Episodic accretion and metamorphism of Jurassic accretionary complex based on biostratigraphy and K-Ar geochronology in the western part of the Mino-Tanba Belt, Southwest Japan

Abstract The low grade metamorphic Jurassic accretionary complex in the western part of the Mino-Tanba Belt, Southwest Japan, is a chaotic sedimentary complex which consists of argillaceous matrices with allochthonous blocks of chert, greenstone, siliceous mudstone, terrigenous sandstone and mudstone. The complex is divided into three distinct geologic units, Units I, II and III, with a tectonic boundary (thrust) between them, forming a pile-nappe structure. They have different features for lithologies, fossil age, metamorphic condition and K-Ar age. Microfossil researches revealed that their timings of accretion were in the early Early Jurassic ( ca 195 Ma) for Unit III, in the early Middle Jurassic ( ca 175 Ma) for Unit II and in the latest Late Jurassic (ca 147 Ma) for Unit I. On the other hand, K-Ar age determinations of white mica separated from pelitic rocks of the three units clarified that the subsequent subduction-related metamorphism was 23 million years after the accretion of each unit. These results strongly suggest that the accretionary and metamorphic process had taken place episodically with an interval of 20 to 28 million years during Mesozoic time in the western part of the Mino-Tanba Belt, Southwest Japan.     

中国东部中生代构造演变与聚煤作用的迁移

中生代期间，中国东部的构造演变具有从西向东不断发展，从南向北不断扩展的特点。正是这种从南向北的构造演变与潮湿气侯带由南向北的不断迁移相结合，决定了中生代聚煤作用不断地从南向北迁移。     

Tempos of Elderly Age and Geographical Concentration*

This paper presents a method to measure two types of tempos: 1) the tempo of elderly age concentration, the rate of change in the elderly proportion in a region; and 2) the tempo of elderly geographical concentration, the rate of change in a region's share of the national elderly population. The two measurements are applied to historical data for the United States. The results reveal that both tempos vary spatially and temporally. Relative tempos are also introduced to compare the indices between different age groups and regions.     

刘家坪杂岩体的岩石特征及时代

刘家坪杂岩体出露于扬子准地台西北边缘。该区扬子期岩浆活动,早期为刘家坪组海底喷发的中～酸性火山岩系,晚期先后侵入有角闪辉长岩—石英闪长岩—斜长花岗岩—辉绿岩脉。岩浆岩具有幔源岩浆及造山带地区火山岩的特征。据震旦系覆于刘家坪杂岩体之上,其底部砂砾岩的砾石中有杂岩体成分,以及杂岩体侵入于同位素年龄值为6.43亿年的刘家坪组之中,认为刘家坪杂岩体时代为前震旦纪。     

赣东北元古代蛇绿岩Sm—Nd同位素年龄及地质意义

江南古陆东南缘赣东北弋阳—德兴—婺源一线有一条蛇绿岩带,普遍认为它是二个一级大地构造单元的分界线,形成于元古代。推断蛇绿混杂岩是元古代古洋壳俯冲时被挤上来的碎片;也有人认为它是中生代二大古陆之间的缝合线。Sm-Nd矿物等时线给出1034±16Ma(MSWD=1.0)的同位素年龄,它符合元古代缝合带的观点,而不利于中生代缝合带。     

齐大山铁矿黑云变粒岩单锆石年龄及意义

齐大山特大型鞍山式铁矿的成矿时代，以往都是根据铁矿的构造和建造特征，以及与铁矿伴生的花岗岩类岩体的同位素年龄数据间接推断的。本文最近获得了该铁矿的黑云变粒岩中，单颗粒锆石Ｕ－Ｐｂ同位素年龄为（２５３３±５３）×１０￣６ａ。这是迄今为止研究该矿床成矿时代较为可靠的直接测年数据。     

滇黔桂三角区微细浸染型金矿床成矿热液一种可能的演化途径:年代学证据

本文主要根据年代学资料和金活化迁移的地球化学属性，探讨了滇黔桂三角区微细浸染型金矿床成矿热液的演化途径。多种定年方法揭示，该区金成矿初始流体（古流体）的年龄为260Ma左右；金成矿年龄约为140～75Ma。考虑到该区燕山晚期岩浆岩富含Cl，本文认为古流体（260～140Ma）向成矿流体（140～75Ma）的转化，可能主要是通过燕山晚期岩浆活动产生的富氯流体加入原已存在的贫氯古流体，从而使这种新的混合流体具有浸取金的能力来实现的。     

金沙江中段元古宙变质岩的Sm-Nd同位素年龄报道

金沙江中段的古老变质岩由多期变质的元古宙变质岩石单元组成。利用Sm-Nd同位素方法获得了雄松群二云母片岩全岩同位素等时线年龄1594Ma和1723Ma、2011Ma两组模式年龄。前者代表了成岩年龄,后者代表金沙江中段地层的基底年龄,这与金沙江流域乃至整个华南地壳最初形成的年龄十分相似。表明,川西、藏东地区雄松群变质岩可能属于华南地壳的组成部分。其形成时间应在早元古代或更早的时期内。     

Cosmic-ray exposure histories of two Antarctic meteorites from Chinese collections and the Guangmingshan and Zhuanghe chondrites

In this paper, we report cosmic-ray exposure ages (3He, 21Ne, 38Ar) and gas retention ages (4He, 40Ar) of two Antarctic meteorites, and the Guangmingshan and Zhuanghe ordinary chondrites. The Antarctic meteorites (GRV98002 and GRV98004) were collected on …     

Denudation rates of crystalline rock in the Huangling anticline of the Three Gorges of the Yangtze River in China since the Eocene

The processes and rates of the weathering denudation of the bare crystalline rock are important problems in the field of geosciences[1—6]. Essentially, the weathering denudation process is the process of rock destruction and transportation. In different …