Subduction,mega-shear systems and Late Palaeozoic basin development in the African segment of Gondwana

 Basins within the African sector of Gondwana contain a Late Palaeozoic to Early Mesozoic Gondwana sequence unconformably overlying Precambrian basement in the interior and mid-Palaeozoic strata along the palaeo-Pacific margin. Small sea-board Pacific basins form an exception in having a Carboniferous to Early Permian fill overlying Devonian metasediments and intrusives. The Late Palaeozoic geographic and tectonic changes in the region followed four well-defined consecutive events which can also be traced outside the study area. During the Late Devonian to Early Carboniferous period (up to 330 Ma) accretion of microplates along the Patagonian margin of Gondwana resulted in the evolution of the Pacific basins. Thermal uplift of the Gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 Ma. At the end of this period the Gondwana Ice Sheet was well established over the uplands. The period 260–300 Ma evidenced the release of the Gondwana heat and thermal subsidence caused widespread basin formation. Late Carboniferous transpressive strike-slip basins (e.g. Sierra Australes/Colorado, Karoo-Falklands, Ellsworth-Central Transantarctic Mountains) in which thick glacial deposits accumulated, formed inboard of the palaeo-Pacific margin. In the continental interior the formation of Zambesi-type rift and extensional strike-slip basins were controlled by large mega-shear systems, whereas rare intracratonic thermal subsidence basins formed locally. In the Late Permian the tectonic regime changed to compressional largely due to northwest-directed subduction along the palaeo-Pacific margin. The orogenic cycle between 240 and 260 Ma resulted in the formation of the Gondwana fold belt and overall north–south crustal shortening with strike-slip motions and regional uplift within the interior. The Gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions. Associated with the orogenic cycle was the formation of mega-shear systems one of which (Falklands-East Africa-Tethys shear) split the supercontinent in the Permo-Triassic into a West and an East Gondwana. By a slight clockwise rotation of East Gondwana a supradetachment basin formed along the Tethyan margin and northward displacement of Madagascar, West Falkland and the Gondwana fold belt occurred relative to a southward motion of Africa. Received: 2 October 1995 / Accepted: 28 May 1996     

The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells

The Silurian of Gotland, Sweden, consists of 440 m of carbonate deposits. Repeatedly, uniform sequences of micritic limestones and marls are interrupted by complex-structured reefs and by adjacent platform sediments. Generally, the alteration of facies is interpreted as the result of sea-level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells. Approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated. The carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than to paleotemperature. Carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions. Consequently, the facies succession on Gotland results from global paleoclimatic conditions. Changes in terrigenous input due to different rates of weathering and freshwater runoff, rather than sea-level fluctuations, control the carbonate formation of the Silurian on Gotland.     

鄂西震旦系上统含磷岩系的划分与对比

上震旦统陡山沱组是鄂西地区最具工业价值的含磷地层。著名的磷矿有荆襄磷矿、宜昌磷矿、兴(山)神(农架)保(康)磷矿。迄今已有20多年的勘探史,但由于各矿区研究程度不一,又缺古生物依据,亦不能依靠同位素年龄来作划分对比,故在地层及磷矿层的划分、对比上认识不一。笔者结合磷矿特征及含磷岩系岩类组合、剖面结构特点、稀土元素特征等,将本区划分为4个磷矿分布区,将陡山沱组划分为3个岩性段、5个含磷层位。     

华南西部晚二叠世碳同位素地层学研究

对黔桂地区７条晚二叠世碳酸盐岩型煤系剖面进行了碳酸盐岩碳氧同位素测定，发现晚二叠世吴家坪阶与长兴阶碳酸盐岩的δ１３Ｃ值有明显差异，前者δ１３Ｃ多大于３．０‰，后者多小于３．０‰，进一步对晚二叠世δ１３Ｃ的变化原因的分析表明，δ１３Ｃ的变化可能与晚古生代末期聚煤作用的减弱密切相关，后者造成自然界碳库中的１２Ｃ的富集，从而使得海水以及在其中沉淀出的碳酸盐岩中的δ１３Ｃ值降低。此外，植物光合作用从大气中吸收ＣＯ２的量也随陆地植被减少而减少，结果造成大气中的ＣＯ２的大量富集，从而进一步产生温室效应，制约生物界的发展。联系到这一阶段生物界的衰亡灭绝，可以认为温室效应是晚二叠世生物逐渐衰亡及晚二叠世末群体灭绝的主要原因之一。     

胶东昆嵛山花岗岩的特征、成因及其与金矿的关系

昆嵛山花岗岩是牟平—乳山金矿成矿带的围岩，在其成因和定位时代上长期存在争议。本文对岩体地质和地球化学特征进行了系统研究，并就岩体成因及其与金矿的关系进行了讨论。作者认为，昆嵛山花岗岩是在中生代由岩浆缓慢冷凝结晶形成的大型复式岩基，并非交代成因。它由垛崮顶片麻状花岗闪长岩（１４７Ｍａ）、八门邹家斑状花岗闪长岩（１３５Ｍａ）、瓦善—水道弱片麻状二长花岗岩及五爪山片麻状含榴二长花岗岩（１２０Ｍａ）组成，成分具贫铝富碱的特点。岩浆来源于中下地壳中酸性变质火山岩系部分熔融形成的岩浆，其形成温度为６５０—７５０℃。岩体经历了韧性和脆性变形作用，不同方向的片麻状构造是岩浆结晶期韧性变形作用的结果。脆性变形形成的断裂和破碎带是本区主要的导矿和容矿构造。花岗岩定位、韧和脆性变形作用与金矿成矿作用有十分密切的关系     

杨溪岩体中黑云母的特征及其地质意义

杨溪岩体黑云母为１Ｍ型的铁质黑云母，其特征说明岩体为壳源改造型花岗岩，但有幔源物质混入。离铌钽矿化点较近的黑云母表现出明显富铝、多锰、贫镁、少硅、少钛的特点。离白钨矿矿区较近的黑云母与远离矿区的黑云母在特征上差别不大，说明杨溪岩体黑云母的特征变化对成矿影响不大，但岩浆活动是重要的成矿因素之一。     

江苏煤层气资源分析

简要地归纳了江苏省煤层气的成矿地质环境，分析了成煤区地质构造特征、煤层及围岩储层的物性特征，在此基础上，对主要地区煤层气资源进行了估算。     

华北地台主要花岗岩-绿岩带含金丰度值的研究

近十余年来，在华北地台太古宙花岗岩－绿岩带金矿的研究中，获取了许多有关地层、岩石金丰度值的资料。早期获取的资料因化验分析方法灵敏度不高且样品中常常包含矿化、蚀变和构造破碎的岩石等原因而使其金丰度值偏高。最新的数据采用了高灵敏度化验分析方法且剔除了矿化、蚀变和破碎的岩石样品，计算出的金丰度值均明显低于或接近于地壳金丰度值。结合其它研究成果，提出绿岩带有关金矿矿源是多源的（变质的、岩浆的），甚至是深源的（下地壳或上地幔）认识。     

江南隆起东端伸展构造的识别及其意义

通过对屯溪流塘盆地典型伸展堆叠构造几何学和运动学的解剖，以及对其它几个伸展构造实例的分析，指出伸展构造在江南隆起的存在，它们的形成主要是重力作用的产物，遍及中、新生代。这种特征构造的解剖有助于深化认识江南隆起的构造特点和演化历史。     

黔中三叠纪地层研究新进展

通过对黔中地区区域地质调查，在二叠系—三叠系过渡层中发现了一系列的韵律层，并通过岩性岩相、矿物学、地球化学和地层古生物学研究，认为过渡层中韵律层是远源火山碎屑重力流沉积产物，并据古生物、沉积学、地层学研究将过渡层归属于三叠系。原安顺组下部“大汪段”之上、下发现大量牙形刺化石，根据岩性及古生物特征，将“大汪段”归入大冶组。黔中—黔南中三叠统陆源碎屑体系的物源来自江南古陆；菊石、双壳类化石层是浊流作用形成的事件沉积层。