南秦岭马道变质杂岩的构造属性及折返机制研究

The Mianlue mélange can provide significant information for the Indosinian collisional process in South Qinling. However, the spatial distribution of the mélange is still in dispute. That is, whether the rocks exposed between the Zhuangyuanbei fault and the Chengxian-Liuba fault belong to the Mianlue mélange or not. This problem, to a great extent, limits the establishment of the orogenic model for the South Qinling during Indosinian. This project focuses on the Madao metamorphic complex in the north of the Zhuangyuanbei-Baocheng fault. Through the observation of geological transects and large-scale mapping in the field, this project will provide rocks assemblage of the Madao metamorphic complex as well as their geological occurrence, distribution, and contact relationships of different types of rocks. The tectonic framework and deformation stages will be founded based on the tectonic superimposition and kinematic symbols of microscope and outcrop-scale structures. Through the analyses of petrography and mineral chemistry, the metamorphic stages and conditions of the Madao metamorphic complex will be acquired and the relationship between the deformation structures and metamorphic crystallization will be identified. Geochronological studies are designed to restrict the metamorphism-deformation age and the formation age of the Madao metamorphic complex. Based on these, the tectonic affinity and exhumation mechanism of the Madao metamorphic complex will be proposed. Through the discussion on the tectonic evolution of the Madao metamorphic complex, new evidences are provided for the Indosinian orogenic process of the South Qinling.

勉略混杂岩带可为印支期南秦岭碰撞造山过程提供重要信息。然而，一直以来，对于状元碑断裂与成县—留坝断裂间的岩石是否属于勉略混杂岩带的争议很大程度上制约了南秦岭印支期造山模式的建立。本项目以状元碑—褒城断裂北侧马道变质杂岩为主要研究对象，通过野外不同剖面观察和大比例尺地质填图，确定马道变质杂岩的岩石组合及不同类型岩石的产出状态、分布范围和接触关系；通过观察岩石内运动学标志、构造叠加关系，分析物质运动方向，建立构造格架并划分变形期次；通过岩相学及矿物化学分析，确定变质阶段及其变质条件，并识别出变形与变质结晶作用的关系；并辅以年代学研究，厘定杂岩的形成时代和各期变质变形作用发生的时代；在此基础上，确定马道变质杂岩的构造属性和折返机制。通过探讨马道变质杂岩的构造演化为南秦岭印支期的造山过程提供了新证据。

马道变质杂岩的构造属性和折返机制的厘定可为印支期南秦岭碰撞造山过程提供重要信息，从而影响南秦岭印支期造山模式的建立。本项目主要对马道变质杂岩进行了宏微观地质调查、剖面测制以及大比例尺填图，通过岩石组合、分布状态、接触关系等确定了马道变质杂岩并非前人所定的变质基底，而是由片岩、片麻岩基质及其内部的不同类型的岩块（大理岩、斜长角闪岩、石英岩等）组成的混杂带，具有典型的“block-in-matrix”结构。地球化学分析结果显示基质的原岩主要为泥质岩，基质的锆石U-Pb测试获得的最年轻年龄为早古生代。基质内部主要经历了Dn南北向挤压，Dn+1由北向南逆冲以及Dn+2近东西向左行走滑三期韧性变形和晚期的两期脆性变形。其中Dn+1期形成了多条近东西向延伸的韧性剪切带，将混杂带分隔为不同的岩片。Dn+1期同变形的花岗岩脉体锆石U-Pb年龄为207Ma，未变形的脉体锆石U-Pb年龄为199Ma，片麻岩中黑云母氩氩年龄为183Ma，这就限定了变形发生及停止的时间。镜下特征表明，Dn期、Dn+1期变形分别对应变斑晶生成及后成合晶阶段。混杂带南、北部分片麻岩样品中石榴子石变斑晶却分别显示出生长环带和扩散环带，且自北向南计算获得的温压条件变化无明显规律性，反应了不同岩片具有独立的变质演化过程。混合岩基体和脉体内深熔锆石的U-Pb年代学数据显示区域总体变质时间介于200-233Ma。综合上述成果，认为马道混杂带内部为不同深度的构造岩片叠置体。混杂带的折返机制与造山带内部发育的板底垫托作用相关，板底垫托伴生的一系列逆冲断层及韧性剪切带，控制了不同深度岩片的抬升及并置。马道混杂带及其折返机制的确定，填补了马道地区增生造山过程研究的空白，揭示了俯冲物质的剥露过程，进而完善了印支期南秦岭地区板块间耦合机制。