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Durham University

Department of Earth Sciences

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Publication details for Professor Yaoling Niu

Zhang, G.B., Song, S.G., Zhang, L.F. & Niu, Yaoling (2008). The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronologymsword. Lithos 104(1-4): 99-118.

Author(s) from Durham

Abstract

Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross
section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol1) and
orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic
protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO2, and high Al2O3 and MgO.
The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO2, low Al2O3 and MgO with incompatible trace
elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite
([87Sr/86Sr]i~0.703–0.704; εNd(T)~5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower εNd(T) value
(1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [87Sr/86Sr]i ratio (0.705–0.716) points to an
additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the
serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle
peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus
thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P≥2.7 GPa). The
harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤3.0 GPa. Zircon
U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516±8 Ma and a metamorphic age of 445±7 Ma. These data indicate the
presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of
subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These
rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.