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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[112-0]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[112-0]Qz//[001]Cpx;(3)11%的石英至少有一个s{112-1}面平行,并且(112-1)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<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、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.
The mineral-depositing structure preserves the physicochemical conditions of the early parent minerals, and studies on them not only help to understand the source depth of host rocks, but also help to study the dynamic evolution process of decompression and reentry.In many high-pressure Eclogites and garnet pyroxenites in the high-pressure zone have been widely found to have acicular or rod-shaped SiO2 precipitates in the clinopyroxene. The mineral phases of the clinopyroxene are mainly α-quartz, sometimes accompanied by calcareous amphiboles These directional acicular or rodlike bodies generally extend parallel to the c [001] axis of the clinopyroxene, with the major axis of the quartz being either its c [0001] axis or a [112-0] axis. The electron backscatter diffraction EBSD) test results show that most of the quartz (96%) precipitates have a crystallographic orientation with the host clinopyroxene: (1) 50% of the quartz c-axis is parallel and [0001] Qz // [001] Cpx; 2) 35% quartz has at least one a-axis parallel and [112-0] Qz // [001] Cpx; (3) 11% quartz has at least one s {112-1} 1) Qz // (100) Cpx. Calcareous amphibolite precipitates also have a close crystallographic orientation relationship with the host clinopyroxene: (100) Amp // (100) Cpx, [010] Amp // [010 ] Cpx, [001] Amp // [00 1] Cpx, [100] Amp ∧ [100] Cpx ≈ 32 °. The above quantitative microstructural evidence indicates that the directional quartz precipitates in clinopyroxene are formed by dissolution and most of the quartz precipitates form in α Quartz stability field.Hydrogen and high pressure experimental data have shown that the formation of vacancies and calcium esterase component (CaEs) content of clinopyroxene are all affected by chemical composition, pressure, temperature and other factors: single The content of CaEs in cristobalite is very sensitive to chemical composition and is co-buffered by free SiO2 and kyanite in the symbiotic mineral system. Under the same chemical composition and isobaric pressure, the content of CaEs generally decreases slowly with increasing temperature; Under chemical composition and isothermal condition, the content of CaEs increases with increasing pressure in the range of <6GPa and decreases with increasing pressure in the range of> 6GPa. The formation of the clinopyroxene precipitates may involve many factors, of which the high pressure is only One of the prerequisites is that the “quartz ± amphibole” precipitates in the eclogitic clinopyroxene are likely to be formed in an open system and are closely related to the melt fluid activity, involving multi-stage material diffusion, crystal nucleation and growth, Recrystallization, degeneration reaction Complex role in the process. Clinopyroxenes directional SiO2 was precipitated microstructural features is not prerequisite UHP rocks, this particular microstructure can not be sufficient as proof of ultra-high pressure conditions.