喜马拉雅东段俯冲折返、变质变形与热史演化——以亚东及错那剖面为例

Among magnitude important problems that related to the tectonic evolution of the Greater Himalayan Sequence, two interesting findings stimulated widespread attention: the formation of the inverted metamorphism that puzzled lots of geologists for a long time and the exposure of (ultra-)high pressure metamorphosed rocks presenting that the subduction and exhumation of the Indian continent should be involved into the evolution model of the Greater Himalayan Sequence. Generally, an acceptable model should be able to explain the two above mentioned geological findings at the same time. This proposal is established in the newly found high-pressure metamorphosed rocks in Yadong and Cuona districts and wide spread middle pressure metamorphic sequence, for the first time, we conduct combined researches of the high-pressure and middle-pressure metamorphisms, as well as the middle-pressure metamorphism and syn-metamorphic deformation. Based on constraints such as the P-T-t conditions for each phase of the high-pressure and middle-pressure metamorphism, the timing and the alteration of the tectonic deformation on the middle-pressure metamorphism, systematic low-temperature thermochronology, we will set up an integrated tectonic framework for the Greater Himalayan Sequence, which includes the early subduction and exhumation in the lower lithosphere, the later metamorphism and deformation in the middle-lower crust, and the finally denudation and exposure to the surface. In this tectonic framework, we will explain the two basic geological findings mentioned above. The practice of this proposal will not only gather critical research data with regard to the high-pressure metamorphism, tectonic deformation and thermal history for the eastern segment of the Himalyan Orogen, but also improve our considerations when proposing a reliable tectonic evolution model for the Greater Himalayan Sequence.

在关于高喜马拉雅结晶岩系构造演化的研究中，有两个发现引起了地质学家的高度关注：一是高压超高压变质岩所反映的大陆俯冲与折返过程，二是长期困扰地质学家的反转变质带的出露，而关于高喜马拉雅带的演化模型必须同时合理地解释上述两个基本地质现象。本课题立足于在喜马拉雅东段亚东和错那两个剖面上新近发现的高压变质岩石和广泛出露的中压变质岩系，以各期次变质作用P-T-t轨迹、构造变形的改造作用及时代、系统的低温热史为主要约束，首次将高压与中压变质作用结合起来，同时将中压变质与构造变形相结合，构建高喜马拉雅结晶岩系从早到晚的演化过程：早期岩石圈深部俯冲、折返- - 中期中下地壳变质与变形改造过程- - 晚期出露地表，力图以此为基础揭示大陆俯冲折返的过程和反转变质带形成的机制。本项目的实施将不仅为国际上关于喜马拉雅东段高压变质、构造变形与热史演化提供重要的研究数据，还将为喜马拉雅构造演化模式的研究提供新的思路。

高喜马拉雅结晶岩系的演化模型必须同时合理解释俯冲折返与后期热史演化的基本地质事实。本项目基于喜马拉雅东部亚东高压（HP）麻粒岩的变质作用及年代学研究和其上下部的花岗岩体多期热年代学研究，喜马拉雅东部亚东段约束了高大喜马拉雅结晶岩系序列（GHS）的新生代构造演化过程。位于高喜马拉雅结晶岩系序列顶部的亚东高压麻粒岩经历了一次峰期高压麻粒岩相变质作用和两期退变质作用，麻粒岩相退变质作用和角闪石相退变质作用分别发生在48.5和31.8 Ma，标志着印度板块俯冲至中下地壳后折返的时间。随后，从亚东高压麻粒岩中获得的平均年轻锆石U-Pb年龄表明俯冲板片该单元在16.9Ma时被其周围部分熔融的围岩所捕获。另外，三个来自高大喜马拉雅结晶岩系序列下部和上部的花岗岩体中黑云母40Ar / 39Ar年龄分别为11.0，11.3和11.5 Ma，这些一致的年龄表明高大喜马拉雅结晶岩系序列沿亚东段是侧向挤压并在~11.3 Ma时同时冷却到〜300°C，。此外，花岗质片麻岩磷灰石裂变径迹年龄约7Ma，标志着高大喜马拉雅结晶岩系序列冷却至~110°C℃的年龄。本项目的研究成果标明，高喜马拉雅结晶岩系经历了两个主要阶段的演化过程模型描述了大喜马拉雅序列新生代构造演化：（1）印度板块俯冲至青藏高原之下早于〜55 Ma，在48.5 Ma折返至下地壳（50-40 km），在31.8 Ma折返至中地壳（22-15 km）；（2）中地壳部分熔融发生在31.8-16.9 Ma，并导致通道流，在后期16.9至~7Ma，高大喜马拉雅序列结晶岩系被韧性中地壳流侧向挤压，以~2mm/y的速率快速冷却速率为特征抬升至地表。依托本项目的研究成果，将俯冲折返模型与通道流模型同时纳入高喜马拉雅结晶岩系的演化过程中，合理解释了前期的俯冲折返与后期的侧向挤出现象。