、火成岩地球化学特征及理论计算均表明，南海北部陆缘从晚白垩世以来发生的张裂作用起始于华夏陆缘造山带的拉伸塌陷，岩石圈拆沉是南海北部陆缘张裂的重要的引发机制。因此，南海北部陆缘张裂既不同于弧后扩张，也不受控于大西洋式的海底扩张，而是该区大陆构造演化和深部壳幔相互作用的结果。     

A sonic well log of the basement complex of the Walvis Ridge

A sonic well log was obtained within the basement complex of the Walvis Ridge during Deep Sea Drilling Project Leg 74. The top of the basement complex is characterized by smooth acoustic reflectors. The rocks recovered within the basement complex consist of basalts with intercalated sediments. According to the log ～-50% of the upper 75 m of basement are igneous rocks and the other 50% sedimentary. Sonobuoy results indicate that the ratio of sediments to basalt increases with depth for an additional 225 m until a typical oceanic velocity structure is observed. Paleontological results suggest that the processes forming this upper 300 m of the basement complex was accomplished within a short time interval.     

冲绳海槽岩浆源的三分量混合模型

综合分析了冲绳海槽玄武岩中微量元素的地球化学特征，发现最少要求三分量混合作用来解释其岩浆源成分，并通过最大方差因子分析估算了3个分量的贡献比例。亏损型地幔是海槽岩浆源的主体，但其中舍有富集型地幔的成分，尤其是海槽中部玄武岩岩浆源中这种富集型地幔成分的特征表现得更为明显。俯冲组分是岩浆源中的另一个主要成分，贡献量为20％左右，是造成海槽区玄武岩中Pb强烈富集的重要原因。     

Prolonged Magmatic and Tectonic Development of the Caribbean Igneous Province Revealed by a Diving Submersible Survey

Using the diving submersible survey NAUTICA we investigated the central part of the Caribbean large igneous province (CLIP) to observe and sample internal portions of this proposed oceanic plateau. Most of the samples are gabbroic and doleritic rocks; basalts are scarce. Radiometric dating by ⁴⁰Ar/³⁹Ar incremental heating experiments indicate that the intrusive rocks are Campanian in age (81–75 Ma). In some places these intrusive rocks underlie older Santonian (85–83 Ma) extrusive basaltic rocks, suggesting that the Campanian rocks represent a sill injection and an underplating episode. Results of the diving program supplemented by information from ODP and DSDP drilling sites document a 20 m.y. period (94–75 Ma) of igneous activity in the submerged portion of the Caribbean large igneous province (CLIP). In the northern part of the Beata Ridge late Campanian and/or post Campanian uplift is documented by prominent Maastrichtian (71–65 Ma) erosion and the establishment of a Paleocene-middle Eocene (65–49 Ma) carbonate platform. During and after the uplift an extensional period is indicated by seismic images and the subsidence (3 km depth) of the carbonate platform. Paleocene ages (55–56 Ma) determined on some volcanic samples are attributed to localised decompression mantle melting that accompanied the extension. We document a prolonged period of magmatic and tectonic events that do not fit with the current models of short-lived plateau formation during mantle plume initiation but shares many similarities with the constructional histories of other oceanic large igneous provinces.     

New perspective of Miocene marine hydrocarbon source rocks in deep-water area in Qiongdongnan Basin of northern South China Sea

Drilling wells reveal that the organic matter abundance of Miocene marine source rocks in shallow water area of the Qiongdongnan Basin is relatively low with poor hydrocarbon generation poten- tial. However, in some drilling wells of deep water area close to the central depression belt, Miocene marine source rocks with better organic matter abundance and hydrocarbon generation have been found, which have achieved better source rock standard based on the analysis of geochemical charac- teristics. Although there are no exploratory wells in deep water area of the research region, through the comparative analysis of geochemical data of several typical exploratory wells respectively from shallow water area in the basin, central depression belt margin in deep-water area of the basin and Site 1148 of deep sea drilling in the South China Sea Basin, it reveals that the tendency of the quality of source rocks becomes positive gradually from delta to bathyal environment, which then becomes negative as in deep oceanic environment. Owing to the lack of terrestrial organic matter input, the important controlling factors of Miocene marine source rocks in the Qiongdongnan Basin are ocean productivity and preservation conditions of organic matter. The element geochemistry data indicate that the tendency of the paleoproductivity and the preservation conditions of organic matter become positive as water depth increase from shallow area to bathyal area close to central depression belt. So it is speculated that there must exist high quality source rocks in the central depression area where the preservation conditions of organic matter are much better. Besides, in theory, in oxygen-poor zone of oceanic environment at the water depth 400-1 000 m, the preservation conditions of organic matter are well thus forming high-quality marine source rocks. The result- ing speculation, it is reasonable to consider that there are high hydrocarbon generation potential source rocks in bathyal environment of the Qiongdongnan Basin, especially at the water depth 400- 1 000 m.     

基于独立成分分析的马里亚纳弧后地幔源区特征研究

对采自太平洋洋中脊(277组)、印度洋洋中脊(159组)、马里亚纳海槽(53组)、马里亚纳岛弧(39组)、中南劳海盆(72组)共600组玄武岩数据进行了独立成分分析,从Sr-Nd-Pb五维同位素比值空间提取出占样本方差99％的3个独立成分(IC1,IC2,IC3),并利用这3个独立成分(ICs)与微量元素比值之间的相关...     

Cenozoic magmatism in the northern continental margin of the South China Sea: evidence from seismic profiles

Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene–Mid Miocene) and after cessation of seafloor spreading (Mid Miocene–Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean–continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to suggest that the northern margin of the SCS is a magma-poor rifted margin.     

北黄海海域的重磁场特征及其地质意义

北黄海是我国近海勘探程度最低的海区,到目前为止只进行了地球物理调查工作,没有进行钻探。根据最新的重磁数据,详细地分析了北黄海海域的重磁场特征、构造特征、基底岩相构造和岩浆岩分布。通过重磁数据处理解释推测认为:两条NW向的次级隆起将海洋岛隆起与刘公岛隆起连接起来,同时将北黄海盆地分成3部分;研究区内断裂非常发育,即有切割较深的深大断裂,又有一般的基底断裂,还存在许多较小的断裂;基底为太古界—元古界变质岩;区内岩浆岩分布广泛,以燕山期中酸性侵入岩为主。     

Characteristics of silicon and oxygen isotopic compositions of basalts near East Pacific Rise 13°N

In this study, 13 groups of silicon and oxygen isotopes and major elements of the basalts near the East Pacific Rise 13°N are used to study the fractionation of silicon and oxygen isotopes. Among these data, δ30Si values of basalts vary from -0.4%o to 0.2%o with a mean value of δ30Si of （-0.18±0.22）%o. The δ180 values range from 4.1%o to 6.4%o with a mean δ180 value of （＋5.35±0.73） %0. Since the δ30Si values increase in the series of basalt-basaltic andesite- andesite, and δ180 values display a positive correlation with the SiO2 content, we propose that the fractionation of silicon and oxygen isotopes is influenced by the SiO2 content in igneous rocks. Compared with the igneous rocks from Manus Basin with clinopyroxene as their dominant mineral phase, MORBs in this study containing olivine and plagioclase as primary minerals have lower δ180 and δ30Si values, indicating that the fractionation of silicon and oxygen isotopes is also affected by different Si-O bridges in silicate minerals. Furthermore, our samples from the EPR are defined as E-MORB based on K/Ti ratios. Probably, the difference in δ30Si and δ30O between our samples and a normal MORB are cause by the enriched components in E-MORBs.     

A sonic well log of the basement complex of the Walvis Ridge

A sonic well log was obtained within the basement complex of the Walvis Ridge during Deep Sea Drilling Project Leg 74. The top of the basement complex is characterized by smooth acoustic reflectors. The rocks recovered within the basement complex consist of basalts with intercalated sediments. According to the log ∼-50% of the upper 75 m of basement are igneous rocks and the other 50% sedimentary. Sonobuoy results indicate that the ratio of sediments to basalt increases with depth for an additional 225 m until a typical oceanic velocity structure is observed. Paleontological results suggest that the processes forming this upper 300 m of the basement complex was accomplished within a short time interval.     

Geochemistry and petrogenesis of Quaternary volcanism from the islets in the eastern Beibu Gulf: evidence for Hainan plum

Some of the islets in the eastern Beibu Gulf are covered by Quaternary volcano strata. The rock samples from these islets mainly consist of quartz tholeiites （at Shenjiandao）, olivine tholeiites （at Linshidao and Xieyang- dao） and alkali basalts （at Yangpubi and Jianshidao）, and basically represent four periods of the Quaternary volcanism of Hainan Island and its adjacent regions. Except for the samples from Shenjiandao, most of the Quaternary volcanics of these islets belong to alkali magma series. The trace element characteristics of all of these samples show they are OIB （oceanic island basalt） -like, which implies that their deep geodynamic setting may be related to a mantle plume. The Sr-Nd-Pb isotopic compositions show that the mantle source beneath the Quaternary strata can be regarded as a result of binary mixing between a depleted, DMM （de- pleted MORB mantle）-like source and an enriched mantle type 2 （EM2）. The EM2 may be originated from the Hainan mantle plume, and has been metasomatized by carbonaceous fluids released from ancient re- cycled oceanic crust at an asthenospheric mantle level. These features, together with typical trace element ratios, reflect that the parent magma was not subjected to crustal contamination during its ascent to the surface. This study provides further petrological and geochemical evidence for the existence of the Hainan mantle plume.     

Hydrocarbon compositions of bitumens from mineralized Devonian lavas and Carboniferous sedimentary rocks, central Scotland

Solid bitumens occur in mineralized veins in the Lower Devonian Ochil Volcanic Formation in the Midland Valley of Scotland. Bitumens are also widespread in the Carboniferous of the Midland Valley. Gas chromatography (g.c.) - mass spectrometry (m.s.) studies undertaken on bitumens from the Devonian rocks and on Devonian and Carboniferous mudrocks show that the bitumens contain biomarkers and that Carboniferous rocks are the more likely source for the bitumens. Hydrocarbons may have migrated into the Devonian rocks along the Ochil Fault zone from a downthrown Carboniferous basin to the south of the fault. The organic geochemistry of bitumen samples from the Carboniferous rocks reflects varying degrees of biodegradation, and heating by igneous intrusions.     

Tectonic control of the oil-rich large igneous-carbonate-salt province of the South Atlantic rift

The Early Cretaceous South Atlantic Magmatic Province (SAMP), which includes the Paraná-Etendeka LIP, produced about 8 million km³ of tholeiitic basalt and diabase over an area of 4 million km². Huge pre-salt oil reserves, discovered in 2007 by Petrobras in non-marine carbonates, are estimated at more than 45 billion barrels. Here we show the close causal relationship of the southward increasing width of the wedge-shaped South Atlantic rift with the similarly southward increase in igneous activity, in the thicknesses of non-marine carbonate and salt, and in the size of oil reserves, all controlled mainly by South America’s early clockwise rotation away from Africa about a pole in its northeast. Large diabase dike swarms transversal to the rift witness to South America’s rotation that opened in its wake the southward widening South Atlantic rift. Westward increasing pressure on the Equatorial margin by South America’s clockwise rotation forced open the Benue trough and created pre-late-Aptian folds in the Demerara Plateau and in Brazil’s Solimões (Upper Amazonas) basin. Prerift and synrift volcanic activity increases southward, culminating in the Parana-Etendeka LIP and in the offshore volcanic SDRSs that continue southward to the Cape Basin. Berriasian-Valanginian rift sediments deposited from about 145 Ma, 10 Ma before the flood basalts of the Parana-Etendeka LIP. The largest transversal dike swarm continued in the proto-Walvis Ridge that separated the central South Atlantic endorheic rift basin from the sea in the south; erosion and leaching of basalts supplied Ca, Mg, and SiO₂ to the endorheic basin for the deposition of non-marine carbonates and authigenic clays. Basalt flows intercalated with carbonates nearly until salt deposition about 113 Ma. Hypogenic leaching of carbonates by mantle-derived CO₂ created optimal reservoirs. Supergiant oil deposits occur where the widest endorheic basin and the volcanic province overlap.     

Rocks from the Gibbs fracture zone and the Minia seamount near 53°N in the Atlantic Ocean

Rocks dredged and drilled from both the rift mountains of the Mid-Atlantic Ridge (Minia seamount) and from the northern wall and the median ridge of the adjacent Gibbs fracture zone near 53°N include tholeiites, serpentinized and mylonitized peridoties, and gabbroic rock.The tholeiites include: (1) pyroxene-tholeiites, commonly without phenocrysts and containing less than 15% Al₂O₃; (2) plagioclase-tholeiites with small (1.1–2.2 mm length) plagioclase phenocrysts and an Al₂O₃ content varying between 14–17%; and (3) high-alumina plagioclase tholeiites with large (> 2.5 mm length) plagioclase phenocrysts and more than 17% Al₂O₃. The apparently transitional differences among the three groups support the possibility that differentiation by crystal fractionation of the high-alumina plagioclase-tholeiites gave rise to the plagioclase-tholeiites with less Al₂O₃ and smaller phenocrysts and to the pyroxene-tholeiites. A small portion of the basalts ampled show effects of low-grade metamorphism. The peridotites may represent evidence of intrusive emplacement of peridotitic material beneath the tholeiitic rocks.     

Geology of an active hot spot: Teahitia-Mehetia region in the South Central Pacific

The Teahitia-Mehetia hot spot region located in the southeastern extension of the Society Islands chain, near 18° S–148° W consists of several active volcanoes. The distribution of recent volcanic activity correlates with seismic epicenters, and covers an area of more than 1000 km². Intermittent volcanic activity has given rise to large (>1000 m high) and small (<500 m high) edifices composed of various types of flows. Several recent volcanic events have produced a suite of alkalic rocks ranging from ankaramites, through alkali basalts to trachy-phonolites. The presence of altered MORB-like tholeiites on one small seamount suggests that a different mantle source material was involved in forming some of the crust in this hot spot region.     

Hydrocarbon Potential of Pre-cenozoic Strata in the North Yellow Sea Basin

The North Yellow Sea Basin ( NYSB ), which was developed on the basement of North China (Huabei) continental block, is a typical continental Mesozoic Cenozoic sedimentary basin in the sea area. Its Mesozoic basin is a residual basin, below which there is probably a larger Paleozoic sedimentary basin. The North Yellow Sea Basin comprises four sags and three uplifts. Of them, the eastern sag is a Mesozoic Cenozoic sedimentary sag in NYSB and has the biggest sediment thickness; the current Korean drilling wells are concentrated in the eastern sag. This sag is comparatively rich in oil and gas resources and thus has a relatively good petroleum prospect in the sea. The central sag has also accommodated thick Mesozoic-Cenozoic sediments. The latest research results show that there are three series of hydrocarbon source rocks in the North Yellow Sea Basin, namely, black shales of the Paleogene, Jurassic and Cretaceous. The principal hydrocarbon source rocks in NYSB are the Mesozoic black shale. According to the drilling data of Korea, the black shales of the Paleogene, Jurassic and Cretaceous have all come up to the standards of good and mature source rocks. The NYSB owns an intact system of oil generation, reservoir and capping rocks that can help hydrocarbon to form in the basin and thus it has the great potential of oil and gas. The vertical distribution of the hydrocarbon resources is mainly considered to be in the Cretaceous and then in the Jurassic.