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Two epochs of eclogite metamorphism link 'cold' oceanic subduction and ‘hot’ continental subduction, the North Qaidam UHP belt, NW China.

Song, Shuguang and Niu, Yaoling and Zhang, Guibin and Zhang, Lifei (2018) 'Two epochs of eclogite metamorphism link 'cold' oceanic subduction and ‘hot’ continental subduction, the North Qaidam UHP belt, NW China.', Geological Society, London, special publications., 474 . pp. 1-15.

Abstract

Eclogites in the high-pressure (HP) and ultrahigh-pressure (UHP) belts record subduction-zone processes; exhumed eclogites of seafloor protoliths record low-temperature (mostly <600°C), high-pressure and ‘wet’ environments: that is, relatively ‘cold’ subduction with highly hydrous minerals such as lawsonite. In contrast, eclogites formed by the continental subduction record relatively ‘hot’ (T > 650°C) and ‘dry’ ultrahigh-pressure metamorphic (UHPM) conditions with syncollisional magmatism. Here, we investigate some eclogites from two ophiolite sequences that intercalated in the North Qaidam UHPM belt, which is genetically associated with continental subduction/collision. The observations of lawsonite pseudomorphs in garnets, garnet compositional zoning, mineral and fluid inclusions in zircons, and zircons with distinct trace-element patterns and U–Pb ages all suggest that these eclogites represent two exhumation episodes of subduction-zone metamorphic rocks: the early ‘cold’ and ‘wet’ lawsonite eclogite and the late ‘hot’ and ‘dry’ UHP kyanite eclogite. The early lawsonite-bearing eclogite gives metamorphic ages of 470–445 Ma and the later kysnite-bearing eclogite gives metamorphic ages of 438–420 Ma, with a time gap of c. 7–10 myr. This gap may represent the timescale for transition from oceanic subduction and continental subduction to depths greater than 100 km. We conclude that evolution from oceanic subduction to continental collision and subduction was a continuous process. In addition, we find that titanium contents in zircons have a positive correlation with U contents. Ti-in-zircon thermometry is likely to be invalid or limited for low-temperature eclogites.

Item Type:Article
Additional Information:Supplementary material: Mineral composition data from the field sites are available at https://doi.org/10.6084/m9.figshare.c.4024774.v1
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1144/SP474.2
Publisher statement:Geological Society, London, special publications 474, 2018, https://doi.org/10.1144/SP474.2 © Geological Society of London [Year].
Date accepted:No date available
Date deposited:10 August 2018
Date of first online publication:23 March 2018
Date first made open access:23 March 2019

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