Petrogenesis of the Mairupt microgranite: A witness of an Uppermost Silurian magmatism in the Rocroi Inlier,Ardenne Allochton

Magmatism in the Rocroi inlier (Ardenne Allochton, southeastern Avalonia during eo-Hercynian times) consists of a swarm of bimodal dykes (diabase and/or microgranite) emplaced in Middle to Upper Cambrian siliciclastics (Revin Group). Felsic volcanites interbedded within the Upper Silurian/Lower Devonian transgressive strata on the eastern edge of the inlier were interpreted as belonging to the same magmatic event. This was subsequently invalidated by zircon U–Pb dating of the Mairupt and Grande Commune magmatic rocks, which yielded an Upper Devonian age. Here we report a reevaluation of the age of the Mairupt microgranite based on LA–ICP–MS in situ U–Pb zircon geochronology, which yields a concordant age of 420.5 ± 2.9 Ma (Late Silurian/Early Devonian). This new dating restores the consistency between the different magmatic occurrences in the Rocroi inlier. The geochemical and petrographical data furthermore indicate a major crustal contribution, which fits well within the context of crust thinning of the Ardenne margin (southeastern Avalonia) in the transtensional Rheno-Hercynian back-arc basin.     

The exposures of Ludlow rocks and associated beds at Tites Point and near Newnham,Gloucestershire

The Leintwardine and Whitcliffe Beds of the Ludlow Series outcropping at Tites Point and near Newnham, Gloucestershire, are described and compared with similar beds exposed at May Hill. Data from the Brookend Borehole, 1 ½ miles (2.41 km) south of Tites Point, reveal that southward thickening affects all subdivisions of the Ludlow Series. The occurrence of the Ludlow Bone Bed in the Tites Point outcrop is also recorded.     

川陕边境广元宁强间的志留系

<正> 李希霍芬(Richthofen 1882)将宁强牢固关、黄坝驿、茅坪沟、广元校场坝、神宣驿(即宣河)和朝天驿(即朝天镇)之间的地层分为8层。珊瑚化石(i层)经Lindstrom鉴定为 Favosites forbesi,Halysites calenularius,Heliolites,Caythophyllum,Amplexus和Alveolites;腕足类经Kayser鉴定为Orthis类(h层),Orthis calligramma 和Spirifer(1层)。葛利普(1924)把宁强广元间的志留系分为朝天组(Chaotien Formation),浅水     

Organic matter in Silurian rocks from the Chernov uplift

This study presents data on the composition of organic matter from the Late Silurian sediments of the Chernov uplift. These sediments are characterized by low C_org contents, which may reach 1–3% in individual layers. A relatively high thermal maturity of organic matter is confirmed by polycyclic biomarker distributions and Rock-Eval pyrolyisis data. Despite its higher thermal maturity level (T _max = 456°C), kerogen in carbonaceous shales from the Padymeityvis River exhibits good preservation of long-chain n-alkyl structures, which are readily identified in the ¹³C NMR spectra and by the molecular analysis of the kerogen pyrolysis products.     

The Longmyndian Rocks of the Old Radnor Inlier,Welsh Borderland

Two formations are named in the presumed Precambrian sedimentary rocks of the Old Radnor Inlier. The Strinds Formation comprises fine- to medium-grained sandstones and pebbly sandstones. Grain-size, texture, clast types, and lithological invariance support correlation with the homogeneous and cross-bedded sandstone facies of the Bayston-Oakswood Formation of the Longmyndian, assigned to a braided alluvial plain environment. The Yat Wood Formation comprises fine-grained sandstones, siltstones, and laminated mudstones. It may be equivalent to some part of the Burway, Synalds, or Lightspout Formation. The Strinds and Yat Wood Formations are petrographically similar to each other and to the type Lonmyndian, and were sourced either directly or indirectly from a magmatic arc. The Strinds and Yat Wood Formations are seen only in faulted contact, and the inlier itself is partly or wholly bounded by strands of the Church Stretton fault zone. Similar faulted relationships are seen further northeast along the system at Pedwardine and in the Church Stretton area. A fault explanation for the juxtaposition of contrasting facies is favoured over that of an original unconformity of Strinds on Yat Wood Formation.     

黔北地区下志留统龙马溪组烃源岩生物标志物

应用气相色谱和色谱-质谱分析技术,对黔北地区下志留统龙马溪组烃源岩的甾烷和萜烷进行测试分析。结果表明,正构烷烃为后单峰型,主峰碳为C22～C30,Pr/Ph值为0.48～0.64,Pr/nC17为1.35～4.22,Ph/nC18为0.92～0.97。甾烷中,具孕烷系列的异常高值,C27、C28和C29呈规则甾烷"V"字型分布(C27C28C29),具明显的C27优势,甾烷/藿烷比值为0.57～0.71。萜烷类中具三环萜烷高值,低升藿烷,高伽马蜡烷指数特征。成熟度方面,OPE1,Ts/(Ts+Tm)分布在0.42～0.48之间,C3122S/(S+R)值为0.6～0.66,C29ββ/(ββ+αα)在0.56～0.58之间,反映出龙马溪期具明显的水体分层和高还原环境。有机质主要来源于浮游水生生物和藻类,并经历了一定的生物降解过程。烃源岩的热演化已进入高成熟-过成熟阶段。     

Boron metasomatism and behaviour of rare earth elements during formation of tourmaline rocks in the eastern Arunta Inlier,central Australia

Tourmaline rocks of previously unclear genesis and spatially associated with W- (Cu)-bearing calc-silicate rocks occur in Palaeoproterozoic supracrustal and felsic intrusive rocks in the Bonya Hills in the eastern Arunta Inlier, central Australia. Tourmalinisation of metapelitic host rocks postdates the peak of regional low-pressure metamorphism (M₁/D₁, ~500 °C, ~0.2 GPa), and occurred synkinematically between the two main deformation events D₁ and D₂, coeval with emplacement of Late Strangways (~1.73 Ga) tourmaline-bearing leucogranites and pegmatites. Tourmaline is classified as schorl to dravite in tourmaline–quartz rocks and surrounding tourmaline-rich alteration zones, and as Fe-rich schorl to foitite in the leucogranites. Boron metasomatism resulted in systematic depletion of K, Li, Rb, Cs, Mn and enrichment of B, and in some samples of Na and Ca, in the tourmaline rocks compared to unaltered metasedimentary host rocks. Whole-rock REE concentrations and patterns of unaltered schist, tourmalinised schist and tourmaline–quartz veins—the latter were the zones of influx of the boron-rich hydrothermal fluid—are comparable to those of post-Archaean shales. Thus, the whole-rock REE patterns of these rocks are mostly controlled by the metapelitic precursor. In contrast, REE concentrations of leucogranitic rocks are low (10 times chondritic), and their flat REE patterns with pronounced negative Eu anomalies are typical for fractionated granitic melts coexisting with a fluid phase. REE patterns for tourmalines separated from metapelite-hosted tourmaline–quartz veins and tourmaline-bearing granites are very different from one another but each tourmaline pattern mirrors the REE distribution of its immediate host rock. Tourmalines occurring in tourmaline–quartz veins within tourmalinised metasediments have LREE-enriched (La_N/Yb_N=6.3–55), shale-like patterns with higher REE (54–108 ppm). In contrast, those formed in evolved leucogranites exhibit flat REE patterns (La_N/Yb_N=1.0–5.6) with pronounced negative Eu anomalies and are lower in REE (5.6–30 ppm). We therefore conclude that REE concentrations and patterns of tourmaline from the different tourmaline rocks studied are controlled by the host rock and not by the hydrothermal fluid causing boron metasomatism. From the similarity of the REE pattern of separated tourmaline with the host rock, we further conclude that incorporation of REEs in tourmaline is not intrinsically controlled (i.e. by crystal chemical factors). Tourmaline does not preferentially fractionate specific REEs or groups of REEs during crystallisation from evolved boron- and fluid-rich granitic melts or during alteration of clastic metasediments by boron-rich magmatic-hydrothermal fluids.Editorial responsibility: J. Hoefs     

Macrofloral biostratigraphy of the Newent Coalfield,Gloucestershire

Boreholes recently drilled by the British Geological Survey provide the first extensive collection of fossil plants from the Newent Coalfield, Gloucestershire. They belong to the Lobatopteris vestita Biozone, indicating a late Westphalian D age. The Newent sequence is thus homotaxial with part of the Forest of Dean Coalfield, and supports the view of Wills (1956) that there was a ‘strait’ extending across St George's Land during the late Westphalian.     

The Carboniferous Limestone of Chipping Sodbury and Wick,Gloucestershire

The rock types and successions encountered at two localities have been described and interpretated with reference to modern analogues. In the Clifton Down Limestone a cycle of sedimentation is recognised passing from intertidal algal mats, through lagoons, barrier and open shelf deposits and back through the same sequence to intertidal algal mats.     

论华南志留系对比的若干问题

<正> 中国志留系的划分与对比,始于20年代。李四光等(1924)和谢家荣等(1925)研究了三峡地区的志留系,将其全部划归下统。孙云铸(1943)从沉积旋迴角度提出了我国志留系划分的三个阶段,后又发现相应的笔石辅以佐证(孙云铸1957、洪友崇1958),将峡区的志留系分别与英国的 Llandovery,Wenlock和 Ludlow三个统对比。这个方案得     

四川盆地东部地区下志留统龙马溪组页岩储层特征

四川盆地是中国西南部重要的含油气盆地,在东部和南部地区下志留统龙马溪组页岩广泛发育。在川东南、鄂西渝东地区的勘探井中志留系具有良好的气显示。研究区龙马溪组厚65~516m,底部为一套海侵沉积的富含笔石的黑色页岩,龙马溪组向上和向东砂质和钙质含量增加,演变为浅水陆棚沉积。龙马溪组主要由层状-非层状泥/页岩、白云质粉砂岩、层状钙质泥/页岩、泥质粉砂岩、层状-非层状粉砂质泥/页岩、粉-细粒砂岩、钙质结核、富含有机质非层状页岩8种岩相组成。总有机碳含量(TOC)为0.2%~6.7%。有机质以II型干酪根为主,Ro为2.4%~3.6%。页岩中石英矿物含量在2%~93%,主要呈纹层状或分散状分布,主要为陆源碎屑外源成因。龙马溪组页岩岩心孔隙度为0.58%~0.67%,渗透率为0.01×10^-3μm²~0.93×10^-3μm²。扫描电镜下龙马溪组页岩微孔隙度为2%左右,主要包括晶间孔和粒内孔,孔隙直径为100nm~50μm。页岩储层的形成机理主要为有利矿物组合、成岩作用和有机质热裂解作用。龙马溪组与美国Barnett页岩具有一定差异,主要表现在龙马溪组页岩埋藏较深、热演化程度较高、含气量较低、储层较致密、以陆源成因石英为主。对于评价下志留统龙马溪组页岩气勘探前景而言,今后须重点加强针对龙马溪组底部黑色硅质岩系石英成因、成熟度、埋藏史、含气量等方面的研究,以及进行详细的古地貌和古环境恢复。     

The engineering geology of the Lower Lias Clay at Blockley,Gloucestershire, UK

Summary The Lower Lias Clay at Blockley, 20 km due south of Stratford-upon-Avon, UK, is thought to be representative of this clay formation over much of the Severn Basin. As far as the mineralogy of the clay is concerned, illite is the dominant clay mineral, kaolinite being subsidiary, with quartz, calcite, pyrite and chlorite/vermiculite present in subsidiary or accessory amounts. Weathering changes the mineralogy, with illite being degraded and calcite and pyrite being removed. Furthermore free iron oxide coatings become important as a result of weathering, with the maximum concentrations being present in the highly weathered material.The unweathered clay possesses a preferred orientation associated with turbostratic structures. At certain horizons microfolding and remoulding occurs in the unweathered clay and silty layers are displaced. Weathering has progressively destroyed the micro-structures present in the weathered clay horizons.The fissures in the Lower Lias Clay occur in five uniplanar sets. In the unweathered clay the intensity is fairly constant but it increases in a narrow zone just below the weathered horizons. The number of curved fissures also increases at this depth. This intense fissuring continues into the lowest zone of weathered clay where weathering is confined to the fissure surfaces. Fissures become less apparent in the more weathered horizons as more and more degradation has taken place. However, new smaller fissure systems are developed in the weathered clay, as are desiccation cracks.Engineering index properties and values of shear strength are given. Relationships between moisture content and strength, liquid limit and iron (Fe) content were observed, and a relationship between weathering zones and the shear strength-depth curve was established.     

张家界地区志留系岩石地球化学与构造背景

张家界地区志留系出露有兰多弗里统及文洛克统,其中兰多弗里统泥岩中SiO₂含量平均61.59%,SiO₂/Al₂O₃值平均3.63,K₂O/ Na₂O值平均4.80,Al₂O₃/(K₂O+Na₂O)值平均3.49,∑REE含量平均238.58×10^-6,LREE含量平均213.89×10^-6, LREE/HREE值平均8.68, B含量平均98.29×10^-6,B/Ga值平均3.99,Th/U值平均5.61。通过构造环境判别图解认为: 兰多弗里统泥岩以活动大陆边缘物源为主,混入部分大陆岛弧物源,其古老物源则可能来自被动大陆边缘; 已经拼合在一起的华夏、扬子陆块,在志留纪时期继续向北西挤压,形成多处隆升(黔中隆起、雪峰隆起等),随着进一步挤压,雪峰隆起升出海面,为张家界地区提供了泥质沉积物,挤压导致岛弧形成并提供了部分物源。     

Petrogenesis and tectonic implications of the middle Silurian volcanic rocks in northern West Junggar,NW China

Zircon U–Pb geochronological and geochemical analyses are reported for a suite of the middle Silurian volcanic rocks from northern West Junggar (NW China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded a concordant weighted mean ²⁰⁶Pb/²³⁸U age of 429 ± 3 Ma, indicating the presence of middle Silurian volcanic rocks in northern West Junggar. The andesite is tholeiite series and characterized by minor variations in compositions (SiO₂ = 55.68–59.17 wt.%, Al₂O₃ = 14.56–17.7 wt.%, TiO₂ = 0.55–1.23 wt.%, Na₂O + K₂O = 3.46–7.16 wt.%, and P₂O₅ = 0.15–0.37 wt.%), with wider MgO content (2.18–6.48 wt.%) and Mg# (57.4–77.9). All andesitic rocks are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Ba, K, and Th) and light rare earth elements (LREEs), but strongly depleted in some high field strength elements (HFSEs; e.g. Nb, Ta and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.8–1). These features suggest that the andesitic magmas were derived from 2–8% partial melting of a garnet lherzolite depleted mantle source with subducted sediments metasomatized by slab-derived fluids. Combining the current study with those data in existing literature, we conclude that the middle Silurian volcanic rocks formed in an intra-oceanic subduction setting during consumption of the Irtysh–Zaysan Ocean, and further confirm the eastern extension of the early Palaeozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.