俯冲带蛇纹岩的变质演化过程是理解俯冲带流体运移、成岩成矿机制,以及岛弧岩浆作用等核心科学问题的关键。西南天山造山带是典型的大洋冷俯冲变质带,其广泛出露的高压-超高压变质岩为研究这一过程提供了理想窗口。前人研究已证实蛇纹...俯冲带蛇纹岩的变质演化过程是理解俯冲带流体运移、成岩成矿机制,以及岛弧岩浆作用等核心科学问题的关键。西南天山造山带是典型的大洋冷俯冲变质带,其广泛出露的高压-超高压变质岩为研究这一过程提供了理想窗口。前人研究已证实蛇纹岩化异剥橄榄岩经历了超高压变质作用,与其空间伴生的蛇纹岩化方辉橄榄岩是否具有相同的变质演化历史,尚存疑问。本研究选取西南天山木扎尔特沟长阿吾子地区出露的蛇纹岩化方辉橄榄岩开展系统研究,通过精细的岩相学观察和矿物地球化学分析,取得如下认识:(1)蛇纹岩化方辉橄榄岩发育了与该区蛇纹岩化异剥橄榄岩相似的含钛斜硅镁石-钛粒硅镁石的矿物组合。峰期变质矿物组合为:钛粒硅镁石+橄榄石+叶蛇纹石+氢氧镁石+绿泥石+尖晶石/铬铁矿+磁铁矿。显微结构中未发现钛铁矿/镁钛矿与橄榄石共生或橄榄石与斜方辉石共生的现象,这表明岩石未经历足以引发钛硅镁石类矿物高温脱水分解的热力学条件,叶蛇纹石也未达到完全脱水分解的温度阈值;(2)识别出了两个关键变质反应:含钛尖晶石/钛铁矿+叶蛇纹石+氢氧镁石=钛粒硅镁石+橄榄石+绿泥石+水(进变质阶段);钛粒硅镁石+橄榄石+叶蛇纹石=钛斜硅镁石+水(退变质阶段)。采用相平衡模拟软件GeoPS(Xiang and Connolly,2021)对两个代表性蛇纹岩化方辉橄榄岩进行了P-T视剖面图计算。叶蛇纹石的Al等值线对温度非常敏感,可以用于限定温压条件。结合矿物组合以及叶蛇纹石成分投图,限定两个样品的变质峰期条件分别为:~460℃、~4.1GPa和~460℃、~4.3GPa。顺时针变质P-T轨迹指示岩石经历深俯冲-折返的动力学过程。本研究首次证实西南天山蛇纹岩化方辉橄榄岩与异剥橄榄岩具有相同的超高压变质演化历史,两者均俯冲至110~140km深部地幔环境。这可能代表了大洋地幔橄榄岩不同层位的岩石组合,整体经历了俯冲作用,后期板片的断离使得不同层位的岩石碎片肢解并进入俯冲隧道中。由于蛇纹岩化橄榄岩密度较轻,更加容易发生折返。此外,这一研究还显示,在冷俯冲带中,叶蛇纹石、钛粒硅镁石和钛斜硅镁石等含水矿物因未达到完全脱水温度,可作为有效载体将大量水输送至深部地幔,这对理解俯冲带水循环及地幔地球化学演化具有重要启示。展开更多
The Gysian ophiolite of NW Iran is located at the intersection of the ophiolite belts of SE Turkey,NE Iraq,and Iran,and provides the opportunity to investigate the preserved subduction and obduction history of an impo...The Gysian ophiolite of NW Iran is located at the intersection of the ophiolite belts of SE Turkey,NE Iraq,and Iran,and provides the opportunity to investigate the preserved subduction and obduction history of an important tectonic site that has not previously been studied.The serpentinized peridotites of the Gysian ophiolite contain the assemblagelizardite+chrysotile+spinel/Mg-spinelwith relict clinopyroxene(diopside)and very rare relict orthopyroxene and olivine.The compositions of clinopyroxenes and spinels are more consistent with the formation of the inferred protolithharzburgitesinafore-arcorsupra-subduction zone instead of an abyssal environment.The Gysian ophiolite is a remnant of the Neo-Tethyan arc-ophiolitic system and records shallow subduction(<50 km,indicated by the absence of antigorite)in the Late Cretaceous to Paleocene beforeobductionalongthrustfaultsoverthecontinental margin.We review the spatial trends of the metamorphic grade of the Neo-Tethyan ophiolites in this region and provide detailed information about the petrology and mineral chemistry of the Gysian ophiolite.展开更多
文摘俯冲带蛇纹岩的变质演化过程是理解俯冲带流体运移、成岩成矿机制,以及岛弧岩浆作用等核心科学问题的关键。西南天山造山带是典型的大洋冷俯冲变质带,其广泛出露的高压-超高压变质岩为研究这一过程提供了理想窗口。前人研究已证实蛇纹岩化异剥橄榄岩经历了超高压变质作用,与其空间伴生的蛇纹岩化方辉橄榄岩是否具有相同的变质演化历史,尚存疑问。本研究选取西南天山木扎尔特沟长阿吾子地区出露的蛇纹岩化方辉橄榄岩开展系统研究,通过精细的岩相学观察和矿物地球化学分析,取得如下认识:(1)蛇纹岩化方辉橄榄岩发育了与该区蛇纹岩化异剥橄榄岩相似的含钛斜硅镁石-钛粒硅镁石的矿物组合。峰期变质矿物组合为:钛粒硅镁石+橄榄石+叶蛇纹石+氢氧镁石+绿泥石+尖晶石/铬铁矿+磁铁矿。显微结构中未发现钛铁矿/镁钛矿与橄榄石共生或橄榄石与斜方辉石共生的现象,这表明岩石未经历足以引发钛硅镁石类矿物高温脱水分解的热力学条件,叶蛇纹石也未达到完全脱水分解的温度阈值;(2)识别出了两个关键变质反应:含钛尖晶石/钛铁矿+叶蛇纹石+氢氧镁石=钛粒硅镁石+橄榄石+绿泥石+水(进变质阶段);钛粒硅镁石+橄榄石+叶蛇纹石=钛斜硅镁石+水(退变质阶段)。采用相平衡模拟软件GeoPS(Xiang and Connolly,2021)对两个代表性蛇纹岩化方辉橄榄岩进行了P-T视剖面图计算。叶蛇纹石的Al等值线对温度非常敏感,可以用于限定温压条件。结合矿物组合以及叶蛇纹石成分投图,限定两个样品的变质峰期条件分别为:~460℃、~4.1GPa和~460℃、~4.3GPa。顺时针变质P-T轨迹指示岩石经历深俯冲-折返的动力学过程。本研究首次证实西南天山蛇纹岩化方辉橄榄岩与异剥橄榄岩具有相同的超高压变质演化历史,两者均俯冲至110~140km深部地幔环境。这可能代表了大洋地幔橄榄岩不同层位的岩石组合,整体经历了俯冲作用,后期板片的断离使得不同层位的岩石碎片肢解并进入俯冲隧道中。由于蛇纹岩化橄榄岩密度较轻,更加容易发生折返。此外,这一研究还显示,在冷俯冲带中,叶蛇纹石、钛粒硅镁石和钛斜硅镁石等含水矿物因未达到完全脱水温度,可作为有效载体将大量水输送至深部地幔,这对理解俯冲带水循环及地幔地球化学演化具有重要启示。
基金The current work is financially supported by Urmia University research branch。
文摘The Gysian ophiolite of NW Iran is located at the intersection of the ophiolite belts of SE Turkey,NE Iraq,and Iran,and provides the opportunity to investigate the preserved subduction and obduction history of an important tectonic site that has not previously been studied.The serpentinized peridotites of the Gysian ophiolite contain the assemblagelizardite+chrysotile+spinel/Mg-spinelwith relict clinopyroxene(diopside)and very rare relict orthopyroxene and olivine.The compositions of clinopyroxenes and spinels are more consistent with the formation of the inferred protolithharzburgitesinafore-arcorsupra-subduction zone instead of an abyssal environment.The Gysian ophiolite is a remnant of the Neo-Tethyan arc-ophiolitic system and records shallow subduction(<50 km,indicated by the absence of antigorite)in the Late Cretaceous to Paleocene beforeobductionalongthrustfaultsoverthecontinental margin.We review the spatial trends of the metamorphic grade of the Neo-Tethyan ophiolites in this region and provide detailed information about the petrology and mineral chemistry of the Gysian ophiolite.
