大别—苏鲁区残余超高压构造及其动力学意义

根据构造学及岩石学研究并结合已有同位素年代学资料地质意义分析证明, 现在观察到的大别-苏鲁超高压变质带的区域构造框架, 主要是在三叠纪中朝与扬子克拉通碰撞期后(200~170 Ma) 伸展体制和角闪岩相条件下形成的. 残余的同碰撞及超高压构造只保留在较大的榴辉岩及其他超高压变质岩体内部. 以湖北省麻城县乘马岗和何家湾、安徽省岳西县碧溪岭及江苏省东海等地区的榴辉岩为例, 分析描述了具弱面理的块状榴辉岩、超高压剪切带和A型褶皱等残余超高压构造. 强调指出, 只是这些早期的构造记录才反映大陆碰撞阶段的构造环境及相对运动学特征. 据此提出一个运动学模型, 表示三叠纪(240~210 Ma) 扬子克拉通向北俯冲于中朝克拉通之下并与其发生斜向碰撞的动力学格局.      

Kanfenggou UHP Metamorphic Fragment in Eastern Qinling Orogen and Its Relationship to Dabie-Sulu UHP and HP Metamorphic Belts, Central China

In the Central Orogenic Belt, China, two UHP metamorphic belts are discriminated mainly based on a detailed structural analysis of the Kanfenggou UHP metamorphic fragment exposed in the eastern Qinling orogen, and together with previous regional structural, petrological and geochronological data at the scale of the orogenic domain. The first one corresponds to the South Altun-North QaidamNorth Qinling UHP metarnorphic belt. The other is the Dabie-Sulu UHP and HP metamorphic belts. The two UHP metamorphic belts are separated by a series of tectonic slices composed by the Qiniing rock group, Danfeng rock group and Liuling or Foziling rock group etc. respectively, and are different in age of the peak UHP metamorphism and geodynamic implications for continental deep subduction and collision. Regional field and petrological relationships suggest that the Kanfenggou UHP metamorphic fragment that contains a large volume of the coesite- and microdiamond-bearing eclogite lenses is compatible with the structures recognized in the South Altun and North Qaidam UHP metamorphic fragments exposed in the western part of China, thereby forming a large UHP metamorphic belt up to 1000 km long along the orogen strike. This UHP metamorphic belt represents an intercontinental deep subduction and collision belt between the Yangtze and Sino-Korean cratons, occurred during the Paleozoic. On the other hand, the well-constrained Dabie-Sulu UHP and HP metamorphic belts occurred mainly during Triassic time (250-220 Ma), and were produced by the intracontinental deep subduction and collision within the Yangtze craton. The Kanfenggou UHP metamorphic fragment does not appear to link with the DabieSulu UHP and HP metamorphic belts along the orogen. There is no reason to assume the two UHP metamorphic belts as a single giant deep subduction and collision zone in the Central Orogenic Belt situated between the Yangtze and Sino-Korean cratons. Therefore, any dynamic model for the orogen must ac-count for the development of UHP metarnorphic rocks belonging to the separate two tectonic belts of different age and tectono-metamorphic history.     

大别山碧溪岭地区超高压变质岩构造分析

大比例尺 (1∶10 0 0 0 )构造制图及构造分析表明 ,碧溪岭地区超高压变质岩石含有丰富的构造演化历史记录。同碰撞或挤压组构只保留于榴辉岩及其它超高压变质岩透镜体内部 ,表现为高角度网络状超高压剪切带与弱应变透镜体域规律组合格式。前者由面理或糜棱岩化榴辉岩组成 ,后者由块状榴辉岩及石榴橄榄岩组成。碰撞期后伸展构造表现为区域性的假单斜状 ,内部呈低缓角度的网络状强应变带及所环绕的透镜状弱应变域组合格式 ,强应变带的岩石为由榴辉岩退变成的角闪岩相高压片麻岩及部分熔融形成的含榴花岗岩 ,透镜状弱应变域的岩石为弱角闪相改造的榴辉岩及石榴橄榄岩。不同尺度上同碰撞或挤压组构及碰撞期后伸展组构所显示的这种残斑基质流变学结构样式 ,虽然与先期原岩成分、结构、流变学的不均一性有关 ,但主要是多期递进应变分解作用的结果 ,支持榴辉岩“原地”成因模式。依据构造学证据和可利用的岩石学及同位素年代学资料 ,分析了超高压变质岩石的形成及折返过程 ,指出碧溪岭地区超高压变质岩石是在 2 45～ 2 10Ma形成的 ,碰撞期后伸展作用主要发生在 2 0 0～ 170Ma。在超高压变质岩石向地壳表层折返过程中 ,张扭作用可能有重要功能 ,不支持碧溪岭地区遭受过多期超高压变质作用的推论。     

桐柏-大别-苏鲁UHP和HP变质带的结构及流变学演化

在岩石圈流变学基本原理指导下，运用现代构造解析学方法，在不同尺度上差别和分析了桐柏－大别－苏鲁UHP和HP变质带内深俯冲，同碰撞构造及UHP和HP岩石折返过程中的变形特征，重点讨论同碰撞形成的高角度网结状榴辉岩切带阵列，高角闪岩相剪切及有关变形组合以及碰撞期后伸展韧性薄化变形样式，强调指出不同地壳层次和物理条件下变形分解作用的重要性，而且，在UHP和HP变质带内最有效的应变体制是剪切作用，并在三维空间上形成不同格式的剪切带网状系统，以构造学记录为主线，结合已有可利用的岩石学，变质作用pT轨迹和同位素年代学资料，提出一个UHP和HP变质带尺度上的流变学演化模式，其中，UHP和HP变质岩石由地幔深度折返到地壳表层，经历了楔状挤出，碰撞期后地壳韧性薄化及晚造山伸展塌陷，揭顶作用等多个阶段的动力学过程。     

大别-苏鲁超高压和高压变质带构造演化

大别—苏鲁是世界上超高压 (UHP) ( >2 .7GPa)和高压 (HP)变质岩石出露最为广泛的地区。通过区域研究 ,尤其是在选择的 30多个关键位置上不同尺度构造记录的深入观察 ,结合已有的可利用的变质、热事件及同位素年代学资料分析 ,揭示出它们曾遭受过一个复杂的从深俯冲到折返构造演化历程 ,识别出 5个主要的构造变质事件 :( 1)由块状榴辉岩中发育的微弱面理和线理所代表的第 1期变形变质事件 (D1) ;( 2 )面状榴辉岩中发育的含拉伸线理的透入性主面理、中小型鞘状褶皱及网络状韧性剪切带 ,代表第 2期构造变质事件 (D2 ) ;( 3)第 3期变形事件主体发生于麻粒岩 /角闪岩相后成合晶形成之后 ,主要构造记录是区域性陡倾斜面理及不均一置换的成分层、榴辉岩透镜体及布丁群、面理内褶皱、网状韧性剪切带系统以及减压部分熔融作用形成的混合岩和含榴花岗质岩石组构 ;( 4)区域性的碰撞期后地壳韧性薄化及剪张作用 (D4)形成缓倾斜角闪岩相主面理及线理、穹状及弧形构造和多层韧性拆离带 ,它们主导了现今观察到的大别—苏鲁超高压和高压变质带的区域构造几何图像 ;( 5 )第 5期构造热事件 (D5)表现为不均一断块抬升、红色沉积盆地发育及大规模的岩体和岩脉就位 ,代表造山晚期的构造揭顶及坍陷作用 ,该期构造控制着造山带     

Triassic Collisional Structures and Post—Collisional Deformation of Bixiling UHP Rock Stack： Insights for Tectonic Evolution of UHP Metamorphic Belt in Dabie Massif, Central China

Detailed three-dimensional structural studies indicate that the Bixiling area,Dabie massif,central Chian shows the deepest exposed levels of the orogenic wedge formed during the Triassic Yangtze0Sino-Korean continental collision.New1:10000 scale structural mapping,combined with detailed petrological analysis in this area,has enabled us to accurately distinguish structures related to the Trias-sic continental collision from those related to post-collisional deformation in the ultrahigh pressure (UHP) metamorphic unit.The collisional or compressional structures include the massive eclogite with a weak foliation,foliated eclogite or UHP ductile shear zones,as well as upper amphibolite facies shear zones,whereas the post-collisional deformation is characterized by a regionally,flat-lying foliation con-taining stretching lineations and common reclined folds .The former is present exclusively in the eclogite lenses and their margins,representing orogenic thickening or syn-collisional events,while the latter was best occurred on variable scales under amphibolite facies conditions,showing sub-vertical,extreme short-ening and ductile thinning of the metamorphic rock stack.The eclogite facies tectonites that have a marked fabric discordance to the penetrative amphibolite facies extension flow fabric are common.It is emphasized that an extensional tectonic setting following the collision-orogenic thickening stage was,at least partly,responsible for exhumation of the UHP metamorphic rocks in the Dabie massif.A new tec-tionic evolution model is proposed for the UHP metamorphic belt on the scale of the Dabie massif.The Bixiling area thus provides a window,from which the dynamic processes concerning the formation and exhumation of the UHP rocks can be observed.Regional studies in the Dabie Mountains have confirmed this interpretation.     

Post-Collisional Ductile Extensional Tectonic Framework in the UHP and HP Metamorphic Belts in the Dabie-Sulu Region, China

The present-day observable tectonic framework of the ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts in the Dabie-Sulu region was dominantly formed by an extensional process, mostly between 200 and 170 Ma, following the Triassic collision between the Sino-Korean and Yangtze cratons. The framework that controls the present spatial distribution of UHP and HP metamorphic rocks in particular displays the typical features of a Cordilleran-type metamorphic core complex, in which at least four regional-scale, shallow-dipping detachment zones are recognized. Each of these detachment zones corresponds to a pressure gap of 0.5 to 2.0 GPa. The detachment zones separate the rocks exposed in the region into several petrotectonic units with different P-T conditions. The geometry and kinematics of both the detachment zones and the petrotectonic units show that the exhumation of UHP and HP metamorphic rocks in the Dabie-Sulu region was achieved, at least in part, by non-coaxial ductile flow in the mul     

Polyphase Deformation of the Weihai-Rongcheng UHP Unit Rocks, NE Sulu： Insights into the Tectonic Evolution of the Dabie-Sulu UHP and HP Belts, China

Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.     

大别—苏鲁区UHP变质岩构造学及流变学演化

在大别—苏鲁区的30个关键位置,对UHP/HP变质岩进行详细构造解析、大比例尺(1∶10000)制图并在区域尺度上进行观察和对比,以便揭示它们的构造几何学、变形条件和流变学演化。初步的研究结果指出,广泛出露的UHP/HP榴辉岩相岩石形成一个巨大的UHP/HP变质带,提供了一个观察中朝与扬子克拉通之间三叠纪大陆深俯冲-碰撞带过程的窗口。观察的显微构造及组构指出,UHP/HP变质带内岩石变形机制,无论是在榴辉岩相阶段还是在榴辉岩相后阶段,都是以塑性流变为主,其力学行为和组构特征都受组成矿物的强度、强度差等流变学特征,以及变形物理环境如压力、温度、应变速率、差异应力和流体含量等的制约。在俯冲/碰撞带内的变形分解作用于岩石圈不同层次及不同的构造阶段都曾发生,而且,在不同尺度上,应变局部化形成具高应变的剪切带网络,且一般显示典型的布丁-基质或碎斑-基质构造及流变学型式。根据构造、岩石、变质作用及地质年代学资料,借助于岩石圈流变学基本原理,提出一个大别—苏鲁区UHP/HP变质岩石流变学演化的工作模式,它涉及早期扬子与中朝克拉通间三叠纪(~250~230Ma)大陆深俯冲/碰撞、UHP/HP变质岩形成,相继深埋岩石的多期折返。特别强调UHP/HP岩石向地壳表层的折返,主要是构造过程,地面侵蚀作用是次要的。     

New Findings in High-Pressure and Ultrahigh-Pressure Metamorphic Belt of Tongbaishan-Dabieshan Regions, Central China

The high- pressure(HP) and ultrahigh- pressure(UHP)metamorphic rocks developed in the Tongbai- Dabie Mountainswere the products of oblique collision between the Yangtzeand Sinokorean cratons in the Triassic.Yetthere are still lotsof controversies about the present tectonic distribution of theHP and U HP metamorphic rocks and their petrogenetic rela-tionships which are crucial to the understanding of the form a-tion and exhumation of the Tongbai- Dabie collisional orogenicbelt(Cong and W…     

大别-苏鲁区超高压(UHP)变质岩的多阶段构造折返过程

追溯和重塑超高压变质岩由100多千米地幔深度折返至上地壳及地表的过程,对理解会聚板块边缘及大陆碰撞带的运动学和动力学是极为重要的.主要依据构造学、岩石学、地球化学和可利用的地质年代学资料,结合区域多期变形分析,大别-苏鲁区超高压变质岩的折返过程至少可分解出4个大的阶段.块状榴辉岩记录了三叠纪(约250~230 Ma)大陆壳岩石的深俯冲/碰撞作用.超高压变质岩早期迅速折返发生于超高压峰期变质作用(P＞3.1~4.0 GPa,T≈800±50 ℃)之后,处于地幔深度和柯石英稳定域,相当于区域D₂变形期阶段.分别与区域变形期D₃、D₄和D₅对应的折返过程,以及后成合晶、冠状体等卸载不平衡结构发育和减压部分熔融作用2个中间性构造热事件,均发生在地壳层次. 网络状剪切带在折返过程的不同阶段和不同层次均有发育,标志着在超高压变质带内的变质和变形分解作用曾重复进行.着重指出,超高压变质岩的折返,主要是由大陆壳的深俯冲/碰撞和伸展作用控制的构造过程,且受到俯冲带内、带外诸多因素的约束,其中水流体就起关键作用.      

Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

INTRODUCTION Anew ultrahigh pressure ( UHP) metamorphicbelt ,the South Altun-North Qaidam-North QinlingUHP metamorphic belt ,has been recently discoveredand widely discussed by different workers (Yang J Set al .,2003 ,2002 ,2001 ,2000 ,1998 ;Zhang J Xetal .,2002 ,1999 ; Zhang G et al .,2001 ; Hu et al .,1996 ,1995 ,1994) . Detailed studies have also beencarried out onthe Dabie-Sulu UHP/ HP metamorphicbelt inthe central orogenic belt (COB) of China (Gaoet al .,2002 ;Sun et al …     

大别地块超高压变质省的构造变形研究

构造解析的基本目的是建立构造事件造成的地质体几何学、运动学、动力学和流变学。大陆碰撞造山带内含柯石英及微粒金刚石等矿物组合的超高压（ＵＨＰ） 变质岩的形成和折返,是极为复杂的地球动力学过程。与世界上已知大多数超高压变质带相似,中国大别地块内超高压变质省现今观察到的主体构造形式,主要是在碰撞或超高压变质峰期后伸展体制下形成的。通过对大别超高压变质省内伸展组构及挤压（ 碰撞） 组构的鉴别、分析,结合有关超高压变质带构造学研究领域的简略综述指出,在揭示超高压变质带的形成及折返动力学过程中,构造解析的思维和工作方法是行之有效的     

浅变质岩在示踪大别—苏鲁造山带大陆板块俯冲与折返过程中的意义

中国大别-苏鲁造山带为大陆板块俯冲形成的碰撞造山带,该带北缘和内部产有原岩时代为新元古代-晚古生代的浅变质岩。这些浅变质岩对应于扬子板块北缘前寒武变质基底和扬子板块北缘古生代大陆架沉积物,形成过程于印支期扬子板块向北俯冲过程中的刮削作用密切相关,与大洋板块俯冲过程中刮削形成的加积楔具有类似的动力学过程。对大别-苏鲁造山带浅变质岩的深入研究,不仅有助于揭示大陆板块俯冲过程中高压-超高压岩石形成与折返过程,而且确定了扬子板块与华北板块之间的缝合线位置位于大别造山带北淮阳带的北部和苏鲁造山带的五莲-蓬莱群的北侧。     

桐柏-大别山区高压变质相的构造配置

作为华北和扬子陆块间的碰撞造山带桐柏大别山区以发育高压、超高压变质带为特征,从南到北变质相从低级到高级,代表俯冲带深度不同的变质产物,整体形成高压变质相系列。不过现今各变质相岩石的分布极受后期地壳规模的伸展构造控制,大别杂岩的穹隆作用更使高压变质相带的空间分布复杂化。超高压变质岩今日多呈大小不等的块体嵌布于相对低压的大别杂岩之内,造山带根部物质的热软化,使许多深层地幔物质得以像挤牙膏一样挤出于大别杂岩内。它们之中广泛发育着减压退变质的显微结构,与大别杂岩内一些麻粒岩相表壳岩所保存的减压退变质证迹一样,同是挤出作用和碰撞后隆升的构造证迹。高压相系的发育使南桐柏山和大别山迥然不同于桐商（ 商丹） 断裂以北的北秦岭北淮阳变质带。新近发表的同位素年代学（４０Ａｒ ３９ Ａｒ） 资料：３１６ ～４３４ Ｍａ ,已证明北秦岭是古生代变质带,它与桐柏－ 大别印支期碰撞造山带差异甚大。这两个变质地温梯度差异甚大的变质地体的拼合,说明华北和扬子陆块碰撞的主缝合带是商丹－ 桐商断裂带     

Multi-Layered Low-Angle Ductile Detachment System in Dabie-Sulu UHPM and HPM Belts,China

INTRODUCTIoNTheDabie-Suluregion,Chinahasbeenrecognizedasthelargestultrahigh-pressuremetamorphic(UHPM)(peakp>2.7GPa)andHPMbeltsintheworld,andattractedexten-siveinterestfrominternationalgeoscientists.AwealthofpublicationsexistsabouttheoccurrenceofUHPMindexmin-erals,thegeochronology,metamorphicPTpaths,andthelarge-scaleconceptualtectonicevolutionofUHPMandhigh-pressuremetamorphic(HPM)rocksintheDabie-Suluregion.Morerecentoverviewsaregiven'byErnstetal.(1995),Liouetal.(l996),andHackere…     

苏鲁超高压变质带东北端多种成因类型变基性岩:来自岩石学、同位素年代学及地球化学属性的制约

苏鲁超高压变质带是扬子板块与华北板块在三叠纪俯冲-碰撞的产物。变基性岩是苏鲁超高压变质带内出露最广泛的岩石类型之一,研究其岩石学、年代学、地球化学属性及成因机制,对于揭示扬子板块与华北板块之间的俯冲-碰撞-折返的动力学过程具有重要的科学意义。以(退变)榴辉岩为代表的超高压变质岩石广泛出露在威海-荣成一带,少量出露在乳山地区。锆石LA-ICP-MS U-Pb定年结果显示,(退变)榴辉岩的原岩时代为792~760Ma,峰期榴辉岩相变质时代为243~226Ma,后期角闪岩相退变质时代为221~207Ma。非榴辉岩相变质的基性岩(麻粒岩和斜长角闪岩)主要出露在乳山地区,其原岩形成时代应不晚于古元古代(1939Ma),峰期麻粒岩相变质时代为1895~1870Ma,后期角闪岩相退变质时代为1848~1806Ma,与胶北地体变基性岩的原岩时代和变质时代十分相似。全岩地球化学研究结果表明,(退变)榴辉岩的原岩显示高Fe拉斑玄武岩的特点,根据其稀土和微量元素特征,可将(退变)榴辉岩进一步划分为A、B和C三组。在球粒陨石标准化稀土配分模式和原始地幔均一化蛛网图解上,A、B和C三组样品分别具有轻稀土弱亏损、轻稀土弱富集和轻稀土富集的特点。轻稀土富集或弱富集型(退变)榴辉岩的原岩地球化学性质与岛弧或大陆玄武岩相似,它们的源区可能与深部富集地幔或受流体交代的地幔楔存在密切的成因关系;而轻稀土亏损型(退变)榴辉岩的原岩可能来自于亏损地幔的部分熔融。由此可见,(退变)榴辉岩的原岩具有成因多样性的特点。乳山地区的基性麻粒岩和斜长角闪岩的原岩也具有高Fe拉斑玄武岩的地球化学属性,Al2O3与Mg O呈正相关变化,TiO_2、P_2O_5与MgO表现出一定程度的负相关性。绝大多数非榴辉岩相变质基性岩的球粒陨石标准化稀土配分模式和原始地幔均一化蛛网配分曲线具有微右倾或明显右倾的特点。上述特征表明,研究区绝大多数非榴辉岩相变质的基性岩原岩来自于富集地幔,少数来自于原始地幔或亏损地幔,并经历了斜长石和辉石的分离结晶以及不同程度的部分熔融过程。由此可见,乳山地区出露的非超高压变质基性岩的原岩具有与胶北地体(高压)基性麻粒岩相近的成因特点。岩石学、同位素年代学和地球化学特征的综合对比研究结果表明,在苏鲁超高压变质带东北端的威海-荣成-乳山地区,既存在与华北板块古老变质基底相关的变基性岩,也存在与华南板块北缘新元古代变质基底相关的超高压榴辉岩,表明三叠纪时期华北板块东南缘胶北地体的部分古老变质基底曾卷入到扬子板块与华北板块之间的俯冲-碰撞造山过程,随后与超高压岩石一起抬升折返,形成当今的构造混杂岩带。     

大别-苏鲁构造带三叠纪碰撞缝合线的位置

大别－苏鲁构造带内超高压（UHP）和高压（HP）变质岩石的分布和不同构造岩石单位间的关系表明, 三叠纪中朝与扬子克拉通碰撞及超高压和高压变质岩石形成时的古缝合线, 位于大别地块的北缘, 沿八里畈－磨子潭－晓天一线展布, 在苏鲁地区, 五莲－烟台断裂代表被强烈改造了的古缝合线的位置.大别山北部霍山县与岳西县交界地区以及舒城、桐城和潜山县交界地区UHP和HP单位岩石的大面积分布, 说明水吼－五河断裂不是UHP和HP榴辉岩相岩石的北限, 因而, 不代表缝合线位置, “南大别”与“北大别”地体的划分地质意义是不明确的.同时, 大别山北部的镁铁质及超镁铁质岩石块体群, 包括变形的方辉橄榄岩、纯橄榄岩组合及未变形的辉石岩、角闪辉石岩和辉长岩组合, 前者与榴辉岩相岩石有相同的变形变质及几何学特征, 后者是燕山期侵入体（123~130Ma）, 因此, 不是“变质蛇绿混杂岩”, 也不代表三叠纪陆－陆碰撞时期古缝合线.强调指出, 准确地鉴别三叠纪碰撞缝合线位置, 是正确理解UHP和HP岩石形成及折返动力学过程的关键.      

秦岭造山带是印支碰撞造山带吗?

国内外一些学者认为秦岭是一个印支碰撞造山带.但迄今为止,秦岭尚未发现三叠纪或古生代延续到三叠纪的洋盆存在的任何痕迹.秦岭泥盆系-三叠系为滨、浅海相沉积,没有远洋沉积,更没有镁铁质和超镁铁质岩石及与之密切相关的放射虫硅质岩组成的蛇绿岩套.泥盆系与下伏地质体之间有一个清楚的区域性角度不整合.商丹断裂并不是印支期,而是加里东期的板块缝合带;其两侧,中朝板块南缘和扬子板块北缘均有十分清楚的加里东造山作用的记录.沉积于扬子板块北缘的中上泥盆统刘岭群的放射性铅同位素组成与北秦岭相近,碎屑锆石年龄谱系亦证明其物质主要来自中朝板块南缘的北秦岭造山带.所谓勉略印支缝合带中的勉略和三里岗蛇绿混杂岩中的镁铁质岩,同位素测年均为元古代之产物,后者又被南华系-震旦系沉积覆盖.所谓勉略缝合带,实为一区域性大断裂带.早古生代,其北侧属扬子板块北部被动边缘;南侧为扬子板块核心部分的扬子准地台(小克拉通).所以,秦岭的印支造山作用,并不是洋盆消失后的陆陆碰撞造山作用,而是海盆消失后的中朝与扬子2个小陆块间逆冲-叠覆造山作用.作为秦岭东延的大别山超高压变质带被认为是秦岭印支碰撞造山的重要证据之一,但大别山超高压变质岩是在造山作用过程中动态超高压条件下形成的,仅用简单的静岩压力来计算其形成深度,显然是不符合实际情况的.野外地质观察、构造地质学、变质岩石学、同位素地质学、地球化学、地球物理学以及物理实验等方面的实际资料和研究结果均说明超高压变质作用并不是在上地幔而是在地壳内进行的.南秦岭-大别山的地壳构造层次,上地壳自上而下依次为:未变质的沉积岩层、绿帘-蓝片岩层、高压变质岩层、超高压变质岩层;下地壳为未卷入超高压变质作用的麻粒岩相-高角闪岩相变质杂岩.含柯石英的超高压单位只是位于上地壳下部的厚约10～12km的席状构造岩片.初步认为上地壳这一从低压到高压再到超高压的构造系统,是印支造山期间,南秦岭-大别山的上地壳以下地壳顶部为主剪切滑动面,多层次剪切作用造成的.上地壳下部的超高压变质岩,则可能是强烈剪切引起的频繁地震的震源区瞬时超高压作用的结果.     

Pb Isotope Mapping in the Tongbai-Dabie Orogenic Belt, Central China

Tongbai-Dabie orogenic belt in Central China is a part of the collisional belt between the Yangtze and North China cratons. It represents one of the most extensive ultrahigh-pressure （UHP） and high-pressure （HP） metamorphic rocks in the world. The Pb isotope mapping in this area is a significant method to constrain the crustal structure and tectonic evolution and to identify the tectonic boundaries within the vertical tectonic stack. Based on the Pb isotope compositions of the Dabie complex （DBC）, the Tongbai complex （TBC）, UHP and HP metamorphic rocks and associated foliated granites, the lower metamorphosed rocks from North Huaiyang （NHY） tectonic belt, and Cretaceous granites in the Tongbai-Dabie orogenic belt, we determined the Pb isotope geochemical map of the Tongbai-Dabie orogenic belt. The Pb isotope map shows that the Pb isotope compositions are similar within each geological body or lithotectonic unit, but the Pb isotope compositions of different lithotectonic units show systematic variations in the Tongbai-Dabie orogenic belt. The NHY tectonic belt contrasts strongly with the Tongbai-Dabie UHP.HP metamorphic belt in Pb isotope compositions. It is suggested that the line along the Xiaotian-Mozitan fault, the north limit of the Tongbai-Dabie UHP and HP metamorphic rocks, represents an important tectonic boundary. Within the Tongbai-Dabie HP -UHP metamorphic belt, to the south of Xiaotian-Mozitan fault, the vertical variations of Pb isotope compositions in different lithotectonic units and the spatial relationship among different major lithotectonic units have been constrained.