Hydrothermal Mineralization on the Mesoproterozoic Passive Continental Margins of China： A Case Study of the Langshan-Zha＇ertaishan Belt, Inner Mongolia, China

Most ore-forming characteristics of the Langshan-Zha‘ertaishan hydrothermal exhalation belt, which consists of the Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulfide deposits, are most similar to those of Mesoproterozoic SEDEX-type provinces of the world. The characteristics include: (1) All deposits of this type in the belt occur in third-order fault-basins in the Langshan-Zha‘ertaishan aulacogen along the northern margin of the North China Platform; (2) these deposits with all their orebodies hosted in the Mesoproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have an apparent stratabound nature; ores display laminated and banded structures, showing clear depositional features; (3) there is some evidence of syn-sedimentary faulting, which to a certain extent accounts for the temporal and spatial distribution and the size of the orebodies in all deposits and the formation of intrabed conglomerates and breccias; (4) they show lateral and vertical zonation of sulfides; (5) The Cu/(Pb Zn Cu) ratio of the large and thick Pb Zn Cu orebodies gradually decreases from bottom to top; and (6) barite is interbedded with pyrites and sometimes with sphalerite. However, some characteristics such as the Co/Ni radio of the pyrites, the volcanism, for example, of the Langshan-Zha‘ertalshan metallogenic belt, are different from those of the typical SEDEX deposits of the world. The meta-basic volcanic rock in Huogeqi, the sodic bimodal volcanic rocks in the Dongshengmiao and potassic bimodal-volcanic rocks with blastoporphyfitic and blasto-glomeroporphyritic texture as well as blasto-amygdaloidal structure in the Tanyaokou deposits have been discovered in the only ore-bearing second formation of the Langshan Group in the past 10 years. The metallogeny of some deposits hosted in the Langshan Group is closely related to syn-sedimentary volcanism based on the following facts: most of the lead isotopes in sphalerite, galena, pyrite, pyrrhotite and chalcopyrite plot on both sides of the line for the mantle or between the lines for the mantle and lower crust in the lead isotope composition diagram; cobalt content of some pyrites samples is much higher than the nickel content (Co/Ni= 11.91-12.19). Some volcanic blocks and debris have been picked out from some pyritic and pyrrhotitic ores. All Zn-Pb-Cu-Fe sulfide orebodies in these deposits occur in the strata overlying metamorphic volcanic rocks in the only ore-bearing second formation. In the Jiashengpan deposit that lacks syn-sedimentary volcanic rocks in the host succession only Pb and Zn ores occur without Cu ore, but in the Dongshengmiao, Tanyaokou and Huogeqi deposits with syn-sedimentary volcanic rocks in the host succession Cu ores occur. This indicates a relatively higher ore-forming temperature. The process of synsedimentary volcanic eruption directly supplied some ore-forming elements, and resulted in secular geothermal anomaly favorable for the circulation of a submarine convective hydrothermal system, which accounts for the precipitation of deep mineralizing fluids exhaling into anoxidic basins along the syn-sedimentary fault system in the Langshan-Zha‘ertai rift. The Dongshengmiao, Tanyaokou, and Huogeqi deposits hosted in the Langshan Group appear to be a transitional type of mineral deposit between SEDEX and VMS-types but with a bias towards SEDEX, while the Jiashengpan deposit hosted in the Zha‘ertai Group is of a characteristic SEDEX type. This evidence, together with other new discoveries of Mesoproterozoic volcanic rocks and the features of lithogeny and metallogeny of the Bayun Obo deposit in the neighborhood emphasize the diversity, complexity and uniqueness of the Mesoproterozoic Langshan-Zha‘ertal-Bayun Obo ore belt.     

Stratigraphic evolution of an aeolian erg margin system: the Permian Cedar Mesa Sandstone, SE Utah, USA

The Permian Cedar Mesa Sandstone of south‐east Utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which, in terms of palaeogeographic setting, reflects a transition from a dry erg centre, through a water table‐controlled aeolian‐dominated erg margin, to an outer erg margin subject to periodic fluvial incursion. The erg margin succession represents a wet aeolian system, accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport. Variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes, and a range of dry, damp and wet (flooded) interdune elements is recognized. Variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre, locally interconnected damp and/or wet interdune ponds in the aeolian‐dominated erg margin and fully interconnected, fluvially flooded interdune corridors in the outer erg margin. Relationships between aeolian dune and interdune units indicate that dry, damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration. Temporal variation in the rates of water‐table rise and bedform migration determined the angle of climb of the erg margin succession, such that accumulation rates increased during periods of rapidly rising water table, whereas sediment bypassing (zero angle of climb) occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table. During these periods in the outer erg margin, the expansion of fluvially flooded interdunes in front of non‐climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass (flood) supersurfaces. A spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the Cedar Mesa erg margin succession.     

Centrifuge modelling of the influence of crustal fabrics on the development of transfer zones: insights into the mechanics of continental rifting architecture

Centrifuge analogue experiments are used to model the reactivation of pre-existing crustal fabrics during extension. The models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones. The typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement. The transfer zone was also characterised by cross-basin faults with both trend and strike-slip component of movement opposite to that displayed by the master faults. Typically, different structural patterns were obtained by changing the offset angle φ between the rift segments, supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre-existing discontinuities with respect to the stretching vector. In the models, the aspect ratio (ratio of length vs. width) of the transfer zone shows a positive correlation with the offset angle (i.e., the more the inherited fabric is parallel to the extension direction, the longer and narrower the transfer zones). In case of staircase offset of the rift segments (φ=90°), the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped. Prominent rise of the ductile lower crust was also observed at the transfer zone. Many of these geometrical features display striking similarities with natural rift systems. The results of the current experiments provide useful insights into the mechanics of continental rift architecture, supporting that rift propagation, width and along-axis segmentation may be strongly controlled by the reactivation of pre-existing pervasive crustal fabrics.     

Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt

Studies of supercontinental cycle are mainly concentrated on the assembly, breakup and dispersal of supercontinents, and studies of continental crustal growth largely on the growth and loss (recycling) of the crust. These two problems have long been studied separately from each other. The Paleozoic–Mesozoic granites in the Central Asian Orogenic Belt have commonly positive Nd values, implying large-scale continental crustal growth in the Phanerozoic. They coincided temporally and spatially with the Phanerozoic Pangea supercontinental cycle, and overlapped in space with the P-wave high-V anomalies and calculated positions of subducted slabs for the last 180 Ma, all this suggests that the Phanerozoic Laurasia supercontinental assembly was accompanied by large-scale continental crustal growth in central Asia. Based on these observations, this paper proposes that there may be close and original correlations between a supercontinental cycle, continental crustal growth and catastrophic slab avalanches in the mantle. In this model we suggest that rapid continental crustal growth occurred during supercontinent assembly, whereas during supercontinental breakup and dispersal new additions of the crust were balanced by losses, resulting in a steady state system. Supercontinental cycle and continental crustal growth are both governed by changing patterns of mantle convection.     

Middle Campanian–lowermost Maastrichtian nannofossil and foraminiferal biostratigraphy of the northwestern Australian margin

The middle–late Campanian was marked by an increase in the bioprovinciality of calcareous microfossil assemblages into distinct Tethyan, Transitional, and Austral Provinces that persisted to the end of the Maastrichtian. The northwestern Australian margin belonged to the Transitional Province and the absence of key Tethyan marker species such as Radotruncana calcarata and Gansserina gansseri has led petroleum companies operating in the area to use the locally developed KCCM integrated calcareous microfossil zonation scheme. The KCCM zonation is a composite scheme comprising calcareous nannofossil (KCN), planktonic foraminiferal (KPF) and benthonic foraminiferal (KBF) zones. This paper presents the definitions and revisions of Zones KCCM8–19, from the highest occurrence (HO) of Aspidolithus parcus constrictus to the lowest occurrence (LO) of Ceratolithoides aculeus, and builds on our previous early–late Maastrichtian study. The presence of a middle–upper Campanian disconformity is confirmed by microfossil evidence from the Vulcan Sub-basin, Exmouth and Wombat plateaus, and the Southern Carnarvon Platform. In the Vulcan Sub-basin and on the Exmouth Plateau (ODP Hole 762C) the hiatus extends from slightly above the LO of common Rugoglobigerina rugosa to above the LO of Quadrum gothicum. On the Wombat Plateau (ODP Hole 761B) it spans from above the LO of Heterohelix semicostata to above the LO of Quadrum gothicum; and in the Southern Carnarvon Platform the disconformity has its longest duration from above the HO of Heterohelix semicostata to above the LO of Quadrum sissinghii. A significant revision of the events which define Zones KCCM18 and 19 was necessary owing to the observation that the LO of Ceratolithoides aculeus occurs below the HOs of Archaeoglobigerina cretacea and Stensioeina granulata incondita and the LO of common Rugoglobigerina rugosa. In the original zonation these events were considered to be coincident.     

Crustal thickening in an extensional regime: application to the mid-Norwegian Vøring margin

The structure of the mid-Norwegian volcanic Vøring margin at the onset of the Maastrichtian–Paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events. Lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between Norway and Greenland. The most important imprints (Moho depth, thermal structure) of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase. We focus on a possible mechanism of formation of the Vøring Marginal High and address the relationship between spatial and temporal evolution of crustal thinning and thickening, uplift of the surface and strength of the lithosphere.It is found that the Vøring Basin formed the strongest part of the margin which explains why the Maastrichtian–Paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations. The modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension. Negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone; convergence of material at the particular part of the margin is mainly taken up by the lower crust. The model shows that during the final phase of extension, the crust in the Vøring Marginal High area was thickened and the surface uplifted. It is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the Marginal High.     

Seafloor spreading, obduction and triple junction tectonics of the Mineoka Ophiolite, Central Japan

The Tertiary Mineoka ophiolite occurs in a fault zone at the intersection of the Honshu and Izu forearcs in central Japan and displays structural evidence for three major phases of deformation: normal and oblique-slip faults and hydrothermal veins formed during the seafloor spreading evolution of the ophiolite at a ridge-transform fault intersection. These structures may represent repeated changes in differential stress and pore-fluid pressures during their formation. The second series of deformation is characterized by oblique thrust faults with Riedel shears and no significant mineral veining, and is interpreted to have resulted from transpressional dextral faulting during the obduction of the ophiolite through oblique convergence and tectonic accretion. This deformation occurred at the NW corner of a TTT-type (trench–trench–trench) triple junction in the NW Pacific rim before the middle Miocene. The third series of deformation of the ophiolite is marked by contractional and oblique shear zones, Riedel shears, and thrust faults that crosscut and offset earlier structures, and that give the Mineoka fault zone its lenticular (phacoidal) fabric at all scales. This deformation phase was associated with the establishment and the southward migration of the TTT Boso triple junction and with the kinematics of oblique subduction and forearc sliver fault development. The composite Mineoka ophiolite hence displays rocks and structures that evolved during its complex geodynamic history involving seafloor spreading, tectonic accretion, and triple junction evolution in the NW Pacific Rim.     

Crustal and mantle strengths in continental lithosphere: is the jelly sandwich model obsolete?

The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition.     

塔里木盆地南缘历史时期气候环境变化的过程与特征

根据塔里木盆地南缘具较高分辨率的湖沼相沉积物碳酸盐δ^13C和粒度等记录，恢复出2162-850BC期间为一相对稳定的温暖干旱时期，之后迅速转冷湿，湿润程度呈持续、阶段式增加，50 BC至500 AD期间呈现的显著冷湿特征于550 AD之后突变转暖干而结束。550 AD和1000 AD前后的具突变性质的气候事件在南疆地区近2．0ka的气候变化中具有重要意义，反映气候状况有过重大调整。850—1300AD期间(相当于中世纪温暖期)冷暖、干湿多变，但温暖特征并不明显。特别是1100—1200AD期间气候快速、频繁变化之后，奠定了本地区现代稳定干旱环境特征。区域对比表明，尼雅剖面记录的气候变化具有广泛的区域一致性。