Metallogenic geodynamic background of Mesozoic gold deposits in granite-greenstone terrains of North China Craton

The spatial distribution map of 65 mid-large gold-deposits hosted in the granite-greenstone terrains of the North China Craton is first drawn. These gold deposits mainly concentrate in the Mesozoic remobilized Yinshan-Yan-shan-Liaoning-Jilin intracontinental collisional orogenic belt, the northern Qinling and the Jiaodong Mesozoic collisional orogenic belts, and the Mesozoic intracontinental fault-magmatic belts developed along the Taihangshan and the Tan-Lu faults; their mineralizing time is predominantly Jurassic-Cretaceous, i. e. the Yanshanian. The metallogenic geodynamic background is exactly the compression-to-extension transition regime during continental collision. The results are partly from the project entitled “The main types of gold mineralizations in China and their metallogenic model” (89-El) supported by the Ministry of Metallurgical Industry of China, and projects “Geology and metallogenesis of the main type gold deposits in East Chinaⁿ” (Grant No. 9488010) and “Study on ore-forming fluids of the Wangfeng gold deposit, Xinjiang” supported by the National Natural Science Foundation of China (Grant No. 49672119).     

中国陆内造山带造山过程地球动力学分析

本文论证了我国陆内造山带岩石圈结构、造山过程岩石圈浅层变形构造样式、盆山变形构造时空耦合关系、造山过程与沉积盆地变形动力学模式等。提出中国西部造山带造山过程岩石圈变形总体是：上部以伸展变形为主,中部以挤压变形为主,下部以伸展变形为主。通过中国造山带造山过程与沉积盆地变形作用时空耦合关系对比研究,首次建立起造山过程与沉积盆地形成时空耦合关系的三类四型动力学模式：单向楔入造山推覆成盆、双向楔入造山推覆成盆、剪切造山反转成盆和伸展造山断陷成盆模式。     

新疆库鲁克塔格地区辛格尔变质核杂岩特征

辛格尔一带的前震旦纪古老变质岩系为一系列的变质核杂岩。其内核由早—中太古代古陆核组成 ,外核为晚太古代、古元古代、中元古代、新元古代早期变质岩系构成的多层次构造增生楔 ,并有多期剥离断层配套。新元古宙代晚期震旦系为盖层。它是地壳多期伸缩构造的产物。变质核杂岩体是一个有利于成矿的构造体系。辛格尔变质核杂岩构造是一个对有色和贵金属等矿产成矿的有利构造     

多岛海型造山作用——以华南印支期造山带为例

大陆造山带大多数形成于弧弧碰撞及其弧后盆地衰缩作用,其古地理格局为多岛海。今日的东南亚是多岛海大地构造的现实模型,其中欧亚大陆和澳大利亚的板块边界位于印度尼西亚的班达—巽它弧以南和西太平洋马里亚纳弧以东。介于前缘弧和欧亚大陆之间的是众多的残余弧和弧后盆地。其中有些盆地仍然是海底扩张的中心,一些是不再活动的海盆,也有些海盆正在遭受挤压作用,而一些海盆则已经完全被弧后衰缩作用所消减。位于这些盆地之间的是残余弧,沉降的残余弧顶部的沉积层序类似于被动陆缘。华南大地构造可用多岛海模式予以解释。华南造山带的大地构造相分析、沉积相分析和古地磁等综合研究结果表明,它们大多数是弧弧碰撞作用所形成的碰撞型造山带,二叠—三叠纪的古地理存在着与东南亚今天类似的多岛海格局。临沧弧和华夏弧可能为华南多岛海的前缘弧,起着与今天欧亚大陆的印度尼西亚弧相类似的作用。多岛海古地理格局可能出现于泥盆纪以后,华南板块发生裂解,所形成的弧后盆地大多数于晚三叠世到早侏罗世发生衰缩。     

Mineralization during Collisional Orogenesis and Its Control of the Distribution of Gold and Other Deposits in the Junggar Orogen, Xinjiang, China

The Junggar orogen, Xinjiang, China, is an important part of the Ural-Mongolian orogen.The collisional orogenesis in this region occurred primarily in the Carboniferous and Permianwith an evolutional process of early compression and late extension. Mineralization of gold andother metals in the Junggar orogen occurred mainly in the Permian and in a few cases in theLate Carboniferous. The deposits are largely distributed in areas where collisional orogenesiswas intensive and formed in a transitional stage from compression to extension. Therefore, goldmineralization in the Junggar orogen is fully consistent with the collisional orogenesis in time,space and geodynamic setting. This indicates that the mineral deposit model of collisionalorogenesis is applicable to prospecting and study of ore deposits in the Junggar orogen.Furthermore, the factual distribution of gold and other deposits in this region is just the same asthe collisional orogenic model presents.     

Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite b_o, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.     

准噶尔造山带碰撞体制的成矿作用及金等矿床分布规律

准噶尔造山带是乌拉尔—蒙古造山带的重要组成部分。该区的碰撞造山作用主要发生在石炭纪和二叠纪,并经历了先挤压后伸展的演化过程。区内的金等矿床主要就位于晚石炭世—二叠纪,集中分布在碰撞造山作用较强烈的地带,矿床形成于碰撞作用的挤压—伸展转变期。因此该区金等矿床的成矿时间、空间和地球动力学背景等与碰撞造山作用发生的时间、空间及动力学背景完全吻合,表明适于用碰撞造山成矿模式指导找矿和研究。而该区矿床的实际分布也确与碰撞造山成矿模式完全一致。     

Cretaceous episodic growth of the Japanese Islands

Abstract The Japanese Islands formed rapidly in situ along the eastern Asian continental margin in the Cretaceous due to both tectonic and magmatic processes. In the Early Cretaceous, huge oceanic plateaus created by the mid-Panthalassa super plume accreted with the continental margin. This tectonic interaction of oceanic plateau with continental crust is one of the significant tectonic processes responsible for continental growth in subduction zones. In the Japanese Islands, Late Cretaceous-Early Paleogene continental growth is much more episodic and drastic. At this time the continental margin uplifted regionally, and intra-continent collision tectonics took place in the northern part of the Asian continent. The uplifting event appears to have been caused by the subduction of very young oceanic crust (i.e. the Izanagi-Kula Plate) along the continental margin. Magmatism was also very active, and melting of the young oceanic slab appears to have resulted in ubiquitous plutons in the continental margin. Regional uplift of the continental margin and intra-continent collision tectonics promoted erosion of the uplifted area, and a large amount of terrigenous sediment was abruptly supplied to the trench. As a result of the rapid supply of terrigenous detritus, the accretionary complexes (the Hidaka Belt in Hokkaido and the Shimanto Belt in Southwest Japan) grew rapidly in the subduction zone. The rapid growth of the accretionary complexes and the subduction of very young, buoyant oceanic crust caused the extrusion of a high-P/T metamorphic wedge from the deep levels of the subduction zone. Episodic growth of the Late Cretaceous Japanese Islands suggests that subduction of very young oceanic crust and/or ridge subduction are very significant for the formation of new continental crust in subduction zones.     

Regional metamorphic belts of the Japanese Islands

Abstract An overview of the regional metamorphic belts of Japan is given in the context of the tectonic evolution of the Japanese Islands. The Japanese Islands were situated on an active margin of the Eurasian continent or its constituent landmass before their assembly during the Phanerozoic. The Japanese Islands are composed mainly of metamorphosed and unmetamorphosed accretionary complexes, granitoids and their effusive equivalents that were formed by the Cordilleran-type orogeny. The metamorphic belts are regarded essentially as a deep-seated portion of an accretionary complex. In spite of continuous subduction of oceanic plates beneath the continents, these orogenic rocks were formed quite episodically, as evidenced by discontinuous matrix ages of the accretionary complexes and a striking concentration of isotopic ages of the granitoids. A systematic along-arc age shift of Cretaceous large-scaled granitic magmatism and regional metamorphism suggests a tectonic control such as ridge subduction, which triggered the episodic orogeny. A tectonic model based on the paired metamorphic belts, combined with the non-steady tectonic control, works well to explain this magmatism and metamorphism in a single arc-trench system as a continental margin process. However, the juxtapositional process of the paired metamorphic belts is still a problem. Two possible cases, namely transcurrent displacement and back-arc overthrusting are discussed.     

Pacific-type orogeny revisited: Miyashiro-type orogeny proposed

Abstract The concept of Pacific-type orogeny is revised, based on an assessment of geologic data collected from the Japanese Islands during the past 25 years. The formation of a passive continental margin after the birth of the Pacific Ocean at 600 Ma was followed by the initiation of oceanic plate subduction at 450 Ma. Since then, four episodes of Pacific-type orogeny have occurred to create an orogenic belt 400 km wide that gradually grew both oceanward and downward. The orogenic belt consists mainly of an accretionary complex tectonically interlayered with thin (<2 km thick), subhorizontal, high-P/T regional metamorphic belts. Both the accretionary complex and the high-P/T rocks were intruded by granitoids ～100 million years after the formation of the accretionary complex. The intrusion of calc-alkaline (CA) plutons was synchronous with the exhumation of high-P/T schist belts. Ages from microfossils and K-Ar analysis suggest that the orogenic climax happened at a time of mid-oceanic ridge subduction. The orogenic climax was characterized by the formation of major subhorizontal orogenic structures, the exhumation of high-P/T schist belts by wedge extrusion and subsequent domed uplift, and the intrusion-extrusion of CA magma dominantly produced by slab melting. The orogenic climax ended soon after ridge subduction, and thereafter a new Pacific-type orogeny began. A single Pacific-type orogenic cycle may correspond to the interaction of the Asian continental margin with one major Pacific oceanic plate. Ophiolites in Japan occur as accreted material and are not of island-arc but of plume origin. They presumably formed after the birth of the southern Pacific superplume at 600 Ma, and did not modify the cordilleran-type orogeny in a major way. Microplates, fore-arc slivers, intra-oceanic arc collisions and the opening of back-arc basins clearly contributed to cordilleran orogenesis. However, they were of secondary importance and served only to modify pre-existing major orogenic components. The most important cause of cordilleran-type orogeny is the subduction of a mid-oceanic ridge, by which the volume of continental crust increases through the transfer of granitic melt from the subducting oceanic crust to an orogenic welt. Accretionary complexes are composed mainly of recycled granitic sediments with minor amounts of oceanic material, which indicate that the accretion of oceanic material, including huge oceanic plateaus, was not significant for orogenic growth. Instead, the formation and intrusion of granitoids are the keys to continental growth, which is the most important process in Pacific-type orogeny. Collision-type orogeny does not increase the volume of continental crust. The name ‘Miyashiro-type orogeny’ is proposed for this revised concept of Pacific-type or cordilleran-type orogeny, in order to commemorate Professor A. Miyashiro's many contributions to a better understanding of orogenesis.