DOI: 10.1016/j.epsl.2020.116664
论文题名: Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust
作者: Feng P. ; Wang L. ; Brown M. ; Johnson T.E. ; Kylander-Clark A. ; Piccoli P.M.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2021
卷: 554 语种: 英语
中文关键词: 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
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. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/165465
Appears in Collections: 气候变化与战略
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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
Recommended Citation:
Feng P.,Wang L.,Brown M.,et al. Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust[J]. Earth and Planetary Science Letters,2021-01-01,554