Two epochs of eclogite metamorphism link 'cold' oceanic subduction and 'hot' continental subduction, the North Qaidam UHP belt, NW China
Posted on 2018-03-07 - 14:30
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' ultrahighpressure
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.
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Song, Shuguang; Niu, Yaoling; Zhang, Guibin; 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 of London. Collection. https://doi.org/10.6084/m9.figshare.c.4024774.v1
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AUTHORS (4)
SS
Shuguang Song
YN
Yaoling Niu
GZ
Guibin Zhang
LZ
Lifei Zhang