Morphology,sedimentary infill and depositional environments of the Early Quaternary North Sea Basin (56°–62°N)

The North Sea Basin has been subsiding during the Quaternary and contains hundreds of metres of fill. Seismic surveys (170 000 km²) provide new evidence on Early Quaternary sedimentation, from about 2.75 Ma to around the Brunhes-Matuyama boundary (0.78 Ma). We present an informal seismic stratigraphy for the Early Quaternary of the North Sea, and calculate sediment volumes for major units. Early Quaternary sediment thickness is > 1000 m in the northern basin and >700 m in the central basin (total about 40 000 km³). Northern North Sea basin-fill comprises several clinoform units, prograding westward over 60 000 km². Architecture of the central basin also comprises clinoforms, building from the southeast. To the west, an acoustically layered and mounded unit (Unit Z) was deposited. Remaining accommodation space was filled with fine-grained sediments of two Central Basin units. Above these units, an Upper Regional Unconformity-equivalent (URU) records a conformable surface with flat-lying units that indicate stronger direct glacial influence than on the sediments below. On the North Sea Plateau north of 59°N, the Upper Regional Unconformity (URU) is defined by a shift from westward to eastward dipping seismic reflectors, recording a major change in sedimentation, with the Shetland Platform becoming a significant source. A model of Early Quaternary sediment delivery to the North Sea shows sources from the Scandinavian ice sheet and major European rivers. Clinoforms prograding west in the northern North Sea Basin, representing glacigenic debris flows, indicate an ice sheet on the western Scandinavian margin. In the central basin, sediments are generally fine-grained, suggesting a distal fluvial or glacifluvial origin from European rivers. Ploughmarks also demonstrate that icebergs, derived from an ice sheet to the north, drifted into the central North Sea Basin. By contrast, sediments and glacial landforms above the URU provide evidence for the later presence of a grounded ice sheet.     

内蒙古科右中旗地区梅勒图组安山岩年代学特征及其地质意义

对内蒙古科右中旗地区梅勒图组安山岩锆石LA--ICP--MS U--Pb年代学、锆石Hf同位素和岩石地球化学测试分析结果表明,梅勒图组安山岩形成时代约为早白垩世中期(123~125 Ma)。锆石εHf(t)=(+5.84~+9.34),一阶段Hf模式年龄为412~553 Ma。岩石地球化学分析表明,安山岩K2O含量较高,SiO2含量和Mg#值中等,富集大离子亲石元素,高场强元素Nb、Ta、Ti等明显亏损。这些特征反映出梅勒图组安山岩的源区为新形成的富集地幔。结合地球化学特征和区域大地构造背景,研究区梅勒图组安山岩的形成可能与太平洋板块俯冲作用后的陆内伸展环境有关。     

135~130 Ma: 大别山第二次“去根”时间？

大别杂岩主要由早白垩世侵入岩和三叠纪变质岩组成。它的四周是四条区域性韧性剪切带:郯城—庐江断裂,商城—麻城断裂,襄樊—广济断裂和晓天—磨子潭断裂。其中,晓天—磨子潭断裂和襄樊—广济断裂在早白垩世具有相反的走滑剪切方向:北侧的边界断裂(晓天—磨子潭断裂)是一个左行剪切断裂,而南侧的边界断裂(襄樊—广济断裂)是一个右行剪切断裂。在大别杂岩内部,早白垩世低角度剪切面理的倾伏向以SE向或NW向为主。这些晚期剪切面理上的拉伸线理的倾伏向同样为SE或NW向。大别杂岩总体具有朝SE向挤出和顶部相对朝NW向剪切的构造特征。这些表明晚中生代是该杂岩演化的重要阶段。该杂岩的边界断裂和内部构造特征指示其晚期抬升是沿造山带方向(SE—NW)以低角度方式进行的。这一过程直接导致高压-超高压变质岩和同构造岩浆岩被抬升至近地表。同时,年代学研究表明:大别杂岩(扬子板块东北缘地壳)在晚侏罗世—早白垩世经历大规模混合岩化的时间为145~135 Ma,同造山岩浆作用的时间为145~135 Ma,后造山火山-岩浆活动的时间为135~120 Ma。因此,该杂岩中三叠纪高压-超高压变质岩所记录的早白垩世抬升过程不是印支事件的后续,而是燕山期陆内造山及随后发生的伸展过程有关。尽管这一陆内造山事件的起始时间至今仍不确定,但大别山未变形岩体(130~120 Ma)的年代学研究结果和我们新测得的同构造伟晶岩脉的锆石U-Pb年龄(130 Ma)为早白垩地壳变形提供了良好的上限制约。这样,大别山经历了三叠纪碰撞造山和伸展,晚侏罗世—早白垩世陆内造山-伸展二次过程。     

青海玉龙滩地区托赖岩群中—早二叠世基性岩墙的发现及地质意义

在玉龙滩地区古元古代托赖岩群变质岩中发现了一套基性岩脉侵入体,岩石类型主要为辉长岩、辉绿岩,侵入于托赖岩群片麻岩、片岩,岩脉长轴走向大多以北西向为主,个别近南北向或北东向,露头规模一般长10~35m,宽5~10m。通过同位素测年分析辉绿岩脉地质时代为早二叠世,结合对地质特征、岩石学特征、岩石化学、地球化学特征的研究,认为其属于拉斑玄武系列,形成于伸展性构造环境。结合本区区域地质分析,该岩脉可能为祁连山造山带加里东期造山运动结束之后,由于陆壳加厚而发生伸展崩塌,造成幔源基性岩浆底侵,形成了该类相当于基性岩墙的产物。反映北祁连地区在早二叠世有一期陆壳加厚伸展背景下的基性岩浆底侵事件。     

Early Paleozoic tectonic evolution of the Xing-Meng Orogenic Belt: Constraints from detrital zircon geochronology of western Erguna–Xing’an Block,North China

To better constrain the Early Paleozoic tectonic evolution of the western part of the Erguna–Xing’an Block, detrital zircon U–Pb dating was applied on the Ordovician to Devonian sedimentary strata along the southeast part of the China–Mongolia border. Most of the zircons from five sedimentary samples display fine-scale oscillatory growth zoning and Th/U ratios higher than 0.1, indicating a magmatic origin. All five Ordovician–Devonian samples display the similar age distribution patterns with age groups at ∼440 Ma, ∼510 Ma, ∼800 Ma, ∼950 Ma, and few Meso- to Paleo-Proterozoic and Neoarchean grains. This age distribution pattern is similar to those from adjacent blocks in the southeastern Central Asian Orogenic Belt. Considering previous tectonic studies, we propose bidirectional provenances from the Erguna–Xing’an Block and Baolidao Arc.Consequently, a new model was proposed to highlight the Early Paleozoic tectonic evolution of the western Erguna–Xing’an Block, which constrains two main Early Paleozoic tectonic events of the Xing-Meng Orogenic Belt: (a) pre-Late Cambrian collision between Erguna–Kerulen Block and Arigin Sum-Xilinhot-Xing’an Block; (b) the Early Paleozoic subduction of Paleo-Asian Ocean and pre-Late Devonian collision between Erguna–Xing’an Block and Songliao-Hunshandake Block.     

青海格尔木哈希牙地区中基性岩墙群地球化学特征与LA-ICP-MS锆石U-Pb年龄

位于东昆仑造山带上的东昆北哈希牙地区发育中基性岩墙群。其岩性主要为辉绿岩、闪斜煌斑岩和闪长玢岩,地球化学数据显示其具有高钾钙碱性玄武岩系列特征。轻、重稀土元素存在较高程度分馏,LREE/HREE值为4.72~8.66,平均值为6.57。δEu值介于0.81~0.98之间,平均值为0.91,显示出微弱的负Eu异常,表明斜长石的分离结晶作用不明显,富集Rb、Ba、Pb、K等大离子亲石元素,贫高场强元素Ta、Nb、Zr、Hf等地球化学特征,主体显示板内玄武岩特征,但有来自俯冲带的部分信息。采用LA-ICP-MS技术对锆石进行了U-Th-Pb同位素测定,结果显示锆石年龄较为分散,其中8颗岩浆锆石给出的206Pb/238U年龄加权平均值为411.5±7.5Ma,代表了格尔木哈希牙地区辉绿岩墙的结晶年龄,反映东昆北地块于晚志留世—早泥盆世已由地体碰撞的挤压状态转为碰撞后伸展状态,标志着该地区加里东期构造旋回的结束。     

内蒙古莫力达瓦旗哈达阳绿泥石白云母构造片岩LA-ICP-MS锆石U-Pb年龄及其地质意义

嫩江—黑河构造带地处松嫩地块与兴安地块的结合部位,一直以来是业内学者关注的焦点。在构造带内内蒙古莫力达瓦旗哈达阳一带野外实地调研过程中,发现一套变质变形程度与周围地质体差异很大的地层。岩石组合为绿泥石白云母构造片岩、二云母微晶片岩、黝帘白云糜棱片岩等。采用LA-ICP-MS技术获得的绿泥石白云母构造片岩中锆石U-Pb年龄为413.9Ma±2.7Ma。通过分析区域地质演化历史及不同时代地质体特征,认为沿红山梁—哈达阳—依克特一带存在一条构造混杂堆积岩带,这套强变形岩石应为构造混杂岩带的一部分,其原岩形成时代为早泥盆世,后期构造定位应与嫩江—黑河构造带最后于早石炭世末拼合作用有关。     

Magmatic activity and its geological significance in Early Jurassic in Mangui area of Inner Mongolia

By investigating the U-Pb zircon isotope geochronology and petrogochemistry of the major magmatic rocks in Mangui area, the authors in this paper discussed the forming era, tectonic background and geological significance. A large amount of intrusive rocks and a small amount of medium-acid volcanic rocks in Early Jurassic were found in this area. U-Pb dating by LA-ICP-MS method shows that the ages are from (199±1)Ma to (184±1)Ma and the rock types can be divided into fine-medium-grained quartz diorite, medium-grained granodiorite, fine-medium-grained monzogranite, medium-fine-grained macrophenocryst monzogranite, rhyolite, dacite and andesite, which didn’t ouur in Neoprotezoic-Paleozoic period as previous researchers thought, revealing the tectonic and magmatic activities during the Early Mesozoic period. The geochemical results show that the rocks are Ⅰ-type magmatic rocks of subluminous-peraluminous high-K calc-alkaline series. The fractionation between light and heavy rare-erath elements((La/Yb)_N= 3.42~32.96) and the Eu depletion degree is not complied with the evaluation from basic to acidic. The large ion lithophile element Ba is relatively rich and Rb, Sr are relatively delicient. The high field strength elements Th and U are relatively rich and Nb, Ti, Y are relatively deficient. The magma origin and tectonic setting show that quartz diorite and medium-fine-grained macrophenocryst monzogranite come from crust-mantle mixed magma, while the medium-acid volcanic rocks, granodiorite and fine-medium-grained monzogranite are from the partial melting of crustal materials, whose formation is connected with the evolution of Mongol-Okhotsk Suture Zone. The geology and geochemistry of the Early Mesozoic magmatic rocks indicate that the middle part of Mongol-Okhotsk Ocean might begin subduction at the end of the middle Triassic and close the Early Jurassic. The peak collision might take place at the Early Jurassic, not in the Late Triassic as previous thought. The Mohe over-thrust nappe system might form in the remote effect of southward extrusion during the closing process of the eastem part of Mongol-Okhotsk Ocean. The middle and eastern Mongol-Okhotsk Ocean closing age is of great importance to reveal the basin-range tectonic formation during the Middle Jurassic to the Late Cretaceous in Northeast China.     

Geochemical characteristics and geological implication of Zijinshan alkaline rocks in Lin County in the west of North China Craton

The Zijinshan rocks in Lvliang area of Shanxi Province, are one of the famous alkaline plutons in China. The petrography and petrogeochemistry of the main rocks of the rock mass were studied to explore the material source and geological significance of Zijinshan alkaline rock mass. The results show that Zijinshan rocks of all stages show characters of low silicon, rich alkali and high potassium, which indicates zijinshan rocks belong to potassium alkaline rocks. These rocks demonstrate a high degree of fractionation and are characterized by the richness of LREE and the poorness of HREE, without obvious Eu anomaly. The trace element pattern exhibits the enrichment of large ion lithophile elements, such as Rb, Ba, K and Sr, and relative depletion of high field strength elements, such as Th, Nb, Ta, La, Ce, Nd, P and Ti. The magma of Zijinshan rocks comes from a deeper source and is closely related to the enriched mantle. The mechanism of magma evolution is mainly partial melting and it may be inter-contamination of continental crust materials. Combined with the characteristics of alkaline and partial alkaline rocks of same age in many places in Shanxi, the authors deduce that the alkaline magma activity in the middle of North China during the Early Cretaceous is closely to the destruction of North China Craton under the background of the subduction of Pacific plate.     

Early Devensian sediments and palaeovenvironmental evidence from the excavations at the Royal Oak Portal Paddington,West London,UK

This paper discusses the results of the investigation of Pleistocene sediments at the Royal Oak Portal (ROP) site on the new Crossrail scheme near Paddington Station, London. The site was sampled and recorded in May 2011 by archaeologists from Oxford Archaeology commissioned by Crossrail Ltd. The investigation revealed a sedimentary sequence associated with cool climate waterlain deposition towards the edge of the River Westbourne floodplain. During excavation an assemblage of around 100 identifiable large mammal bones was recovered, dating to the Late Pleistocene. The major concentration of bones, from bison and reindeer, was located and excavated from a shallow sequence of sediments. Analysis of the bones indicates that they represent a natural death assemblage, scavenged and subsequently disarticulated, transported by water, exposed and further dispersed and broken by trampling. The site is of regional and national importance because the assemblage derives from a well-constrained geological context, with associated dating evidence suggesting accumulation during the later parts of Marine Isotope Stage (MIS) 5 and continuing within MIS 4. The site is also of significance because it is one of a growing number of recently discovered sites away from the main fluvial archive for the British Middle and Upper Pleistocene. These sites have the potential to add significantly to our understanding of parts of the Pleistocene record that remain difficult to document through the investigation of the more active systems associated with major rivers such as the Thames, Severn or Trent.