气候变化领域集成服务门户(CCPortal): Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust

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DOI	10.1016/j.epsl.2020.116664
	Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust
	Feng P.; Wang L.; Brown M.; Johnson T.E.; Kylander-Clark A.; Piccoli P.M.
发表日期	2021
ISSN	0012821X
卷号	554
英文摘要	Results from numerical modelling and experimental petrology have led to the hypothesis that partial melting was important in facilitating exhumation of ultrahigh-pressure (UHP) metamorphic rocks from mantle depths. However, the melting reactions responsible are rarely well-documented from natural examples. Here we report microstructural features and compositional data that indicate in situ partial melting dominated by breakdown of omphacite in UHP eclogite from the Sulu belt, China. Diagnostic microstructures include: (i) the presence of in situ leucosome pockets composed of plagioclase, euhedral amphibole, minor K-feldspar and epidote within host zoisite- and phengite-bearing eclogite; (ii) skeletal omphacite within the leucosome pockets that has a lower jadeite content (25–45 mol.%) than rock-forming omphacite (39–54 mol.%); and, (iii) seams of Na-rich plagioclase that extend along grain boundaries separating phengite, quartz and zoisite, and which commonly exhibit low dihedral angles where they terminate at triple grain-boundary junctions. Major oxide proportions of 57 leucosome pockets, calculated using mineral modes and compositions, yield leucodiorite bulk compositions characterized by intermediate SiO2, high Al2O3 and Na2O, and low K2O contents. In primitive mantle-normalised trace element diagrams, the leucosome pockets show enrichment in large ion lithophile elements, U, Pb, Zr, Hf and Ti, but depletion in Th and Ta, patterns that are similar to those of rock-forming omphacite. Rather than forming predominantly by breakdown of phengite and/or zoisite, as widely proposed in the literature, the leucosome pockets have petrographic characteristics and major oxide and trace element compositions that are consistent with partial melting dominated by omphacite breakdown. Based on conventional thermobarometry, the eclogite was exhumed from pressure–temperature (P–T) conditions of 3.6–3.1 GPa and 900–840 °C. Partial melting led to the formation of the leucosome pockets, which equilibrated with the rims of surrounding rock-forming garnet and pyroxene during crystallisation. Conventional thermobarometry using rim compositions yields P–T conditions of 1.6–1.2 GPa and 780–690 °C, broadly consistent with calculated phase equilibria and Ti-in-zircon temperatures from zircon overgrowths. Weighted mean ages of ca 217–214 Ma from thin overgrowths on zircon are interpreted to record melt crystallisation. This study provides insight into an overlooked mechanism by which eclogites partially melt during exhumation from UHP conditions, and permits a better understanding of the processes that assist deeply-subducted continental crust to return to shallower depths. © 2020 Elsevier B.V.
关键词	exhumation leucosome omphacite-breakdown partial melting ultrahigh-pressure eclogite
英文关键词	Alumina; Aluminum oxide; Dihedral angle; Feldspar; Grain boundaries; Melting; Phase equilibria; Semiconductor junctions; Silica; Sodium compounds; Trace elements; Zircon; Grain boundary junctions; Large ion lithophile elements; Microstructural features; Petrographic characteristics; Subducted continental crust; Trace element composition; Ultra-high-pressure eclogite; Ultra-high-pressure metamorphic rocks; Metamorphic rocks; continental crust; eclogite; exhumation; leucosome; omphacite; P-T conditions; partial melting; petrology; ultrahigh pressure metamorphism; China; Sulu Belt; Trachinotus falcatus
语种	英语
来源期刊	Earth and Planetary Science Letters
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/203087
作者单位	State Key Laboratory of Geological Processes and Mineral Resources, Centre for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China; Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College ParkMD 20742, United States; School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia; Department of Earth Science, University of California, Santa Barbara, CA 93106, United States
推荐引用方式 GB/T 7714	Feng P.,Wang L.,Brown M.,et al. Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust[J],2021,554.
APA	Feng P.,Wang L.,Brown M.,Johnson T.E.,Kylander-Clark A.,&Piccoli P.M..(2021).Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust.Earth and Planetary Science Letters,554.
MLA	Feng P.,et al."Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust".Earth and Planetary Science Letters 554(2021).