西藏羌塘中部蓝岭高压变质带的构造剥露过程

This study focuses on the Lanling high-pressure metamorphic belt in Central Qiangtang of Tibet, and aims to find out its tectonic exhumation process based on studies on the structural sequence and the relationship between deformation and metamorphism. On the basis of conducting structure-lithology mappings (on 1:10 000 scale ) and geological sections (on 1:2000 or 1:1000 scale), and figuring out the superimposition or replacement relationship of different structure types, the general structural style will be constructed. The deformation sequence will be established after microscopic analyses on the deformation behaviors and structural fabrics of metamorphic rocks and minerals. According to the forming sequence of metamorphic minerals and their composition changes from EPMA analyses, the metamorphic process or stages will be established. After finding out the relationship between origin of foliation & lineation and arrangement characteristics of metamorphic minerals, the coupling relevance of deformation and metamorphism will be made out. Ar-Ar dating data will constrain ages of the metamorphic-deformation stages. Ascertaining the contacting relationships of rocks of different tectonic levels, metamorphic facies series and ages helps to understand the forming tectonics of whole high-pressure metamorphic belt. In general, the above researches will led to the construction of forming and lateral tectonic exhumation process of the Lanling high-pressure metamorphic belt, which will then provide geological proofs for knowing the tectonic attribute and origin of the Central Qiangtang metamorphic complex.

本课题拟以西藏羌塘中部的蓝岭高压变质带为研究对象，通过研究构造变形的过程及其与变质作用的关系，查明高压变质带的构造剥露过程。拟运用1:1万构造—岩性解析填图与1:2000或1:1000构造—岩石剖面测量，利用中小尺度构造解析的理论与方法，查明各类型构造的叠加、置换关系，建立高压变质带的构造样式；拟运用显微构造分析的相关方法，确定岩石、矿物的变形行为与变形组构，建立构造变形的期次；依据变质矿物组合的生成顺序，结合电子探针成分分析，划分变质作用的阶段；根据面理、线理与变质矿物生长排列的关系，厘定构造变形与变质作用的耦合关系；依据变质矿物的同位素年代约束变质变形作用的过程；厘定不同层次、不同变质相系、不同时代变质岩的接触关系。在上述研究内容的基础上，建立高压变质带的形成和后期构造剥露的过程，为厘定羌塘中央变质杂岩带的构造属性提供有利佐证。

青藏高原中部的羌塘盆地分布有一条东西向延伸＞500 km的三叠纪高压变质带，蕴含着青藏高原早期生长和东特提斯洋演化的重要信息。选取蓝岭高压变质地体及其围岩—玛依岗日中高压变质杂岩为解剖窗口，综合利用地质填图、构造解析、变质作用分析和Ar-Ar热年代学等方法，查明了两个变质体的构造成因、变形样式和期次，定性、定量分析了代表性岩石的P-T-D-t过程。厘定蓝岭高压变质地体为洋壳俯冲后期弧前“两阶段伸展剥露”的产物，发现玛依岗日以中高压变质地体的形成与陆陆碰撞期板片断离所诱导的局部伸展相关。蓝岭高压变质地体具有区域尺度的“核-幔-边”对称结构，核部的退变质榴辉岩具有升温降压的折返P-T轨迹，幔部绿帘石蓝片岩经历了升温脱水的折返过程。对同位素定年结果的统计和多重扩散域的模拟，揭示蓝岭高压变质体的剥露过程为：核部（退变质）榴辉岩由俯冲通道深部的折返始于244–230 Ma，在227–223.4 Ma到达幔部蓝片岩层次，经历硬柱石向绿帘石的相变之后，两者自223–220 Ma 向上快速折返，222–217 Ma之后侵位于低绿片岩相增生杂岩之中。玛依岗日穹窿构造中，中高压变质岩和上盘低绿片岩相混杂岩之间为右行平移正断层所分隔。角闪石的韭闪石核、阳起石幔和蓝透闪石边的温压条件分别为：524 °C 和0.88 GPa、386 °C 和0.34 GPa、~404 °C和0.76 GPa. 白云母片岩峰期变质条件为~400 °C和~0.7–1.1 GPa。角闪石Ar-Ar定年结果与其三重生长环带一致，具有递减式年龄谱：~242.4–241.2 Ma、≥202.6–196.8和192.9–189.8 Ma。穹窿体变形期次表明羌塘中部古特提斯洋碰撞造山存在明显的四个变形幕：(a)洋壳北西俯冲到北羌塘之下(~244–220 Ma)、(b)碰撞早期板片断离作用(223–202 Ma)、(c)碰撞期俯冲通道低密度物质上涌引起的局部伸展变形(~202.6–197 Ma)、(d)后碰撞期挤压变形和岩片叠覆 (195.6–188.7 Ma)。研究成果印证了板块碰撞带幕式演化特征，发现羌塘中部存在洋壳俯冲后期和陆陆碰撞期两次高压变质岩的伸展折返事件，支持“龙木错–双湖古特提斯洋缝合带”的观点，初步建立了羌塘中部中三叠世—早侏罗世古特提斯洋碰撞造山的变形过程。