【作者】 李建波；

【导师】 王涛；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2010， 博士

【摘要】 华北克拉通北缘发育有一系列中生代变质核杂岩,被认为是了解地壳伸展构造特征的窗口。本文选取位于华北克拉通楼子店、云蒙山、医巫闾山和辽南变质核杂岩韧性拆离带作为研究对象,以有限应变测量和运动学涡度为研究方法,精细确定拆离带的应变型式、剪切作用类型以及韧性减薄量；以此为基础,探讨华北克拉通北缘中生代以来主要核杂岩的形成机制、韧性减薄量,并为探讨地壳伸展减薄的机制、减薄量及其地球动力学背景提供依据。楼子店变质核杂岩位于赤峰南部,为—NE—南西向、SE倾的低角正断层系,下盘由韧性剪切带、韧-脆性剪切带以及脆性断裂带组成。韧性剪切带自下而上依次发育糜棱岩化岩石、糜棱岩和初糜棱岩。长石应变标志体的Rf/φ法有限应变测量显示,在韧性剪切带内核部糜棱岩的应变强度最大(Es=0.67－0.96),下部接近花岗岩体的糜棱岩化岩石与上部的初糜棱岩次之(Es分别为0.59－0.62和0.47－0.56)。对数付林图解与霍塞克图解显示初糜棱岩的应变类型为接近于平面应变的拉长应变(lod参数v=-0.228--0.535),糜棱岩应变类型为平面压扁应变(lod参数v-0.14－0.87)。用极摩尔圆、刚性颗粒网法测得的运动学涡度值分别为初糜棱岩0.81-0.90,平均0.85；糜棱岩0.51-0.80,平均0.66；依据最大有效力矩准则获得伸展褶劈理形成时的运动学涡度值为0.63－0.37,平均0.5。由最大有效力矩准则确定的主压应力轴σ1与拆离带边界的夹角角经历了61°、69°到75°的递进增大过程,最后趋于与拆离带边界垂直。云蒙山变质核杂岩发育—NE走向、SE倾向的低角拆离正断层系,其中韧性拆离断层带由靠近核部岩体的糜棱岩化岩石、初糜棱岩、糜棱岩组成。长石碎斑的Rf/φ法有限应变测量显示：糜棱岩的应变强度最大(Es=0.72－1.0),初糜棱岩次之(Es=0.70－0.86),糜棱岩化岩石最小(Es=0.66－0.70)。付林图解与霍塞克图解显示应变类型为压扁型(lod参数v=0.29－0.91)。极摩尔圆获得的平均运动学涡度为0.76,显示早期糜棱岩面理与线理记录了以单剪为主的一般剪切作用；演化后期伸展褶劈理记录的平均运动学涡度为0.613,显示以纯剪为主的一般剪切作用。依据运动学涡度与有限应变获得的瓦子峪拆离带内韧性剪切带的韧性减薄量介于330－430m之间。医巫闾山变质核杂岩为—NE走向、NW-NNW倾向的低角正断层系,瓦子峪拆离断层带自下而上依次由韧性剪切带(糜棱岩带)、韧脆性剪切带(碎裂岩带)以及脆性拆离断层面组成。其中韧性剪切带由下而上由糜棱岩化花岗片麻岩以及糜棱岩组成。韧—脆性剪切带主要表现为脆性构造发育并对先存韧性剪切带的叠加改造。脆性拆离断层面主要表现为断层泥与断层角砾岩发育。基于石英颗粒的Rf/中有限应变测量显示糜棱岩的应变强度Es=0.51-1.0。付林图解显示应变类型为压扁应变(lod参数v=0.47－0.90)；霍塞克图解显示随应变强度增大罗德参数也逐渐加大。基于极摩尔圆、石英条带斜交面理以及有限应变法估算的运动学涡度显示,韧性剪切带内糜棱岩面理和线理形成时记录了以单剪为主的一般剪切作用,运动学涡度Wk=0.61－0.96,平均为0.80；伸展褶劈理记录了以纯剪为主的一般剪切作用,Wk=0.24－0.53,平均为0.37。依据运动学涡度与有限应变获得的瓦子峪拆离带内韧性剪切带的韧性减薄量介于320－520m之间。我们选择石英作为应变标志体,主要因为在医巫间山变质岩区,样品手标本及薄片下中长石的含量较少、石英的变形强烈成拔丝状,更能代表岩石应变。辽南变质核杂岩为—NNE走向、NWW倾向的低角正断层系,其组成从下往上依次为糜棱岩化花岗片麻岩、韧性剪切带、绿泥石化带、脆性拆离断层面以及上盘未变质的元古代-古生代-中生代岩石组成。其中韧性剪切带由位于下部的糜棱岩化花岗片麻岩以及糜棱岩组成。糜棱岩中的所有运动学指向标志都指示上盘向NWW的拆离。基于长石的Rf/Φ有限应变测量显示糜棱岩的应变强度介于0.36－1.0、平均为0.75。付林图解与霍塞克图解显示应变类型为压扁型(lod参数v=0.11－0.98)。用极摩尔圆法与有限应变法获得的糜棱面理与线理形成时的运动学涡度介于0.74－0.96之间,平均为0.85；依据最大有效力矩准则获得伸展褶劈理或C′形成时的运动学涡度介于0.10－0.84,平均0.58。依据运动学涡度与有限应变获得的拆离带的韧性减薄量为水源地395m、董家沟705m、大李家屯212m、三十里铺387m、普兰店437m、瓦房店原台130m。野外观察发现,上述拆离带内的韧性剪切带(糜棱岩带)内的糜棱岩发育两种组构,其一为由石英与长石碎斑定向排列构成S－C组构和拉伸线理。其二为以小角度切割和错断透入性糜棱面理并使之发生弯曲且剪切指向与糜棱状岩石的剪切指向一致的伸展褶劈理或C′。研究发现,伸展褶劈理或C′带内残存的早期面理发生了转动,而带外面理则保持其初始方位；这些说明伸展褶劈理或C′是在同一递进变形过程中的较晚的增量应变期间形成的。基于不同的运动学涡度估算方法对糜棱面理与伸展褶劈理或C′进行计算可知：糜棱面理与线理记录了单剪为主的一般剪切作用、伸展褶劈理或C′记录了纯剪为主的一般剪切作用；结合糜棱面理与伸展褶劈理或C′发育的先后顺序,认为研究区拆离带内糜棱岩经历了早期单剪为主、晚期纯剪为主的一般剪切作用。该研究结果精细刻画了拆离带形成的剪切变形机制,并从变形机制角度进一步证明,研究区地壳伸展作用发生后,以糜棱面理与线理发育为代表的早期单剪为主的一般剪切、以伸展褶劈理发育为代表的晚期纯剪为主的一般剪切的剪切模式在变质核杂岩及其拆离带的形成中可能具有普遍意义：与之对应,以简单剪切为代表的地壳伸展与以纯剪切为代表的岩浆底侵是制约变质核杂岩及其拆离带形成的主导因素。本研究表明,华北克拉通核杂岩拆离带演化过程可能是地壳缩短加厚到伸展减薄这一构造体制转折的反映。依据应变与运动学涡度估算的韧性减薄量表明,拆离带在形成演化过程中存在韧性减薄,减薄量因剪切带的厚度不同而有差异,虽然韧性减薄对地壳的总体减薄量贡献不大,但韧性减薄确实存在,且对于丰富中生代以来华北克拉通伸展减薄研究仍具有重要的启示意义。需要说明的是,以长石为应变标志体时其应变小于全岩应变,以石英为应变标志体时其应变可能大于全岩应变；再者,由于没有获得拆离带内局部细小的强变形岩石的应变参数：因此,我们估算出的拆离带内的减薄量为最小的韧性减薄量。本研究说明,在估算变质核杂岩伸展拆离对地壳减薄量贡献时,不应仅仅依据主拆离带位移(主要为刚性位移)量,也应考虑韧性变形的效应和减薄量,以便更准确估算构造变形对华北岩石圈和地壳减薄的贡献。本文与国内外诸多研究成果相一致,比较典型的有中蒙边界区中生代亚干大型拆离断层系以及希腊西奈山Chelmos剪切带,共同特征是早期单剪为主、伴随伸展引发岩浆侵入逐步转化到晚期纯剪为主的构造体制,且两者共同作用制约着研究区变质核杂岩及其拆离带的形成与演化。目前,以有限应变测量为基础,对变质核杂岩及其拆离带进行精细的应变与运动学涡度解析,在国际上类似的研究还不多见,由此推算拆离带韧性减薄量的研究更少。本文是这方面研究的初步探讨,说明该研究思路、方法可行。更多还原

【Abstract】 A series of metamorphic core complex are located at the northern margin of the North China Craton, and regarded as a window to understand the crust extension. We choose the Louzidian, Yunmengshan, Yiwulvshan and Liaonan as the research objects; and select strain measurement and kinematic vorticity as research methods; and determine the strain types, shear types and ductile thinning of metamorphic core complex and its detachment fault zone. Based on these, we can investigate the formation mechanism, ductile thinning of these metamorphic core complex, and further provide the evidences of mechanism of crust extension and thinning, quantification of thinning and geodynamic background of the North China Craton.The Louzidian low-angle ductile shear detachment zone at the south of Chifeng is a SE-dipped, low angle normal fault system. It is composed mainly of ductile shear zone, ductile-brittle shear zone and brittle fault zone. The ductile shear zone consists of, from bottom to top, mylonitic rocks, protomylonites and mylonites. Finite strain measurement of feldspar strain markers from those rocks using the Rf/φmethod shows strain intensities (Es) increase from 0.47-0.56 to 0.59-0.62 and to 0.67-0.96, respectively, and the strain types of the protomylonites and mylonties are flattening strain (lod parameter of protomylonites v=-0.228——0.535; lod parameter of mylonites v=0.14-0.87), respectively. The kinematic vorticity values (Wk) estimated by the Polar Mohr diagram and the Rigid Grain Net range from 0.81 to 0.90 with an average of 0.85 for the protomylonites, and from 0.51 to 0.80 with 0.66 on average for the mylonites; W(?) values of the extensional crenulation cleavage (i.e. C’) estimated by C method range from 0.63 to 0.37 with an average of 0.50. Dip angles of the maximum principle stress determined using the Maximum effective moment criterion evolved from 61°to 69°and to 75°, and finally normal to shear direction.The Yunmengshan metamorphic core complex is a NE trend, SE-dipped, low-angle normal fault system. It is composed mainly of, from bottom to top, mylonitic rocks, protomylonites and mylonites. Finite strain measurement of feldspar strain markers f using the Rf/φmethod suggests strain intensities increase from mylonitic rocks (Es=0.66-0.70), proto mylonite (Es=0.70-0.86) and mylonite (Es=0.72-1.0). Fulin and Hossack diagrams indicate strain types is close to flattening strain(lod parameter v=0.29-0.91). Kinematic vorticity measurements using Mohr diagram method suggest that foliations and lineation of mylonite (0.63<Wk<0.85) record a bulk simple-dominated general shearing at the beginning of evaluation of Hefangkou detachment fault; and Extensional crenulation cleavage (0.34<Wk<0.77) recorded a bulk pure-dominated general shearing at the end of evaluation of Hefangkou detachment fault. Based on the strain measurement and kinematic vorticity, we estimate the quantification of thinning ranging from 330 to 430m.The Yiwulvshan metamorphic core complex is a NE trend, NW-NNW-dipped, low-angle normal fault system. It is composed mainly of, from bottom to top, ductile shear zone (mylonite zone), ductile-brittle shear zone and brittle detachment fault plane. Ductile shear zone consists of mylonitic gneiss which lies at the bottom, and mylonite which locates at the top. Ductile-brittle shear zone performs as brittle structure overprinting the pre-existed ductile shear zone. Brittle detachment fault plane registers as fault gouge and fault breccia. Finite strain measurement of quartz strain markers using the Rf/φmethod suggests strain intensities of mylonite chang from 0.51 to 1.12. Fulin and Hossack diagrams indicate strain types is close to flattening strain (lod parameter v=0.47-0.90). Kinematic vorticity measurements using Mohr diagram method, oblique foliation in quartz ribbons and finite strain method suggest that foliations and lineation of mylonite record a bulk simple-dominated general shearing, the Kinematic vorticity ranges from 0.61 to 0.96 with an average of 0.80; and Extensional crenulation cleavage recorded a bulk pure-dominated general shearing, and its Kinematic vorticity changes from 0.24 to 0.53 with 0.37 on average. Based on the strain measurement and kinematic vorticity, we estimate the quantification of thinning ranging from 320 to 520m.The Liaonan metamorphic core complex is a NE trend, NNW-dipped, low-angle normal fault system. It is composed mainly of, from bottom to top, mylonitic granitic gneiss, ductile shear zone, chloritized zone, brittle detachment fault plane and the non-metamorphosed rocks within upper plate ranging from Proterozoic to Paleozoic, and to Mesozoic. Ductile shear zone consists of mylonitic granitic gneiss and mylonites. Finite strain measurement of feldspar strain markers using the Rf/φmethod suggests strain intensities of mylonite changing from 0.36 to 1.0 with an average of 0.75. Fulin and Hossack diagrams indicate strain types is close to flattening strain(lod parameter v=0.11-0.98). Kinematic vorticity measurements using Mohr diagram method, and finite strain method suggest that foliations and lineation of mylonite record a bulk simple-dominated general shearing, the Kinematic vorticity ranges from 0.74 to 0.96 with an average of 0.85; and Extensional crenulation cleavage recorded a bulk pure-dominated general shearing, and its Kinematic vorticity changes from 0.10 to 0.84 with 0.58 on average. Based on the strain measurement and kinematic vorticity, we estimate the quantification of thinning of Shuiyuandi, Dongjiagou, Dalijiatun, Sanshilipu, Pulandian and Yuantai, and their quantification of thinning are 395m,705m,212m, 387m,437m and 130m, respectively.Based on the field observations, we find there are two types of fabrics in the studying mylonites, one is S/C fabric defined by orientational quartz and feldspar; the other is extensional crenulation cleavage (ecc) or C’, and it is characterized by discrete sub-parallel minor shear bands; they transect and displace the mylonitic foliation and stretching lineation at a small angle and cause previous foliations rotation. These suggest that the ecc formed during relatively later increments of the same progressive deformation.The research of kinematic vorticity indicate that foliation and lineation of mylonites record a bulk simple-dominated general shearing, and extensional crenulation cleavage recorded a bulk pure-dominated general shearing; together with the sequence of foliation and extensional crenulation cleavage of mylonites, we consider that the mylonites in study area experience simple-dominated at early period, and pure-dominated at later period during mylonites formation. This result depicts subtly the mechanism of deformation of detachment fault zone, and further demonstrates that simple shear at the early stage and pure shear at the late stage in the formation of metamorphic core complex has probably general significance. Crust extension represented by simple shear and magma intrusion represented by pure shear are dominated factors that control the formation of metamorphic core complex and its detachment fault zone. Strain and kinematic vorticity researches are the effective methods to reveal the dynamics of tectonic regime transition and lithospheric thinning in North China during Late Mesozoic. The quantification of ductile thinning estimated by strain and kinematic vorticity indicate that there really existed ductile thinning during the formation and evolution of detachment fault zone; although the quantification of ductile thinning is a little, it helps us understand preferably the extension and thinning of the North China Craton. There are some problems need to express clearly that the strain of feldspar is lower than the whole rock strain, whereas the strain of quartz can represent the whole rock strain; the veins which are characterized by locality and strong deformation are not consideration; so, the quantification of ductile thinning estimated by us is the minimum.This paper further confirm that there is a tectonic regime transition from crust thickening to crust extension thinning in the North China Craton at Mesozoic, and demonstrate the Maximum effective moment criterion is an effective method to interpret the low angle normal fault system.This conclusion coincides with other classical studies, such as the Yagan detachment fault in the Sino-Mongolian border area and the Chelmos shear zone of External Hellenides Greece. These shear zone are characterized by a transition of tectonic regime from simple-dominated shear at early stage to pure-dominated shear at a late stage which produced by magma intrusion.更多还原