【作者】 李阳；

【导师】 张国伟；

【作者基本信息】 西北大学 ， 构造地质学， 2017， 博士

【摘要】 印支期碰撞造山奠定了秦岭造山带的基本构造格架,是中国大陆拼合的重要时期,同时也是秦岭造山带由古生代板块构造体制向中、新生代的陆内造山体制过度的重要阶段。然而,目前关于秦岭印支期碰撞造山的结束时间依然不够明确,主要表现为晚三叠花岗岩形成的构造环境存在巨大争议。由此引申的其他问题也长期得不到有效解决,如“环斑结构”花岗岩能否指示碰撞造山的结束等。针对秦岭晚三叠世花岗岩,前人的工作大多集中在岩相学和岩石地球化学方面,而花岗岩的地球化学特征能否指示其侵位时的大地构造环境仍是值得商榷的问题。本次论文以拟解决秦岭造山带晚三叠世岩浆作用的构造环境问题为选题依据,选取具有代表性的东江口岩体及其围岩为主要研究对象,结合前人研究成果和进展,开展包括野外构造观察、显微构造研究、内部组构分析、同位素年代学以及岩体定位深度等在内的岩体构造研究,结果表明:(1)东江口岩体及其围岩内记录了大量的同岩浆期构造变形,说明秦岭造山带晚三叠世仍处于同碰撞挤压的构造环境。晚三叠世东江口岩体内发育了大量的同岩浆期构造,而并非前人所认为的无任何变形。岩体中暗色包体以及花岗岩脉的同岩浆期构造变形均说明在其形成阶段剪切变形作用广泛存在。岩体北界沙沟街韧性剪切带表现出左行走滑挤压的运动学特征,为NNE向汇聚挤压沿商丹构造带变形分解作用所致,变形时间限定为219～210 Ma。岩体南界的山阳—凤镇于晚三叠世发生逆冲推覆构造,其次级断裂发育的鞘褶皱群、叠瓦状逆冲推覆构造、不对称旋转碎斑、挤压构造透镜体一致表明,断裂主活动期为SSW向的逆冲推覆,兼具左行走滑特征。此外,围岩接触带内岩石的较高温度变质变形为岩浆作用与构造作用共同作用的结果。而围岩刘岭群中同构造花岗岩脉(左行走滑挤压)的形成时代为210～201 Ma,进一步表明晚三叠世仍然存在强烈的构造变形作用。因此认为秦岭造山带于晚三叠世仍处于同碰撞挤压的构造环境,并由此推断同期的“环斑结构”花岗岩不能代表印支期造山的结束。(2)东江口岩体具有复式岩体特征,在横向上由两个不同时期形成的单独的岩体拼和而成,在纵向上岩体的形成具有增量生长特征,其同心环状的岩相学分带不是岩浆房分异演化的结果,而是不同批次岩浆作用的体现。岩体内部组构研究表明,东江口岩体内部呈现两个明显不同的同心环状分带。位于营盘附近的小环带内花岗岩获得210 Ma的锆石U-Pb年龄,而较大环带内花岗岩的结晶年龄为200.6～199.2 Ma。结合前人定年成果综合认为,东江口岩体是由两个分别形成于～210Ma和～200Ma的岩体在横向上拼合而成。同时,较大同心环带内从内到外岩石有变年轻趋势,角闪石全Al压力计的研究结果也反映了各岩相单元形成于不同深度上,表现出从内到外形成压力(深度)逐渐变大的特征。因此,年代学和侵位深度的研究共同说明该岩体的形成具有增量生长的特征,其同心环状的岩相学分带应是不同批次岩浆作用的反映。(3)秦岭造山带晚三叠世的构造作用可能控制了同期的岩浆作用,并且该阶段的碰撞造山作用导致了东西秦岭双向挤出构造。东江口岩体的侵位生长机制的研究结果表明,构造变形作用控制了岩体岩浆侵入以及结晶晚期阶段组构的形成。此外,出露于商丹构造带及附近的八里坪岩体、侵位于南秦岭的老城岩体和胭脂坝岩体以及西秦岭地区发育的糜署岭岩体的侵位均受到同期区域构造的约束。同时,秦岭造山带晚三叠世花岗岩的空间分布也显示与大型断裂带有明显的关联。因此,晚三叠世的构造作用很可能控制了同期的岩浆作用的发育。再者,晚三叠世的碰撞造山导致了东、西秦岭形成完全反向的剪切运动学,在商丹带和勉略带内均有所体现,据此推测秦岭晚三叠世碰撞造山发育了双向挤出构造。更多还原

【Abstract】 The Indosinian Orogeny played a special role in the whole evolutionary history of the Qinling orogen.It not only laid a foundation for the formation of the Qinling but also represented an important transition stage between the Paleozoic plate tectonics and the mesozoic-cenozoic intra-continental tectonic regime.However,the end time of this orogenic episode is uncertain due to the debate on the tectonic setting for those Late Triassic plutons with proposals of syn-subduction,syn-collision,and post-or late-collision.In spite of those plausible models,it should be noted that most previous works have focused on petrological and geochemical studies and neglected to detect the structural information in these granites.However,it remains to be demonstrated if the geochemistry of granites can discriminate the tectonic setting in which they emplace.In this study,multidisciplinary studies of the Late Triassic Dongjiangkou granite pluton and its country rocks have been conducted to explore the emplacement mechanism of the pluton and further to constrain the tectonic setting of the Late Triassic magmatism in the Qinling Orogen.The following salient insights can be drawn from this study:(1)The Qinling orogen was still under the syn-collisional convergent setting during the Late Triassic orogeny,according to evidences of extensive syn-plutonic deformation that were recognized both in the Dongjiangkou pluton and its country rocks.There are extensive syn-plutonic deformations can be recognized within the Late Triassic Dongjiangkou pluton and its country rocks.In the pluton,syn-plutonic shear deformations that can be inferred from the mafic microgranular enclaves and granitic dikes attest to the existing of regional deformation during the growth pcocess of the pluton.At the immediately north of the pluton,the sinistral transpressional kinematics recorded in the Shagoujie ductile shear zone is deformation partitioning of the oblique convergence along the Shangdan belt,and the timing of deformation can be limited at the interval of ca.219-210 Ma.At the immediately south of the pluton,the Shanyang-Fengzhen fault is characterized by thrust and nappe tectonics combining with sinistral movement that were indicated by sheath folds,imbricate thrust-nappe structure,asymmetry rotated porphyroclast and structural lenticle.Meanwhile,the high-temperature deformation in the contact between the pluton and its country rocks also suggests the coexistence of the regional deformation and the magmatism.Furthermore,formation ages(210-201 Ma)of the syntectonic granitic dikes emplaced within the Liuling group can provide the upper limit on the sinistral transpressional deformation.Therefore,we argue that the Qinling orogen was still under the syn-collisional convergent setting during the construction of the Late Triassic Dongjiangkou pluton,and the coeval"rapakivi-textured" granites in Qinling could not be used to mark the end of the Late Triassic orogeny.(2)Laterally,the Dongjiangkou pluton is assembled by amalgamation from two separated plutons of different ages.Vertically,it is characterized by incremental growth mechanism.The concentric petrologic zoning of the pluton cannot be interpreted as having resulted from in situ magma differentiation but rather have been inherited from the source region before emplacement.Internal fabrics of the Dongjiangkou pluton show two different concentric zonings.Granites collected from the smaller zoning close to the Shagoujie ductile shear zone yielded a crystallization age of 210 Ma,while formation age of granites from the bigger zoning close to the Shanyang-Fengzhen fault is ca.200.6～199.2 Ma.Taken together,our structural and geochronological studies indicate a composite origin of the Dongjiangkou pluton that it is laterally assembled by amalgamation from two separated plutons of different ages(～210 Ma and～200 Ma).Meanwhile,crystallization age of granites from the bigger zoning show a trend of getting younger from margin to core.Moreover,results of Al-in-hornblende geobarometer show each petrological facies emplaced within different level of the crust,i.e.intrusion level of the granites are getting deeper from core to margin.Therefore,the emplacement of the Dongjiangkou pluton can be characterized by incremental growth mechanism,and the concentric petrologic zoning of the pluton is probably resulted from multiple magma batches rather than in situ magma differentiation.(3)Regional tectonics probably controlled the emplacement of Late Triassic granite magmatism.The bilateral extrusion(escape)tectonics model is proposed for better understanding the coeval opposite kinematics in the West and East Qinling which were developed in response to the Late Triassic collision.Structural studies of the Dongjiangkou pluton demonstrate that regional tectonics controlled the emplacement of the pluton as well as the formation of the internal fabrics during late stage of the crystallization process.Moreover,accroding to previous studies,emplacement of many other Late Triassic plutons also show close relationship with regional tectonics,such as the Baliping pluton,the Laocheng pluton,the Yanzhiba pluton and the Mishuling pluton that were developed within the Shangdan belt,the South Qinling and the West Qinling,respectively.It is also noteworthy that almost all these Late Triassic plutons expose in/near the narrower segment,and mainly bounded by the crustal-scale fault zones.Therefore,we argue that regional tectonics probably controlled the emplacement of granite magmatism during the Late Triassic orogeny.Furthermore,the coeval opposite kinematics in the West and East Qinling is probably resulted from the bilateral extrusion(escape)tectonics in response to the Late Triassic oblique convergence.更多还原