Source and possible tectonic driver for Jurassic–Cretaceous gold deposits in the West Qinling Orogen,China

The West Qinling Orogen (WQO) in Central China Orogenic Belt contains numerous metasedimentary rock-hosted gold deposits (>2000 t Au), which mainly formed during two pulses: one previously recognized in the Late Triassic to Early Jurassic (T3–J1) and one only recently identified in the Late Jurassic to Early Cretaceous (J3–K1). Few studies have focused on the origin and geotectonic setting of the J3–K1 gold deposits.Textural relationships, LA-ICP-MS trace element and sulfur isotope compositions of pyrites in hydrothermally altered T3 dykes within the J3–K1 Daqiao deposit were used to constrain relative timing relationships between mineralization and pyrite growth in the dykes, and to characterize the source of ore fluid. These results are integrated with an overview of the regional geodynamic setting, to advance understanding of the tectonic driver for J3–K1 hydrothermal gold systems. Pyrite in breccia- and dyke-hosted gold ores at Daqiao have similar chemical and isotopic compositions and are considered to be representative of J3–K1 gold deposits in WQO. Co/Ni and sulfur isotope ratios suggest that ore fluids were derived from underlying Paleozoic Ni- and Se-rich carbonaceous sedimentary rocks. The geochemical data do not support the involvement of magmatic fluids. However, in the EQO (East Qinling Orogen), J3–K1 deposits are genetically related to magmatism. Gold mineralization in WQO is contemporaneous with magmatic deposits in the EQO and both are mainly controlled by NE- and EW-trending structures produced by changes in plate motion of the Paleo-Pacific plate as it was subducted beneath the Eurasian continent. We therefore infer that the J3–K1 structural regime facilitated the ascent of magma in the EQO and metamorphic fluids in the WQO with consequent differences in the character of contemporaneous ore deposits. If this is correct, then the far-field effects of subduction along the eastern margin of NE Asia extended 1000's of km into the continental interior.     

中国侏罗纪年代地层学研究的现状

“国际地层表”依据菊石带建立起来的侏罗纪年代地层系统在全球海相侏罗系的划分和对比中有着广泛的应用,但却很难直接应用于非海相侏罗纪地层系统中。中国的侏罗系多属非海相,近年来我国地质工作者们不但将中国的海相侏罗系与全球侏罗纪年代地层系统进行了较合理的对比,发现了穿越海相三叠系-侏罗系界线的连续沉积的剖面,而且建立了非海相侏罗系的阶。但是中国非海相侏罗系区域性阶的时代和不同阶之间的界线有待海相化石和地层测年来确定或检验     

西藏羌塘盆地北部托纳木地区上侏罗统雪山组沉积相

羌塘盆地地处藏北高原,其南、北边界分别为班公错一怒江缝合带与拉竹龙一金沙江缝合带,盆地面积约18×10⁴km²,可进一步分为羌北坳陷、中央隆起带和羌南坳陷3个二级构造单元.双湖托纳木地区地处羌北坳陷南部,与中央隆起带毗连,广泛发育上侏罗统雪山组沉积,垂向上由3个沉积旋回组合构成,总体显示为一个向上变粗的充填序列.雪山组发育扇三角洲相和冲积扇相,其中雪山组一段为前扇三角洲亚相沉积;二段为扇三角洲前缘亚相和扇三角洲平原亚相沉积,扇三角洲前缘亚相分为水下分流河道、河口坝和远砂坝沉积,扇三角洲平原亚相发育天然堤和分流河道沉积;三段为冲积扇相沉积,包括扇端和扇中沉积.雪山组沉积的物源来自西南部的中央隆起带.     

晚三叠世—中侏罗世羌塘盆地的形成与演化

晚三叠世-侏罗纪是羌塘盆地大型海相沉积盆地形成时期,是研究羌塘盆地形成过程、判别盆地性质的重要时期.本文通过区域构造、盆地充填建造和岩相古地理分析,提出了该时期的羌塘盆地并非晚三叠世前陆盆地(或弧后盆地)和侏罗纪弧后盆地(前陆盆地),而是晚三叠世早期(肖茶卡期)的陆表海盆地和晚三叠世诺利期-中侏罗世巴柔期(那底岗日期-雀莫错期)的坳陷-裂陷盆.肖茶卡期内陆盆地主要受可可西里-金沙江活动带的控制,沉积期后出现的海退事件可能与冈瓦纳大陆与欧亚大陆的碰撞作用有关;那底岗日期-索瓦期羌塘盆地的强烈拉张断陷可能反映冈瓦纳大陆边缘的总体构造背景.     

Sedimentology, petrography and geochemistry of a limestone breccia (Pietra di Billiemi) from north-west Sicily, Italy: implications for evolution of the Tethyan basins around the Triassic/Jurassic boundary

In this study, the Pietra di Billiemi , a famous dimension stone, is investigated because it records the tectonic evolution of the south Tethys continental margin and preserves a record of major environmental changes occurring near the Triassic/Jurassic boundary. The Pietra di Billiemi is a grey, coarse-grained and clast-supported limestone breccia cropping out in an area of the Palermo Mountains representing a segment of the Apennine–Maghrebian chain in western Sicily. The rock consists of metre-sized to centimetre-sized angular clasts, derived from Upper Triassic sponge boundstones and rudstones, with a differently coloured, silt-grained matrix. Fitted fragments are observed commonly which suggest an in situ origin for the bulk of the breccia. The matrix is characterized by the absence of biogenic components and by variable mineralogy and geochemistry. Petrographic features and Sr-isotope values indicate that the most important and earliest fillings of the breccia consist of black matrix and white matrix temporally referable to Hettangian–Sinemurian times. Clotted micrite, carbonate fluorapatite and abundant pyrite, in addition to relatively high contents of redox-sensitive elements (V, Ni, Zn and S), are consistent with deposition in anoxic conditions that favoured microbial mediation for authigenic carbonate (calcite and dolomite) precipitation in the matrix. As a whole, the Billiemi breccia can be considered a product of tectonic fragmentation of a Tethyan carbonate platform edge around the Triassic/Jurassic boundary, formed when the drowned platform edge was covered by hemipelagic mudstones recording the anoxic conditions existing during Early Jurassic times.     

Controls and predictability of carbonate facies architecture in a Lower Jurassic three-dimensional barrier-shoal complex (Djebel Bou Dahar, High Atlas, Morocco)

Spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.     

The discovery of Isocrinus cf. robustus from the Lias Group (Lower Jurassic) near Dunrobin Castle, Sutherland, Scotland

A single stem section (pluricolumnal) belonging to a post-Palaeozoic crinoid (sea lily) is reported from a small outcrop of Lower Jurassic Lias Group strata exposed in low cliff near Dunrobin Castle. This is the first Jurassic crinoid recorded from Eastern Scotland and the small fragment has enough diagnostic characters to be assigned to the species Isocrinus cf. robustus; a crinoid found commonly in the Lower Jurassic of England. The Scottish form collected has unusual morphology that is atypical of the genus.     

A New Theropod Dinosaur from the Middle Jurassic of Lufeng,Yunnan, China

A new theropod dinosaur, Shidaisaurus jinae gen. et sp. nov., has been described on the basis of an incomplete skeleton. The specimen was found near the base of the Upper Lufeng Formation (early Middle Jurassic) in Yunnan, China. It is the first theropod dinosaur from the Middle Jurassic of Yunnan. Shidaisaurus jinae is distinguishable from other Jurassic theropods by certain features from the braincase, axis, and pelvic girdle. The absence of any pleurocoels in the axis or in any anterior dorsal vertebrae suggests that the new Lufeng theropod is relatively primitive and more plesiomorphic than most of the Middle to Late Jurassic theropods from China. Most Chinese taxa of Jurassic theropod dinosaurs have not been well described; a further detailed study will be necessary for us to determine their phylogenetic relationships with Shidaisaurus jinae.     

Structure,Orientation and Finite Element Analysis of the Tail Club of Mamenchisaurus hochuanensis

Abstract: The structure and orientation of the posterior extremity (tail club) of the caudal vertebrae of Mamenchisaurus hochuanensis Young and Chao, 1972 from the Upper Jurassic Shangshaximiao Formation has been analyzed to determine the tail club function using Finite Element Analysis. Of the four caudal vertebrae composing the tail club, the second largest (C"1") was probably the most proximal, and is fixed with the preceding sequence of the caudal vertebrae, whereas the smallest (C"4") is free and forms the termination of the tail club. Our analysis also suggests that the tail club is more efficient in lateral swinging rather than up-and-down motion, and that the best region for the tail club to impact is at the spine of the largest of the four caudals (C"2"), with a maximum load for impact at about 450 N. The tail club of Mamenchisaurus hochuanensis probably also had limitations as a defense weapon and was more possibly a sensory organ to improve nerve conduction velocity to enhance the capacity for sensory perception of its surroundings.     

长江中下游成矿带深部找矿思路探讨

长江中下游成矿带的探明资源已开采殆尽,深部找矿（-500--2000m）应是当务之急。根据勘查与开采资料的综合研究,该区的控矿条件可概括为：岩石圈减薄事件是成矿作用的重要前提,燕山期强烈的构造-岩浆活动是区域成矿的主要关键,地表断裂构造样式是深部构造的响应,前陆盆地沉积建造是成矿作用的优越围岩条件,铁、铜矿床的岩浆岩蚀变特点明显不同,埃达克质岩可视为找铜矿床的间接标志。对于今后深部找矿提出4个思路：①火山岩盆地下部的深部找矿思路;②目标地层的深部找矿思路;③复合类型矿床的深部找矿思路;④接触带“多台阶”的深部找矿思路。