Crust–Mantle Interaction in a Continental Subduction Channel: Evidence from Orogenic Peridotites in North Qaidam, Northern Tibet

A combined study of rock-forming mineral and O isotope compositions, zircon U–Pb ages, trace element compositions, mineral inclusions and Hf–O isotopes was made on orogenic peridotites from North Qaidam in northern Tibet to provide insights into crustal metasomatism at the slab–mantle interface in a continental subduction channel. Rock-forming mineral O isotope analyses yield olivine δ
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O values of 5·0–5·6‰ for lherzolites and 3·5–5·3‰ for dunites; orthopyroxene has δ
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O values of 4·5–5·3‰ for lherzolites, 4·5–5·3‰ for dunites, and 7·8‰ for pyroxenites. It appears that the protolith of these ultramafic rocks underwent high-temperature metasomatism by both low- and high-δ
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O fluids. The occurrence of zircon in thin section and the presence of olivine inclusions in zircon from these rocks explicitly indicates the growth of metasomatic zircon during continental subduction zone processes. Two distinct groups of zircon domains are distinguished. Intermediate- to high-luminescent zircon domains have U–Pb ages of 417 ± 4 to 426 ± 3 Ma, variable Th/U ratios, low rare earth element (REE) contents, flat heavy REE (HREE) patterns with a relative lack of negative Eu anomalies, and inclusions of olivine, clinopyroxene, orthopyroxene and graphite. These features are interpreted as zircon growth through metasomatic reaction by addition of fluids in the early exhumation stage of deeply subducted continental crust. The Hf–O isotope compositions of these zircon domains suggest the same origin for these fluids. In contrast, low-luminescent zircon rims mostly exhibit younger U–Pb ages of 404 ± 3 to 405 ± 8 Ma, low Th/U ratios < 0·1, high REE contents, and steep HREE patterns with negative Eu anomalies, and contain calcite inclusions as well as multiphase solid inclusions of Cal + Kfs and Qtz + Kfs + Pl. Such observations indicate their growth through metasomatic reaction by infiltration of hydrous silicate melts during the late exhumation stage. All the zircon domains have variable Hf isotope compositions with a large difference between samples, but a narrow δ
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O range from 5·4 to 6·6‰. Their Hf–O isotope compositions are similar to those of UHP metamorphic rocks in North Qaidam, suggesting that the metasomatic fluids originated from deeply subducted continental crust. Whereas the high-δ
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O fluids originated from deeply subducted continental crust, the low-δ
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O fluids were derived from dehydration of the subducting oceanic crust that preceded the subduction of continental crust. The orogenic peridotites thus record the metasomatic reaction of mantle wedge peridotite with fluids of different origins.