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。     

鄂尔多斯盆地南缘中晚三叠世物源演变及其地质意义*

针对鄂尔多斯盆地南缘中晚三叠世物源构成转化及盆山耦合机制不清的问题,选取了铜川地区金锁关剖面和周至柳叶河剖面的延长组砂岩为研究对象,运用岩石学、碎屑锆石U-Pb年代学、地球化学的方法,探讨鄂尔多斯盆地南部延长组的物源构成和变化,并探寻其构造耦合机制。结果表明,金锁关剖面上三叠统碎屑锆石可分为5个年龄段,分别是237-330 Ma、390-480 Ma、870-1230 Ma、1740-1980 Ma、2070-2732 Ma,中三叠统碎屑锆石共具有4个年龄段,分别是240-290 Ma、1760-1840 Ma、2250-2300 Ma、2350-2700 Ma;盆地南端柳叶河地区上三叠统碎屑锆石共具有5个年龄段,分别是244-310 Ma、360-600 Ma、800-1300 Ma、1700-2100 Ma和2450-2550 Ma。通过物源对比发现,中三叠世鄂尔多斯盆地南缘的物源来自于华北克拉通、兴蒙造山带和阿拉善地区,晚三叠世沉积砂体的物源来自华北克拉通、阿拉善、兴蒙造山带、西秦岭、北秦岭以及祁连造山带,且岩石学特征和源区构造背景的转变均支持这一认识。这种物源的转变,与中三叠世盆地南部秦岭造山带的活化以及盆地样式的转变有关。     

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。     

鄂尔多斯盆地西、南缘奥陶纪地质事件群耦合作用

北祁连造山带和北秦岭造山带在早古生代经历了相似的洋陆转化过程,于奥陶纪时发育了汇聚板块边缘的沟-弧-盆体系,分别形成了北西向展布的北祁连造山带走廊南山北缘早古生代岛弧及弧后盆地和东西向展布的北秦岭造山带早古生代岛弧及弧后盆地。期间,秦岭-祁连洋的俯冲造山作用和鄂尔多斯盆地西南缘沉积类型和内陆湖盆的发展演化之间存在有机的联系,构成了盆-山耦合体系,引发一系列构造事件、火山喷发事件和多种类型的事件沉积等。它们之间存在着一系列成因机制上的联系,有着共同的宏观背景。鄂尔多斯盆地西、南缘在几乎相同时期存在一次构造背景的转变,由被动大陆边缘转化为主动大陆边缘,并诱发了多期火山喷发事件,在盆地西南缘奥陶系形成多套斑脱岩夹层,这些斑脱岩可能为同时期或者稍后的钾盐矿(包括含钾卤水)的形成提供了重要物源。同时,鄂尔多斯盆地南缘由浅水碳酸盐台地陷落为深水斜坡,在盆地西、南缘奥陶系有规律的集中发育重力流沉积(海底扇、浊积岩等)、滑塌沉积和震积岩等事件沉积。从形成机制上,华南板块向北俯冲触发了火山活动和地震,火山喷发在奥陶系集中沉积了多套凝灰岩夹层,地震活动导致同时期大套重力流沉积,并触发相对深水区沉积物向深水区移动,使得重力流沉积转化为浊流沉积,形成了具有良好储层的浊积岩。统计表明,上述事件发育的时间与秦岭地区构造活动相对最活跃的时期基本一致。因此这些分布稳定的凝灰岩薄层和中奥陶世集中有规律分布的重力流沉积砂体为华南板块向华北本快俯冲背景下形成的,它们之间存在耦合关系。     

Petrography and Geochemistry of the Upper Triassic Sandstones from the Western Ordos Basin,NW China: Provenance and Tectonic Implications

Petrographic and geochemical characteristics of the Upper Triassic sandstones in the western Ordos Basin were studied to provide insight into weathering characteristics, provenance, and tectonic implications. Petrographic features show that the sandstones are characterized by low-medium compositional maturity and textural maturity. The CIA and CIW values reveal weak and moderate weathering history in the source area. The geochemical characteristics together with palaeocurrent data show that the northwestern sediments were mainly derived from the Alxa Block with a typical recycled nature, while the provenance of the western and southwestern sediments were mainly from the Qinling-Qilian Orogenic Belt. The tectonic setting discrimination diagrams signify that the parent rocks of sandstones in the western and southern Ordos Basin were mainly developed from continental island arc, which is closely related to the evolution of the Qinling-Qilian Orogenic Belt. However, the sandstones in the northwestern Ordos Basin show complex features, which may be resulted from a typical recycling process. Overall evidence from petrography, geochemistry and sedimentology, together with previous researches suggest the Kongtongshan and Helanshan areas were the southwestern and northwestern boundary of the Ordos Basin, respectively, and there was no clear boundary between the Hexi Corridor Belt and Ordos Basin, where a large, uniform sediment dispersal system developed during the Late Triassic.     

鄂尔多斯盆地中—晚三叠世盆地原型及构造古地理响应

中—晚三叠世的鄂尔多斯盆地沉积了一套优质的砂岩储层,目前对该套砂岩的成因及其空间分布已经有了成熟的认识,然而在中—晚三叠世鄂尔多斯原型盆地的确切边界位置、盆内古地理演化的构造成因机制等问题上依然存在不少争议。本文通过对鄂尔多斯盆内及周缘57个露头及165口钻井的层序地层学与沉积学研究,厘定了鄂尔多斯盆地在中—晚三叠世的边界位置并在层序格架下开展了古地理演化研究,同时探讨了古地理演化的构造成因。研究表明:中—晚三叠世鄂尔多斯盆地的北部边界从内蒙古的达拉特旗向东延伸至山西大同,东部边界应在山西宁武—太原—太谷—永和—河南安阳—开封—登封一线附近,南部边界为北秦岭(NQT)与华北板块的缝合带(陕西西安—洛南—河南栾川—南召沿线以南),西南以六盘山的西部断层边缘为界,西北界位于贺兰山西部断层边缘带,西部边界延伸至河西走廊盆地的西部边界(甘肃马良沟附近)。在中—上三叠统延长组识别出4个沉积旋回(SQ1-SQ4),代表了从起始阶段(SQ1)到最大沉降阶段(SQ2和SQ3)再到后期关闭阶段(SQ4)的湖泊演化过程。中—晚三叠世鄂尔多斯盆地内呈现出北部/东北部的曲流河-三角洲沉积体系和南部/西南部冲积扇-辫状河-三角洲沉积体系汇聚的沉积格局,在空间上表现出明显的南北差异,在时间上呈现出沉积中心的东-西破坏分异的演化特征。这种古地貌差异和演化主要受控于秦岭—大别山造山带(QDOB)与兴安岭—蒙古造山带(XMOB)的不同构造演化过程。盆地南部的古地理演化主要受控于QDOB的活动,中—晚三叠世勉略洋闭合驱动的北秦岭造山带活化不仅导致盆地南部陡坡带的形成和盆地东南部古地貌的突变,也导致晚三叠世盆地西南部发育一个分隔内克拉通盆地及西南缘类前陆盆地的水下低隆。盆地西部的古地貌演化受控于多种构造机制,中三叠世现今六盘山地区发育一个南北向的低隆区,很可能是盆地东南部的强烈挤压下的远端效应;晚三叠世中期后该低隆区发生下沉,现今鄂尔多斯盆地与河西走廊地区连通,这很可能是由盆地西南方特提斯构造域挤压作用下的盆内挠曲沉降导致的。这些认识不仅是对盆山耦合理论的补充,也对鄂尔多斯盆地石油和天然气的后期勘探具有重要的现实意义。     

河南南召盆地上三叠统碎屑岩地球化学特征及物源示踪

三叠纪是秦岭造山带全面碰撞造山的关键时期,随着扬子、秦岭和华北板块分别沿勉略、商丹缝合带的汇聚拼合, 秦岭造山带逐渐形成并从板块构造体制向陆内造山体制转化,同时强烈的造山作用控制着周缘盆地的形成与演化。文章通 过研究区的碎屑岩元素地球化学分析,对河南南召盆地上三叠统的物源区及构造背景特征进行探讨。结果表明,上三叠统 源岩成分主要为上地壳长英质火山岩;源岩经历了中等的化学风化强度,校正后CIA值指示其形成于温暖潮湿的气候和相 对较强的构造活动环境;太山庙组源区构造背景主要为大陆岛弧与活动大陆边缘,太子山组源区构造背景主要为大陆岛弧 与被动大陆边缘。根据南召盆地近源沉积特征和秦岭造山带构造演化过程推断,秦岭造山带和华北南缘是南召盆地晚三叠 世的重要物源区,前期太山庙组物源主要由北秦岭隆升基底提供,后期太子山组物源可能来自南秦岭、北秦岭和华北南缘 沉积再循环。南召盆地上三叠统物源区的转变是晚三叠世秦岭造山带逆冲推覆作用逐渐增强的体现,对研究恢复秦岭构造 带造山隆升过程和周缘盆地盆山系统演化具有重要的意义。     

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.     

Early Mesozoic basin development in North China: Indications of cratonic deformation

We integrated a systematic sedimentary data into a regional Early Mesozoic stratigraphic framework which demonstrated a detailed picture of spatiotemporal variations in basin deposition and formation in the North China Craton. The Early Mesozoic basin sedimentary evolution is utilized to interpret polyphase tectonism and to unravel the craton deformation. The Late Triassic, nearly WNW-trending, giant intracratonic Ordos basin was widely distributed across most of North China Craton, with a southern wedge-top depozone along the northern East Qilian–Qinling orogenic belt and a northwestern rift depozone along the Helanshan. The continuous subsidence and deposition within the basin were dominantly related to the thrust load of the East Qilian–Qinling belt and inferred mantle flow effects associated with paleotethys plate subduction, and the rift in the northwestern Ordos was driven by nearly north-vergent compression of the eastern North Qilian–North Qinling active margins with the stable North China Craton. This intracratonic Ordos basin formation initiated the deformation of the North China Craton. Formation of the Jurassic NNE-trending walled intracratonic Ordos basin and the broken flexural basins indicates the North China Craton underwent the second, even more abroad nearly NNE trending crustal deformation, with lithosphere thickening in the eastern part of the North China Craton, and dynamic subsidence in the west, which may have been driven by nearly northwestward subduction of the Izanagi plate and the eastward extrusion and underthrusting of the western North China Craton crustal basement.     

内蒙古阿拉善西南缘早前寒武纪地质研究进展

阿拉善地块西南缘的早前寒武纪变质基底主要分布在内蒙古阿拉善右旗与甘肃河西走廊相交界的龙首山—北大山地区,在大地构造上该区是华北克拉通、塔里木克拉通和祁连造山带3个地体的交汇处,因此长期以来备受关注。笔者综合了近十年来中外地质学家对阿拉善地块早前寒武纪地质的研究成果,对已取得的资料成果进行分析整理,从岩石单元划分、年代学、构造边界及大地构造归属4个方面进行论述,总结了近年来对以龙首山岩群为代表的阿拉善地块西南缘早前寒武纪变质基底的研究进展,提出目前争议性较大的问题,并探讨今后进一步研究的方向。     

青藏高原东北缘祁连山三叠系砂岩碎屑锆石U-Pb定年及其物源分析

三叠系沉积物广泛覆盖青藏高原东北缘,其中松潘—甘孜地区三叠系的沉积物得到了较系统的研究,但是青藏高原北缘的祁连山三叠系盆地的研究却较为缺乏。为了丰富相关研究和揭示区域构造演化的特点,通过古水流方向统计、砂岩中碎屑矿物统计和碎屑锆石U-Pb测年等方法对祁连山三叠纪盆地物源进行系统研究。结果表明,祁连山三叠系盆地的古流向主要有南东向、正南向、南西向,物源来自岩浆弧和大规模褶皱造山作用的混合区。祁连山三叠系砂岩中的碎屑锆石的年龄谱主要峰值集中在250~290 Ma、360~460 Ma、1 600~2 000 Ma和2 200~2 600 Ma这4个年龄段。通过对比分析华北板块、华南板块中和秦祁昆中央造山带中岩浆锆石年龄谱特征可知:1 600~2 000 Ma和2 200~2 600 Ma年龄段的锆石来自华北板块,360~460 Ma年龄段的锆石来自北祁连造山带,250~290 Ma年龄段的锆石来自东昆仑的火山岛弧。此外,600~1 000 Ma年龄段锆石很少,这些锆石来自扬子板块,表明在三叠纪扬子克拉通和华北克拉通发生碰撞形成了秦岭造山带,阻断了来自扬子克拉通的物源。     

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。     

Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework,kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt

Based mainly on field geological observation and geochronologic data, six tectonic units have been recognized in western Inner Mongolia (China), including, from south to north: North China Craton (NCC), Southern Orogenic Belt (SOB), Hunshandake Block (HB), Northern Orogenic Belt (NOB), South Mongolia microcontinent (SMM), and Southern margin of Ergun Block (SME), suggesting that the tectonic framework of the CAOB in western Inner Mongolia is characterized by an accretion of different blocks and orogenic belts. The SOB includes, from north to south, fold belt, mélange, arc-pluton belt, and retroarc foreland basin, representing a southern subduction–collision system between the NCC and HB blocks during 500–440 Ma. The NOB consists also of four units: arc-pluton belt, mélange, foreland molasse basin, and fold belt, from north to south, representing a northern subduction–collision system between the HB and SMM blocks during 500–380 Ma. From the early Paleozoic, the Paleo-Asian oceanic domains subducted to the north and the south, resulting in the forming of the SOB and the NOB in 410 Ma and 380 Ma, respectively. This convergent orogenic system, therefore, constrained the consumption process of the Paleo-Asian Ocean in western Inner Mongolia. A double subduction–collision accretionary process is the dominant geodynamic feature for the eastern part of the CAOB during the early to middle Paleozoic.     

秦岭-大别新元古代-中生代沉积盆地演化

秦岭-大别造山带处于中央造山带的东部,经历了复杂的构造-沉积历史.在系统分析研究区4个二级和13个三级构造单元岩石地层、化石组合、同位素年代学及构造学等资料的基础上,划分出18个沉积盆地类型,并讨论新元古代-中生代构造-沉积演化:(1)新元古代-早古生代:商丹洋以北的北秦岭为岩浆弧和弧前盆地;南秦岭为陆内裂谷-台盆、台地-陆缘裂谷发育阶段;大别-苏鲁为陆内裂谷-台盆台地发育阶段;(2)晚古生代:北秦岭为海陆交互陆表海;勉略洋于泥盆纪开启;南秦岭为弧后陆棚与台盆台地并存发育阶段;(3)三叠纪:陆陆碰撞造山,全区进入前陆盆地发育阶段;(4)侏罗纪-白垩纪:断陷盆地和压陷盆地发育阶段.      

鄂尔多斯盆地西南部上三叠统延长组碎屑锆石U-Pb定年及其物源意义

由于构造位置的特殊性和勘探开发工作的局限性,鄂尔多斯盆地西南部上三叠统延长组6段的物源分析一直存在较大的分歧。本文选取鄂尔多斯盆地西南部里78井、庄33井、演40井以及汭水河地区上三叠统长6段砂岩为研究对象,运用LA-ICP-MS U-Pb方法对样品中的碎屑锆石进行了年代学研究。结果表明,4个样品都存在250~350 Ma、400~660 Ma、920~1200 Ma、1750~1950 Ma和2200~2600 Ma 5个基本相近的年龄区间,与之对应的峰值年龄主要集中在284~330 Ma、430~460 Ma、950~1000 Ma、1850~1915 Ma以及2250~2450Ma。通过与邻区碎屑锆石年龄分布特征对比并结合重矿物资料,对研究区长6段物源的方向、母岩时代及岩性进行了系统全面的分析,认为研究区物源具有多方向、多时期、多岩石类型的特点,主要来自于盆地西南缘、南缘分布的北秦岭西段、北祁连东段及西秦岭的加里东期岩浆岩、晋宁期岩体和位于盆地东北部的古元古代孔兹岩系,盆地西北缘阿拉善地块的华力西期岩浆岩、鄂尔多斯地块东南部的变质基底也提供了部分物源。这一研究结果将有助于进行原盆沉积环境的恢复和指导下一步油气勘探的部署。     

Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin,North China: Evidences for multi-cycle sedimentation

The western Ordos Basin(WOB), situated in a tectonic transition zone in the North China Craton, acts as an excellent example for studying the Mesozoic intraplate sedimentation and deformation in Asia. In this study, U-Pb ages for 1203 detrital zircons of 14 sandstone samples collected from 11 sections are presented to unravel the sediment source locations and paleogeographic environments of the Early-Middle Jurassic coal-bearing Yan'an Formation in the WOB. Data show that there are five prominent age groups in the detrital zircons of the Yan'an Formation, peaking at ca. 282 Ma, 426 Ma, 924 Ma, 1847 Ma, and2468 Ma. Samples from the northern, middle, and southern parts of the WOB contain these five age categories in various proportions. In the northern region, the Yan'an Formation exclusively contains Early Permian detrital zircons with a single age group peaking at 282 Ma, matching well with the crystallizing ages of the widespread Early Permian granites in the Yinshan Belt to the north and the Alxa Block to the northwest. While in the southern region, the Yan'an Formation mainly contains three groups of detrital zircons, with age peaks at 213 Ma, 426 Ma, and 924 Ma. These zircon ages resemble those of the igneous rocks in the Qilian-Qinling Orogenic Belt to the south-southwest. Samples in the middle region, characterized by a mixture age spectrum with peaks at 282 Ma, 426 Ma, 924 Ma, 1847 Ma and 2468 Ma, are previously thought to have mixed derivations from surrounding ranges. However, by referring to the detrital-zircon age compositions of the pre-Jurassic sedimentary successions and combining with paleontological and petrographic analysis, we firstly propose that the sediments of the Yan'an Formation in the middle region were partly recycled from the Triassic and Paleozoic sedimentary strata in the WOB.The occurrence of recycled sedimentation suggests that the Late Triassic-Early Jurassic intraplate compressional deformation was very intense in the WOB, especially for regions in front of the Qilian Orogenic Belt.     

Source-to-sink of Late carboniferous Ordos Basin: Constraints on crustal accretion margins converting to orogenic belts bounding the North China Block

The Upper Carboniferous Benxi Formation of the Ordos Basin is the lowest strata overlying Middle Ordovician above the major ca. 150-Myr sedimentary gap that characterizes the entire North China Block (NCB). We apply an integrated analysis of stratigraphy, petrography, and U–Pb dates and Hf isotopes on detrital zircons to investigate its provenance and relationships to the progressive collisions that formed the Xing’an-Mongolia Orogenic Belt to the north and the Qinling Orogenic Belt to the south. The results show that, in addition to regional patterns of siliciclastic influx from these new uplifted sources, the Benxi Formation is composed of two sequences corresponding to long-term glacial-interglacial cycles during the Moscovian to lower Gzhelian stages which drove global changes of eustatic sea level and weathering. The spatio-temporal distribution of sediment isopachs and facies indicate there were two sediment-infilling pulses, during which the southern and the northern Ordos Basin developed tidal-reworked deltas. The age spectra from detrital zircons, trace element patterns and ε_Hf(t) values reveal that the siliciclastics forming the southern delta was sourced in the Qinling Orogenic Belt, whereas the northern delta was derived from the Xing’an-Mongolia Orogenic Belt. The source-to-sink evolution of this Upper Paleozoic system records the progressive development of orogenic belts and uplifts forming on the southern and northern margins of the NCB prior to its collisions with the South China and the Siberian plates, respectively.     

鄂尔多斯盆地西南缘下-中侏罗统碎屑锆石U-Pb年代学及其地质意义

改造盆地主要发育时期原始沉积边界的厘定具有重要的油气地质和区域构造意义。物源分析是厘定盆地原始沉积边界的重要内容和有效手段。对鄂尔多斯盆地西南缘邻区下-中侏罗统砂岩碎屑锆石LA-ICP-MS U-Pb年龄谱的研究,为厘定盆地西南沉积边界提供了新的依据。盆地西南缘六盘山地区上流水和炭山剖面的下-中侏罗统延安组砂岩碎屑锆石年龄谱较为相似,而与靖远地区红会四矿剖面同期地层的碎屑锆石年龄谱相差较大。将这三区碎屑锆石的年龄谱与周邻构造单元结晶岩体的年龄对比表明,早-中侏罗世期间靖远地区沉积物来源于西南侧的祁连造山带,而六盘山地区沉积物来源于西北侧阿拉善地块和北侧的华北克拉通北缘。对物源、地层厚度、岩性、煤层特征等差异的综合研究认为,这两个地区在早-中侏罗世期间属不同沉积域,鄂尔多斯盆地的西南沉积边界为海原断裂带。     

Decoding Provenance and Tectonothermal Events by Detrital Zircon Fission‐Track and U‐Pb Double Dating: A Case of the Southern Ordos Basin

Multi-dating on the same detrital grains allows for determining multiple different geo-thermochronological ages simultaneously and thus could provide more details about regional tectonics. In this paper, we carried out detrital zircon fission-track and U-Pb double dating on the Permian-Middle Triassic sediments from the southern Ordos Basin to decipher the tectonic information archived in the sediments of intracratonic basins. The detrital zircon U-Pb ages and fission-track ages, together with lag time analyses, indicate that the Permian-Middle Triassic sediments in the southern Ordos Basin are characterized by multiple provenances. The crystalline basement of the North China Craton (NCC) and recycled materials from pre-Permian sediments that were ultimately sourced from the basement of the NCC are the primary provenance, while the Permian magmatites in the northern margin of NCC and Early Paleozoic crystalline rocks in Qinling Orogenic Collage act as minor provenance. In addition, the detrital zircon fission-track age peaks reveal four major tectonothermal events, including the Late Triassic-Early Jurassic post-depositional tectonothermal event and three other tectonothermal events associated with source terrains. The Late Triassic-Early Jurassic (225–179 Ma) tectonothermal event was closely related to the upwelling of deep material and energy beneath the southwestern Ordos Basin due to the coeval northward subduction of the Yangze Block and the following collision of the Yangze Block and the NCC. The Mid-Late Permian (275–263 Ma) tectonothermal event was associated with coeval denudation in the northern part of the NCC and North Qinling terrane, resulting from the subduction of the Paleo-Asian Ocean and Tethys Ocean toward the NCC. The Late Devonian-early Late Carboniferous (348±33 Ma) tectonothermal event corresponded the long-term denudation in the hinterland and periphery of the NCC because of the arc-continent collisions in the northern and southern margins of the NCC. The Late Neoproterozoic (813–565 Ma) tectonothermal event was associated with formation of the Great Unconformity within the NCC and may be causally related to the Rodinia supercontinent breakup driven by a large-scale mantle upwelling.     

Mesozoic intracontinental orogeny in the Qinling Mountains,central China

The Qinling Orogenic belt has been well documented that it was formed by multiple steps of convergence and subsequent collision between the North China and South China Blocks during Paleozoic and Late Triassic times. Following the collision in Late Triassic times, the whole range evolved into an intracontinental tectonic process. The geological, geophysical and geochronological data suggest that the intracontinental tectonic evolutionary history of the Qinling Orogenic Belt allow deduce three stages including strike-slip faulting during Early Jurrassic, N-S compressional deformation during Late Jurassic to Early Cretaceous and orogenic collapse during Late Cretaceous to Paleogene. The strike-slip faulting and the infills in Early Jurassic along some major boundary faults show flower structures and pull-apart basins, related to the continued compression after Late Triassic collision between the South Qinling Belt and the South China Block along the Mianlue suture. Late Jurassic to Early Cretaceous large scale of N-S compression and overthrusting progressed outwards from inner of Qinling Orogen to the North China Block and South China Block, due to the renewed southward intracontinental subduction of the North China Block beneath the Qinling Orogenic Belt and continuously northward subduction of the South China Block, respectively. After the Late Jurassic-Early Cretaceous compression and denudation, the Qinling Orogenic Belt evolved into Late Cretaceous to Paleogene orogen collapse and depression, and formed many large fault basins along the major faults.