Petrological studies of Jilang eclogite in the Lhasa terrane and its constraint on the subduction and exhumation processes of the Paleo-Tethys oceanic crust
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P588.34;P581

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    Abstract:

    The (ultra-)high pressure eclogites from Sumdo area in eastern Lhasa terrane recorded the subduction and exhumation process of the Paleo-Tethys oceanic crust. Previous studies reported four main outcrops in Sumdo area, i.e., Sumdo, Xindaduo, Bailang and Jilang. Studying the peak metamorphic conditions and p-T path of Sumdo eclogite is of great significance for revealing the subduction and exhumation mechanism of Paleo-Tethys Ocean. In this paper, the authors chose Jilang eclogite for a case study, which has a mineral assemblage of garnet, omphacite, phengite, hornblende, rutile, epidote, quartz and symplectite (diopside+amphibole+plagioclase), and minor biotite. Garnet has a "dirty" core with abundant mineral inclusions and a "clear" rim with less mineral inclusions, showing typical growth zoning. From the core to the rim, Prp content in garnet increases while Grs content decreases, and the rim of garnets is wrapped by pargasite+plagioclase (An=28) corona, showing an amphibolite facies superimposition during retrogression. p-T pseudosection calculated with Domino contoured with isopleths of grossular and pyrope values in garnet and Si content in phengite constrained peak p-T conditions of Jilang eclogite as 563℃, 2.4 GPa. Combined with petrographical observation, the authors recognized four stages of metamorphism:① prograde metamorphism represented by the core of garnet and mineral inclusions therein; ② peak metamorphism represented by the rim of garnet, omphacite, phengite, glaucophane, lawsonite, rutile and quartz; ③ early stage retrograde metamorphism characterized by decomposition of lawsonite to epidote; ④ late stage retrograde metamorphism characterized by symplectites surrounding omphacite and cornona rimmed garnet. Jilang eclogite shows a clockwise p-T path, near isothermal decompression during exhumation, and is recorded as a high pressure, low temperature elcogite. Different from eclogites in other areas which are hosted by garnet-bearing mica schists or serpentinites, Jilang eclogites are enclosed in metamorphic quartzites, with relatively low p-T conditions. On the basis of mixture of rocks with various p-T conditions in this area, the authors infer that Jilang eclogite was derived from the shallow part of the subduction zone, and was exhumed by low density materials in the subduction channel.

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申婷婷,张聪,田作林,李杨,杨经绥,2018,拉萨地块吉朗榴辉岩的岩石学研究及其对古特提斯洋壳俯冲折返过程的限定[J].岩石矿物学杂志,37(6):917~932. SHEN Ting-ting, ZHANG Cong, TIAN Zuo-lin, LI Yang, YANG Jing-sui,2018,Petrological studies of Jilang eclogite in the Lhasa terrane and its constraint on the subduction and exhumation processes of the Paleo-Tethys oceanic crust[J]. Acta Petrologica et Mineralogica,37(6):917~932.

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History
  • Received:May 10,2018
  • Revised:August 29,2018
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  • Online: November 27,2018
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