青岛灵山岛南背来石下白垩统大型砂脉的发现及其形成动力学研究

青岛灵山岛南背来石剖面发育了两层大型不规则砂脉。其主要特征如下：上层大型不规则砂脉呈分散团块状顺黄褐色凝灰质泥岩分布,断续延伸约60 m,宽数米;有17个侵入砂团块。砂团块多呈浅灰色,主要岩性为砂岩、砂砾岩;砂团块直径在1~2 m。砂团块的形态非常复杂,侧向极易尖灭或突然中止。边界与围岩多突变,某些地方与节理发育方向相同。下层大型不规则砂脉浅灰色,主要底部为砂岩、顶部为砂砾岩,呈透镜状顺层产于灰黑色薄层砂泥岩中,均一团块状和边缘含大量泥砾;厚18 m,宽度和高度不详;上层砂脉发育在浅湖中;下层大型不规则砂脉发育在三角洲前缘。与此同时还探讨了上层砂脉的形成的动力学机制,建立了顺层砂脉的形成深度与液化层的深度之间的关系式（h=0.29H）。该公式可以用来预测顺层的侵位深度和寻找砂脉,为含油气盆地砂脉储层预测提供理论依据和新方法。通过流化公式计算获得了形成上层砂脉的流体上侵速度为1.26 m/s。通过本文的论述,进一步为理解灵山岛地区下白垩统的沉积环境和沉积动力学特征提供了新的岩石学证据。     

塔里木盆地西北缘震旦系玄武质岩墙和西南缘奥陶系中辉绿岩脉地质年代及构造环境的对比研究

本文对塔西北震旦系中的苏盖特布拉克组玄武质岩墙以及塔西南奥陶系玛列兹肯群中辉绿岩脉的区域地质、岩石学、地球化学等进行了对比研究,同时对锆石的微区 U-Pb 同位素年代学及其构造环境等进行了研究。 前者为顺层侵入、包裹了中酸性的捕虏体, 发育了枕状构造、球形风化和柱状节理等玄武岩喷发等特征;后者穿插于韧性剪切带中碳酸盐质糜棱岩化中。 它们的常量组分变化较大：不包括 SiO₂ 外其它 9 种常量组分的含量为 36.3%～45.3%, 全碱 （Na₂O+K₂O）为 3.83%～7.16%;TiO₂ 含量较低, 为 0.63%～1.89%;MgO 为 3.72%～6.74%;前者中的常量组分、微量元素 Sr 、Ta、Nb、Ti、Yb、Ni、Cu、Zn、Y 含量明显高于后者对应值, 但大离子亲石、造岩元素中 Cs、Rb 等元素、La/Nb、K/T 元素比相对较低。 相比于原始地幔, 两者均相对富集造岩元素 Li、Rb、Ba、Ga、Sr 和稀有元素 Nb、Ta, 轻稀土中等富集;相对亏损铁族元素 Cr、Co 及 Mo、Pb、Bi、U 等;前者 ΣREE 为 227.72×10^-6～426.32×10^-6 、L/H = 2.51～2.61、δEu = 0.48～0.52、δCe = 0.56;后者的 ΣREE 为 142.37×10^-6 ～187.92×10^-6 、L/H = 2.34～3.99、δEu = 0.63～0.84、δCe = 0.52～0.54;综合判识表明, 前者主要来自于未混染或较弱混染的幔源、发育于新元古（878～808.6 Ma）板内伸展的构造环境, 变质和残留核锆石谐和年龄为 2 090 Ma, 记录了早元古的联合大陆的形成时代, 蚀变锆石 455.4±10.8 Ma 代表了加里东中晚期的岩浆热事件;后者岩浆锆石的 U-Pb 年龄为晚奥陶世 478±48 Ma, 发育于弧后盆地、与韧性剪切带有关的加里东中晚期岩浆活动。     

桂西大新-钦甲地区辉绿岩脉地球化学与锆石U-Pb同位素年代学及对碳硅泥岩型铀矿床成因的启示

碳硅泥岩型铀矿床是我国铀矿地质工作者建立的铀矿床类型,是我国四大铀矿工业类型之一。桂西地区是我国华南地区一个重要的碳硅泥岩型铀矿床产出区域,包括大新(373)铀矿床等多个典型铀矿床(点)。研究辉绿岩与矿床成因的联系,对重新认识矿床成因有重要意义。本文在对桂西大新-钦甲地区碳硅泥岩型铀矿床和辉绿岩脉野外地质研究的基础上,系统研究了辉绿岩的地质特征、岩石学、地球化学特征以及其中锆石的U-Pb同位素年代学特征,在此基础上,探讨了辉绿岩的成岩与碳硅泥岩型铀矿床成矿间的关系。元素地球化学特征显示Rb、U、Th、Ba等大离子亲石元素的含量都高于MORB值,高场强元素Ta、Nb、Zr、Hf等均相对于MORB有所富集,而HREE元素中等亏损,表明本区辉绿岩属于富集地幔特征的板内碱性玄武岩系列(WPB),属于华南陆内伸展构造背景下软流圈上涌导致富集岩石圈地幔部分熔融形成的铁镁质岩浆发生侵位的产物。辉绿岩脉3个样品的锆石LA-ICP-MS U-Pb年龄为86. 7±0. 9Ma、89. 05±0. 96Ma、91. 6±8. 3Ma,表明形成于晚白垩纪早期,与华南地区与中基性岩脉有关的铀矿床成矿时代具有较好的对应关系,也与华南地区广泛分布的中生代基性岩脉时代一致,对应于华南白垩纪构造应力场中四次重要的拉张活动时代中的一期。辉绿岩脉锆石特征指示了辉绿岩成岩过程对矿源层中铀的活化所产生的重要作用以及成岩与成矿之间的热动力联系;结合元素地球化学、矿床成矿年龄和辉绿岩锆石UPb同位素年龄研究结果表明,区内铀矿床具有多期成矿特点,即"沉积期铀预富集、辉绿岩作用下二次预富集、后期热液再次富集成矿",可能是该类型矿床成矿作用的重要形式。     

Dating synmagmatic folds: a case study of Schlingen structures in the Strona-Ceneri Zone (Southern Alps, northern Italy)

The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3  Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298  Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294  Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.     

Radial patterns of bitumen dykes around Quaternary volcanoes,provinces of northern Neuquén and southernmost Mendoza,Argentina

Where the Neuquén Basin of Argentina abuts the Andes, hundreds of veins of solid hydrocarbon (bitumen) are visible at the surface. Many of these veins became mines, especially in the last century. By consensus, the bitumen has resulted from maturation of organic-rich shales, especially the Vaca Muerta Fm of Late Jurassic age, but also the Agrio Fm of Early Cretaceous age. To account for their maturation, recent authors have invoked regional subsidence, whereas early geologists invoked magmatic activity. During 12 field seasons (since 1998), we have tracked down the bitumen localities, mapped the veins and host rocks, sampled them, studied their compositions, and dated some of them. In the provinces of northern Neuquén and southernmost Mendoza, the bitumen veins are mostly sub-vertical dykes. They tend to be straight and continuous, crosscutting regional structures and strata of all ages, from Jurassic to Palaeocene. Most of the localities lie within 70 km of Tromen volcano, although four are along the Rio Colorado fault zone and another two are at the base of Auca Mahuida volcano. On both volcanic edifices, lavas are of late Pliocene to Pleistocene age. Although regionally many of the bitumen dykes tend to track the current direction of maximum horizontal tectonic stress (ENE), others do not. However, most of the dykes radiate outward from the volcanoes, especially Tromen. Thicknesses of dykes tend to be greatest close to Tromen and where the host rocks are the most resistant to fracturing. Many of the dykes occur in the exhumed hanging walls of deep thrusts, especially at the foot of Tromen. Here the bitumen is in places of high grade (impsonite), whereas further out it tends to be of medium grade (grahamite). A few bitumen dykes contain fragments of Vaca Muerta shale, so that we infer forceful expulsion of source rock. At Curacó Mine, some shale fragments contain bedding-parallel veins of fibrous calcite (beef) and these contain some bitumen, which is geochemically of low grade. In contrast, a large crosscutting bitumen dyke is of higher grade and formed later. At other localities, near basement faults, bitumen dykes have cap-rocks of hydrothermal calcrete. Other dykes or their wall rocks contain hydrothermal minerals. Finally, some dykes splay upward towards the current land surface. We conclude that (1) the bitumen dykes formed during volcanic activity in Pliocene–Pleistocene times, and that (2) heat advection by hydrothermal fluids helped to generate oil, which migrated upwards or downwards from the source rock and filled intrusive veins, before solidifying to bitumen, by loss of volatile elements. This unconventional hydrocarbon system may have significant implications for regional exploration in the foothills of the Andes.     

Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina,USA

ABSTRACT

Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al₂O₃ and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.     

北阿尔金拉配泉米兰群片麻岩及其中两期基性岩墙群LA-ICP-MS锆石U-Pb年龄及地质意义

北阿尔金拉配泉北侧米兰群混合角闪斜长片麻岩被2期基性岩墙群(辉绿玢岩和棕闪煌斑岩)侵位。本次分别对三者进行了LA-ICP-MS锆石U-Pb同位素侧年。混合岩化角闪斜长片麻岩中锆石结晶年龄为2007Ma±25Ma,结合西北地区2.0Ga左右广泛的构造-热事件,可能与Columbia超大陆汇聚有关。辉绿玢岩和棕闪煌斑岩的年龄结构相似,锆石较老年龄组分分别为1962～2093Ma和1804～2038Ma,系在侵位时从围岩(混合角闪斜长片麻岩)中捕获的锆石的年龄;最年轻的年龄组分(79～94Ma)代表基性岩墙群的侵位时间,与阿尔金北缘断裂伸展活动有关;古生代年龄(288～504Ma)为岩墙群上侵过程中同化混染北阿尔金蛇绿混杂岩带部分岩石的年龄。     

SHRIMP Zircon U‐Pb Dating of Alkaline Dykes in the Pobei Area,Beishan Rift,Xinjiang Autonomous Region,China: Implications for Tectonic Setting and Mantle Plume Events

In the Beishan rift in the eastern Tianshan orogen, Xinjiang Province, a N-S-trending dyke swarm is present in the Pobei area. The swarm cuts through the 270–290 Ma mafic-ultramafic intrusions associated with Ni-Cu sulphide mineralization. These mafic-ultramafic intrusions are typically found along E-W major faults in the Tianshan orogenic belts. We report SHRIMP U-Pb dating of zircons from a dyke of alkaline composition, which yielded a mean age of 252±9 Ma. Alkaline dykes of the same age are found in the Altay region of Siberia. This age is younger than the 270–290 Ma intraplate magmatic events that produced the mafic-ultramafic intrusions in the region, but in general agreement with the 250–260 Ma Permian plume event that gave rise to the Siberian traps and the Emeishan flood basalts in SW China. We suggest that there is a link between the Emeishan event and the dyke swarm in the Beishan rift and that the intraplate magmatism at 270–290 Ma reflects an early stage of mantle plume activity. The N-S trending dyke swarm in the Beishan rift may represent a later stage in the evolution of mantle plume activity in the NW and SW of China. We also speculate that in Beishan rift and possibly elsewhere in the Tianshan region, the dykes fed basaltic volcanism, whose products have since been eroded due to the strong uplift of the Tianshan orogen as a result of the India-Eurasia collision in the Cenozoic.     

冀北与角闪石岩相关铁钛磷灰岩的特征及成因-磷灰石矿物化学的证据

铁钛磷灰岩在超基性岩中很少发育,笔者野外工作发现冀北铁马哈叭沁超基性岩杂岩体的角闪石岩中发育少量团块状铁钛磷灰岩。本研究选择岩浆演化不同阶段的辉石角闪石岩、角闪石岩和铁钛磷灰岩中的磷灰石进行了电子探针和LA-ICP-MS激光探针分析,对比分析了磷灰石矿物主微量元素特征。不同岩石类型中磷灰石微量元素蛛网图形态十分相似,但铁钛磷灰岩中磷灰石的F、U、Th、Sr、∑REE含量明显更高,连续变化的趋势与岩体岩相序列相符,表明在岩浆演化的晚期阶段,随着分异程度的增强,角闪石岩母岩浆中的Fe-Ti-P组分逐渐富集并达到饱和,从而在角闪石岩中发育团块状的铁钛磷灰岩。     

Brachinite meteorites: Partial melt residues from an FeO-rich asteroid

Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO₂ (∼36–39 wt.%), high in MgO (∼27–30 wt.%) and notably high in FeO (∼26–37 wt.%), and low in Al₂O₃ (∼0.2–2.5 wt.%) and combined alkalis Na₂O and K₂O (∼0–0.7 wt.%). They consist mostly of olivine (∼71–96 vol.%; ∼Fo_64–73), major clinopyroxene (minor to ∼15 vol.%; ∼En_40–63Wo_36–48), with variable small amounts of plagioclase (0 to ∼10 vol.%; ∼An_15–33), and minor to trace amounts of orthopyroxene (none to ∼20 vol.%; En_69–73Wo_2–4), Fe-sulfides (trace to ∼7 vol.%), chromite (none to ∼5 vol.%), phosphates (none to ∼3 vol.%) and metallic Fe,Ni (trace to ∼2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (∼0.1–1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from ∼0.01 to ∼0.9 CI. Oxygen isotope composition (Δ¹⁷O) ranges from ∼−0.09 to −0.39‰, with the mean = −0.23 ± 0.14‰. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess ¹²⁹Xe from the decay of short-lived ¹²⁹I (half-life 17 Ma) and of fission tracks from the decay of ²⁴⁴Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using ⁵³Mn–⁵³Cr systematics, relative to the Pb–Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most ∼4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <∼100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts.