Multi-Stage Tectonic Exhumation Processes of Ultrahigh-Pressure (UHP) Metamorphic Rocks in the Dabie-Sulu Area, East-Central China
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摘要: 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.Abstract: Exhumation processes of ultrahigh-pressure (UHP) metamorphic rocks, by which these deep-seated rocks were rapidly returned from mantle depths of more than 100 km within deep subduction zones into the upper crust and to the surface, are of paramount importance for the understanding of the kinematics and dynamics of convergent plate margins and continental collision belts. At least four large successive stages of the exhumation process of UHP metamorphic rocks in the Dabie-Sulu area, east-central China, mainly based on structural, petrological, geochemical data and available geochronological data, combined with previously regional polyphased deformation analyses, can be distinguished. Massive eclogites recorded the deep continental subduction/collision between the Sino-Korean and Yangtze cratons during the Triassic (~250-230 Ma).The early exhumation occurred after the peak of UHP metamorphism at conditions of up to > 3.1-4.0 GPa and 800±50 ℃, which may correspond to the D2 regional deformation phase and took place at mantle depths, displaying the model of vertical ductile extrusion, within the coesite stability field. The subsequent exhumation stages, corresponding with D3, D4 and D5 regional deformation phase, respectively, and two intermediate tectono-thermal events, i.e., the formation of the granulite/amphibolite facies symplectite or corona and the generation of extensive partial melting, occurred in crustal levels. Shear zones of an anastomosing rheological type were developed at diffterent stages and depths, indicating that the partitioning of deformation and metamorphism within the UHP metamorphic belt repeated. These imply that the exhumation of the UHP rocks is a complex tectonic process, in response to subduction/collision or extension of the continental crust, and is strongly constrained by a number of internal and external factors of the belt. In particular, fluids play important roles in the exhumation process of UHP rocks in the Dabie-Sulu area, east-central China.
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0. 引言
自20世纪80年代初在西阿尔卑斯Dora Maira地块及挪威西片麻岩区变质的上地壳岩中发现含柯石英矿物包裹体的超高压(UHP)变质岩以来(Chopin, 1984; Smith, 1984)，全世界已识别和研究了多达近20个具超高压变质岩石的大陆板块会聚或碰撞造山带.近30年来，经大陆壳物质的深俯冲到地幔的超高压变质岩形成及相继深位岩石折返到中上地壳乃至地表的地球动力学过程，一直是地球科学的重要研究热点之一，而且对经典的板块学说及变质岩石学等提出了严峻的挑战(Masago et al., 2010). 我国的大别-苏鲁造山带，由扬子克拉通与中朝克拉通间主要在三叠纪印支期(约250~230 Ma)的大陆深俯冲/碰撞作用，形成了世界上最大的高压和超高压变质带.Xu(1987)首先在大别山古老变质杂岩中发现柯石英假象包裹体，相继Xu et al.(1992)又在造山带核部变质岩中发现了微粒金刚石包裹体，并初步讨论了其形成的构造背景和地球动力学意义.自此开始，国内外众多地质学者云集大别-苏鲁区，在高压和超高压变质岩的矿物学、岩石学、地球化学、地球物理学、同位素地质年代学、实验岩石学及不同尺度的构造学等多学科领域，开展了长期和深入的研究，取得了大量的有创新意义的成果(Wang et al., 1993; Cong et al., 1995; Liou et al., 1997; Zhang and Liou, 1998; Webb et al., 1999; Li et al., 2000; Suo et al., 2000, 2007；游振东等，2000; Jin et al., 2001; Liu et al., 2004；刘祥文等，2005; Hacker et al., 2009; Zhang et al., 2009; Qiu et al., 2010；王世明等，2010)；一些综述性研究结果和超高压变质地体形成及折返的概念性模式也相继提出(Liou et al., 1996, 1998; Maruyama et al., 1996；游振东等，1998; Wallis et al., 1999; Hacker et al., 2000；许志琴等，2003；钟增球等，2007).但是，许多学者在超高压变质岩经典地区(阿尔卑斯Dora Maira地块、挪威西片麻岩区、法国华力西构造带及哈萨克斯坦的柯切塔夫地块等)的长期研究实践指出，超高压变质岩的形成和折返演化历史，是一个复杂的地球动力学过程.虽然学者们已经提出了许多理论或工作模型(Dewey et al., 1993; Platt, 1993; Chemenda et al., 1996; Thompson et al., 1997; Rubatto and Hermann, 2001; Chopin, 2003; de Sigoyer et al., 2004; Jolivet et al., 2005; Bellot and Roig, 2007; Hacker et al., 2010)，但是，这些设想的模型及驱动机制大多是建立在对超高压变质岩的岩相学、变质作用演化研究及所追溯的P-T-t轨迹样式基础上，仍存在颇多争议，见仁见智.而且比较起来，涉及超高压变质岩石由地幔深度折返至地壳浅层次的模式更为不成熟，正如著名学者Platt(1993)明确指出的，没有单一的机制或模型能解释所有高压和超高压变质岩的折返过程.
本文运用解析构造学的基本原理和方法(马杏垣，2004)，主要依据我们长期对大别-苏鲁高压和超高压变质带观察研究获得的构造学、岩石学和地球化学等资料，包括不同尺度的详细制图、岩石变形的构造学、流变学和变质岩石学解析，结合可利用的同位素年代学测试成果(Ames et al., 1996; Li et al., 2000; Liu et al., 2004；石永红和王清晨，2006; Wawrzenitz et al., 2006; Hacker et al., 2009; Liu et al., 2009)，讨论和分析大别-苏鲁区由大陆壳深俯冲/碰撞形成的超高压变质岩多期折返历史；侧重点在于分析不同折返阶段(特别是处于地幔层次的早期折返阶段)的几何学和运动学图像.强调指出，超高压变质岩的折返演化，主要是一个复杂的构造过程，且受深俯冲/碰撞带内外多种因素和边界条件(如水流体)的约束.
1. 地质背景和造山带结构
我国的所谓中央造山带内，发育2个不同时代的高压-超高压变质带.一个是南阿尔泰-北柴达木-北秦岭带，主要发育于早古生代(500~400 Ma)(Yang et al., 2003)；另一个是本文讨论的大别-苏鲁带，其高压-超高压峰期变质年龄可能是穿时的(约250~230 Ma)(刘敦一和简平，2004；石永红和王清晨，2006; Hacker et al., 2009).2个高压-超高压变质带虽都位于扬子和中朝克拉通间，但它们沿走向并不能连成一个长度大于4 000 km的深俯冲/碰撞带(Yang et al., 2003; Suo et al., 2005；钟增球等，2007)，两者的内部结构、构造和变质演化历史以及远场碰撞动力学背景都有明显差异.向东，大别-苏鲁高压-超高压变质带越过黄海，可能与朝鲜半岛的Imjingang构造变质带或Gyeongg地块衔接(Ree et al., 1996; Oh et al., 2005)，构成世界上最大的高压-超高压变质带.
大别-苏鲁高压-超高压变质带虽然主要是三叠纪扬子克拉通与中朝克拉通间大陆壳深俯冲/碰撞的结果(Li et al., 1993; Okay, 1993; Cong et al., 1994; Liou et al., 1994; Wang et al., 1995；刘敦一和简平，2004; Wawrzenitz et al., 2006; Hacker et al., 2009)，但现今观察到的变质带三维结构样式，主要展示其隆升和折返至中地壳层次的基本格局(图 1).突出的几何学特点，类似于北美科迪勒拉型变质核杂岩.桐柏-大别区的娘娘庙-八里畈-磨子潭-晓天剪切(断裂)带及苏鲁区的五莲-烟台剪切(断裂)带，构成高压-超高压变质带的北缘构造边界，相当于被强烈改造了的扬子克拉通向北俯冲于中朝克拉通之下的原深俯冲/碰撞带的顶部边界.在该构造边界以南的高压-超高压变质岩分布区域，相当于大别-苏鲁造山带的核部或内带，5个岩石构造单位被4个地壳尺度的缓倾斜韧性伸展拆离带或变质不连续带分隔开，共同构成垂向上叠置的地壳构造柱，并显示基本的正向变质层序.若不计位于构造柱最下部的核杂岩单位(CC)，垂直区域绿帘-钠长-角闪岩相主面理估测的构造柱视总厚度约35~37 km.其中超高压单位(UHP)厚度约8~12 km，高压单位(HP)10~15 km，绿帘-蓝片岩单位(EB)5~7 km.最上部的未变质或仅具极低级变质的沉积盖层岩系(SC)，在南侧的前陆褶皱-逆冲带很发育，而在造山带的核部或内带，仅只残留于桐柏-大别山南坡的局部地段.通常，超高压和高压单位及下拆离带(LDZ)，中拆离带(MDZ)等，环绕核杂岩单位展布，显示不同规模的穹窿、半穹窿或短轴背形构造.含金刚石、柯石英(假象)和镁铝榴石等超高压变质标型矿物及有关矿物岩石组合，主要分布于大别山和苏鲁地区，是我们和国内外学者重点研究的地段，其详细的区域地质背景及造山带结构，可参阅Liou et al.(1996)、Webb et al.(1999)、Hacker et al.(2000, 2009)、Ratschbacher et al.(2003)、Faure et al.(2003)、许志琴等(2003)、Suo et al.(2005)和钟增球等(2007)相关论著.
图 1 大别-苏鲁区构造略图(据Suo et al., 2005修改)CC.核杂岩单位；UHP.超高压单位；HP.高压单位；EB.绿帘-蓝片岩单位；SC.沉积盖层单位；LDZ.下拆离带；MDZ.中拆离带；UDZ.上拆离带；TDZ.顶拆离带；SDF.商丹断裂带；GMF.龟梅断裂带；NBMXF.娘娘庙-八里畈-磨子潭-晓天剪切(断裂)带；NHY.北淮阳构造带；XGF.襄樊-广济断裂带；WYF.五莲-烟台剪切(断裂)带；TLF.郯庐断裂带.右下方插图表示大别-苏鲁造山带核部地壳构造柱Fig. 1. Simplified tectonic map of the Dabie-Sulu area showing major petrotectonic units, structural framework and detachment zones formed by extensional thinning, as well as tectonic pile of the UHP-HP metamorphic belt2. 块状榴辉岩——大陆深俯冲终端产物
野外直接观察、中国大陆科学钻探(CCSD)的揭露以及光学和透射电镜的鉴定研究，均可识别出矿物组合、结构构造特征明显不同的2类新鲜榴辉岩，即块状榴辉岩和面理化榴辉岩(刘祥文等，2005; Suo et al., 2005；钟增球等，2007).
块状榴辉岩(图 2a, 2b)的矿物组合较为简单，主要有名义上不含水矿物石榴子石、绿辉石、金红石等，含微量副矿物锆石、磷灰岩、黄铁矿等.突出特点是富含金红石(2%~8%)，故Zhang et al.(1995a)曾称其为富含金红石榴辉岩.在石榴子石、绿辉石中多见柯石英(或假象)及罕见的微粒金刚石包裹体(Xu et al., 1992; Okay, 1993)，不同的温压计估测块状榴辉岩形成的P-T条件是＞3.1~4.0 GPa和800±50 ℃，处于柯石英-金刚石稳定域，代表三叠纪大陆深俯冲/碰撞的终端产物之一，也是深埋于地幔的岩石学和构造学主要记录.手标本及薄片尺度上，块状榴辉岩多呈现中细粒粒状变晶结构，均一块状构造，仅绿辉石略呈定向排列，显示微弱的面状和线状组构，表明块状榴辉岩的流变学行为，主要是由作为主动相的辉绿石控制的(van der Klauw et al., 1997; Suo et al., 2005; Zhao et al., 2005)，推测其微观变形机制以伴有颗粒边界迁移重结晶作用的位错蠕变为主.
图 2 大别-苏鲁区代表性的超高压变质岩显微及露头尺度照片a.块状榴辉岩(标本号r-2-11-7-2)，单偏光，视域宽约4 mm，大别山碧溪岭；b.块状榴辉岩中绿辉石晶体内具放射状结构的多晶柯石英假像(标本号Ⅰ-2-2)，正交偏光，视域宽约1.3 mm，江苏省东海县碱场；c.面理化榴辉岩(r-205)，正交偏光，视域宽约4 mm，大别山碧溪岭；d.面理化榴辉岩形成的鞘状褶皱，地质锤把长约35 cm；e.发育麻粒岩-角闪岩相卸载不平衡后成合晶及冠状体结构的退变质面理化榴辉岩(标本号S-r-19-3)，单偏光，视域宽约5 mm，山东省荣城大疃，其中S2表示面理化榴辉岩的主面理；f.层状糜棱岩化斜长角闪岩(样品号S-r-17-4)，单偏光，视域宽约5 mm，山东省文登泽库，其中S、C分别为糜棱岩的片理面和剪切面；g.围岩片麻岩及面理化含榴花岗岩形成的平卧褶皱，地质锤把长约35 cm，安徽省潜山三祖寺；h.近水平对称面理布丁构造，布丁间隙由含石英的绿泥石物质充填，铁锤把长约35 cm，湖北省麻城何家湾.Grt.石榴子石；Omp.绿辉石；Rt.金红石；Coe.柯石英(假象)；Qtz.石英；Phe.多硅白云母；Ky.蓝晶石；Zo.黝帘石；Sym.后成合晶；Bt.黑云母；Hbe.角闪石Fig. 2. Representative photomicrographs and outcrop-scale photographs of ultrahigh-pressure metamorphic rocks in the Dabie-Sulu area青岛仰口出露的超高压变质岩中，保留有从变辉长岩经部分变质到完全变质重结晶的含柯石英榴辉岩化的完整岩石学和构造学记录(Zhang and Liou, 1997).榴辉岩体核部初始超高压变质的辉长岩，还残留着岩浆岩矿物组合及结构特征.证明形成块状榴辉岩的温度和压力是重要控制因素，差异应力较小，变形(D1)强化变质反应动力学是有限的.桐柏地区大阜山含金红石变辉长岩向高压榴辉岩的过渡和转换行为，与青岛仰口含柯石英榴辉岩的形成过程，有类似的特点(索书田等，2001).
3. 多阶段折返过程
Froitzheim et al.(2003)及Kurz and Froitzheim(2002)结合对西阿尔卑斯超高压变质带减压退变折返几何学和运动学研究，总结了现有的不涉及驱动力或机制的主要折返模式，包括侵蚀、伸展、挤出和抽拉等(图 3)；对于折返演化过程，也可分为单一阶段的、两阶段的(Liou et al., 2004; Štípská et al., 2008)、三阶段的(Wang and Cong, 1999)和幕式的(Bellot and Roig, 2007)等.笔者结合对大别-苏鲁区高压-超高压变质带多期区域构造叠加变形演化研究(Suo et al., 2005；钟增球等，2007)，认识到超高压变质岩由地幔层次快速折返到中上地壳及至地表，是复杂的多阶段构造过程.
图 3 碰撞造山带内高压-超高压变质岩可能的折返运动学模式(据Froitzheim et al., 2003)a.侵蚀作用；b.伸展作用；c.挤出作用；d.上覆地幔楔向下抽拉作用.星号代表高压-超高压变质地体Fig. 3. Possible kinematic models of exhumation of high- and ultrafigh-pressure metamorphic rocks3.1 面理化榴辉岩——从100多千米地幔深处折返至壳幔边界层
面理化榴辉岩的矿物组合较为复杂，除石榴子石、绿辉石、金红石(＜2%)外，尚有大量含水矿物多硅白云母、蓝晶石、滑石、尼泊闪石、黝帘石(斜黝帘石)等.柯石英(或假象)呈包裹体形式.面理化榴辉岩与块状榴辉岩相比，颜色较淡，故程裕淇等(2002)分别称之为浅色榴辉岩和暗色榴辉岩.面理化榴辉岩，顾名思义，由矿物优选定向排列和形组构显示特征的面理和线理(S2、L2)组构(图 2c).在XZ切面统计，石榴子石、绿辉石、金红石、多硅白云母和蓝晶石的形组构长短轴比分别为10~14、10~20、3~5、4~6和10~15.大别山双河区面理化榴辉岩内石榴子石的轴比弗林指数(k)约为1.18，显示近简单剪切条件下平面应变(徐树桐等，1999).详细露头至薄片尺度观察，面状组构具SC复合面理特点，S面受多晶石榴子石、绿辉石及金红石等矿物的定向排列约束，而多硅白云母、蓝晶石、黝帘石等含水相矿物，则多沿C面定向排列.据S与C间夹角估测，考虑到临界剪应变，剪应变值(r)一般大于6.3.表明与块状榴辉岩相比较，面理化榴辉岩的塑性变形强度和水解弱化作用(hydrolysis weakening)要高得多(钟增球等，2007).透射电镜观察进一步证实面理化榴辉岩主要组成矿物的晶格塑性变形组构更为清楚，其中石榴子石位错构造有自由位错、位错环、位错壁及位错网等，位错密度大于块状榴辉岩中石榴子石约3个数量级(徐树桐等，1999; Su et al., 2004；刘祥文等，2005)，因而，面理化榴辉岩实质上是在超高压条件下退变质或应变局部化带内的岩石和构造标志，属于榴辉岩相糜棱岩(Zhao et al., 2005).
在大比例尺(如1∶10 000)制图尺度上，大的榴辉岩体主要由面理化榴辉岩组成，其中大小不等的块状榴辉岩透镜体群，一般只占总体积的5%~10%，两者一起构成布丁-基质或残斑-基质流变学型式(Handy, 1990)，是在超高压柯石英稳定域退变质和变形分解作用的结果.块状榴辉岩相当于弱应变域，面理化榴辉岩代表网络状剪切带或线状强应变带.面理化榴辉岩形成的变质条件P＞2.7 GPa，T≈610~700 ℃(Zhang et al., 1995a, 1995b)或P≈1.5~2.0 GPa，T≈770~810 ℃(Xiao et al., 1995)，考虑到温压测试的误差及不确定性，与块状榴辉岩代表的超高压峰期变质环境比较，至少体现出明显等温减压的事实.在众多研究大别-苏鲁超高压变质岩的国内外学者中，魏春景等(1996)较早认识到超高压峰期变质与普遍发育卸载不平衡后成合晶结构为特征的中晚期退变质阶段之间，尚存在着一个早期折返退变质阶段，其矿物岩石学及退变质反应记录，与Schreyer(1988)的实验研究结果和这里所讨论的面理化榴辉岩的发育是相同的.Chavagnac and Jahn(1996)用碧溪岭地区6个面理化榴辉岩样品(石榴子石+绿辉石+全岩)的Sm-Nd方法获得220~210 Ma数据，大体代表面理化榴辉岩的形成年龄，也大体与Li et al.(2000)提出的UHP变质岩第一次快速冷却事件时限(226~219 Ma)相当.
图 4 露头尺度块状榴辉岩透镜体与面理化榴辉岩间几何学和运动学关系Fig. 4. Outcrop sketches showing the local geometric and kinematic relationships between lenses of massive eclogite and foliated eclogite在超高压构造单位不同的榴辉岩体内部，面理化榴辉岩的面理(S2)多是陡倾斜的，线理(L2)多是陡倾伏的(现今方位).由面理作形面的鞘状褶皱，如在山东桃行及荣城大疃等地所见(图 2d)，他们的X轴也是陡倾伏的.其他运动学和剪切方向标志，包括面理化榴辉岩主面理与所包围的块状榴辉岩体形状、初始剪切破裂面间几何学配置(图 4)、面理化榴辉岩SC复合面理的锐夹角大小及指向等，均表示深俯冲/碰撞带内的UHP岩石，沿着陡倾斜的剪切带面理，剖面上显示向上分散剪切运动，平面上伴有左行或右行剪切运动分量的特征(Suo et al., 2005).总体效应是，非共轴剪切作用导致部分超高压变质地体从地幔深处，近等温减压快速(约20~31 mm/a)折返到增厚了的地壳底部或壳-幔边界层(Wawrzenitz et al., 2006; Suo et al., 2009)，类似于同碰撞楔状挤出样式(Maruyama et al., 1996；许志琴等，2003).
3.2 后成合晶及冠状体——中间性构造(1)
榴辉岩最显著减压退变质作用，由环绕组成矿物发育的后成合晶或冠状体等卸载不平衡结构记录下来(图 2e).山东威海由紫苏辉石、次透辉石、斜长石和角闪石等组成的后成合晶及冠状体，重结晶条件为P≈0.7~1.3 GPa，T≈760±50 ℃或P≈0.7~1.2 GPa，T≈700~800 ℃(Zhang et al., 1995b; Nakamura and Hirajima, 2000)，属麻粒岩相.大别山碧溪岭区后成合晶结构形成参考条件为P≈0.9~1.3 GPa，T≈600~700 ℃，相当高角闪岩相(Xiao et al., 1995)，反映不同构造部位的含水条件及其影响有所差别.
后成合晶多成纤维状、针状、栅状和指纹状，其合晶长轴多垂直于主(母)晶边界延长，代表超高压岩石无应变的退变质显微组构(Carswell, 1986).块状及面理化榴辉岩均有相同成分及反应结构的后成合晶和单层或双层冠状体发育，标志着大陆深俯冲物质超高压变质作用的终结，麻粒岩-角闪岩相减压递退变质作用开始.因此，后合成晶及冠状体发育是超高压变质岩构造演化及折返过程中一个重要转变环节，也标志着脱水或水化反应等水流体的积极功能.
3.3 韧性陡立构造带——于深部地壳层次的折返
大别山区石马、四道河及苏鲁区文登泽库、荣城岳家、东海禹山等地，超高压单位岩石出现宽约几百米至几千米的陡立构造带(图 5).删除未变形的晚中生代基性和酸性岩墙(床)，构造带多由各种岩石经不均一构造置换和糜棱岩化的成分层组成，包括层状糜棱岩化含榴斜长角闪岩(富石英榴闪岩)、层状二云片麻岩、榴辉岩质片麻岩及含榴角闪黑云斜长片麻岩等.递进解体成的串珠状榴辉岩和退变质榴辉岩布丁、透镜体平行陡立的成分层及其面、线组构(S3、L3)定向排列，与围岩片麻岩一同构成布丁-基质或残斑-基质流变学结构型式(Handy, 1990；钟增球等，2007).大部分围岩片麻岩含有超高压及退变质榴辉岩残余矿物如石榴子石、金红石和黝帘石等，露头尺度上即可观察到超高压变质岩与不同成分层围岩间的过渡现象.因此，至少一部分围岩片麻岩是超高压榴辉岩等的退变质重结晶产物，其温压条件介于后成合晶和冠状体卸载不平衡组构发育与钠长绿帘角闪岩相退变质作用之间(P≈1.0 GPa，T≈572~700 ℃；游振东等，2000).陡立构造带不仅发育陡倾斜的成分层、面理和线理，还发育大量的面理内有根及无根的褶皱，许多还具有复杂的紧闭等斜形态、力学不稳定性产生的叠加褶皱干扰型式以及A型褶皱特点.多种运动学及剪切方向标志显示(图 2f)，该阶段的折返过程，亦具有挤压体制下近似直立的韧性挤出和逆冲特征.在糜棱岩化的层状片麻岩中，角闪石和斜长石等多具塑性变形行为，属主动相，而超高压榴辉岩和退变质榴辉岩残余矿物，如石榴子石和金红石等，则属被动相，多呈残斑形式.
图 5 山东省荣城岳家陡立构造带剖面简图Fig. 5. Simplified cross-section showing a steep structural belt in Yuejia, Rongcheng, northeastern Shandong3.4 面理化含榴花岗岩——中间性构造(2)
高压及超高压单位中，约40%~60%的岩石为面理化含榴花岗岩或花岗质片麻岩(许志琴等，2004；游振东等, 2004, 2005).其中除部分原岩属于前寒武纪大陆壳组成部分的古老变花岗岩和花岗闪长岩(Wallis et al., 1999)外，相当大的部分是高压和超高压岩石退变质形成的围岩片麻岩减压选择性部分熔融的产物(Wallis et al., 1999, 2005; Zhong et al., 1999, 2001；游振东等, 2004, 2005; Zhang et al., 2009).推测在中下地壳层次超临界流体的钾交代作用下，尤其是含黑云母和角闪石的斜长片麻岩，通过钾长石的逐渐增加而过渡为二长片麻岩或花岗质片麻岩.所形成的岩石普遍含有高压和超高压变质或退变质残余矿物，如石榴子石、多硅白云母、黝帘石、金红石、柯石英等(Carswell et al., 2000; Ye et al., 2000).在部分熔融作用形成的岩体内部，常可以见到属于陡立构造带成分的榴辉岩、退变质榴辉岩和围岩片麻岩残留体，它们之间的切割穿插接触关系也很明显，表明减压部分熔融作用并非与陡立构造带的发育同步.不过，如图 5所示，在不少情况下，面理化含榴花岗岩的面、线组构，与陡立构造带显示的总体组构是平行的或协调的，证明减压部分熔融作用在超高压变质岩递进挤出过程晚期就开始了.
减压选择性部分熔融作用，是高压-超高压变质岩石折返过程的重要组成部分，不仅被世界上经典高压和超高压变质岩分布区不同尺度的野外直接观察所证实，而且也得到岩石地球化学、同位素地球化学、岩石实验学等研究的支持(杨启军等，2003；Auzanneau et al., 2006; Zhang et al., 2009).Auzanneau et al.(2006)实验证明，在温度为800~850 ℃条件下，只要地质体内有少许含水流体，即可导致10%~30%的岩石发生熔融作用.因而，除了俯冲板片断离和软流圈物质上涌等岩石圈尺度因素外，富水流体的注入是引起高压-超高压岩石减压部分熔融作用的一个关键因素(Regard et al., 2008).
减压部分熔融的热弱化(thermal weakening)结果，大大改变了俯冲/碰撞带内岩石物理特征(密度、强度和流变性)，提高可变性和浮力效应，促使构造体制由挤压(碰撞)、地壳增厚向伸展薄化的转换，(Ranalli, 1997; Brown et al., 2011)，因而，面理化含榴花岗岩，是重要的中间性构造.虽然Zhang et al.(2001)及Wawrzenitz et al.(2006)对大别-苏鲁区高压-超高压单位内减压部分熔融事件年代学进行过研究，但所得结果有不同的解释，争议较多.通过多方判斟，Jahn et al.(1994)曾一度使用的面理化含榴花岗岩的全岩-黑云母Rb-Sr法获得的181~188 Ma矿物冷却年龄数据，似乎可作为面理化含榴花岗岩形成时代的参考，亦可作为构造体制转换的年代学标尺，此与Hacker et al.(2009)提出的210~170 Ma作为角闪岩相退变质作用终结时限的观点非常近似.
3.5 科迪勒拉变质核杂岩型构造——中地壳韧性伸展薄化作用
大别-苏鲁高压-超高压变质带尺度上的基本构造格架类似于北美不对称科迪勒拉型变质核杂岩(Davis, 1983; Ames et al., 1996; Ratschbacher et al., 2000).笔者曾详细讨论过其组成、结构及形成过程(Suo et al., 2000, 2001, 2005；钟增球等，2007).在此仅概括其如下特征：(1)发育4个地壳尺度的低缓角度韧性伸展拆离带.下拆离带为底基拆离带，橫过中、上和顶拆离带各显示一个明显压力降(pressure drop)或间断，表示地壳构造柱中相当厚的岩石被构造作用删除掉了(Carswell et al., 1997)，说明高压-超高压岩石的折返，不是依赖于高压-超高压岩石上覆地质体的顶蚀，而是取于复杂的构造作用过程.(2)高压-超高压单位岩石区域缓倾斜主面理(S4)、线理(L4)与伸展拆离带环绕核部杂岩单位形成穹窿、半穹窿形构造.穹窿的中部，以垂向共轴压缩的纯剪变形为主，先期陡立构造带发育平卧叠加褶皱.在穹窿的翼部及周缘，则以正向剪切滑动和非共轴剪切占优势(图 6).由于存在流变性能的差异，不同岩性岩石的垂向薄化及水平方向伸展流动量差别很大.(3)根据普通角闪石和斜长石(An20)的共存及奥长石的不稳定性等，估测主面理发育的温压条件为P≈0.5~0.7 GPa，T≈500~550 ℃(游振东等，2000)，或P≈0.6~0.8 GPa，T≈470~570 ℃(Cong et al., 1995)，大体相当钠长-绿帘-角闪岩相，局部可退变至绿片岩相(P≈0.4 GPa，T≈250~300 ℃)，面理布丁间隙或细颈化部位绿泥石脉的发育可作为标志(图 6c，6d).上述特征证明，在造山带尺度上，碰撞期后中地壳层次的韧性伸展薄化作用，是导致超高压岩石进一步退变质重结晶和向上地壳抬升折返的主要原因.中晚侏罗世特别是早白垩世的大规模未变形花岗质岩体及相关岩脉体系，与区域性钠长-绿帘-角闪岩相变形组构，是呈明显侵入接触关系的.因之，Ratschbacher et al.(2000)对有关白垩纪大别山形成科迪勒拉型伸展核杂岩的推测不够确切.
图 6 韧性伸展薄化作用的应变和运动学标志a, b.非共轴剪切变形且沿着向南或南西缓倾斜面理正向剪切滑动(四道河)；c, d.共轴纯剪切变形和近水平方向伸展流动(分别位于麻城大河背及何家湾)Fig. 6. Field sketches showing strain and kinematic indicaters for the structures acquired by ductile extensional thinning3.6 盆岭构造——造山期后构造揭顶作用
高压-超高压单位岩石在地壳伸展体制下抬升折返到上地壳层次后，受板块运动产生的远场效应影响，进一步的向地面的折返和抬升，主要受地壳较高构造层次摩擦或摩擦-粘塑性过渡变形机制下的构造揭顶及坍陷作用控制，表现为主要的长命构造边界断裂再造、不均一断块抬升或沉降、大规模高层侵入体就位和盆岭地貌格局形成等(D5).最终导致大面积高压-超高压变质岩及其退变质岩石出露地表(王世明等，2010).在造山带尺度上，受变形和应变分解作用约束，内带总体表现为伸展、抬升和侧向扩展，外带以挤压缩短为主，表现为褶皱与逆冲断裂组合.大别-苏鲁造山带整体显示一个双侧造山带结构样式(索书田等，1993).在上地壳层次，除了沿着网状剪切带或碎裂岩带发生局部绿片岩相和更低级的退变质作用外，高压-超高压变质岩不会再产生深刻的区域性减压退变质重结晶作用.在大别山北麓的晚侏罗世砾岩层中曾发现过榴辉岩的砾石，证明大致在150 Ma时期，就可能有部分高压-超高压变质岩石折返裸露地表，显示顶蚀作用在高压-超高压变质岩折返过程中，有一定的辅助功能，但改变不了构造过程的主导地位.
4. 讨论
4.1 关于超高压变质岩的折返模式
所谓大陆壳深俯冲带岩石的折返，是指它们相对地面的向上运动或隆升(England and Molnar, 1990)，大别-苏鲁高压-超高压岩石向地面的折返，是一个多阶段复杂的地球动力学过程.由于建立动力学模型，涉及到对主导驱动力的推测，在现今科技水平和研究程度情况下困难颇多，争议较大.我们主要考量折返过程中不同阶段形成的特征性构造样式及岩石学记录，构建其几何学和运动学模式，进而推断不同阶段的温压条件、岩石的流变学行为和变形场(deformation field)(马杏垣，2004)，为建立动力学模式奠定基础.
图 7是大别-苏鲁区超高压变质岩折返构造过程的解释性略图，强调碰撞/挤压构造体制和伸展构造体制，在复杂的构造折返过程的不同阶段、不同层次和不同尺度上，分别作为主导的构造体制发挥作用.无可置疑，在许多情况下，还受到俯冲带内外多种因素及变量的制约.
图 7 大别-苏鲁区超高压变质岩的减压退变构造折返示意减压退变P-T轨迹示意据张泽明等(2006)；D1，D2，D3，D4和D5，分别为UHP变质峰期变形及不同折返阶段变形记录示意.变质相代号：PP.葡萄石绿纤石相；PA.绿纤石阳起石相；GS.绿片岩相；BS.蓝片岩相；EA.绿帘角闪岩相；AM.角闪岩相；GR.麻粒岩相；EC.榴辉岩相Fig. 7. Synoptic illustrations explaining the tectonic exhumation of UHP metamorphic rocks in the Dabie-Sulu area4.2 关于变形和变质分解作用的意义
大别-苏鲁区高压-超高压变质带的原岩，主要是前寒武纪大陆壳成分.在高压-超高压变质作用前，就存在着固有的矿物岩石成分、含水量和流变性等的不均一性.在大陆壳深俯冲/碰撞、高压-超高压变质重结晶作用及减压退变质折返过程中，受到组成的不均一性和力学不稳定性(mechanical instability)制约，变形和变质作用都是非透入性和可分解的(Lister and Williams, 1983; Burg, 1999; Bellot and Roig, 2007)，应变、减压部分熔融及冷却退变质水化反应等局部化明显，域行为(domainal behaviour)极为特征，即由线(网)状强应变韧性剪切带与透镜状弱应变域共同组成残斑-基质流变学格式.这种格式在超高压岩石折返过程不同阶段、不同层次和不同尺度上都会出现，其中，面理化榴辉岩与块状榴辉岩的组合图象最为典型.这充分表明，剪切作用也是高压-超高压变质带演化，尤其是构造折返过程中有效的应变机制.充分认识和理解变形和变质分解作用，便于从透镜状弱应变域内识别先阶段残余变形组构和岩石学特征，在线状强应变带即剪切带内，对糜棱状岩石进行运动学和流变学解析.自然，在现今出露的高压-超高压变质岩石地区，常见进变质和退变质差异明显的地质体或岩片构造拼贴在一起，它们是差异性深俯冲或折返形成的，还是不同构造演化阶段变形和变质分解作用的结果，需要深入鉴别和论证.在进行地震测深等地球物理信息解释时，也应严格加以区分(Yuan et al., 2003).
4.3 关于水流体及其作用
超高压变质岩的形成与折返过程，是受深俯冲/碰撞带内部外部多种因素和变量制约的，含水流体的成分、量级和活动性等就是其中之一.Liou et al.(1998)曾依据苏鲁区榴辉岩中发现粒间柯石英及变辉长岩不彻底的榴辉岩化等现象，推测在超高压变质作用演化和折返初期，是不存在水流体的.因为按Mosenfelder and Bohlen(1994)的分析，在T＞300 ℃条件下.只要有重约450×10-6的粒间水流体，柯石英就会全部转变为石英.
随着主要组成矿物及副矿物多种流体包裹体研究(游振东等，2001；沈昆等，2006)、红外光谱及透射电镜等现代高科技测试分析(苏文等，2001; Su et al., 2002, 2004; Katayama et al., 2006; Masago et al., 2010)，有关水流体在超高压变质岩的形成尤其是在减压退变质重结晶作用和折返过程中功能的理解，发生了很大的变化，且把流体演化作为超高压变质带地球动力学演化一个重要组成部分(Austrheim, 1998；张泽明等，2006).
大陆壳物质深俯冲及递进变质重结晶作用过程中，无疑以脱水反应为主，因此，块状榴辉岩和硬玉石英岩等为代表的超高压变质岩组合，可能是在“干”的峰期变质环境下形成的.不过，Katayama et al.(2006)运用现代测试技术对哈萨克斯坦的柯切塔夫地块含金刚石的榴辉岩研究结果发现，名义上不含水的矿物绿辉石.金红石及石榴子石分别含有870×10-6，740×10-6和130×10-6的微量水，估算大部分榴辉岩全岩含水量在300×10-6至3 070×10-6之间.证明，曾有一定量的水流体参与深俯冲并贮存于上地幔，可作为折返退变质流体的一个来源.Su et al.(2002, 2004)、苏文等(2001)和刘祥文等(2005)通过对大别-苏鲁区榴辉岩及硬玉石英岩样品研究，确定自由水(H2O)及结构水(OH-)等为主要的水流体赋存形式.在强变形或面理化榴辉岩和硬玉石英岩的组成矿物中，除了大量的含水矿物外，名义上无水矿物也含微量水.大别山双河硬玉石英岩中强变形的硬玉，从晶体核部到边缘的含水量变化为44×10-6~404×10-6~721×10-6(Su et al., 2004)，显示水解弱化作用对矿物的塑性变形及超高压剪切带的发育制约是明显的.研究还指出，随着压力增加，结构水溶解度增大，随着压力降低而逸出来，形成退变质流体.TEM分析还证实变形硬玉具有较高的位错密度，且位错环和位错壁等塑性变形组构发育，均与含结构水和水分子团相关，推测其可能是硬玉中部分Si-O键被Si-OH-OH-Si键所替代的结果.变形的石榴子石等水的载体矿物也有类似的情况.这些事实证明，超高压变质岩早期在地幔层次快速折返阶段，水流体起着重要的促进作用.
超高压变质岩折返到中下地壳层次后，会发生更为广泛的退变质水化作用.Masago et al.(2010)估算出哈萨克斯坦的柯切塔夫地块超高压榴辉岩和泥质岩，在水化退变质过程中，分别从下伏呈构造接触的低级变质Daulet岩套中吸取了约1.0%及0.2%重量水.广而言之，在地壳层次不同的折返和隆升阶段，源于深俯冲带内部或外部具不同盐度的H2O或CO2-H2O流体，对后成合晶及冠状体等卸载不平衡结构的发育、角闪岩相条件下减压选择性部分熔融作用、岩石及其组合流变性能变化、不同物理化学条件下含矿和不含矿热液脉体的发育等(徐珏等，2004；汤倩等，2006)都有关键性的约束作用.总之，要全面重塑超高压变质岩的构造折返过程，分析P-T-t-d轨迹故然很重要，而获取含水流体的成分、含量、活动性等信息也是重要的.研究指出，即使只有变质地体0.1%的水渗入，也足以影响广泛的退变质重结晶作用和折返过程(张泽明等，2006; Masago et al., 2010).
5. 结论
(1) 块状榴辉岩代表的超高压变质岩，是大别-苏鲁区三叠纪大陆深俯冲/碰撞的终端产物，深埋于100多千米的地幔层次，是超高压变质岩构造折返的起始位置，记录了折返过程的减压、冷却退变和变形的开始.
(2) 超高压变质岩石的折返是一个复杂的多阶段构造过程，每一个阶段都留下了特征性的构造学和岩石学记录.其中面理化榴辉岩反映地幔层次的折返，相继几个阶段的折返都发生在地壳不同层次.
(3) 超高压岩石的折返过程主要受构造体制的约束.同碰撞挤压体制与碰撞后伸展体制均曾主导过超高压岩石的折返过程.但相应的几何学、运动学及物理学特征是不同的，且受俯冲/碰撞带内部及外部多种因素的影响.其中水流体的含量、成分、活动性，对超高压岩石的减压退变质重结晶作用、变形和变质作用分解、部分熔融作用以及构造体制反转等，都起着关键性作用.
(4) 超高压和高压岩石构造单位内的岩石原岩是不均一的，经过深俯冲递进变质作用的改造，流变学不均一性更为明显.在构造应力及力学不稳定性作用下，变形和(退)变质作用分解现象，在折返过程的不同阶段和不同层次显示更为明显，域行为更为清楚，表明剪切作用是超高压和高压变质带内部的有效应变体制.
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图 1 大别-苏鲁区构造略图(据Suo et al., 2005修改)
CC.核杂岩单位；UHP.超高压单位；HP.高压单位；EB.绿帘-蓝片岩单位；SC.沉积盖层单位；LDZ.下拆离带；MDZ.中拆离带；UDZ.上拆离带；TDZ.顶拆离带；SDF.商丹断裂带；GMF.龟梅断裂带；NBMXF.娘娘庙-八里畈-磨子潭-晓天剪切(断裂)带；NHY.北淮阳构造带；XGF.襄樊-广济断裂带；WYF.五莲-烟台剪切(断裂)带；TLF.郯庐断裂带.右下方插图表示大别-苏鲁造山带核部地壳构造柱
Fig. 1. Simplified tectonic map of the Dabie-Sulu area showing major petrotectonic units, structural framework and detachment zones formed by extensional thinning, as well as tectonic pile of the UHP-HP metamorphic belt
图 2 大别-苏鲁区代表性的超高压变质岩显微及露头尺度照片
a.块状榴辉岩(标本号r-2-11-7-2)，单偏光，视域宽约4 mm，大别山碧溪岭；b.块状榴辉岩中绿辉石晶体内具放射状结构的多晶柯石英假像(标本号Ⅰ-2-2)，正交偏光，视域宽约1.3 mm，江苏省东海县碱场；c.面理化榴辉岩(r-205)，正交偏光，视域宽约4 mm，大别山碧溪岭；d.面理化榴辉岩形成的鞘状褶皱，地质锤把长约35 cm；e.发育麻粒岩-角闪岩相卸载不平衡后成合晶及冠状体结构的退变质面理化榴辉岩(标本号S-r-19-3)，单偏光，视域宽约5 mm，山东省荣城大疃，其中S2表示面理化榴辉岩的主面理；f.层状糜棱岩化斜长角闪岩(样品号S-r-17-4)，单偏光，视域宽约5 mm，山东省文登泽库，其中S、C分别为糜棱岩的片理面和剪切面；g.围岩片麻岩及面理化含榴花岗岩形成的平卧褶皱，地质锤把长约35 cm，安徽省潜山三祖寺；h.近水平对称面理布丁构造，布丁间隙由含石英的绿泥石物质充填，铁锤把长约35 cm，湖北省麻城何家湾.Grt.石榴子石；Omp.绿辉石；Rt.金红石；Coe.柯石英(假象)；Qtz.石英；Phe.多硅白云母；Ky.蓝晶石；Zo.黝帘石；Sym.后成合晶；Bt.黑云母；Hbe.角闪石
Fig. 2. Representative photomicrographs and outcrop-scale photographs of ultrahigh-pressure metamorphic rocks in the Dabie-Sulu area
图 3 碰撞造山带内高压-超高压变质岩可能的折返运动学模式(据Froitzheim et al., 2003)
a.侵蚀作用；b.伸展作用；c.挤出作用；d.上覆地幔楔向下抽拉作用.星号代表高压-超高压变质地体
Fig. 3. Possible kinematic models of exhumation of high- and ultrafigh-pressure metamorphic rocks
图 4 露头尺度块状榴辉岩透镜体与面理化榴辉岩间几何学和运动学关系
Fig. 4. Outcrop sketches showing the local geometric and kinematic relationships between lenses of massive eclogite and foliated eclogite
图 5 山东省荣城岳家陡立构造带剖面简图
Fig. 5. Simplified cross-section showing a steep structural belt in Yuejia, Rongcheng, northeastern Shandong
图 6 韧性伸展薄化作用的应变和运动学标志
a, b.非共轴剪切变形且沿着向南或南西缓倾斜面理正向剪切滑动(四道河)；c, d.共轴纯剪切变形和近水平方向伸展流动(分别位于麻城大河背及何家湾)
Fig. 6. Field sketches showing strain and kinematic indicaters for the structures acquired by ductile extensional thinning
图 7 大别-苏鲁区超高压变质岩的减压退变构造折返示意
减压退变P-T轨迹示意据张泽明等(2006)；D1，D2，D3，D4和D5，分别为UHP变质峰期变形及不同折返阶段变形记录示意.变质相代号：PP.葡萄石绿纤石相；PA.绿纤石阳起石相；GS.绿片岩相；BS.蓝片岩相；EA.绿帘角闪岩相；AM.角闪岩相；GR.麻粒岩相；EC.榴辉岩相
Fig. 7. Synoptic illustrations explaining the tectonic exhumation of UHP metamorphic rocks in the Dabie-Sulu area
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