滇西南涧地区无量山岩群变质英安岩锆石U-Pb年龄及其构造意义

滇西南涧—云县一带广泛出露的无量山岩群是一套与三江特提斯造山带密切相关的中低变质沉积岩夹火山岩系,其形成时代、沉积充填序列及大地构造属性一直存在争议。出露于南涧县公郎乡一带的无量山岩群发育有厚数厘米的变质英安岩夹层。对变质英安岩进行LA-ICP-MS锆石U-Pb定年,分别获得428.7±8.7Ma的岩浆结晶年龄和338～387Ma、250.5±8.1Ma、146.0±6.0Ma的变质年龄。认为无量山岩群原始沉积岩形成于志留纪,在古特提斯洋俯冲过程中,经历古生代造弧而发生区域变质作用,在白垩纪受热事件影响而发生热变质作用;同时还获得2034±22Ma、1140±16Ma、731±15Ma及469～522Ma的单颗粒锆石年龄,这些年代峰值与整个扬子陆块西缘获得的前寒武纪、古生代构造热事件年龄及分布特征基本可以对比;记录了Columbia、Rodinia和Gondwana超大陆的形成与裂解过程。     

Phanerozoic magmatism in the Proterozoic Cuddapah Basin and its connection with the Pangean supercontinent

Magmatic pulses in intraplate sedimentary basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin.The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe.Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation,Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma(Late Carboniferous) and 227 Ma(Triassic).Further,these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian,Panjal Traps,Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature.During the Phanerozoic Eon,the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.     

Assembly and Breakup of Rodinia (Some results of IGCP project 440)

The principal results of project 440 “Assembly and Breakup of Rodinia” of the International Geological Correlation Programme (IGCP) are reviewed in this work. A map of that supercontinent compiled using geological and paleomagnetic data describes global paleogeography 900 Ma ago. The assembly of Rodinia, which comprised most of Precambrian continental blocks, lasted ca. 400 m.y. (from 1300 to 900 Ma). Its breakup presumably triggered by mantle superplume took place between 830 and 650 Ma. The correlation between tectonic events in different continental blocks is considered. Some problems concerning the Rodinia reconstruction and history, e.g., the slow growth of juvenile crust and effects of mantle-plume events during the amalgamation period and of glaciations at the breakup time, are discussed. The latter caused changes in the biosphere and climate, whereas postglacial periods stimulated progress in biota evolution.     

青海都兰地区前泥盆纪古陆块的物质组成和重大地质事件

通过对青海都兰地区前泥盆纪地块的地质、地球化学和同位素年代学研究,将其由北向南划分为欧龙布鲁克微陆块、沙柳河超高压碰撞带和柴达木陆块三个构造单元.基本查明了欧龙布鲁克古陆块主要由德令哈片麻岩、莫河片麻岩、达肯大坂岩群、万洞沟群和全吉群组成,柴达木盆地北缘高压碰撞带主要为中新元古代沙柳河岩群、花岗闪长质片麻岩、英云闪长质片麻岩、钾长花岗片麻岩、榴辉岩、滩间山群和蛇绿岩,柴达木古陆块主要由古元古代金水口岩群和夕线堇青花岗片麻岩组成.详细厘定了本区前泥盆纪重要地质事件,特别是与Rodinia超大陆有关的汇聚与裂解事件.同时,利用先进的同位素地质年代学手段,确定了本区重要地质事件的地质时代和序列,为研究我国中-新元古代古陆块汇聚与裂解关系,以及它们在整个超大陆中的位置提供了证据.     

塔里木克拉通东北缘Columbia超大陆裂解事件：库鲁克塔格地区辉绿岩床的地球化学、锆石U-Pb年代学和Hf-O同位素证据

研究塔里木克拉通东北缘库鲁克塔格地区发育的中元古代早期基性岩浆活动,对深入了解Columbia超大陆裂解过程具有重要意义。本文报道了库鲁克塔格地区侵入于兴地塔格群的阿斯廷布拉克辉绿岩床SHRIMP锆石U-Pb年龄,结合岩石学、地球化学和Nd-Hf-O同位素资料,对该基性岩的形成时代、岩浆起源和源区类型以及岩浆作用的动力学背景进行讨论。研究表明：辉绿岩锆石具有高Th/U比值(>1),CL图像带状分区,显示基性岩浆锆石特点,207Pb/206Pb 加权平均年龄(1 551±8) Ma代表辉绿岩的形成时代,锆石δ18O值为5.52‰~6.53‰(正态分布的峰值为5.8‰),略高于地幔锆石的变化范围。辉绿岩高FeOT(11.4%~13.4%),低MgO(5.46%~7.11%)和TiO2(1.51%~2.45%),具有拉斑质属性,轻、重稀土元素分馏明显((La/Yb)N=4.6~5.4),具弱的Eu正异常(δEu=1.05~1.27),富集大离子亲石元素(LILE),亏损Nb、Ta等高场强元素(HFSE),微量元素组成与大陆溢流玄武岩(CFB)类似,全岩Nd(εNd(t)=-3.8~-1.8)和锆石Hf(εHf(t)=-3.7~1.9)同位素均显示岩浆来自富集地幔。阿斯廷布拉克辉绿岩是被交代的大陆岩石圈地幔部分熔融的产物,形成于板内的拉张环境,与该时期全球构造演化体制相吻合,属于中元古代早期Columbia超大陆裂解事件。     

柴北缘欧龙布鲁克地块中元古代晚期麻粒岩相变质作用——来自石榴夕线堇青石片麻岩的岩石学、相平衡模拟和U-Pb年代学的制约

在欧龙布鲁克地块乌兰北部地区察汗河岩群中识别出麻粒岩相石榴夕线堇青石片麻岩,其矿物组合为石榴子石、夕线石、堇青石、黑云母、斜长石、钛铁矿和少量钾长石等。岩相学观察显示,M1阶段矿物组合有斜长石±钾长石+石榴子石+夕线石+石英,M2阶段矿物组合有斜长石±钾长石+石榴子石+夕线石+石英+钛铁矿+黑云母,M3阶段矿物组合有堇青石+黑云母+钛铁矿+石英+石榴子石+斜长石±钾长石。相平衡模拟计算结果显示,该岩石的峰期温压条件为p=0.92～1.08 GPa,峰期温度t>790℃,峰期之后经历升温降压的p-T演化轨迹。锆石和独居石LA-ICP-MS U-Pb年代学研究获得的变质年龄分别为1 133±14 Ma和1 125±37 Ma, 1 133～1 125 Ma应代表了该期麻粒岩相变质作用的时代。结合区域地质资料和已有的研究成果,我们认为乌兰北部察汗河岩群的石榴夕线堇青石片麻岩可能形成于大洋俯冲作用下的弧或弧后构造环境,乌兰北部的岩浆-变质杂岩带经历了从中元古代晚期-新元古代早期俯冲增生到碰撞造山的演化过程,是全球Rodinia超大陆汇聚过程的响应。     

浙江章村地区中元古代岛弧火山岩的地球化学及构造意义

浙江章村地区中元古代双溪坞群火山岩是江南造山带的重要组成部分,下部北坞组以中基性至中酸性凝灰岩类为主,中部岩山组以沉凝灰岩类占优势,上部章村组以中酸性至酸性熔结凝灰岩类为特色,形成较完整的中基性-酸性火山岩演化序列。岩石化学和稀土元素、微量元素地球化学均指示其形成于岛弧环境,反映为大陆边缘岛弧挤压背景下同源岩浆连续分异过程中火山作用的产物,自下而上显示了岛弧逐步增大并不断成熟的过程。从所见物质成分分析,它经历了北坞期火山岛弧陆相喷发-岩山期火山间歇弧内盆地沉积-章村期火山岛弧陆相爆溢3个阶段,在经历了持续的火山活动之后,区内之龙门山岛弧处于挤压体制之中并褶皱造山,其起因与Rodinia超大陆发生聚合造山作用有关。     

Extensional collapse of the Gondwana orogen: Evidence from Cambrian mafic magmatism in the Trivandrum Block,southern India

The assembly of Late Neoproterozoice Cambrian supercontinent Gondwana involved prolonged subduction and accretion generating arc magmatic and accretionary complexes, culminating in collision and formation of high grade metamorphic orogens. Here we report evidence for mafic magmatism associated with post-collisional extension from a suite of gabbroic rocks in the Trivandrum Block of southern Indian Gondwana fragment. Our petrological and geochemical data on these gabbroic suite show that they are analogous to high Fe tholeiitic basalts with evolution of the parental melts dominantly controlled by fractional crystallization. They display enrichment of LILE and LREE and depletion of HFSE with negative anomalies at Zre Hf and Ti corresponding to subduction zone magmatic regime. The tectonic affinity of the gabbros coupled with their geochemical features endorse a heterogeneous mantle source with collective melt contributions from sub-slab asthenospheric mantle upwelling through slab break-off and arc-related metasomatized mantle wedge, with magma emplacement in subduction to post-collisional intraplate settings. The high Nb contents and positive Nbe Ta anomalies of the rocks are attributed to inflow of asthenospheric melts containing ancient recycled subducted slab components and/or fusion of subducted slab materials owing to upwelling of hot asthenosphere. Zircon grains from the gabbros show magmatic crystallization texture with low U and Pb content. The LA-ICPMS analyses show ²⁰⁶ Pb/²³⁸ U mean ages in the range of 507-494 Ma suggesting Cambrian mafic magmatism. The post-collisional mafic magmatism identified in our study provides new insights into mantle dynamics during the waning stage of the birth of a supercontinent.     

东北地块群:构造演化与古大陆重建

东北地区位于西伯利亚板块、华北板块和太平洋板块之间,为"中亚造山带"的东段和太平洋构造域的叠加部位,因此东北地块群构造属性和背景的研究对深入探讨二大构造域的叠加与转化背景具有重要的理论意义。东北地块群从东到西可细分佳木斯兴凯、松辽、兴安和额尔古纳四大地块,这些地块具有相同的新元古代泛非期变质基底,而古生代沉积岩也存在一定的可比性,表明这些地块存在相同或者相似的构造演化背景。分割这些地块的构造边界特征为:1）额尔古纳与兴安地块的缝合带为早古生代头道桥-新林缝合带,而非中生代德尔布干断裂;2）兴安地块与松辽地块之间的贺根山黑河缝合带形成时代为晚石炭世（330~300 Ma）,而非最近报道的中生代;3）古亚洲洋分布在东北陆块群与华北板块之间,沿西拉木伦-长春缝合带闭合,时代为三叠纪;4）佳木斯兴凯地块与松辽地块之间的吉黑高压带形成于古亚洲构造域与环太平洋构造域转换的关键时期（210~180 Ma）;5）那丹哈达增生杂岩为中国境内古太平洋板块俯冲增生的唯一直接证据,并记录了晚三叠早白垩世古太平洋板块向欧亚大陆俯冲增生的过程。在此基础上,分析了东北地块群发育的典型古生物和年代学标志,重建了东北地块群从Gondwana 大陆到Pangea大陆的位置与模型。     

Detrital zircon geochronology of quartzites from the southern Madurai Block,India: Implications for Gondwana reconstruction

Detrital zircons are important proxies for crustal provenance and have been widely used in tracing source characteristics and continental reconstructions. Southern Peninsular India constituted the central segment of the late Neoproterozoic supercontinent Gondwana and is composed of crustal blocks ranging in age from Mesoarchean to late Neoproterozoic–Cambrian. Here we investigate detrital zircon grains from a suite of quartzites accreted along the southern part of the Madurai Block. Our LA-ICPMS U-Pb dating reveals multiple populations of magmatic zircons, among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic (ca. 2980–1670 Ma, with peaks at 2900–2800 Ma, 2700–2600 Ma, 2500–2300 Ma, 2100–2000 Ma). Zircons in two samples show magmatic zircons with dominantly Neoproterozoic (950–550 Ma) ages. The metamorphic zircons from the quartzites define ages in the range of 580–500 Ma, correlating with the timing of metamorphism reported from the adjacent Trivandrum Block as well as from other adjacent crustal fragments within the Gondwana assembly. The zircon trace element data are mostly characterized by LREE depletion and HREE enrichment, positive Ce, Sm anomalies and negative Eu, Pr, Nd anomalies. The Mesoarchean to Neoproterozoic age range and the contrasting petrogenetic features as indicated from zircon chemistry suggest that the detritus were sourced from multiple provenances involving a range of lithologies of varying ages. Since the exposed basement of the southern Madurai Block is largely composed of Neoproterozoic orthogneisses, the data presented in our study indicate derivation of the detritus from distal source regions implying an open ocean environment. Samples carrying exclusive Neoproterozoic detrital zircon population in the absence of older zircons suggest proximal sources in the southern Madurai Block. Our results suggest that a branch of the Mozambique ocean might have separated the southern Madurai Block to the north and the Nagercoil Block to the south, with the metasediments of the khondalite belt in Trivandrum Block marking the zone of ocean closure, part of which were accreted onto the southern Madurai Block during the collisional amalgamation of the Gondwana supercontinent in latest Neoproterozoic–Cambrian.