Late devensian ice sheet in the western Grampians,Scotland

A radial pattern of ice flow of the last ice sheet in the largest source area of ice (c. 5000 km²) in the British Isles in western Scotland is demonstrated by the dispersal of indicator erratics and by patterns of striae, friction cracks and ice-moulded landforms. Three major ice domes and the principal ice divide are identified in the western Grampians. The ice domes coincided with the highest mountain blocks while it is inferred that the alignment of many of the pre-existing valleys controlled much of the outflow of ice, forming ice streams within the ice sheet. The importance of the Rannoch Moor basin as a radial provider of ice to surrounding areas was apparently less significant than has hitherto been considered. Ice flowed into and across the southern part of the basin from the principal ice divide located to the west and south. No evidence has yet been found that would support a model of an eastward-migrating ice divide either during the build-up or during the deglaciation of the Late Devensian ice sheet.     

The metallic ore-deposits of the Guerrero Terrane,western Mexico: an overview

The Guerrero Terrane has produced Ag, Au, Pb, Zn, Cu, and Fe from Cretaceous, Paleocene, and mid-Tertiary ore bodies, which here are considered to be those deposits hosted by Guerrero Terrane rocks regardless of their ages or genetic links. The deposits formed during the three metallogenetic epochs are described. The most important, economically, are the mid-Tertiary deposits hosted in rocks of the Guerrero Terrane, which show a characteristic distribution. Fe-skarn deposits are close to the Pacific coast. Ag–Au and base metal ore bodies are generally located along the eastern border of the Guerrero Terrane. Copper deposits are present between these two zones. Important precious and base metal epithermal deposits are aligned along the suture zone between the Guerrero Terrane and the Sierra Madre Oriental Terrane, where mid-Tertiary age igneous bodies intruded shallow levels.     

大巴山西段高川地体的构造变形特征及其意义

勉略缝合带是华北与华南大陆于印支期完成拼合的主要拼接带,在中国大陆的形成与演化中占据重要地位.但是.勉略带的东、西延伸至今没有确定的结论,尤其陕西省勉县以东到湖北随州一带,还没有找到确凿的缝合带的证据.已发表的文章多认为勉略带在勉县以东到达宁陕县两河口之后向南转折,沿着高川岩片的位置顺大巴山弧形构造带向东延伸到湖北随州一带.本文的研究表明,高川岩片是一个独立于大巴山构造带的外来地质体,称之为高川地体.高川地体是一个由近南北向右行走滑断层与轴向近南北向的走滑剪切褶皱构造组成的右行走滑构造系统.是大巴山冲断推覆构造向南推覆过程中从原勉略缝合带中挤过来的一片,仅仅是被挟持在大巴山冲断-推覆构造带与大巴山前陆构造带之间的地壳浅层地质体.因此.高川地体现在的位置不能代表勉略带向东的延伸.大巴山弧形构造带也不是中国南北大陆最终碰撞拼贴的位置.     

西藏中拉萨地块西段左左乡晚侏罗世-早白垩世花岗岩年代学、地球化学与岩石成因

近年来对青藏高原中拉萨地块西部中生代岩浆作用的源区、成因和演化等问题的研究较少。本文针对中拉萨地块西部噶尔县左左乡两个中酸性岩体进行了岩石学、锆石U-Pb年代学、微量元素和锆石Hf同位素,以及主量元素和微量元素地球化学研究。左左乡北侧岩体寄主花岗闪长岩和闪长质包体获得同期(163Ma和160Ma)的年龄,属于晚侏罗世;南侧岩体花岗岩年龄为142～147Ma,属于早白垩世。北侧与南侧的中酸性岩石均属于准铝质-弱过铝质高钾钙碱性I型花岗岩,富集大离子亲石元素和轻稀土元素,亏损高场强元素。晚侏罗世岩体寄主岩具有较为富集的锆石Hf同位素成分(ε_(Hf)(t)=-16.8～-13.6),可能来源于古老下地壳的部分熔融;包体具有相似的Hf同位素成分(ε_(Hf)(t)=-15.7～-13.6),结合晚侏罗世寄主岩与包体较高的MgO(4.13%～6.90%)、Cr(146×10~(-6)～370×10~(-6))和Ni(31×10~(-6)～113×10~(-6))含量,说明包体代表的中基性岩浆可能源于古老富集地幔的熔融,并且在晚侏罗世寄主花岗闪长岩与闪长质包体所代表的两种岩浆间发生过充分的岩浆混合作用。南侧早白垩世花岗岩具有较负且变化范围大的ε_(Hf)(t)值(-8.2～-4.8和-3.8～+0.2),指示早白垩世岩浆活动中有更多亏损地幔物质或新生地壳物质的加入,并且亏损地幔物质或新生地壳物质与拉萨地块古老地壳物质熔融形成的酸性岩浆发生了岩浆混合,经历岩浆混合后中酸性岩浆又发生了角闪石、长石的分离结晶,最终形成南侧早白垩世岩体的一系列中酸性岩石。研究区早白垩世亏损地幔物质或新生地壳物质的增多可能与南向俯冲的班公湖-怒江板片的回转有关。     

Contact metamorphosed dunite-harzburgite complex in the Chugoku district,western Japan

The Tari-Misaka ultramafic complex, which is emplaced into the Paleozoic sediments and thermally metamorphosed by two younger granitic masses, is divided into four zones on the basis of the mineral assemblage. They are, in order of increasing metamorphic grade: Zone I antigorite-olivine-orthopyroxene-clinopyroxene. Zone II olivine-talc. Zone III olivine-anthophyllite. Zone IV olivine-orthopyroxene. Strongly serpentinized clinopyroxene-bearing harzburgite in Zone I is similar to ordinary Alpine-type harzburgite. In Zonne II, two kinds of olivine are recognized. One is Mg-rich olivine (Fo₉₃ to Fo₉₇) with opaque inclusions and is probably a recrystallization product of serpentine with talc. The other is Fe-rich olivine (Fo₈₈ to Fo₉₃) free of opaque inclusions and is probably a relic of the primary peridotite. Olivine in Zone III and Zone IV is also relatively Mg-rich (Fo₉₁ to Fo₉₅). Chromitite in Zone IV commonly has an assemblage, olivine+cordierite+Mg-Al spinel (Mg/Mg+Fe²⁺, more than 0.9). Enstatite is rare and coexists with less magnesian Mg-Al spinel (Mg/Mg+Fe²⁺, less than 0.9). Petrological and mineralogical characters of the ultramafic rocks can be well explained by thermal metamorphism of strongly serpentinized peridotite by granitic intrusion. Metamorphic zones are consistent with the experimental results in the system MgO-SiO₂-H₂O. The assemblage olivine+cordierite indicates that the metamorphism occurred at relatively low pressures (<3kb).     

Chromian spinel in the Ivrea-Verbano layered igneous complex,western Alps,Italy

Summary Spinel occurs in the pyroxenitic, peridotitic and gabbroic layers of the Ivrea-Verbano layered igneous complex. Its composition varies between picotite and Mg-hercynite. Maximum Cr₂O₃ contents are found in the spinels of the dunitic cumulitic layers occurring at some height above the base of the complex. Chromium deposits are absent. The lack of chromite and of chromium deposits is attributed to the early fractionation of clinopyroxene (which depleted the residual liquid in chromium) instead of olivine, as a consequence of the relatively high pressure prevailing during crystallization (8 kb). The relationships betweenfO₂-composition of spinel-composition of the silicate phases indicate thatfO₂ exerted a major control on the internal stratigraphy of the single layers and on the pattern of fractionation. The variation offO₂ are on their turn related to accidental variations of the pressure acting on the magma.It is finally suggested that chromium deposits are limited to low pressure layered intrusions, where the magma has been emplaced rapidly into a shallow magma chamber. No chromium concentration is to be expected in those complexes that crystallized into deep-seated magma chambers and that fractionated at relatively high pressure, not markedly different from that at which the magma was produced. One of the major controls on the presence of chromium deposits results, therefore, to be the geotectonic environment of intrusion.

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Chrom-Spinell im geschichteten vulkanischen Komplex von Ivrea-Verbano, Westalpen, Italien

Zusammenfassung Für die Spinelle der Pyroxenit-, Peridotit- und Gabbro-Lagen des Ivrea-Verbano-Intrusivkomplexes wurde eine Zusammensetzung im Bereich Picotit und Mg-Herzynit bestimmt. Die Dunit-Lagen im basalen Anteil des Komplexes führen Spinelie mit Cr₂O₃-Gehalten bis zu 30 Gew-%. Chromerz-Konzentrationen fehlen jedoch, was auf die fraktionierte Kristallisation von Klinopyroxen (und Entstehung Cr-Restschmelzen) an Stelle von Olivin, als Folge der bei relativ hohem Druck (8 kbar) erfolgten Intrusion, zurückzuführen ist. Die Beziehungen zwischen Sauerstoffpartialdruck und Spinell- und Silikat-Zusammensetzung zeigen den bedeutendenfO₂-Einfluß auf den stratigraphischen Aufbau der einzelnen Gesteinslagen und auf die fraktionierenden Kristallisationsvorgänge. Die schwankendenfO₂-Werter stehen in Zusammenhang mit den ebenfalls variierenden auf das Magma wirkenden Drücken.Es ist anzunehmen, daß Chromerzkonzentrationen nur in geschichteten Intrusivkomplexen vorkommen, die sich unter niederem Druck bzw. in einer oberflächennahen Magma-Kammer bildeten, und nicht in jenen, die tiefer intrudierten und bei hohem Druck kristallisierten. Von großer Bedeutung für die Bildung von Chromerzlagerstätten ist also das geotektonische Milieu der Intrusion.

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With 7 Figures     

Plagioclase zoning in the setogawa ash-flow sheet of the Nohi rhyolite complex,central Japan

Single whole crystals of calcic augite from M. Etna, Sicily, which display both sector and oscillatory zoning, have been serially sectioned and the form of the sectors mapped out. The principal intersectorial chemical variation is shown to be that of Ti and Al by as much as 39% (cation %) change in each, and the least variation, that of Ca (showing a 0.77 to 3.20% change). For each of three isochronous surfaces the ratio of Ti∶Al is shown to be individually constant no matter what the sector and this is thought to indicate near equilibrium conditions at the crystal/liquid interface. In addition to sector zoning these crystals display two types of concentric banding, having dimensions of about 0.25 mm, 20 μm. The 20 μm banding is shown to be both optically and chemically oscillated. Relative diffusion rates of cationic species in the melt, and growth rates of crystal faces are thought to play an important role in the formation of the oscillations, and the sector zoning.     

Tectonic position and geological manifestations of the 2006 olyutor earthquake in Koryakia

The results of seismotectonic study of the epicentral zone of the major earthquake in northern Kamchatka oblast are presented. Primary and secondary surface seismic dislocations were revealed. The exposed seismic source as a complex system of seismic faults up to 140 km in total length was found and mapped in detail. The system consists of three en echelon arranged NE-oriented segments about 16, 45, and 75 km long. The general strike of the fault system coincides with the orientation of the ridges in the Koryak Highland. The kinematics of the longest northeastern segment is reverse faulting of the southeastern wall combined with right-lateral strike-slip faulting. The maximum vertical and horizontal separations are 3 and 1.5 m, respectively. Vibration fractures, griffons, landslides, and rockfalls were revealed and documented as secondary seismic dislocations. The indications of paleoseismic dislocations were studied and documented as well. The age of paleoseismic events was determined with radiocarbon method from soil samples. The seismic source is confined to the boundary of the North American and Bering Sea lithospheric plates and exhibits its internal structure for a long distance. Seismic events testify to recent geological activity of the zone of interaction between the lithospheric plates. The collected data provide insights into the structure of the seismic source and its tectonic setting at the active continental margin of Asia.     

Early Cretaceous exhumation of the Qiangtang Terrane during collision with the Lhasa Terrane,Central Tibet

The timing of the closure of the Bangong Ocean between the Lhasa and South Qiangtang Terranes in central Tibet and the resulting crustal thickening are still under debate. We integrate published apatite fission track and (U–Th)/He thermochronometer data with new zircon (U–Th)/He ages from eight samples and with structural profiles to document that the South Qiangtang Terrane experienced slow exhumation between 200 and 150 Ma, associated with the opening of the Bangong Ocean. Accelerated exhumation (around 0.2–0.3 mm/a) of the South Qiangtang Terrane was initiated at around 150 Ma. This exhumation event is interpreted to reflect collision between the Lhasa and South Qiangtang Terranes after closure of the Bangong Ocean, associated with crustal thickening via thick‐skinned folding and thrusting within the South Qiangtang Terrane. The amalgamation of the Lhasa and South Qiangtang Terranes recorded here may represent the first stage of crustal thickening in the central Tibetan Plateau.     

豫西银家沟杂岩体年代学、地球化学和岩石成因

银家沟杂岩体位于河南省灵宝市,主要岩石类型为二长花岗斑岩、钾长花岗斑岩和石英闪长斑岩,并有少量呈脉状产出的花岗闪长斑岩及闪长玢岩.该杂岩体是银家沟硫铁多金属矿床的成矿母岩.杂岩体的元素地球化学和锆石SHRIMP U-Pb年代学研究结果表明,银家沟杂岩体二长花岗斑岩、钾长花岗斑岩和花岗闪长斑岩的侵位年龄分别为147.5±2.1Ma、147.8±1.6Ma和142.0±2.0Ma,为晚侏罗世-早白垩世的产物.二长花岗斑岩和钾长花岗斑岩具有高的SiO2(65.17％ ～73.94％)和Al2O3含量(13.53％ ～ 15.96％)、非常高的K2O含量(5.03％ ～9.89％,多数大于6.0％),但具有低的Na2O(0.24％～1.86％,多数小于1.0％)、Fe2O3(0.82％～3.71％)、FeO(0.02％ ～ 1.62％)、MgO(0.23％～1.47％,多数小于1.0％)和CaO含量(0.08％～ 1.98％,多数小于0.4％),铝指数(ASI)为1.18～2.04,全部大于1.1,应为热液蚀变引起.杂岩体的稀土元素总量介于71.38×10-6～276.4×10-6,具有中等-微弱的Eu负异常(δEu介于0.57 ～0.88),稀土元素分馏强烈,其(La/Yb)N介于6.93 ～ 30.1之间,为轻稀土富集型.岩石具有低Sr(104×10-6～461×10-6,多数＜300×10-6)和Yb含量(0.90×10-6～2.10×10-6,绝大多数＜1.9×10-6).在原始地幔标准化的微量元素蜘蛛网图中,银家沟杂岩体富集Rb、Ba、Th、U、K、La、Ce、Nd、Hf、Zr等,强烈亏损Sr、P、Ti等,具有中等Nb、Ta亏损.主量、稀土和微量元素特征表明,岩石具后碰撞花岗岩类的地球化学特征,属后碰撞花岗岩.杂岩体的锶初始比值(Isr)为0.7069～0.7091,多数＜0.7085,εNd(t)值为-14.99～-10.57,亏损地幔单阶段Nd模式年龄为1.41～1.76Ga,两阶段Nd模式年龄为1.51～1.81Ga;钾长石206Pb/204Pb、207 Pb/204Pb和208Pb/204Pb比值变化范围分别为17.376 ～17.628、15.455～ 15.502和37.867～38.090,Nd、Sr和Pb同位素组成指示银家沟杂岩体最可能源自宽坪群、二郎坪群和太华群的混合物.银家沟杂岩体形成于晚侏罗世-早白垩世期间的EW向构造体制向NNE向构造体制转换环境,构造体制转换使东秦岭地区处于减压环境,下地壳大规模部分熔融,导致银家沟杂岩体形成.     

Terrane assembly and geodynamic evolution of central–western Hoggar: a synthesis

After a review of the rock sequences and evolution of the eastern and central terranes of Hoggar, this paper focusses on the Neoproterozoic subduction-related evolution and collision stages in the central–western part of the Tuareg shield. Rock sequences are described and compared with their counterparts identified in the western and the eastern terranes exposed in Hoggar and northern Mali. The Pharusian terrane that is described in detail, is floored in the east by the Iskel basement, a Mesoproterozoic arc-type terrane cratonized around 840 Ma and in the southeast by Late Paleoproterozoic rock sequences (1.85–1.75 Ga) similar to those from northwestern Hoggar. Unconformable Late Neoproterozoic volcanosedimentary formations that mainly encompass volcanic greywackes were deposited in troughs adjacent to subduction-related andesitic volcanic ridges during the c. 690–650 Ma period. Abundant arc-related pre-collisional calc-alkaline batholiths (650–635 Ma) intruded the volcanic and volcaniclastic units at rather shallow crustal levels prior to collisional processes. The main E–W shortening in the Pharusian arc-type crust occurred through several stages of transpression and produced overall greenschist facies regional metamorphism and upright folding, thus precluding significant crustal thickening. It was accompanied by the shallow emplacement of calc-alkaline batholiths and plutons. Ages of syn-collisional granitoids range from 620 Ma in the western terranes, to 580 Ma in the Pharusian terrane, thus indicating a severe diachronism. After infill of molassic basins unconformable above the Pan-African greenschists, renewed dextral transpression took place in longitudinal domains such as the Adrar fault. The lithology, volcanic and plutonic suites, deep greenschist facies metamorphism, structures and kinematics from the Adrar fault molassic belt previously considered as Neoproterozoic are described in detail. The younger late-kinematic plutons emplaced in the Pharusian terrane at 523 Ma [Lithos 45 (1998) 245] relate to a Cambrian tectonic pulse that post-dates molasse deposition. The new geodynamic scenario presented considers several paleosubductions. The major east-dipping subduction, corresponding to the closure of a large Pan-African oceanic domain in the west (680–620 Ma) post-dates an older west-dipping “Pharusian” subduction (690–650 Ma?) to the east of the eastern Pharusian terrane. Such a diachronism is suggested by the 690 Ma old eclogites of the western part of the LATEA terrane of central Hoggar [J. African Earth Sci. this volume (2003)] that are nearly synchronous with the building up of the Pharusian terrane, thus suggesting that the 4°50^′ lithospheric fault represents a reactivated cryptic suture.     

The Yanbian Terrane (Southern Sichuan Province,SW China): A Neoproterozoic arc assemblage in the western margin of the Yangtze Block

The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important elements in the reconstruction of Neoproterozoic plate tectonics. The Yanbian Terrane in the western margin of the Yangtze Block is a typical arc assemblage composed of a flysch-type sedimentary sequence intruded by gabbroic and granodioritic plutons. The sedimentary sequence consists chiefly of tuffaceous material with interlayered chert, sandstone, and pillow basalts. Laser ablation ICP-MS U–Pb dating of detrital zircons from the sandstones yield ages as young as 840 Ma. The Gaojiacun and Lengshuiqing mafic intrusions are dated at 812 ± 3 Ma and 806 ± 4 Ma, respectively, using the SHRIMP zircon U–Pb technique. Geochemical data show that both the Gaojiacun and Lengshuiqing intrusions have arc signatures, with ɛNd(t) values of +1.5 to +6.0, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.705–0.706 and pronounced negative Nb–Ta and Zr–Hf anomalies. Their geochemical variations are best explained by fractional crystallization without major crustal contamination. The Yanbian Terrane represents a typical arc assemblage formed on the western edge of the Yangtze Block during Neoproterozoic time. The sedimentary sequence was deposited in an oceanic setting, probably in a back arc basin environment. The depleted, subduction-modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block.     

Significance of mineral chemistry of syenites and associated rocks of Elagiri complex,Southern Granulite Terrane of the Indian shield

The Elagiri complex (12°31′ N: 78°35′ E) represents one of the important silica — oversaturated syenite plutons of the Southern Granulite Terrane of the Indian shield. This article for the first time reports the mineral chemistry of the Elagiri complex and brings out important petrogenetic significance. The litho-members of Elagiri complex are intrusive into high grade country rocks viz. granite gneiss, amphibolite and pyroxene granulite. The country rocks are foliated bearing evidences of multiple folding and deformation. On the other hand, the constituent litho members of the Elagiri complex (syenites, gabbro and later intrusives marked by lamprophyre and carbonatite) show preservation of igneous layering in terms of discernible parallelism of the constituent minerals. The Elagiri complex shows presence of sharp contacts among litho members and marked absence of chilled facies peripheral to the margin. Electron microprobe data have been critically used to systematize the constituent mineral-phases of the different lithomembers of the complex. Geothermobarometric data indicate a temperature of equilibration in the range of ∼700° to 500°C at ∼2.0 to 5.2 kb which corresponds to shallow level (cf. 18.2 km) equilibration-depth of the complex. Field observations and mineral chemistry data suggest that liquid immiscibility plays an important role during the evolution of the Elagiri complex.     

青藏高原拉萨地块发现古元古代地体

拉萨地块东部呈断块状分布的松多岩群包括岔萨岗岩组、马布库岩组和雷龙库岩组, 对其形成时代存在较大分歧.通过高精度的LA-ICP-MS(激光剥蚀等离子体质谱仪)锆石微区原位U-Pb同位素测年, 西藏工布江达县一带岔萨岗岩组变基性火山岩中获得2 450.3±9.9 Ma的不一致线上交点年龄, 为原岩形成时代, 属于古元古代早期, 是目前拉萨地块获得的最老基底同位素年龄值.依据野外调研和新获得的同位素年代学数据, 结合新近完成的区域地质调查资料, 建议将原"松多岩群"解体, 新建工布江达岩群(Pt₁Gb), 松多岩群仅保留马布库岩组(AnOm)和雷龙库岩组(AnOl).拉萨地块古元古代地体的发现, 对重新厘定该区前寒武纪基底格架、区域对比、构造归属划分以及寻找与前寒武纪相关的矿产具有重要意义.      

鲁西新泰地区孟家屯岩组岩石学、地球化学及原岩特征

孟家屯岩组为鲁西新太古代早期形成的一套表壳岩,代表性的岩石类型为石榴石英岩、黑云母石英片岩、斜长角闪岩等.对这些岩石的岩相学、稀土元素、微量元素地球化学的研究和斜长角闪岩Nd同位素的初步分析,表明孟家屯岩组的原岩建造为一套砂岩、富泥质砂岩和岛弧拉斑玄武岩组合.这一认识对探讨鲁西早前寒武纪地壳的形成和演化特征具有重要意义.     

Records of Indosinian Orogenesis in Lhasa Terrane, Tibet

Based on the deformation characteristics of the ductile shear zones in Sumdo (松多) Group, the quartz fabric by EBSD (electron backscatter diffraction), the data of muscovite 40Ar-39Ar geochronology (220-230 Ma) from ductile shear zones and the zircon SHRIMP U-Pb chronology (190 Ma) of granites in Snmdo region, Lhasa (拉萨) terrane is thought to have experienced an important Indosinian orogenic event at 220-230 Ma, which caused the closure of the paleo-Tethys Ocean along the tectonic zone of eclogite and the collision between northern part and southern part of the Lhasa terrane. The zircon SHRIMP U-Pb chronology of 190 Ma for biotite adamellite, with the distributing characteristics of the granite massif intruding in Sumdo Group, indicates that the biotite adamellitc should be the late orogenic or post-orogenic granite resulting from the lndosinian orogenesis. The discovery of Indosinian orogenic belt in Lhasa terrane expansed the southern boundary of lndosinian orogenic belt in Qinghai (青海)-Tibet plateau to Lhasa terrane from Qiangtang (羌塘) terrane, which changed the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet plateau and extended the "T" type lndosinian orogenic belt in China. The study is very important for the formation and distribution of paleo-Tethys Ocean in Tibet. The ancient terrane framework and evolution of Qinghai-Tibet plateau need further research.     

印支地体泥盆纪牙形石、放射虫和竹节石组合

Devonian faunas including conodonts, radiolari-ans, tentaculitids and other important fossils have re-cently been found at several localities in the Indochina Terrane. These faunas are biostratigraphically and tectonically important.