河西走廊东部香山群时代和物源讨论

香山群是一套经历轻微区域变质作用的陆源碎屑岩,夹有少量碳酸盐岩和硅质岩,主要位于河西走廊东部香山地区。迄今为止,前人已经做过大量工作,但是对香山群与该区奥陶纪米钵山组的关系、香山群的时代以及香山群的物源等问题还存在较大争议。笔者对香山群的碎屑锆石做了定年(LA-ICP-MS)工作,通过香山群与米钵山组锆石年龄的对比,发现米钵山组最年轻锆石(451±8)Ma比香山群最年轻锆石(525±3)Ma年轻,且香山群不含米钵山组其他年轻锆石成分,香山群时代应该晚于(525±3)Ma,而早于中奥陶世米钵山组,应属中—晚寒武世。香山群碎屑锆石年龄谱与华夏地块、阿拉善基底、华北西部和北祁连岩浆变质事件年龄差异显著,华夏地块、阿拉善基底、华北西部和北祁连不是香山群物源,而与澳大利亚西南部地区相似,同时根据香山群古水流分析结果,推测澳大利亚西南部可能是香山群的沉积物源。     

Sedimentary Paleoenvironment of the Eastern Hexi Corridor,NW China: Constraints from Chert Geochemistry and Sedimentary Analysis of Early Paleozoic Strata

The eastern Hexi Corridor, northwest China, is located at the tectonic junction of the Alxa Block, the North China Craton, and the Qinling‐Qilian Orogen. The early Paleozoic Xiangshan Group record critical information regarding paleoenvironment, paleoclimate and paleotectonic setting, from which we here present a focused study on the chert beds within the Xiangshan Group. Through field mapping, microstructural observation, whole‐rock geochemistry analyses and detrital zircon dating, we suggest that the Xiangshan Group chert was deposited along a passive continental margin, formed primarily through biological activity with minor hydrothermal influence and terrestrial input. The characteristics of the chert support a low latitude sedimentary paleoenvironmental origin, and reveal the fact that the Alxa Block was separated from the North China craton, while emerged some paleogeographic affinity with the Qilian region in the Middle‐Late Cambrian.     

阿拉善地块性质和归属的再认识

阿拉善地块被认为是一个前寒武纪微陆块,其归属和演化历史长期存在争议。一部分学者认为其为华北克拉通的组成部分,是华北克拉通的西延;一部分学者认为与扬子或塔里木克拉通具有亲缘性;还有一些学者认为具有独立的地质演化历史。本文综合近年来阿拉善地块的研究进展,结合我们新的研究资料,梳理了阿拉善地块自新太古代到显生宙的岩石组成、构造热事件及年代格架,得出以下主要认识:1)阿拉善地块的早前寒武纪变质基底记录了2.7~3.0Ga陆壳生长、~2.5Ga TTG岩浆-变质事件,2.0~2.3Ga岩浆事件,以及1.9~1.95Ga和1.80~1.85Ga两期重要的变质事件,具有与华北克拉通相似的岩石组成和地壳演化历史;2)阿拉善地块发育与华北克拉通相似的中-新元古代沉积岩系,其碎屑锆石具有来源华北克拉通本身的物源区特征,反映出阿拉善地块直到中元古代-新元古代早期仍可能是华北克拉通的一部分;3)阿拉善地块显生宙的多期岩浆-构造-变质事件可能主要是中亚造山带早古生代以来造山作用的响应,而不是阿拉善地块与华北克拉通碰撞的产物。从奥陶纪到早二叠世,阿拉善地块可能处在古亚洲洋向南俯冲的活动大陆边缘环境;4)河西走廊-贺兰山南部早古生代沉积岩系(大黄山群和香山群)不属于阿拉善地块,而是祁连造山带的组成部分,其碎屑锆石组成和代表的物源特征不能作为阿拉善地块与华北克拉通在早古生代彼此分离的证据。     

早古生代阿拉善地块与华北地块之间的关系:来自阿拉善东缘中奥陶统碎屑锆石的信息

阿拉善东缘奥陶纪地层位于鄂尔多斯(华北地块)与北祁连早古生代造山带之间的过渡地区,该区的构造背景一直是长期争论的问题,它涉及到阿拉善地块是否与华北地块相连、奥陶系的物源以及"贺兰拗拉槽"是否存在等问题。分布于阿拉善地块东缘的中奥陶统米钵山组的碎屑锆石LA-ICP-MSU-Pb年龄测试表明,样品中数量最多的锆石年龄为900～950Ma,Alxa-1的峰值年龄为916Ma,Alxa-2的峰值年龄为953Ma,次者在494～623Ma之间,这个区间内存在多个峰值,如Alxa-1存在505Ma和588Ma两个主要峰值,Alxa-2则存在494Ma、517Ma、623Ma等几个峰值。在2.5Ga左右两个样品都存在一个弱的峰值,Alxa-1峰值为2517Ma,而Alxa-2峰值为2552Ma和2670Ma。除此之外,两个样品都有个别大于3.0Ga的成分,Alxa-1样品中最年轻的锆石为451±8Ma,Alxa-2样品则为483±4Ma。这些年龄以及沉积特征表明:(1)传统认为的奥陶纪"贺兰拗拉槽"并不存在,鄂尔多斯西南缘地区以及阿拉善东部地区当时属于北祁连早古生代周缘前陆盆地系统;(2)早古生代主要物源来自北祁连造山带,新元古代物源来自阿拉善地块;(3)鄂尔多斯西缘整个米钵山组的锆石年龄分布及其变化,指示出北祁连造山带(岛弧)逐渐靠近阿拉善地块,其间洋盆逐渐消失的过程;(4)阿拉善地块基底与华北有明显差别,阿拉善地块明显受到新元古代和古生代构造热事件的影响,两者可能是在中奥陶世或之后才拼贴在一起。     

贺兰山及周边地区加里东运动研究

基于对贺兰山及周边地区下古生界详细的野外地质调查,通过碎屑锆石年龄谱的物源分析、地层接触关系追踪、岩性岩相突变特征分析、残留地层分布、古生物组合及亲缘性分析,讨论了阿拉善地块与华北地块和中祁连地块的亲缘关系,探讨了研究区加里东运动的幕次,认为早古生代阿拉善地块和华北地块相互独立,俯冲造山特征建造不清晰,两者之间可能为转换型陆缘。阿拉善地块和中祁连地块之间的加里东运动幕次清晰。加里东运动一幕发生在中-晚寒武世,香山群深水复理石沉积和张夏组浅水碳酸盐台地相沉积呈拼合式接触,生物组合为亲华北-亲祁连混合型,彼时中祁连地块从Rodinia大陆裂离,逐步靠近阿拉善地块;加里东运动二幕发生在早-中奥陶世,下奥陶统碳酸盐岩在全区稳定分布,与下伏香山群不整合接触,与阿不且亥组整合接触,生物相统一,生物组合为华北型,中祁连地块-阿拉善地块-华北地块平和成为一个统一的块体,整体为浅水台地沉积环境;加里东运动三幕发生在奥陶纪末,中-上奥陶统与泥盆系角度不整合接触,商丹洋闭合,扬子地块与阿拉善地块和华北地块靠近,商丹缝合带南部的宽坪洋打开,导致研究区中、晚奥陶世亲华北-亲扬子混合型生物组合类型。     

Tectonic evolution of the western Ordos Basin during the Palaeozoic-Mesozoic time as constrained by detrital zircon ages

ABSTRACT

The Ordos Basin has experienced a complicated tectonic evolution since the Palaeozoic. Its multi-stage evolution was closely related to the tectonic events that occurred along plate boundaries. The detrital zircon ages and crystallization age (CA)-deposition age (DA)/cumulative proportion curves obtained from Palaeozoic-Mesozoic strata from different tectonic units in and around the western Ordos Basin demonstrate that during the early Palaeozoic, the so-called Helan Aulacogen did not develop along the western Ordos Basin, the Alxa Block was an independent unit from the North China Craton, and the southern Ordos Basin was a foreland basin of the North Qinling Orogenic Belt. During the early Palaeozoic, the western Ordos Basin and its vicinity belonged to three different tectonic units (i.e. the North China Craton, the Alxa Block, and the North Qilian Orogenic Belt). At the end of the early Palaeozoic, the Alxa Block amalgamated with the Ordos Basin. From the Silurian to the Middle Devonian, the southern Alxa Block was a foreland basin of the North Qilian Orogenic Belt and underwent regional extension during the Late Devonian. During the late Palaeozoic, the western Ordos Basin and its vicinities were located in a back-arc extensional setting of the western Qinling Orogenic Belt. The southern part of the western Ordos Basin may have been a retro-arc foreland basin of the western Qinling Orogenic Belt during the Late Triassic, and the northern part of the western Ordos Basin experienced large-scale left-lateral strike-slip at the same time. The CA-DA/cumulative proportion curves can adequately explain the evolution of the western Ordos Basin during the Palaeozoic; however, the settings indicated by the CA-DA/cumulative proportion curves in intraplate evolutions are different from those proposed in other studies, which may be due to the number and distribution of samples and rapid lateral changes in sedimentary facies.     

华北盆地三叠纪物源特征及其沉积—构造演化

华北盆地三叠纪沉积厚度大,分布广泛,其地层沉积特征很好地记录了周缘造山带或隆起区在该时期的构造演化过程。目前,前人已经对华北各地区三叠纪碎屑物源进行了大量研究,而对于物源区的认识仍存在分歧,对于盆缘地区沉积—构造演化过程的研究也相对较少。通过整理前人对华北各地区三叠纪碎屑物源研究的锆石年龄数据,并结合造山带构造演化过程和地层沉积特征,对华北盆地三叠纪碎屑物源及沉积—构造演化过程进行了整体研究。结果表明:华北北部三叠纪沉积物源均来自北缘的内蒙古隆起,锆石年龄和地层沉积特征记录了源区逐渐增强的岩浆活动和隆升过程。华北南部地区在该时期主要接受来自华北南缘二叠纪沉积盖层和北秦岭造山带的碎屑物质供给,华北南缘伴随着秦岭造山过程可能在中三叠世就已经逆冲隆升并遭受剥蚀,两者的协同演变共同控制着盆地南部沉积演化过程。鄂尔多斯盆地西北部碎屑物源主要来自阿拉善地块和北祁连造山带,西南部地区物源则主要来自盆地西南缘再旋回沉积盖层和北祁连造山带,分别为伸展和挤压状态下的内陆盆地沉积。早—中三叠世,华北盆地为统一的大型内陆沉积盆地,晚三叠世,盆地南、北缘发育沿褶皱逆冲带分布的陆内前陆盆地系统。     

东亚原特提斯洋(Ⅴ):北界西段陆缘属性及微陆块拼合

早古生代原特提斯洋在祁连造山带的分支本文称为古祁连洋。其洋内及邻区存在中祁连、阿拉善、柴达木、华北、扬子、塔里木等多个陆块、微陆块,处在一个复杂的多岛洋的环境中。祁连地区早古生代经历了较为复杂的俯冲拼合、碰撞造山过程。本文探讨了祁连造山带的几个构造单元构造属性,认为早古生代阿拉善微陆块南缘为被动大陆边缘,中祁连北缘为活动大陆边缘。阿拉善南部与之平行的龙首山构造单元为俯冲造山形成的增生楔体;北祁连构造带为一套俯冲增生杂岩,包含高压变质岩带、蛇绿岩带、岛弧岩浆和部分洋壳残片等,记录了古祁连洋壳从大陆裂解,洋壳形成,俯冲拼合,碰撞造山的造山过程。495Ma左右南祁连南部柴达木微陆块向北俯冲的影响,古祁连洋壳俯冲受阻,俯冲带向北后退,形成大岔大坂岛弧。弧前地区发生洋-洋俯冲事件,堆积增生大岔大坂、白泉门、九个泉等SSZ型北祁连蛇绿岩北带,并伴随第二期清水沟、牛心山、野牛滩等地岩浆事件。460Ma左右阿拉善微陆块和中祁连微陆块开始碰撞拼合,古祁连洋开始闭合。值得注意的是拼合过程不是均一的,存在自西向东斜向"剪刀式"的拼合方式,产生了由西向东年代变新的"S"型同碰撞岩浆岩。约440Ma古祁连洋闭合,进入陆内造山阶段。440Ma之后,拼合陆块处在一种拉伸的构造环境之下,金佛寺、牛心山、老虎山等地产生碰撞后岩浆岩。422~406Ma发生俯冲折返、高压榴辉岩和高压低温蓝片岩退变质作用,形成以紧闭不对褶皱为特征的第二幕变形。根据各陆块、微陆块碎屑锆石年龄谱分析对比,中祁连基底应与华北不同,而可能与扬子有关。Rodinia超大陆聚合之前,中祁连微陆块作为一个独立的微陆块与华北、扬子保持一定距离。1.0~0.8Ga Rodinia超大陆聚合过程中祁连微陆块与冈瓦纳北缘拼贴在一起,而距华北较远。随着Rodinia超大陆裂解,中祁连微陆块远离冈瓦纳,逐渐向华北靠近,500~400Ma原特提斯洋闭合,华北、阿拉善与中祁连拼合,并整体拼合到冈瓦纳大陆北缘。     

贺兰山地区中奥陶统樱桃沟组物源及构造背景分析

鄂尔多斯盆地西缘贺兰山地区的樱桃沟组发育一套夹有碳酸盐滑塌重力流沉积的陆源碎屑岩。由于缺少可靠的物源数据,对早古生代鄂尔多斯盆地西缘的构造背景尚未有定论。本文对樱桃沟组碎屑岩进行了地球化学及岩石学分析,砂岩碎屑组分特征表明,该组源区大地构造背景为再旋回造山带。常量元素和微量元素地球化学特征表明樱桃沟组的物源呈现双物源的特点。更进一步,樱桃沟组的稀土元素配分模式对比表明了樱桃沟组碎屑岩的物源来自阿拉善古陆和/或北祁连造山带。综合分析砂岩的常量元素、微量元素和稀土元素的地球化学特征,经构造判别图解,认为樱桃沟组物源区主要为主动大陆边缘,也有来自被动大陆边缘的信息,表明贺兰山地区业已受到北祁连早古生代造山带的影响。樱桃沟组与研究区周缘造山带稀土元素配分模式对比研究显示,前者与其北西部阿拉善古陆和南西部北祁早古生代连造山带花岗岩露头稀土元素配分模式一致,均表现为轻稀土富集,重稀土亏损的右倾模式,具有轻微的Eu负异常和明显的Tm负异常,结合中奥陶世鄂尔多斯西缘古水流和碎屑锆石年龄分布特征,认为樱桃沟组的物源主要来自祁连造山带,部分来自阿拉善古陆。     

A missing link of the Proto-Tethys Ocean between the Qinling and Qilian orogens,China:Insights from geochronology and structural geology

The Qinling-Qilian connection zone(QQCZ) is a key area to reveal the relationship and to make a link of the North Qinling and the North Qilian orogens,China.Here we present U-Pb dating data of detrital zircons from four sedimentary/metasedimentary rocks in the QQCZ and the southwestern North China Block(NCB) and detailed regional structural data.Three episodes of fold deformation(D_1,D_2 and D3) are distinguished in the QQCZ,with the former two occurred during the early Paleozoic.The D_1 deformation is mainly characterized by regionally penetrative schistosity and some residual rootless intrafolial folds due to the intensive superpositions by the subsequent D_2 and D3 deformations.The D_2 deformation characterized by tight folds,associated axial plane foliations and crenulation lineations indicates a stress field characterized by NNE-SSW-directed compression,which may be induced by the collision between the NCB and the southern blocks.The D3 deformation which might occur during the Mesozoic is marked by upright open folds and kink bands.The similarity of the detrital zircon age spectra of the Huluhe Group in the North Qilian Orogen and the Erlangping Group in the North Qinling Orogen suggests that the two groups have similar provenance,which may indicate that the North Qilian Orogen corresponded to the North Qinling Orogen in a regional tectonic framework.In addition,the remarkable age peak at～435 Ma of the detrital zircon age spectrum of the Duanjiaxia Formation in the southwestern NCB indicates that this formation obtained the provenance of the North Qilian and North Qinling orogens,which may be generated by the collage of the southwestern NCB and the QQCZ during the Late Ordovician-Early Silurian.Based on structural,detrital zircon and metamorphic data,we suggest that the North Qilian and North Qinling orogens underwent similar evolution during the early Paleozoic due to the closure of the North Qilian and the Kuanping oceans which located at the northern boundary of the Proto-Tethys Ocean.     

Lower Paleozoic detrital zircon U-Pb geochronological characteristics of the Baoshan Block in western Yunnan and its constraints on the Proto-Tethys tectonic evolutionSCIEI北大核心CSCD

滇西保山地块是东特提斯构造域的主要微陆块之一,但对其物源和古地理位置仍存在较大争议。本文通过对保山地块西缘早古生代地层进行碎屑锆石U-Pb定年来约束其物源及古地理位置,并进一步探讨原特提斯洋早古生代构造演化模式。保山地块西缘早古生代地层具有相似的年龄分布模式,主年龄峰期为-0.95Ga、次级年龄峰期为-1.2Ga和-2.5Ga。寒武系公养河群最小锆石年龄为526Ma,结合其上部年龄为499.2Ma的火山岩夹层,约束其沉积时代为早寒武世早期。对比保山地块不同区域早古生代地层的碎屑锆石年龄数据,它们都具有相似的锆石年龄分布模式和年龄峰值。-0.95Ga主年龄峰期和-2.5Ga的次级年龄峰期指示保山地块早古生代的沉积物主要来自于印度大陆,而-1.2Ga的次级年龄峰期表明有部分沉积物来自于西澳大利亚,其早古生代古地理位置位于印度和西澳大利亚之间。结合沉积学证据及滇西地区广泛发育的早古生代岩浆作用,本文认为早古生代冈瓦纳大陆北缘为活动大陆边缘。     

兴蒙造山带南缘早古生代增生造山带内前寒武纪地层的识别及其地质意义

兴蒙造山带南缘早古生代增生造山带内是否存在陆壳一直缺少直接证据,制约了对增生带结构及演化的认识.在达茂旗东北部兴蒙造山带南缘增生带内识别出一套原地的变质地层,野外观察及锆石U-Pb年代学、Hf同位素地球化学研究表明,该套地层形成时代为中元古代晚期,可能代表了增生造山带内的古老地壳.地层的碎屑锆石峰值年龄集中于~1.8 Ga,并具有~3.0 Ga、~2.8 Ga、~2.5 Ga、~1.6 Ga等次级峰值,与华北北缘白云鄂博、化德、狼山群等地层碎屑锆石年龄组成一致;表明其应为华北陆块的一部分,物源则主要来自于华北克拉通内部不同时代地质体的剥蚀.该套地层的识别及其与华北陆块的亲缘性表明,早古生代兴蒙造山带南缘增生带内存在陆壳,构造环境上应为古亚洲洋向华北克拉通俯冲形成的活动陆缘;同时,其与车根达来构造带的关系显示后者并非蛇绿岩,更可能为弧后伸展引起的超基性岩对陆壳的侵入形成.      

Geochronology and geochemistry of metasedimentary rocks from the Dongnancha Formation in the Huadian area,central Jilin Province,Northeast (NE) China: Implications for the tectonic evolution of the eastern segment of the Paleo-Asian Ocean

To constrain the tectonic evolution of the eastern segment of the Paleo-Asian Ocean, we conducted zircon U–Pb-Hf dating and whole-rock geochemical analyses for metasedimentary rocks from the Dongnancha Formation in the Huadian area in central Jilin Province, Northeastern (NE) China. Most detrital zircons from the metasedimentary rocks display clear oscillatory zoning and striped absorption in cathodoluminescence (CL) images and have Th/U ratios of 0.1–1.8, thus indicating a magmatic origin. U–Pb isotopic dating using LA-ICP-MS method for zircon samples from the metasedimentary rocks reveals that the depositional age can be constrained to the period between 250 and 222 Ma. Geochemical data reveal low to intermediate degrees of weathering of the source material and compositionally low to intermediate maturity. Detailed analyses of detrital zircon U–Pb-Hf geochronology and geochemistry show that these metasedimentary rocks are derived from a bidirectional provenance. The predominant derivation is from Permian–Early Triassic felsic-intermediate igneous rocks of central Jilin Province and adjacent regions in the northern margin of the North China Craton, although felsic-intermediate igneous rocks and continental material in the eastern segment of the Central Asian Orogenic Belt from the Cambrian–Carboniferous represent additional sources and minor amounts of Paleoproterozoic–Neoproterozoic material have been input from the North China Craton. A number of geochemical indicators and tectonic discrimination diagrams collectively indicate a continental island arc-active continental margin setting for the deposition of the protoliths of the metasedimentary rocks. The results of geochemical and geochronological analyses of the provenance and tectonic setting of the metasedimentary rocks indicate that the Dongnancha Formation was likely deposited in an intermountain basin in a post-orogenic fast uplift setting, suggesting that the final closure of the eastern segment of the Paleo-Asian Ocean in the Huadian area of central Jinlin Province likely occurred between the Early Triassic and Middle Triassic.     

宗务隆构造带西段三叠系隆务河组碎屑锆石特征及其构造意义

宗务隆构造带位于柴北缘构造带与南祁连构造带之间,总体呈北西西向展布。构造带东段丰富的岩浆活动记录了该构造带晚古生代—中生代期间的裂解和闭合过程,而西段岩浆活动的记录较为稀少,对于其东、西段是否具有相同的构造演化尚不清晰。通过分析构造带西段三叠系隆务河组碎屑岩的地球化学特征、碎屑锆石U-Pb年龄及Hf同位素组成,认为隆务河组的碎屑沉积物的源区古风化程度轻微,不具备沉积再循环的特征,原岩主要为长英质岩石,南祁连新元古代花岗质片麻岩和早古生代大陆弧型花岗岩为隆务河组碎屑岩的主要物源;碎屑岩可能沉积于早中三叠世挠曲型盆地中;锆石U-Pb年龄分析表明宗务隆构造带东西段构造演化具有不同的历程,东段发育有限洋盆,而西段并未出现,转换地带可能位于生格至罗根郭勒之间。     

Tectonic affinity and significance of the Qilian Block: Evidence from river sediments in the Central Qilian Belt

The tectonic affinity of the Qilian Block is crucial to reconstructing the ocean-continent configuration of Rodinia. This study integrates previous studies with new whole-sample Nd and zircon U-Pb-Hf isotopic data from modern river sediments to identify the origin of the Qilian Block and its tectonic role in the Rodinia supercontinent cycle. Zircon U-Pb-Hf data in combination with whole-sample/whole-rock Nd and Pb isotopic data indicate that the Central Qilian Belt (CQB) shows close tectonic affinity with the Hualong and North Wulan terranes and together constituted a coherent Qilian Block crustal domain, whereas the Quanji Block likely underwent an independent evolution prior to the early Paleozoic orogeny. A comprehensive assessment of geochronological and whole-rock geochemical-isotopic data reveal that the Precambrian basement of the Qilian and Alxa blocks experienced similar historical evolutions and possibly formed a coherent Qaidam-Altyn-Qilian-Alxa continent before the breakup of Rodinia. Moreover, the Qaidam-Altyn-Qilian-Alxa continent was probably once part of the Yangtze Block, laying along the periphery of the supercontinent Rodinia. Therefore, the northwestern boundary of the Yangtze-affinity block can extend to the Alxa Block and the Qilian Ocean is a branch of the proto-Tethys Ocean. A micro-block can serve as a window into reconstructing the ocean-continent configurations of supercontinents during certain periods.     

Provenance and paleogeography of the Early Paleozoic basin in the northern Yangtze Block: Constraints from detrital zircon U-Pb-Hf data

The Proto-Tethys was a significant post-Rodinia breakup ocean that eventually vanished during the Paleozoic. The closure timing and amalgamation history of numerous microblocks within this ocean remain uncertain, while the Early Paleozoic strata on the northern margin of the Yangtze Block archive valuable information about the evolution of the Shangdan Ocean, the branch of the Proto-Tethys. By comparing the detrital zircon U-Pb-Hf isotopic data from Cambrian, Ordovician, and Silurian sedimentary rocks in the northern Yangtze Block with adjacent blocks, it was found that detrital zircons in Cambrian strata exhibit a prominent age peak at ∼ 900–700 Ma, which indicates that the primary source of clastic material in the basin was the uplifted inner and margin regions of the Yangtze Block. In the Silurian, abundant detrital material from the North Qinling Block was transported to the basin due to the continuous subduction and eventual closure of the Shangdan Ocean. This process led to two distinct age peaks at ∼500–400 Ma and ∼900–700 Ma, indicating a bidirectional provenance contribution from both the North Qinling Block and the Yangtze Block. This shift demonstrates that the initial collision between these two blocks occurred no later than the Silurian. The northern Yangtze Basin transitioned from a passive continental margin basin in the Cambrian to a peripheral foreland basin in the Silurian. Major blocks in East Asia, including South Tarim, North Qilian, North Qinling, and North Yangtze, underwent peripheral subduction and magmatic activity to varying degrees during the late Early Paleozoic, signifying the convergence and rapid contraction of microplates within northern Gondwana and the Proto-Tethys Ocean. These findings provide new insights on the tectonic evolution of the Proto-Tethys Ocean.     

秦-祁构造结合部位陇山岩群中碎屑锆石年代学研究及其地质意义

秦-祁构造结合部位以新阳-元龙大型剪切带为界,北侧的北祁连造山带和南侧的西秦岭造山带的构造线呈斜截关系,致使两侧岩石单元的对比存在困难。陇山岩群位于秦-祁结合部位的北祁连构造单元东端,是一套岩性复杂的中深程度变质杂岩,其形成年代和构造属性一直存在较大的争论。本文重点以陇山岩群中黑云母石英片岩为研究对象,对其中的碎屑锆石进行LA-ICP-MS U-Pb测年。其测年结果明显分为4组,两个主要区间为1 097～795 Ma （49%,峰期年龄约为929 Ma）和2 713～2 265 Ma （21.5%,峰期年龄约为2 435 Ma）,另有两个次级年龄组为575～471 Ma （12%,峰期年龄约为541 Ma）与1 864～1 539 Ma （14%,峰期年龄约为1 717 Ma）。通过最小岩浆峰期年龄和陇山岩群内侵入体的最老年龄初步限定其形成时代介于寒武纪早期-早-中奥陶世（539～454 Ma）,与已报道的晚太古代-早元古代陇山岩群TTG片麻岩形成于不同时代,不属于华北板块南缘基底的沉积岩系。通过与周边构造单元年龄特征峰值的对比研究发现,其新太古代-古元古代（2 713～2 265 Ma）和中元古代（1 864～1 539 Ma）的碎屑物质主要来自华北板块南缘基底,新元古代（1 097～7 95 Ma）的碎屑物质主要来自于北秦岭造山带和中祁连陆块,早古生代（575～471 Ma）的碎屑物质来自于天水-武山构造带,与该洋盆形成过程有关。因此,陇山岩群中黑云母石英片岩的碎屑物源既包括北侧的华北板块南缘基底,又包括南侧的秦岭-祁连造山带,可能形成于早古生代红土堡弧后盆地的形成扩展阶段,与北秦岭东段宽坪岩群副变质岩和二郎坪岩群变沉积岩形成构造环境相似。     

Jurassic detrital zircon U–Pb and Hf isotopic geochronology of Luxi Uplift,eastern North China,and its provenance implications for tectonic–paleogeographic reconstruction

Relatively successive sequences of Late Mesozoic are preserved and exposed in Luxi Uplift (LU), eastern North China block (NCB), which is an important region to study the late Mesozoic tectonic evolution of the eastern NCB. In this study, in situ U–Pb ages and Hf isotopic analyses on detrital zircons from the sandstones of Jurassic Fangzi and Santai Formations in LU combining the analysis of sandstone detrital modes were performed, with an aim to trace the Jurassic sediment provenances and the tectonic–paleogeographic configuration of eastern NCB. Three sandstone samples (one from Fangzi Formation and two from Santai Formation) have very similar U–Pb age spectrums which can be divided into three major groups: Phanerozoic (I), Paleoproterozoic (II), and Neoarchean (III). Detrital zircons of Group II and Group III broadly match the age spectra of the basement of NCC which exposed extensively in the northern part. No middle Neoproterozoic magmatic zircons or Triassic metamorphic zircons were found in this study, ruling out the clastic provenance transported from the Sulu orogen to LU. Dominant zircon populations of Group Iare Late Paleozoic (250–393 Ma) recording the corresponding magmatic activities which are not found both in LU and its peripheral tectonic terranes, but can be well compared with that of the northern NCB (NNCB) and the Xing-Meng Orogenic Belt (XMOB). Furthermore, Hf isotope compositions of the Phanerozoic detrital zircons can be distinctly divided into two clusters with ε_Hf(t) values ranging from −1.0 to +12.7 and −21.9 to −3.0, respectively resemble those from the XMOB and NCB (mainly from NNCB). Sandstone detrital modes analysis indicates the provenance came from the areas that have been eroded deeply to expose the basement rocks which accords with the tectonic setting of the NNCB. This research proposes that an evident mountain or provenance region once increasingly developed along NNCB during Early to Late Jurassic (182–155 Ma) due to the continuous collision of the Siberia and North China–Mongolian plates, easily shed mass clastic materials southward into the inner NCB and became the major provenance of Jurassic sediments in LU.     

Detrital zircon U-Pb-Hf isotopes and provenance of Late Neoproterozoic and Early Paleozoic sediments of the Simao and Baoshan blocks,SW China: Implications for Proto-Tethys and Paleo-Tethys evolution and Gondwana reconstruction

Early Paleozoic evolution of the northern Gondwana margin is interpreted from integrated in situ U-Pb and Hf-isotope analyses on detrital zircons that constrain depositional ages and provenance of the Lancang Group, previously assigned to the Simao Block, and the Mengtong and Mengdingjie groups of the Baoshan Block. A meta-felsic volcanic rock from the Mengtong Group yields a weighted mean ²⁰⁶Pb/²³⁸U age of 462 ± 2 Ma. The depositional age for the previously inferred Neoproterozoic Lancang and Mengtong groups is re-interpreted as Early Paleozoic based on youngest detrital zircons and meta-volcanic age. Detrital U-Pb zircon analyses from the Baoshan Block define three distinctive age peaks at older Grenvillian (1200–1060 Ma), younger Grenvillian (~ 960 Ma) and Pan-African (650–500 Ma), with εHf(t) values for each group similar to coeval detrital zircons from western Australia and northern India. This suggests that the Baoshan Block was situated in the transitional zone between northeast Greater India and northwest Australia on the Gondwana margin and received detritus from both these cratons. The Lancang Group yields a very similar detrital zircon age spectrum to that of the Baoshan Block but contrasts with that for the Simao Block. This suggests that the Lancang Group is underlain by a separate Lancang Block. Similar detrital zircon age spectra suggest that the Baoshan Block and the Lancang Block share common sources and that they were situated close to one another along the northern margin of East Gondwana during the Early Paleozoic. The new detrital zircon data in combination with previously published data for East Gondwana margin blocks suggests the Early Paleozoic Proto-Tethys represents a narrow ocean basin separating an “Asian Hun superterrane” (North China, South China, Tarim, Indochina and North Qiangtang blocks) from the northern margin of Gondwana during the Late Neoproterozoic-Early Paleozoic. The Proto-Tethys closed in the Silurian at ca. 440–420 Ma when this “Asian Hun superterrane” collided with the northern Gondwana margin. Subsequently, the Lancang Block is interpreted to have separated from the Baoshan Block during the Early Devonian when the Paleo-Tethys opened as a back-arc basin.     

辽东岫岩地区下古生界碎屑锆石定年:罗迪尼亚和冈瓦纳超大陆在华北遗留的痕迹？

超大陆演化是地质研究的重要内容,华北克拉通与不同地质历史时期超大陆汇聚与裂解的联系对反演华北克拉通构造演化历史具有重要意义。本文在辽吉古元古代造山带中段的原划分为辽河群地层中首次识别出一套早古生代沉积建造。这套沉积建造与华北克拉通以浅海相的碳酸盐岩为主的早古生代沉积并不一致,以发育大量的陆源碎屑沉积为特征。我们对2件细砂岩分别进行了锆石LA-ICP-MS和SHRIMP U-Pb定年。定年结果显示,2件样品的最小年龄分别为～482Ma和～498Ma,反映了它们的最大沉积时代。2件样品的碎屑锆石年龄主要介于1600～500Ma,缺乏亲华北的物源信息,表明它们的物源主要来自于华北之外。2件样品年龄谱中最重要的峰值出现在格林威尔期和泛非期,表明华北克拉通曾与罗迪尼亚超大陆和冈瓦纳大陆存在联系。格林威尔期碎屑锆石可能来自于罗迪尼亚超大陆时期波罗地古陆物质在华北克拉通东缘的再循环;泛非期碎屑锆石可能来自于东冈瓦纳大陆的北缘造山带。