超高压变质岩单斜辉石中石英出溶体的形成机制及其构造物理意义

In recent years an increasing number of ultrahigh pressure (UHP) metamorphic localities have been described where clinopyroxene contains oriented needles or rods of quartz, in part coexisting with calcic amphibole, phlogopite or phengite. However, there is still heated debate on its mechanism and dynamic process, expecially on whether it can be used as UHP-indicators. In this project, we plan to combine qualitative research and quantitative study on such microstructure in natural rocks, to reveal the detailed microstructure, crystallographic orientation and chemistry of quartz rod from clinopyroxene. The main methods and techniques include the electron backscatter diffraction (EBSD), electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). Then, the representative samples will be selected and prepared for high pressure and high temperature experiments using multianvil apparatus to reveal the P-T condition of the pre-exist super silicpyroxene. In order to understand the pressure implications of silica exsolution from clinopyroxene in UHP metamorphic rocks, experiments will be conducted in the P-T range of 2-12GPa and 600-1200℃ using bulk compositions close to natural eclogites collected from Dabie-Sulu region. Then, we will integrate exsolution microstructure and chemical analyses in natural rocks with experimental results at high pressure and temperature to investigate the silica solubility of clinopyroxene as a function of P, T and water content. This study can provide important restrictions on the mechanism, dynamic process and tectonic implication of quartz rods in clinopyroxene from the UHP belts, and will supply an explanatory key for the debatable problem of whether quartz exsolution in clinopyroxene can be used as UHP-indicators.

单斜辉石中发育定向石英棒状体的显微结构在超高压变质岩区广泛发现，有些还伴生有定向钙质角闪石。目前还不清楚其成因机制和动力学过程，对其是否能作为超高压变质的可靠识别标志还存在很大争议。我们拟采用电子背散射衍射、扫描电镜和电子探针等多种微区测试分析手段，分析天然单斜辉石中石英棒状体的显微拓扑结构及其与主晶的结晶学取向关系，研究其与全岩化学组成以及单斜辉石Ca-Eskola含量的关系；并选择典型天然样品，粉碎后开展化学平衡的高温高压实验岩石学研究，查明石英棒状体完全溶解进入辉石的平衡温压条件。同时，模拟天然榴辉岩化学体系的初始样品，开展2-12GPa、600-1200℃范围的高温高压实验，研究单斜辉石中硅含量随温度、压力和含水量的变化关系。在此基础上结合前人研究成果，探讨这种显微结构的形成机制、关键控制因素和动力学过程，解析其所指示的构造物理条件以及能否作为超高压变质作用的可靠识别标志。

在世界许多高压-超高压带的榴辉岩和石榴辉石岩中，普遍发现单斜辉石中有定向排列的针状或棒状SiO2析出物，其矿物相主要为α石英，有时会伴生钙质角闪石等含水矿物．本项目研究数据表明：这些定向针状或棒状体通常平行于单斜辉石c[001]轴方向延伸，石英长轴可以为其轴c[0001]或a[11-20]轴．电子背散射衍射(EBSD)测试结果表明，多数石英(96%)析出物与寄主单斜辉石具有结晶学取向关系：(1) 50%石英c轴平行，并且[0001]Qz//[001]Cpx；(2) 35%石英至少有一个a轴平行，并且[11-20]Qz//[001]Cpx；(3) 11%石英至少有一个s{11-21}面平行，并且(11-21)Qz//(100)Cpx．钙质角闪石析出物与寄主单斜辉石也具有密切结晶学取向关系：(100)Amp//(100)Cpx、[010]Amp//[010]Cpx、[001]Amp//[001]Cpx、[100]Amp∧[100]Cpx ≈ 32°．上述定量显微构造证据表明，单斜辉石中定向石英析出物是由出溶作用所形成，并且多数石英出溶体形成于α石英稳定域．已有高温高压实验研究数据表明，单斜辉石中空位的形成和钙埃斯科拉组分(CaEs)的含量受化学组成、压力和温度等多种因素综合影响：单斜辉石中CaEs含量对化学组成非常敏感，并受到共生矿物体系中自由SiO2相和蓝晶石的共同缓冲；相同化学组成和等压条件下，CaEs含量总体上随温度升高缓慢降低；相同化学组成和等温条件下，CaEs含量在<6GPa区间随压力升高而增加，在>6GPa区间随压力升高而降低．单斜辉石定向SiO2析出物的形成可能涉及多种因素，高压只是其中必要条件之一．榴辉岩质单斜辉石中“石英±角闪石”析出物很可能形成于开放体系，与熔流体活动密切相关，涉及多阶段物质扩散、晶体成核生长、重结晶和退变质反应等复杂作用过程．单斜辉石中定向SiO2析出物的显微结构特征并非超高压岩石的必要条件，这种特殊显微结构也不能作为证明超高压的充分条件．