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Durham University

Department of Earth Sciences

Profile

Publication details for Professor Yaoling Niu

Song, S.G., Zhang, L.F., Niu, Yaoling, Wei, C.J., Liou, J.G. & Shu, G.M. (2007). Eclogite and carpholite-bearing meta-pelite in the North Qilian suture zone, NW China: implications for Early Paleozoic cold oceanic subduction. Journal of Metamorphic Geology 25(5): 547-563.

Author(s) from Durham

Abstract

Low-temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as
blocks within foliated blueschist that was originated from greywacke matrix; they formed a highpressure
low-temperature (HPLT) subduction complex (me´ lange) in the North Qilian oceanic-type
suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the
basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet
from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic
conditions of 460–510 C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 C and 1.75–
1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–
phengite–glaucophane schist and various chloritoid-bearing schists. Mg-carpholite was identified in
some high-Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 C for Grt–Omp–
Phn–Gln schist, and 2.45–2.50 GPa and 525–530 C for the Mg-carpholite schist. Mineral assemblages
and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to
semi-pelagic sediments have been subducted to the mantle depths (‡75 km) before 460 Ma. Blueschist
facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of
lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian
suture zone demonstrate the existence of cold subduction-zone gradients (6–7 C km)1), and this cold
subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times.