西藏蓬湖西镁质榴辉岩中的出溶物及其地质意义
Exsolutions in a magnesian eclogite and the geologic significance from western Peng Lake,Tibet
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文摘
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西藏蓬湖西发现了有别于一般榴辉岩和石榴石辉石岩的超高压镁质榴辉岩,具有平衡变晶结构,广泛发育复杂多期的出溶结构.根据矿物出溶关系可分出三个世代的单斜辉石(Cpx)和三个世代的石榴石(Grt)和对应地三个单斜辉石(Cpx)石榴石(Grt)对,此外还有出溶的斜方辉石、铝铬铁矿和钠长石.对具代表性的单斜辉石石榴石和单斜辉石斜方辉石矿物对,得到三个阶段温度压力:阶段Ⅲ1的平衡条件为4GPa和1113℃~1129℃;阶段Ⅲ2的平衡条件为3GPa和863℃~1033℃;阶段Ⅲ3平衡条件为2GPa和502℃~667℃.蓬湖西镁质榴辉岩P T路径经历了三个阶段:阶段Ⅰ,蓬湖西榴辉岩形成,压力范围可能为0.5~1.5GPa;阶段Ⅱ,特提斯洋关闭,洋壳向深部俯冲,致使镁质榴辉岩从尖晶石二辉橄榄岩稳定域穿过石榴石二辉橄榄岩稳定域并最终到达1400℃和5GPa的柯石英稳定域,形成了在超高压条件下结构均一的、稳定的"单斜辉石+石榴石+斜方辉石"固溶体;阶段Ⅲ,洋壳折返过程中温度和压力降低使得原来稳定的"单斜辉石+石榴石+斜方辉石"固溶体发生出溶作用,形成多期次、多种类型的出溶物.阶段Ⅲ再分为三个次阶段,其温压变化范围大,为2~4GPa,502℃~1129℃.镁质榴辉岩中矿物出溶肯定与压力温度降低有关. |
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其他语种文摘
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Ultrahigh-pressure magnesiann eclogite was found in western Peng Lake,Tibet.Different from common eclogite and garnet pyroxenite,it containes equilibrium crystalloblastic texture and complex multi-phase exsolution texture.According to the exsolution relationship,three generations of clinopyroxenes(Cpx)and garnets(Grt)can be identified:1.The bigger clinopyroxene and garnet are under equilibrium with a granular equilibrium crystalloblastic texture,and form a Cpx1 Grt1pair.2.The host crystal forms second generation rodlike Cpx2 Grt2pair within the exsolution crystal of Cpx1and Grt1respectively.3.In host crystal Cpx1,the two Magnesian eclogite parallel exsolution Grt3shold several Cpx3sparallelly,and they form Grt3-Cpx3pair.There are also orthopyroxene,alumochromite and albite exsolutions.We work out the equilibrium condition,by choosing“clinopyroxene-garnet”and“clinopyroxeneorthopyroxene” pair and get the temperature and pressure of the 3stages:StageⅢ 1,the equilibrium condition of Cpx1 Grt1is 4GPa and 1113℃~1129℃;StageⅢ 2,the equilibrium condition of Cpx2 Grt2is 3GPa and 863 ℃~1033℃;StageⅢ 3,the equilibrium condition of Cpx3 Grt3is 2GPa and 502℃~667℃.The P T path of magnesiann eclogite of Western Peng Lake underwent three 3stages.StageⅠ:the eclogite is formed in a 0.5~1.5 GPa pressure range.StageⅡ:when Tethys ocean closed,the oceanic crust were subducted into the depth,sequentialy the magnesiann eclogite go from spl-lherzolite stable field across Grt-lherzolite stable field into Coesite stable field of 1400℃and 5GPa,and forming homogeneous and a stable“clinopyroxene+garnet+orthopyroxene”solid solution under ultrahigh pressure.StageⅢ:The decline of temperature and pressure with exhumation of the subducted oceanic crust makes the original stable“clinopyroxene+garnet+orthopyroxene”solid solution to exsolve and to form polyphasic and polytypic exsolutions.This stage can be divided into 3secondary stages in which the range of the temperature and pressure is wider(2~4GPa,502℃~1129℃).It is certain that the mineral exsolution of magnesiann eclogite do have relation with the pressure-temperature decline. |
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来源
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南京大学学报. 自然科学版
,2013,49(3):356-386 【核心库】
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关键词
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西藏
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班公湖-怒江缝合带
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蓬湖西镁质榴辉岩
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矿物出溶
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深俯冲折返
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地址
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1.
中山大学海洋学院, 广州, 510275
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南京大学地球科学系, 南京, 210093
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中国科学院广州地球化学研究所, 广州, 510640
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语种
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中文 |
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文献类型
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研究性论文 |
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ISSN
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0469-5097 |
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学科
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地质学 |
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基金
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国家973计划
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国土资源部中国地质调查局项目
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文献收藏号
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CSCD:4871724
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