Depositional Age,Provenance Characteristics and Tectonic Setting of the Ailaoshan Group in the Southwestern South China Block

The depositional and metamorphic ages and provenances of the Ailaoshan(ALS) Group in the Ailaoshan-Red River(ALS-RR) shear zone, southwestern South China Block(SCB), were investigated to constrain the tectonic history of the southwestern SCB. In this study, we use petrology, geochemical analysis, zircon cathodoluminescence imaging and UPb geochronology to analyse samples of quartzite, garnet-bearing two-mica schist and metapelite. The age spectra of detrital zircon grains from these metasediments show two dominant age peaks at 550–424 Ma and 876–730 Ma and two subordinate peaks at 970–955 Ma and ～2450 Ma. The youngest peak, corresponding to the early Palaeozoic, accounts for more than 20% of the total dates and constrains the deposition of the ALS Group to the Palaeozoic rather than the Palaeoproterozoic as traditionally thought. Moreover, two peaks of metamorphic ages corresponding to the Permo-Triassic and Cenozoic were also identified, and these ages document the tectonothermal events associated with the Indosinian collision between the Indochina Block and the SCB and the Himalayan collision between the Indian and Asian plates. Geochemical data suggest that the provenances of the ALS Group were dominated by continental arc and recycled metasedimentary rocks. The comparison of probability density distribution plots of the detrital zircon U-Pb age data indicates that the Neoproterozoic detritus in the ALS Group was probably derived from the arc-related Neoproterozoic intrusive bodies in the northwestern and southwestern SCB. Furthermore, the early Palaeozoic detritus might have been sourced from eroded early Palaeozoic strata and magmatic plutons in Cathaysia and volcanic rocks in the western Indochina Block.     

Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen: Implications for Crustal Evolution of the South China Craton

The Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China. As a volcanic-sedimentary sequence developed in the Nanhua rift, the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent. In this study, we report the results from whole-rock major-and trace-element concentrations, with zircon LA-(MC)-ICP-MS U-Pb ages, trace elements and Lu-Hf isotopic comp...     

华南双桥山群和河上镇群凝灰岩中的锆石SHRIMP U-Pb年龄—对江南新元古代造山带演化的制约

通过西澳科庭大学离子探针中心的远程测试,在双桥山群横涌组和安乐林组斑脱岩中获得大量锆石,其SHRIMP U-Pb加权平均年龄为831Ma±5Ma(横涌组)、829Ma±5Ma(安乐林组),在河上镇群上墅组中获得加权平均年龄767Ma±5Ma.锆石SHRIMP U-Pb年龄表明华南地区广为发育的双桥山群应归入新元古界,该年龄为标定双桥山群在地层柱中的位置提供了准确的年代学依据.     

华南双桥山群和河上镇群凝灰岩中的锆石SHRIMP U—Pb年龄——对江南新元古代造山带演化的制约

通过西澳科庭大学离子探针中心的远程测试,在双桥山群横涌组和安乐林组斑脱岩中获得大量锆石,其SHRIMP U—Pb加权平均年龄为831Ma±5Ma（横涌组）、829Ma＋5Ma（安乐林组）,在河上镇群上墅组中获得加权平均年龄767Ma±5Ma。锆石SHRIMP U—Pb年龄表明华南地区广为发育的双桥山群应归入新元古界,该年龄为标定双桥山群在地层柱中的位置提供了准确的年代学依据。     

Geochronology,Geochemistry, Whole Rock Sr‐Nd and Zircon Hf‐O Isotopes of the Early Neoproterozoic Volcanic Rocks in Jiangshan,Eastern Part of the Jiangnan Orogen: Constraints on Petrogenesis and Tectonic Implications

This paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr-Nd and zircon Hf-O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan–Shaoxing suture. SHRIMP zircon U-Pb dating of samples in the three areas yielded weighted mean 206 Pb/238 U ages of 842.8 ± 6.9 Ma and 850.0 ± 7.3 Ma, 839 ± 9 Ma and 832.2 ± 8.1 Ma, 82...     

江南造山带新元古代中期(830～750Ma)岩浆活动及对构造演化的制约

连接扬子地块和华夏地块的江南造山带是华南前寒武纪最重要的构造单元,其形成和演化长期以来备受关注。在江南造山带的范围内广泛发育了新元古代岩浆岩,它们是探讨江南造山带构造演化的重要对象,但其成因和形成的构造背景却备受争论。本文系统收集和分析了江南造山带830~820 Ma花岗岩、800~780 Ma酸性岩和800~750 Ma基性岩的地球化学数据。研究表明,不同时间段的岩石成因类型存在系统差异,830~820 Ma的花岗岩主要为S型花岗岩,800~780 Ma的酸性岩主要为A型酸性岩,而800~750 Ma的基性岩以拉斑系列和碱性系列为主,并在构造判别图中显示了板内玄武岩(WPB)和洋中脊玄武岩(MORB)的特征。综合同位素年代学、岩石地球化学和沉积学等学科领域的研究成果我们认为:扬子北缘和西缘应先于东南缘在1000~900 Ma期间发生碰撞,而此时的东南缘仍为活动大陆边缘;直到~830 Ma,扬子地块与华夏地块沿江南造山带发生拼贴,但只是陆-弧-(微)陆之间的"软碰撞",而无山脉隆升和高级变质作用,各个块体之间处于"联而不合"的状态,大洋岩石圈拆沉之后的软流圈上涌和由拆沉所引起的拉张作用导致了上覆岩石圈和陆壳发生部分熔融,产生了江南造山带830~820 Ma的S型花岗岩;随着全球Rodinia超大陆的裂解,~820 Ma,华南裂谷盆地开启,并在随后的裂解过程中发育了大量与伸展有关的800~780 Ma A型酸性岩和基性岩脉/墙,而其明显高于同时代MORB源区的地幔潜能温度显示,导致Rodina超大陆裂解的地幔柱可能对该时期岩浆岩的地幔源区有一定影响;随着拉张作用的不断加强,出现了760~750 Ma碱性系列和具MORB特征的基性岩,此时的软流圈地幔既提供热量又有物质供应。     

Petrology, Geochemistry and Zircon U-Pb Geochronology of the Xiangshan Group in the Eastern Hexi Corridor Belt: Implications for Provenance and Tectonic Evolution

The eastern Hexi Corridor Belt (HCB) is located in the transitional belt among the Alxa Block, the Qilian Orogenic Belt and the North China Block. Because of its unique tectonic location, the tectonic setting, provenance, and even the age of the sedimentary strata in the eastern HCB during the Early Paleozoic remain controversial. This study analyzes the provenance of the poorly studied Xiangshan Group, discusses its age of development and tectonic setting in the eastern HCB using a combination of petrological, geochemical and LA-ICP-MS U-Pb zircon dating methods. Based on the youngest age peaks and the fossil evidence, we suggest that the Xiangshan Group is Middle Cambrian to Late Ordovician in age. The complexity of the geochemical characteristics and associated diagrams suggests that the early stage of the Xiangshan Group developed in a passive continental margin environment, late in the back-arc basin of the eastern HCB. Based on the sandstone detrital composition, whole-rock geochemistry and detrital zircon ages, we conclude that the Xiangshan Group had an early provenance that was mainly from the Qilian Block and a late provenance from the Qilian Block and the western Alxa Block. The eastern HCB and its northern and southern blocks have similar palaeontology, lithology and basement age characteristics to the South China Block. This indicates that the eastern HCB might not have formed in the intra-continental aulacogen of the North China Block during the Early Paleozoic but has a close affinity to eastern Gondwana.     

Geochronology and Tectonic Evolution of the West Section of the Jiangnan Orogenic Belt

As an important part of South China Old Land, the Jiangnan Orogenic Belt plays a significant role in explaining the assembly and the evolution of the Upper Yangtze Block and Cathaysia, as well as the structure and growth mechanism of continental lithosphere in South China.The Lengjiaxi and the Banxi groups are the base strata of the west section of the Jiangnan Orogenic Belt.Thus, the research of geochronology and tectonic evolution of the Lengjiaxi and the Banxi groups is significant.The maximum sedimentary age of the Lengjiaxi Group is ca.862 Ma, and the minimum is ca.822 Ma.The Zhangjiawan Formation, which is situated in the upper part of the Banxi Group is ca.802 Ma.The Lengjiaxi Group and equivalent strata should thus belong to the Neoproterozoic in age.The Jiangnan Orogenic Belt consisting of the Lengjiaxi and the Banxi groups as important constituents is not a Greenville Orogen Belt(1.3 Ga–1.0 Ga).The Jiangnan Orogenic Belt is a recyclic orogenic belt, and the prototype basin is a foreland basin with materials derived from the southwest and the sediments belong to the active continental sedimentation.By combining large amounts of dating data of the Lengjiaxi and the Banxi groups as well as equivalent strata, the evolutionary model of the western section of the Jiangnan Orogenic Belt is established as follows: Before 862 Ma, the South China Ocean was subducted beneath the Upper Yangtze Block, while a continental island arc was formed on the side near the Upper Yangtze Block.The South China Ocean was not closed in this period.From 862 Ma to 822 Ma, the Upper Yangtze Block was collided with Cathaysia; and sediments began to be deposited in the foreland basin between the two blocks.The Lengjiaxi Group and equivalent strata were thus formed and the materials might be derived from the recyclic orogenic belt.From 822 Ma to 802 Ma, Cathaysia continued pushing to the Upper Yangtze Block, experienced the Jinning-Sibao Movement(Wuling Movement); as result, the folded basement of the Jiangnan Orogenic Belt was formed.After 802 Ma, Cathaysia and the Upper Yangtze Block were separated from each other, the Nanhua rift basin was formed and began to receive the sediments of the Banxi Group and equivalent strata.These large amounts of dating data and research results also indicate that before the collision of the Upper Yangtze Block with Cathaysia, materials of the continental crust became less and less from the southwest to the east in the Jiangnan Orogeneic Belt; only island arc and neomagmatic arc were developed in the eastern section.Ocean-continent subduction or continent-continent subduction took place in the western and southern sections, while intra-oceanic subduction occurred in the eastern section.Comprehensive analyses on U-Pb ages and Hf model ages of zircons, the main provenance of the Lengjiaxi Group is Cathaysia.     

江南古陆中段沧水铺群锆石U-Pb年龄和构造演化意义

江南古陆中段湖南益阳地区沧水铺群发育在冷家溪群和板溪群之间,其在地层柱中的位置一直是中国地质学家解疑江南造山带何时启动的关键层位。笔者在沧水铺群中的火山集块岩中获得锆石SHRIMP U-Pb年龄(821±13)Ma,再次验证了沧水铺群中的火山岩是820 Ma武陵运动之前的火山事件的产物,与武陵运动的启动有关,也说明该期火山岩与冷家溪群中大量的火山凝灰岩为同一构造运动的产物。     

下江群沉积地质特征及其对华南新元古代构造演化的约束

华南西部新元古代裂陷纪大地构造属性和原型盆地性质是一个涉及到扬子古陆在全球构造中的定位、且争议较大的基础地质问题.新元古代下江群发育于强变形变质的盐边群和弱变形变质的板溪群之间,处于以滨浅海沉积为主的板溪群向深海沉积转化的过渡带内.下江群以浊流沉积的杂砂岩为主,沉积物扩散方向指向西.下江群清水江组砂岩骨架颗粒统计分析表明,物源来自再旋回造山带.常量元素地球化学特征显示,下江群碎屑岩系形成于活动陆缘区的弧后盆地靠大陆一侧.结合盐边群、苏雄组大地构造属性及其与下江群的空间关系,推测当时的华南西部处于汇聚型板块边缘内,总体属弧后伸展型盆地.     

辽东弓长岭—岫岩地区古元古界辽河群变沉积岩的物源及构造背景

辽东弓长岭—岫岩地区位于胶—辽—吉造山带东段,区内发育厚层古元古代变质沉积岩系,本文对这些变质沉积岩进行了岩相学和U-Pb年代学等方面的研究。岩相学特征显示这些岩石变质程度普遍偏低,多为绿片岩相变质条件,未发现高级特征变质矿物。岩石中主要矿物颗粒为石英,其中北辽河群碳酸盐矿物较为发育。根据辽东弓长岭-岫岩地区古元古代变沉积岩碎屑锆石阴极发光（CL）图像,这些锆石普遍具有清晰的振荡环带和较高的Th/U值（>0.3）,表明其原岩来自岩浆岩。结合LA-ICP-MS U-Pb年代学,得到里尔峪组、高家峪组、大石桥组和盖县组碎屑锆石分别呈现出2 159、2 500、2 189/2 530和2 044/2 197 Ma的年龄峰值,表明其沉积物来源于古元古代花岗质岩石和相邻的太古宙杂岩。结合前人资料,本文认为南、北辽河岩群形成于相同的环境并经历了相同的构造演化过程,古元古代弧-陆碰撞模式可能更适用于胶—辽—吉造山带的构造演化。     

The U-Pb Geochronology and Lu-Hf Isotope Compositions of Detrital Zircons from the Nanhua Group of the Longsheng Region, South China and their Implications for Pan-African Events

It is unclear whether the South China blocks have an affinity with continental Gondwana due to a lack of direct Pan-African magmatic and metamorphic features. In this study, we conducted U-Pb geochronological and Lu-Hf isotopic analyses for detrital zircons from a sandstone of the Chang’an Formation of the Nanhua Group in the Longsheng region of northern Guangxi, with the aim of constraining the timing of sedimentation and information as to its source, as well as seeking evidence for Pan-African events in the South China blocks. The results show that the ages of detrital zircons peaked at 654.7 ± 6.2 Ma, 773.2 ± 4.1 Ma and 821.9 ± 6.5 Ma, with some at 920–870 Ma; the youngest age indicates the existence of the Pan-African thermal event. The εHf(t) and TDM2 values demonstrate that the study area has experienced three stages of crustal growth at 3.0–2.4 Ga, 2.1–1.5 Ga and 1.3–0.9 Ga. With intensively distributed Neoproterozoic mafic-ultramafic and granitic plutons emplaced at 830–810 Ma along the southwestern section of the Jiangnan Orogenic Belt and positive εHf(t) values from a large group of zircon grains, it is proposed that the sediments of the Chang’an Formation (of Nanhua Group) were largely sourced from the southeastern margin of the Yangtze block. Comparison with the zircon age spectra of the Cathaysian block shows that about 79% of the Pan-African aged detrital zircon grains that have TDM2 = 1352–1031 Ma and εHf(t) = 3.68–8.79, were sourced from the recycled Grenvillian crust of the Cathaysian block, suggesting that the Cathaysian block had a close connection with Gondwana.     

碎屑锆石年代学在沉积物源研究中的应用及存在问题

近年来,碎屑锆石U-Pb定年技术的不断突破极大地促进了沉积盆地物源分析的发展,该方法被进一步应用于分析古地理环境、沉积盆地演化以及物源区剥蚀速率等。在广泛应用的同时,国际学者逐渐意识到诸多不确定因素可影响分析结果、导致偏差。在系统总结碎屑锆石年代学在沉积物源研究中值得关注的几个问题的基础上,分析结果认为:①采样过程中要注意采样间隔和水动力对锆石的分选作用;②实验过程中合理地选择U-Pb定年方法,根据特定科学问题选定待测锆石和确定分析数量;③物源对比过程中不应简单地将沉积岩的碎屑锆石年龄谱与周缘造山带结晶岩体（包括岩浆岩和变质岩）的年龄进行对比,还应关注较老沉积岩提供的再旋回锆石、不同源岩的锆石产出能力、剥蚀区适当的隆升时间及碎屑锆石的“滞后时间”等因素。     

Provenance analysis of the Upper Miocene turbidite sand-bodies in deep-sea basin of northern South China Sea

The International Ocean Discovery Program (IODP) Expedition 367/368 reported massive Upper Miocene deep-sea turbidite in the northern South China Sea basin. The Upper Miocene turbidite sand-bodies at Site U1500 were examined with detrital zircon U-Pb dating to conduct the source-to-sink analysis. This study shows that the U-Pb age spectrums of Site U1500 sample are similar to those detrital zircons from the Miocene Qiongdongnan Basin and the Pearl River Mouth Basin. Multidimensional scaling (MDS) plot also shows that the turbidite sand-bodies at Site U1500 are closely related to the sediments in the Pearl River Mouth Basin and Qiongdongnan Basin. It is likely that the thick deep-sea turbidite succession in the deep-water basin of northern South China Sea was formed by a mixed provenance pattern during the late Miocene. On the one hand, terrigenous sediments from the west of the South China Sea were transported along the Central Canyon to the eastern South China Sea deep-sea basin in the form of turbidity current. On the other hand, terrigenous sediments were also transported from the Pearl River through the slope canyon system to the northern South China Sea in the form of gravity flow . Those mixed sediments from two different source areas have collectively deposited at the deep-sea basin and thus, give rise to turbidite sequence of hundred meters. Provenance analysis of the thick turbidites sand-bodies in the deep-sea basin is of great significance to the profound understanding of the tectonic evolution, filling processes, provenance evolution, and the palaeogeographic characteristics of the Cenozoic basins of the South China Sea.     

南海北部深海盆地上中新统浊积砂体物源分析*

IODP367/368航次在南海北部深海盆地多个站位发现上中新统厚达数百米的大规模深海浊积岩。采用碎屑锆石U-Pb年龄谱系分析方法对U1500站上中新统浊积砂体进行源汇对比分析。研究结果表明U1500站上中新统浊积岩碎屑锆石年龄谱系与其西侧琼东南盆地和北侧珠江口盆地中新世沉积物特征类似。多维排列分析(MDS)结果也显示,该站位样品与珠江口盆地、琼东南盆地沉积物关系密切,表明南海北部深海盆地内厚达数百米的上中新统浊积砂体为南海北部物源和南海西部物源混合堆积形成。南海西部陆源输入物质以浊流搬运的方式,沿中央峡谷从西到东搬运至南海东部深海盆地;南海北部珠江物源以重力流的形式,经南海北部陆坡峡谷搬运至深海盆地中,两种来源的沉积物在深海盆地发生混合沉积,形成U1500站厚达数百米的浊积砂体。南海北部深海盆地厚层浊积砂体物质来源的准确识别,对深刻理解南海新生代盆地的构造演化、沉积物充填过程、物源演变以及古地理特征均具有重要意义。     

黔东南地区新元古代下江群的地层年代及其地质意义

黔东南地区下江群是分布在江南造山带西段的一套前寒武纪浅变质陆源碎屑岩夹火山碎屑岩组合, 其地层时代归属与划分特别是区域地层对比一直未能得到很好解决.在对该区前寒武纪地层广泛而详细的剖面测制和地质调查基础上, 运用LA-ICP-MS技术, 对地层中沉凝灰岩、含凝灰质碎屑岩和碎屑岩进行锆石U-Pb年代学测试, 获得一批高精度的年龄.四堡群河村组顶部的碎屑岩和下江群乌叶组第1段顶部的含凝灰质碎屑岩、清水江组底部与中部的沉凝灰岩、平略组中上部的含凝灰质碎屑岩、隆里组中下部的碎屑岩等样品中锆石U-Pb最小年龄组的加权平均年龄分别为819.8±6.4Ma和779.5±4.7Ma、764.0±6.3Ma与(756.8±7.6Ma、756.0±13.0Ma)、733.9±8.8Ma及725.0±10.0Ma.这些数据将四堡群和下江群的时代限制为新元古代.结合研究区隆里组与长安组整合接触关系和全球低纬度南华系冰期起始年龄(717.4Ma)的地质事实, 将下江群沉积时限约束在815~717Ma之间.根据区域岩浆事件进一步约束了下江群各组段的地层时限, 并开展江南造山带下江群及其相当层位的地层划分与对比, 认为新元古代岩浆作用制约着黔东南下江群盆地的演化.      

华南新元古代花岗岩的锆石U-Pb年龄及其构造意义

SHRIMP锆石U Pb年龄测定结果表明 ,云南峨山、江西九岭和安徽许村等 3个岩体的形成年龄分别为818± 10Ma、818± 10Ma和 82 9± 11Ma ,与广西北部的本洞、三防、元宝山、湖北的黄陵岩体以及的广西北部的基性岩墙的年龄完全一致 ,表明在 82 0～ 830Ma期间 ,扬子克拉通在广大区域内几乎同时发生了广泛的地壳重熔和幔源基性岩浆活动。虽然这些花岗岩的类型不同 ,但它们的形成时代相当一致 ,表明这些不同类型的花岗岩应形成于相同或相关的构造环境 ,很可能是地幔柱活动导致下地壳重熔的产物。     

Provenance of the Late Permian Xuanwei Formation in the Upper Yangtze Block: Constraints from the Sedimentary Record and Tectonic Implications

Xuanwei Formation is composed of mudstone, siltstone, and sandstone, with local conglomerate. However, its provenance and tectonic setting have been scarcely studied. In this paper, we use sedimentology, electron probe microanalysis(EPMA), and detrital zircon dating to investigate its source area and depositional tectonic setting. The facies assemblages indicate that it formed in alluvial fan and fluvial river sedimentary environments. The strata thicknesses and facies distribution indicate that the sediment supply was from the west. The results of EPMA show that chromian spinels within the sediments are characterized by high Cr# and varying Mg#. Discrimination plots suggest that these spinels were sourced from large igneous province(LIP) magmatic rocks. The laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) U–Pb chronology of detrital zircons suggests that the sediments were derived from intermediate–acid igneous rocks dating back to 251–260? Ma. We could, therefore, conclude that the provenance of the Xuanwei Formation is from Emeishan basalt and synchronous felsic igneous rocks, which is consistent with the composition of the detrital framework. The detrital zircon dates also suggest that felsic magmatism occurred during the Late Permian, not after the eruption of the Emeishan basalt. Based on the sedimentary successions and provenance analysis, the tectonic setting for Xuanwei Formation deposition was a volcanic rifted margin.     

桂北地区丹洲群锆石U-Pb年代学及对华南新元古代裂谷作用期次的启示

桂北地区丹洲群是南华裂谷盆地南段的一套连续裂谷充填沉积,厘定各组沉积时限及区域地层关系,对理解华南新元古代裂谷作用期次具有重要意义。本文利用LA-ICP-MS锆石U-Pb同位素测年,获得丹洲群合桐组二段和拱洞组底部凝灰岩夹层形成年龄分别为801±4 Ma和781±5 Ma。研究表明,丹洲群白竹组和合桐组一段与下江群甲路组和乌叶组、板溪群沧水铺组和马底驿组、西乡群孙家河组及陆良组一段相当,沉积时限为820~800 Ma;合桐组二段与下江群番召组相当,沉积时限为800~780 Ma;拱洞组可与下江群清水江组、平略组和隆里组,板溪群五强溪组中上部和牛牯坪组,西乡群大石沟组中上部和三郎铺组,陆良组二段及澄江组、开建桥组、莲沱组、虹赤村组和上墅组的中上部直接对比,沉积时限为780~725 Ma。华南新元古代裂谷盆地系统的典型地层锆石年龄存在5组高峰,峰值年龄分别为818±2 Ma、802±1 Ma、780±4 Ma、756±4 Ma及728±5 Ma。综合华南新元古代岩浆活动特征及盆地沉积演化过程,确定华南新元古代裂谷作用可分为两期:820~800 Ma和800~725 Ma。此外,华南新元古代岩浆活动与裂谷作用之间存在明显的耦合关系,但各期岩浆活动对各裂谷盆地的影响程度存在差异。