高压变质作用与地震的关系研究

High-pressure metamorphism is one of the major subjects for the study of orogenics. It is generally believed that high-pressure metamorphic rocks were formed deep in subducted slabs and then exhumed back in the crust. Those displaying close relations to earthquakes are interpreted to have been subjected to high-pressure metamorphism in response to fluid infiltration caused by deep faulting. This hypothesis, however, does not explain the transient nature and brittle deformation as well as the major role played by stress rather than fluid in the coseismic high-pressure phase transformations shown by our recent work. Our observations suggest that the earthquakes may have actually occurred in the crust. On the other hand, whether or not earthquake can generate sufficiently high pressure and temperature, and the mechanism by which the high-pressure phase transformation takes place at the timescale of earthquakes, are important theoretical concerns yet to be elucidated. The aim of the proposed project is (1) on the basis of our existing work, to carry out more detailed and systematic field and petrographic observations of several major high-pressure metamorphic belts, micro-Raman and electron microscopic analyses of the high-pressure minerals, in order to unveil the relations of high-pressure metamorphism to seismicity; (2) to perform thermodynamic modelling of the stable assemblages in the high-pressure metamorphic rocks and diffusion modelling of chemical zoning in the high-pressure minerals, in order to retrieve the information about pressure, temperature, and timescale of the process; (3) to reveal the mechanism of coseismic high-pressure phase transformation, and (4) to propose a mechanical model for the coseismic high-pressure metamorphism.

高压变质带是研究造山作用的主要对象之一。一般认为，高压变质岩形成于俯冲带深部之后折返到地壳，其中一些与地震密切相关的岩石被认为是因深部地震引起流体浸入而发生高压变质作用。这一假说无法解释申请人等发现的一些同地震高压变质岩相转变的瞬时性和脆性变形以及应力而非流体在高压相变的主要作用。这些现象指示地震可能就发生在地壳。另一方面，地震能否产生足够高的压力和温度引起高压－超高压变质作用，高压相转变能否在地震的时间尺度完成等重要理论问题尚未解决。本项目拟在已有工作结果基础上，通过进一步系统的野外地质、岩相学、高压矿物的激光喇曼微区分析和电子显微镜观测揭示中国几个高压变质带与地震的关系；对稳定矿物组合进行热力学模拟以获得其温压条件，对高压矿物的组成环带进行扩散模拟以获得其记录的地质过程的时间尺度；揭示同震高压相转变机理；提出同震高压变质作用的力学模型。

本项目对对山东仰口、大别山碧溪岭和港河以及南天山的高压变质岩及其部分围岩做了较深入的野外和岩石学工作。其中以仰口变质辉长岩的工作最为系统深入。从西北向东南部约20米内，仰口变质辉长岩逐步由变辉长岩过渡为含石英榴辉岩和含柯石英榴辉岩。变质压力和温度从约1.2 GPa和520±50 ºC增高到约3.3 GPa和710±40 ºC。全岩化学分析和不同变质程度辉长岩中斜长石假象的相图模拟工作表明，各类榴辉岩的含水量都很低，远在水饱和线之下。因此它们的变质程度差异并非流体不同程度参与的结果，而是变质压力的差异所致；按照深俯冲模式，造成2 GPa的压力差需要有大约60 千米的深度差，因此它们之间在20米内的压力差不可能是俯冲造成的；岩石中发育大量细小的星射状、树枝状和骨架状石榴石，针状绿帘石和蓝晶石，球状绿辉石+石英合晶等淬火结构。普通辉石普遍显示机械双晶，粗粒石英发生碎粒化形成细粒集合体，黑云母发生扭折和破裂并且被没有变形的骨架状石榴石包围。这些显微构造指示了一个在较低温的地壳环境中瞬时的升温升压过程。之后迅速降温降压使得这些脆性变形的辉长岩原岩矿物没有来得及重结晶。因此仰口的高压变质作用不是区域变质作用而是冲击变质作用;因为仰口榴辉岩的峰变质温度不超过750 ºC，所以矿物淬火结构不是熔体结晶的产物。根据变辉长岩中易熔而密度较大的黑云母较好地保存，难熔而密度较小的斜长石完全被高压矿物构成的假象所取代这一事实，上述淬火结构更可能是固态非晶质即玻璃结晶形成的；玻璃相是地震应力波引起的击致非晶化的结果。基于这一结果本项目提出了震击高压变质作用模式以取代俯冲折返模式。