阿尔金构造带西段加里东期山根的初步研究

【作者】 张建新；

【导师】 许志琴；

【作者基本信息】 中国地质科学院 ， 构造地质学， 1999， 博士

【副题名】榴辉岩及麻粒岩相岩石的变质变形及山根形成演化的探讨

【摘要】 在阿尔金构造带西段，在原认为主要由高绿片岩相—低角闪岩相(下元古界)的阿尔金群中，发现有榴辉岩及麻粒岩相的孔兹岩系，它们分别分布在阿尔金山的北坡和南坡。本论文通过对榴辉岩及孔兹岩系的岩石学、矿物学、地球化学、同位素年代学及变形作用的研究，提出它们为阿尔金构造带西段加里东期陆陆碰撞造山作用的产物，并探讨了加里东山根的形成和演化过程，榴辉岩及孔兹岩系的区域构造意义及阿尔金断裂可能的位移量。阿尔金构造带西段的榴辉岩呈透镜状产于长英质片麻岩中，它主要经历了进变质的角闪岩相(P=6.04-8.70Kb，T=615-690℃)、峰期的榴辉岩相(P＞15Kb，T=700-850℃)及减压过程中的麻粒岩相(P=11-14Kb，T=750℃)及角闪岩相(P=6.3-9.5，T=619-738℃)的变质作用，构成一个顺时针的PT轨迹。榴辉岩的稀土元素、微量元素及Sm-Nd同位素研究显示其原岩主要为过渡型的大洋拉斑玄武岩。选择保存较好的榴辉岩分别进行Sm-Nd和U-Pb的同位素年代学测定，其中测得全岩—石榴石—绿辉石的Sm-Nd等时线年龄为500±10Ma；四个颗粒的锆石U-Pb同位素测定获得的表面年龄很好地落在一致线上，并得出其权重平均值为503.9±5.3Ma。两种方法获得的基本一致的年龄数据代表了榴辉岩的峰期变质时代。榴辉岩及相关地质体的韧性形作用可分为三期，第一期变形以从N向S逆冲为特征，变形机制为简单剪切，代表造山带根部榴辉岩相—高压麻粒岩相条件下的变形作用；第二期变形作用为SN向挤压、近EW向拉伸的共轴变形，形成近EW向的拉伸线理，其变形条件为榴辉岩减压过程中的中高压角闪岩相，且在不同构造层次上产生不同的变形构造；第三期变形在含榴辉岩的深部单元表现为垂向挤出，变形条件为中低压角闪岩相，而在滑脱面之上的浅部单元为SN向的伸展作用。阿尔金构造带西段的孔兹岩系的主要由夕线石榴黑云二长片麻岩、石榴黑云二长片麻岩、含石墨夕线石榴黑云片岩等富铝的片麻岩(片岩)类和夹于片麻岩(片岩)中的石榴角闪二辉麻粒岩，并伴随有夕线榴长石石英岩及含石墨大理岩。岩石学及地球化学的初步研究表明富铝的片麻岩(片岩)的原岩为富铝的泥质和泥砂质沉积岩，所夹基性麻粒岩的原岩可能为大陆拉斑玄武岩，这套孔兹岩系的原岩可能形成于大陆边缘环境。孔兹岩系经历了麻粒岩相的变质作用，矿物的温压估算得到其峰期变质温度为700-850℃，压力为8-12Kb。岩石中变质锆石的U-Pb及Pb-Pb同位素测定获得447-462Ma的年龄值，代表其麻粒岩相的变质作用时代，富铝片麻岩中含继承组分的锆石还获得较老的上交点年龄，它可能代表其沉积岩原岩源区的年龄。孔兹岩系的变形作用可分为三期：第一期变形以SN向挤压、近EW向拉伸的共轴变形为特征；第二期变形表现为为从NW向SE的逆冲剪切作用；第三期变形则可能与阿尔金主断层的左行走滑活动有关。榴辉岩及孔兹岩系的447-503Ma的峰期变质年龄显示了阿尔金西段加里东期山根的存在，它的形成和演化经历了a：早期与榴辉岩形成有关的板块的俯冲和山根的形成，b：与榴辉岩的减压作用及孔兹岩系的形成有关的SN向挤压、EW向拉伸及c：晚期的折返作用——造山带根部的垂向挤出及较浅层次的伸展及扩展作用。通过阿尔金构造带西段的榴辉岩及孔兹岩系与柴北缘构造带的榴辉岩及麻粒岩相岩石的初步对比，认为它们可能同为加里东期的陆陆碰撞带的一部分，后来被阿尔金断裂的左行平移作用所切割，结合阿尔金构造带其它地质体与祁连造山带的对比，粗略地估计其位移大约在400—500km之间。更多还原

【Abstract】 At western segment of Altyn Tagh tectonic belt, The eclogites and khondalite series were recognized in Altyn Group (Early Proterozoic?) Which had been thought prviously that it consists of high greenschist facies and lower amphibolitic facies metamorphic rocks. They crop out at the northern slope and southern slope of Altyn Tagh Mountains respectively. The purpose of this thesis is twofold. One is to present the Petrology, geochemistry , geochronologyl, and ductile deformation on eclogites and Khondalite series. On the basis of these, the second objective is to propose that the eclogite and Khondalite series resulted from Caledonian continent-continent collision, and discuss the formation and evolution of Caledonian orogenic root, the regional tectonic significance of eclogite and khondalite series and possible displacement of Altyn Tagh fault.The eclogites at western segment of Altyn Tagh tectonic belt occur as lens or boundins within quratz-feldspathic gniesses characterized by amphibolite-facies parageneses. It mainly experienced the prograde amphibolite-facies stage (P=6.06-8.70kb, t=615-690℃) ,peak eclogite-facies stage (P＞15kb,T=700-850℃), granulitic facies stage (P=11-14kb,T=750℃) and ampbiolite-facies stage (P=6.3-9.5kb,T=619-738℃) during decompression. The trace element, rare earth element and Sm-Nd isotopic data suggest that most of the eclogites have protolith feature resembling T-type (transitional type) MORB (mid-ocean ridge basalt) . The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield an Whole rock-garnet-omphacite isochron of 500±10Ma age.The U-Pb isotopic measurements of zircons show that the four grain populations are near concordant and are well plotted on concordia curve, giving a weighted mean age 503.9±5.3Ma. Two kinds of methods obtain a similar age which reveals the peak metamorphic age of eclogites. Three stages of ductile deformation were recognized in eclogites and terrane related to eclogites . The first stage of deformation is characterized by thrusting from north to south with a deformation mechanism of simple shear, representing the deformation of orogenic root under the condition of eclogite-facies to high pressure granulite facies metamorphism. The second stage of deformation show coaxial deformation mechanism with SN compression and parallel-orogen EW stretching under the condition of middle-high pressure ampbiolite facies metamorphism. The third stage of deformation caused vertical extrusion of the under unit including eclogites and SN extension and thining of upper units.The Khondalite series are characterized by aluminum-rich gneisses(schists) consisting of sillimanite-garnet-biotite-monzonite gneiss, garnet-biotite-monzonite gniess, graphite-sillimanitebiotite schist, and garnet-amphibole two-pyroxene granulites occuring as lenses and layers within gneisses (schists). The petrology and geochemistry indicate that the protoliths of aluminum-rich gneisses(schists) are aluminum-rich pelitic and pelitic arenaceous sedimentary rocks, the protoliths of basic granulites are continental tholeiitic basalts. Therefore, the khondalite series may be produced at continental margin. They had suffered granulitic facies metamorphism with the peak temperatures of 700-850℃and the pressures of 8-12Kb.The U-Pb and Pb-Pb isotopic dating of zircons provided the ages of 447-462Ma representing the ages of peak granulitic metamorphism. The U-Pb dating of detrital zircons from aluminum-rich gneisses yielded older upper intercept ages which reflect the times of older materials derived from sourse rocks of the gneiss protoliths. Three stages of deformation can be observed in khondalite series.The first stage of deformation is characterized by co-axial SN compression and EW stretching under the condition of granulite facies metamorphism ; The second stage of deformation is represented by thrusting from NW to SE. The third stage of deformation may be related to sinistral strikeslipping of main Altyn Tagh fault.The ages of 447-503Ma for eclogite and khondalite series imply the existence of Caledonian orogenic root. The formation and evolution of orogenic root include: a: the subduction, continentcontinent collision and formation of orogenic root related to the formation of eclogite; b: SN compression and EW stretching related to early exhumation of eclogite and formation of khondalite series, and c: the vertical extrusion of orogenic root and simultaneous extension and spreading of upper crust.The tentative comparsion between eclogites and granulite facies rocks at western segment of Altyn Tagh, and North Qaidam Mountains imply that the western segment of Altyn Tagh tectonic belt is part of the Same Caledonian continent-continent collision orogenic belt as the North Qaidam tectonic belt, and is truncated by sinistral strike-slip Altyn Tagh fault. In combination with comparsion between other terraneain Altyn tagh Mountains and Qilian Mountains,the displacement of Altyn Tagh fault is about 400-500km.更多还原