Subduction of the Bangong–Nujiang Ocean: constraints from granites in the Bangong Co area,Tibet

We report geochronologic, whole‐rock geochemical and Sr–Nd isotopic analyses of the granites that are exposed to both the north and the inside of the Bangong–Nujiang Suture (BNS) zone as well as the implications for the Mesozoic history of Tibet. To the north of the BNS, the Larelaxin pluton consists of I‐type quartz diorite and highly fractionated I‐type biotite granite. The Larelaxin pluton is enriched in large‐ion lithophile elements (LILE) but depleted in high‐field‐strength elements (HFSE); therefore, it exhibits the features of volcanic arc rocks. The initial Sr (0.7102 to 0.7215) and negative εNd (t) (−2.91 to −5.20) values imply a mixture of depleted mantle and continental crust. The mean ²⁰⁶Pb/²³⁸U zircon age is 168 Ma; we therefore propose that the Bangong–Nujiang Ocean (BNO) had already been subducted beneath the Qiangtang terrain by the middle Jurassic. Inside the BNS, the Rutog granites intruded into the Lagongtang and Duoren formations, which show a continental margin and a forearc basin sedimentary facies, respectively. The mean ²⁰⁶Pb/²³⁸U zircon age is 101 Ma. The Rutog granites are monzogranites with a high Na/K ratio (Na₂O/K₂O > 1) and a high LILE/HFSE ratio, and A/CNK < 1.1. The high Sr/Y ratio (22 to 56) implies that these granites are adakitic. The low initial Sr (0.7044 to 0.7055) and positive εNd (t) (+1.46 to +2.70) values indicate that the protolith of the Rutog granites originated mainly from a depleted source. We attribute the Rutog plutonism to the development of an oceanic arc during the continuing northward subduction of the BNO and propose that the Rutog adakitic granites were formed by melting of the subducted BNO crust with limited crustal contamination. Copyright © 2013 John Wiley & Sons, Ltd.     

蛇绿岩研究进展、存在问题及思考

20世纪70年代建立的蛇绿岩（Ophiolite）概念认为出露在缝合带中的一套镁铁一超镁铁岩组合是大洋岩石圈的残留，其岩性单元可以与现代大洋岩石圈各个层圈一一对应，所以蛇绿岩是确定古板块边界的重要证据，该认识促进了板块构造学说的发展。近年来完善的MOR型和SSZ型蛇绿岩理论体系认为MOR型蛇绿岩形成于洋中脊（MOR），SSZ型蛇绿岩形成于俯冲带上（Supra—Subduction Zone），二者的地幔橄榄岩、堆晶岩组合及上部熔岩在岩石学、矿物学和地球化学方面均有不同的特征，洋陆俯冲和洋内俯冲是形成SSZ型蛇绿岩的两种机制，较为合理地解释了蛇绿岩的多样性及其与大洋岩石圈的差异。由于大洋板块的俯冲作用，在缝合带中MOR型蛇绿岩很少被保存下来，保存较好的大多数为SSZ型蛇绿岩。本文探讨了蛇绿岩研究中经常遇到的问题并提出了解决的思路，同时认为Rodinia超大陆前是否存在蛇绿岩、太古宙绿岩是否为蛇绿岩均为有待深入研究的问题。     

Geochronology of Intermediate-Acid Intrusive Rocks from Tongling, Anhui

1. Introduction Tongling district is situated in the southern part of the middle-lower reaches of the Yangtze River, Anhui province, China (Fig.1). It is one of the most important metal base in China, being rich copper material resources, also named "ancient copper capital of China". Many geologists have been working in petrology, tectonics and ore-deposits in the area for many years. Studies show none of the ore-deposits is not bound up with the intermediate-acid intrusion in the area (Ma…     

Constraints of Fe Isotopic Compositions on the Origin of the Luobusa Podiform Chromite Deposit, Tibet

<正>We measured Fe isotopic compositions of mineral separates of harzburgite,dunite and chromitite from Luobusa,Tibet as well as two harzburgites from Zedang for comparison to constrain the origin of podiform chromite deposits.The olivine(Ol)and orthopyroxene(Opx)of harzburgites from both localities have δ~(56) Fe values of     

南苏鲁晚中生代花岗岩的形成年龄及源区

本文对苏鲁造山带西南端(南苏鲁)的中生代花岗岩采用激光探针等离子体质谱(LA-ICP-MS)方法进行了年代学测定,李埝岩体为122±5Ma(6个测点的加权平均值),抗日山岩体的年龄为119±2Ma(5个测点的加权平均值),这些结果表明,南苏鲁地区的花岗岩主要形成于燕山期,相当于早白垩世。两个花岗岩体中还分别检测出新元古代的锆石,年龄范围为751~639Ma,表明有新元古代的岩石参与了中生代花岗岩的形成。花岗岩与围岩花岗质片麻岩地球化学和Sr-Nd同位素以及锆石特征的对比表明,经历超高压变质的花岗质片麻岩不是中生代花岗岩的主要源岩。花岗岩中存在暗色包体,结合该地区存在大量同时代的幔源岩浆活动,推测壳幔岩浆的混合可能是南苏鲁花岗岩形成的主要方式。     

含柯石英锆石的SHRIMP U-Pb定年:胶东印支期超高压变质作用的证据

苏鲁超高压变质带的形成时代究竟是印支期还是新元古代争议始终很大。对山东胶南地区超高压变质带中超镁铁岩和榴辉岩的锆石激光拉曼、阴极发光和离子探针原位定年的研究获得超高压变质作用发生的时代为印支期。其中超镁铁岩含柯石英锆石的年龄为221±12Ma,该深成岩侵位时代为新元古代(581±44Ma)。此外,锆石中另有约400Ma年龄记录,可能代表岩石形成后另有一期热事件。榴辉岩的下交点年龄为228±29 Ma,与超镁铁岩含柯石英锆石年龄一致,代表超高压变质时代;上交点为中元古代(1821±19Ma),代表原岩年龄,后者与其片麻岩围岩时代相一致,说明榴辉岩是原位俯冲。     

Discovery of metamorphic diamonds in central China: an indication of a > 4000-km-long zone of deep subduction resulting from multiple continental collisions

The Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra‐high‐pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.     

阿尔金山米兰红柳沟的席状岩墙群：海底扩张的重要证据

米兰红柳沟一带的蛇绿岩位于青藏高原的北部边缘,属于北阿尔金蛇绿岩带的西段,由变质橄榄岩、辉长岩、辉绿岩和基性熔岩组成。野外调查发现约50m厚的均由岩墙组成的岩墙群,其产状近直立,单个岩墙厚约30～40cm。岩墙和与其相伴的熔岩成分相同,属拉斑玄武岩系列,可与典型的洋底玄武岩对比。成分特点:SiO2含量为45wt%～49wt%,TiO2为～1wt%,K2O小于0.3wt%,P2O5为0.1wt%);LREE轻微富集到平坦型,(La/Sm)N0.86～1.74;Cr、Ni、Co和V等过渡金属元素含量高。岩墙群的发现为古海底扩张形成洋壳提供了关键性证据,并可进一步研究洋壳的形成过程和构造背景。     

青海都兰榴辉岩的发现及对中国中央造山带内高压—超高压变质带？…

在青海都兰北东４０ｋｍ野马滩一带元古宙变质杂岩中,新发现走向近东西的榴辉岩;可分为南北两带,北带宽约５ｋｍ,南带宽３～５ｋｍ。北带榴辉岩相矿物组合为石榴石－绿辉石－多硅白云母－金红石,它们的退变质矿物主要为角闪石及少量高钠的斜长石类。而南带峰期变质矿物为石榴石－绿辉石－蓝晶石－金红石－黝帘石（？）,退变质矿物中则出现大量高钠质斜长石。带另一个明显的区别是北带铁铝榴石端元组分含量较南带的略高,而镁铝     

中央碰撞造山带中两期超高压变质作用：来自含柯石英锆石的定年证据

沿中央造山带存在一条巨大的超高压变质带,其西起阿尔金-祁连,往东经秦岭,延至大别苏鲁,全长超过4000 km.柴北缘片麻岩中含柯石英锆石的SIMS离子探针原位微区U-Pb定年获得超高压变质年龄452±1 3.8 Ma,锆石的退变质年龄419±6.7 Ma.SHRIMP U-Pb定年获得秦岭含金刚石片麻岩中锆石的下交点年龄502±45 Ma,上交点年龄1545±100Ma,认为前者代表超高压变质年龄,后者为原岩岩浆锆石年龄;获得榴辉岩锆石的上交点年龄1381±82 Ma和下交点493±170 Ma,认为上交点代表榴辉岩原岩年龄,下交点代表超高压变质年龄;获得江苏东海县青龙山榴辉岩含柯石英等超高压矿物锆石的年龄为441±9 Ma,449±9 Ma,和442±9Ma,平均444±9 Ma,核部含斜长石+磷灰石锆石年龄为761±13 Ma,认为前者代表超高压变质年龄,后者代表榴辉岩原岩结晶年龄.认为中国中部沿中央造山带中存在两期超高压变质作用,第一期为加里东期,第二期为印支期,两期超高压变质事件在时空分布方面是不同的,加里东期超高压变质事件由西部阿尔金-柴北缘延至东部大别-苏鲁,印支期超高压变质事件没有在大别以西发现.认为中央造山带应是一个多期活动的造山带,较早形成罗德尼亚大陆的格林威尔造山运动可能留下了10亿年左右的构造岩浆事件记录,如中央造山带中大量10亿年左右的花岗岩及基性超基性岩类;罗德尼亚大陆之后第一次裂解作用可能发生在8亿年左右;其后早古生代加里东期的洋盆裂开,蛇绿岩和超高压变质岩石的大量出现是一次十分强烈的板块构造事件,从东到西,沿中央造山带均有分布;加里东期造山事件之后印支期沿该造山带又有一次大的板块裂解和俯冲碰撞作用,表现在勉略蛇绿岩洋壳及大别-苏鲁印支期超高压变质带的存在.中央造山带保留和记录了多期裂解、会聚事件,通过对其解剖,不仅可以认识中国大地构造格局和演化,并由此理解全球的大陆漂移、一系列大裂解和大会聚等重大地质事件.