Fast Variscan anticlockwise P-T-t path in the Eastern Alps: evidence from metasedimentary rocks and metamorphic veins of the Silvretta thrust sheet

Abstract

In the South-Western part of the Silvretta thrust sheet (Pischa area), metasedimentary rocks and metaso-matie veins show evidence of a Variscan anticlockwise P-T-t path, which differs from previously proposed P-T-t paths in the pre-Mesozoic basements of the Alps. The prograde path corresponds to a HT-LP metamorphism illustrated by sillimanite blastesis after andalusite. Maximum temperature (ca. 550 °C) is constrained by the stability of staurolite and muscovite. After temperature climax, pressure increased quasi-isothermally to P max of ca. 0.5–0.6 GPa. P max is limited by phase relations and fluid inclusion data. The prograde path is characterized by the development of an S2 schistosity and mineral lineation L2 oriented E-W, and by the formation of a first generation of quartz-andalusite veins (type 2 quartz veins) parallel to the S2. The decompression path ends in the stability field of andalusite and is characterized by extensional structures such as symmetric and asymmetric foliation boudinage showing a stretching direction still oriented E-W. The extensional structures also result in the formation of quartz-andalusite-muscovite aggregates. Shear bands affecting both metapelites and type 1 and 2 quartz veins indicate a top to the W-SW sense of shear. The last generation of quartz-andalusite veins (type 3) strikes N-S and vertically cross cuts the foliation. Phase relations suggest that the Silvretta thrust sheet underwent an anticlockwise P-T path, probably a result of magmatism and/or nappe emplacement. The prograde part of this P-T loop developed between 320 and 300 Ma, and cooling already occurred at ca. 300 Ma. Therefore, the Variscan cycle was fast in this part of the Alps.     

青藏高原南部拉萨地块中新世超钾质岩石中的锆石记录

幔源岩浆上升的过程中捕获的锆石为揭示深部地壳"隐藏"的岩浆作用事件提供了宝贵机会。本文对采自南部拉萨地块学那地区的超钾质脉岩中的锆石进行了U-Pb年代学、微量元素和Hf同位素研究。研究结果表明,学那超钾质岩石中的锆石主要展示出4个主要的年龄峰值,分别是:<100Ma、300~400Ma、450~500Ma以及700~850Ma。这些锆石高U/Yb比值、低Y含量的特征暗示起源于大陆地壳。而新生代-中生代(<100Ma)和晚古生代(300~400Ma)的岩浆活动在南部拉萨地块上广泛发育,这表明南部拉萨地块新生地壳物质对学那超钾质岩浆活动的贡献。但是超钾质脉岩中早古生代和元古代(450~500Ma和 700~850Ma)锆石捕掳晶的存在则证实印度大陆地壳物质的加入。此外,从大约55Ma左右开始,锆石颗粒的(Dy/Yb)_N比值开始逐渐增高,ε_Hf(t)值则从+10~+5迅速下降至-10~-25。考虑到南部拉萨地块新生地壳的同位素组成特征,超钾质脉岩中的这些锆石颗粒可能记录了印度-亚洲陆陆汇聚过程中地壳的显著加厚以及俯冲的印度大陆地壳物质对南部拉萨地块后碰撞岩浆作用的贡献。     

Lateral variations in the lithology and organic chemistry of a black shale sequence on the Mesoarchean seafloor affected by hydrothermal processes: The Dixon Island Formation of the coastal Pilbara Terrane,Western Australia

The Dixon Island Formation of the coastal Pilbara Terrane, Western Australia is a 3.2 Ga volcanic–sedimentary sequence influenced by syndepositional hydrothermal activity formed in an island‐arc setting. We documented lateral variations in stratigraphy, hydrothermal alteration, and biological activity recorded in the sedimentary rocks (over several kilometers), with the aim of identifying areas of biological activity and related small‐scale structures. The Dixon Island Formation comprises volcaniclastics, black chert, and iron‐rich chert within seven tectonic blocks. Based on detailed geological mapping, stratigraphic columns, carbon isotope composition, and organic carbon (C_org) content, we found lateral (>5 km) variations in stratigraphy and carbon isotope compositions in a black chert sequence above the Mesoarchean seafloor with hydrothermal activity. Two felsic tuff layers are used as stratigraphic marker beds within a black chert sequence, which was deposited on altered volcanic rocks. The black chert sequence in each tectonic block is 10–20 m thick. Thickness variations reflect topographical undulations in the paleo‐ocean floor due to faulting. Early‐stage normal faults indicate extensional conditions after hydrothermal activity. Black chert beds in the topographically subsided area contain higher C_org contents (about 0.4 wt%) than in areas around the depression (<0.1 wt%). Carbon isotope compositions for the black chert vary from ?40 to ?25‰, which are similar to values obtained for a black chert vein within the komatiite–rhyolite tuff sequence (underlying the black chert sequence). Those for other rock types in the Dixon Island Formation are ?33 to ?15‰. Results indicate that deformation occurred soon after the final stages of hydrothermal activity. After this early‐stage deformation, organic‐rich sediments were deposited over an area several kilometers across. The organic‐rich sediments indicate stagnant anoxic conditions that resulted in the deposition of siliceous and organic matter from hydrothermal vein systems. When hydrothermal activity terminated, normal faulting occurred and organic matter was deposited from the sea surface and silica from the seafloor.     

粤西河台金矿床富硫化物含金石英脉锆石U-Pb 测年及成矿意义

河台金矿是一个强烈受韧性剪切带控制的金矿床,金的成矿主要发生在热液蚀变成矿期的金石英脉阶段和金硫化物阶段,形成的矿石有蚀变糜棱岩型和富硫化物石英脉型.富硫化物含金石英脉中单颗粒锆石U-Pb同位素测年显示早加里东期是河台金矿床主成矿期,成矿年龄为492±16Ma;矿区燕山期伍村花岗岩的单颗粒锆石年龄为153.6±2.1Ma;结合已有的同位素资料,认为河台金矿床是华南加里东期构造-变质-岩浆作用及成矿作用的一个组成部分,是一个早加里东期造山作用同期的造山带型金矿床,并可能有燕山期的热液叠加成矿作用.     

论玄武岩底侵作用与长英质火成岩形成的关系--以华东南花岗质火山-侵入杂岩为例

地球层圈热结构特征表明,大陆地壳位于力学边界层（MBL）范围内,其热传递效应受地壳岩石热传导率制约。本文根据热传导理论,采用与前人相同的热物理参数,计算得出1200℃、500m厚玄武岩浆侵位于初始温度为500℃的地壳岩石中,由底侵作用引起地壳部分熔融产生的长英质岩浆（850℃）厚度≤250m,且产生长英质岩浆的时间限制在玄武岩侵位到地壳中后短暂的2700年内。江西南部龙南县临江地区中侏罗世余田群菖蒲组双峰式火山岩地质-地球化学特征及同位素年代学研究证实,该火山岩组合中的流纹质火山岩是拉班玄武质岩浆底侵作用的产物,符合上述数量比例和时间条件。我国东南部中生代陆壳（厚度≤50km）位于热传导起主导作用的MBL层内,由玄武岩浆底侵作用产生的长英质火成岩,在形成的时间及数量上应受地壳岩石热传导机制制约。但在我国东南部大面积出露的中生代火成岩中,花岗岩、流纹质火山岩类占90％以上,而玄武岩仅有局部零星分布,两者在数量上不匹配,在形成时间上也不一致,因而,我国东南部大面积出露的长英质火成岩可能并不是玄武岩浆底侵作用产物,其形成的热动力学背景和机制有待进一步研究。     

新疆乌苏地区新近系石膏脉特征及形成过程

基于野外地质调查及采集样品的微观分析,文中详细研究了新疆乌苏地区新近系发育的石膏脉特征,并从古沉积环 境、成岩作用、风化作用和溶盐水注入期次等方面分析了石膏脉的形成过程。该区石膏脉发育形态特殊,主要是由两横两 纵4条线状主控石膏脉派生出的各种产状石膏脉组成,呈龟壳状。通过分析认为该石膏脉的形成过程共经历过4个阶段,炎 热的古沉积环境为沉积大量的石膏矿物奠定了基础,先成岩后胶结的成岩作用为石膏脉形成提供了必要条件,加上强烈的 风化作用和多期地表溶盐水注入和析出,形成了现今的龟壳状结构石膏脉。该套石膏脉的存在为推测盆地中心可能存在天 然储层裂缝改造区提供线索,并对西北地区新近纪干旱沉积环境具有一定的指示意义。     

A Low‐Sulfidation Epithermal Mineralization in the River Reef Zone,the Poboya Prospect,Central Sulawesi,Indonesia: Vein Textures,Ore Mineralogy,and Fluid Inclusions

The Poboya Prospect lies along the North Northwest ‐ South Southeast Palu‐Koro Fault Zone in the central part of the West Sulawesi Arc. The geology of the area consists of the Palu Metamorphic Complex overlain by the Paleogene‐Neogene Tinombo Formation of volcanosedimentary rocks, the Celebes Molasse sediment, and Late Cenozoic granitic rocks. Petrography, scanning electron microscope with energy‐dispersive spectrometry (SEM‐EDS), electron probe microanalyzer (EPMA), and fluid inclusion microthermometry were carried out to examine vein textures, ore mineralogy, and characteristics of the ore‐forming fluid responsible for mineralization in the River Reef Zone, the Poboya Prospect. Textures of quartz‐carbonate veins in the River Reef Zone include massive micro‐comb, moss, colloform, crustiform, mosaic, feathery, flamboyant, lattice bladed, ghost bladed, parallel bladed, and saccharoidal textures representing primary growth, recrystallization, and replacement. The homogenization temperature and fluid salinity are 240–250°C and 0.3–0.7 wt% NaCl eq., respectively. Ore minerals precipitated in the early stage consist of electrum, naumannite‐aguilarite, chalcopyrite, pyrite, marcasite, sphalerite, and pyrrhotite. Apart from pyrrhotite, these ore minerals were also precipitated in the late stage along with selenopolybasite, freibergite, argyrodite, pyrargyrite, and galena. Selenium more preferably occurs as the crystallographic replacement of sulfur in naumannite‐aguilarite, argyrodite, pyrargyrite, selenopolybasite, and freibergite instead of as independent selenide minerals. The low‐sulfidation epithermal deposit in the River Reef Zone, the Poboya Prospect, illustrates the potential of the West Sulawesi Arc, particularly along the Palu‐Koro Fault Zone, to host epithermal gold mineralization.     

周围型腰椎管狭窄及椎间静脉压迫症的临床CT分析

目的探讨周围型腰椎管狭窄及椎间静脉压迫症的CT表现,为骨科手术方式的选择和术中的处理提供依据.方法分析经手术病理证实的周围型腰椎管狭窄及椎间静脉压迫症的患者52例,全部行CT平扫,部分有CT增强扫描.分析其各种类型的CT表现和不同术式的选择及术中所见.结果大多数病例术中所见与CT表现相一致.CT诊断引起侧隐窝狭窄、椎间孔狭窄及椎间静脉压迫症的原因有:椎间盘的病理性膨突、侧方椎间盘突出、上关节突增生肥大、椎体后外缘骨质增生、黄韧带肥厚、腰椎滑脱等.结论选择合理的CT扫描方法可以提高诊断符合率.正确的CT诊断有助于手术方式的选择和术中的处理,能有效的减少术后并发症.     

石英脉型钨矿成矿研究进展

我国钨矿的储量、产量、消费量和出口量均处于全球领先地位,尤其是石英脉型钨矿床研究成果最为丰富。本文通过大量查阅国内外有关石英脉型钨矿的文献资料,对石英脉型钨矿的成矿研究成果和观点进行综合评述。石英脉型钨矿在我国各个成矿带均有分布,其空间分布和区域构造单元息息相关,所处的区域构造活动一般都很强烈,经历多次叠加构造运动。钨矿床产在震旦寒武系地层中,特别是在寒武系地层最多。成矿物质主要来源是岩浆热液,岩浆活动频繁为成矿热液的形成创造了必要的前提条件,但也有部分成矿物质源于围岩。石英脉型钨矿成矿模式以"五层楼+地下室"成矿模式最具影响力和实际意义,开创了模式找矿的先河。