| 单斜辉石中石英出溶体的显微结构和成因机制 |
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| 引用本文： | 徐海军,赵素涛,武云. 单斜辉石中石英出溶体的显微结构和成因机制[J]. 地球科学, 2016, 41(6): 948-970. DOI: 10.3799/dqkx.2016.080 |
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| 作者姓名： | 徐海军 赵素涛 武云 |
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| 作者单位： | 1.中国地质大学地球科学学院，湖北武汉 430074 |
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| 基金项目： | 国家自然科学基金项目41172070国家自然科学基金项目41272080国家自然科学基金项目41204060高等学校博士学科点专项科研基金项目20120145120003 |
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| 摘 要： | 矿物出溶结构保存有早期母体矿物的物理化学条件信息，对其开展研究不仅有助于了解寄主岩石的来源深度，而且有助于研究减压折返的动力学演化过程.在世界许多高压-超高压带的榴辉岩和石榴辉石岩中，人们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.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c[0001]轴或a[1120]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%的石英c轴平行，并且[0001]Qz//[001]Cpx；(2)35%的石英至少有一个a轴平行，并且[1120]Qz//[001]Cpx；(3)11%的石英至少有一个s{1121}面平行，并且(1121)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)的含量均受化学组成、压力、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＜6 GPa区间随压力升高而增加，在＞6 GPa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、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.
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| 关 键 词： | 超高压变质作用 单斜辉石 石英出溶体 电子背散射衍射 拓扑结构 岩石学 |
| 收稿时间： | 2016-01-25 |
Microstructure and Mechanism of Quartz Exsolution in Clinopyroxene |
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| Abstract: | Exsolution lamellae-bearing minerals preserve information on the physicochemical conditions of the precursor homogeneous host and are helpful in understanding the subduction depth as well as the processes of decompression recorded in the host rocks during exhumation. Oriented silica precipitates in clinopyroxene have been reported widely in eclogite and garnet pyroxenite from high pressure and ultrahigh pressure metamorphic terranes around the world.Most of such silica precipitates are identified as α-quartz which in part coexist with hydrous minerals such as calcic amphibole.Such oriented precipitates are elongated parallel to the c-axis of host clinopyroxene, while the long axes of quartz being either c[0001]or a[1120]. Electron backscatter diffraction (EBSD) analyses demonstrate that the majority (96%) of quartz precipitates have topotactic relationships with their host clinopyroxenes.Three types of crystallographic topotactic relationships have been identified between quartz and host clinopyroxene: (1) 50% quartz precipitates share the same orientation for the c-axes with [0001]Qz//[001]Cpx; (2) 35% quartz precipitates share the same orientation for the a-axes with [1120]Qz//[001]Cpx; and (3) 11% quartz precipitates share the same orientation for the s-planes with (1121)Qz//(100)Cpx.Other quartz axes and planes disperse in large or small girdles around the shared axes or planes.Calcic amphibole precipitates also have a strong crystallographic relationship with host clinopyroxene, i.e., (100)Amp//(100)Cpx, [010]Amp//[010]Cpx, [001]Amp//[001]Cpx, [100]Amp∧[100]Cpx≈32°. The results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite.Integrated analyses of published high pressure and high temperature experiments show that the cation vacancy and Ca-Eskola (CaEs) component in clinopyroxene are affected by bulk chemistry, pressure and temperature. The solubility of SiO2 in clinopyroxene are sensitive to bulk chemistry, and the CaEs content in clinopyroxene at high pressure conditions is buffered by free silica phase and kyanite. The CaEs contents in clinopyroxene depend strongly on pressure, which shows rapid increase with pressure up to 6 GPa and then decrease with pressure. By contrast, the CaEs contents in clinopyroxene decrease slightly with temperature which indicates that the effect of temperature is relatively week.On the basis of the above integrated analyses on high pressure experiments and observations on mineral association and microstructural results from natural samples, we suggest that the formation mechanism of the oreiented silica in clinopyroxene is more complicated than we might initially assume. The oriented precipitates of α-quartz and calcic amphiboles in host clinopyroxene are probably formed during multi-stage mechanism involving exsolution, diffusion and exchange of multiple substances, nucleation and growth, recrystallization and some retrograded reactions, which are probably promoted by supercritical fluid or partial melting during exhumation. This study suggests that the texture of oriented quartz precipitates in clinopyroxene is neither necessary nor sufficient for UHP rocks, i. e., it cannot be used as an indisputable UHP-indicator. |
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