Two contrasting eclogite cooling histories, North Qaidam HP/UHP terrane, western China: Petrological and isotopic constraints

Abstract

Felsic gneisses near Yuka, western North Qaidam Mountains (NQD), and (60 km SE) near Xitieshan, central NQD, enclose eclogite-bearing mafic boudins. Inclusions in prograde-zoned garnets from a Yuka eclogite record pre-eclogite facies conditions of 12.1 ± 2.1 kbar, 603 ± 36 °C. The phengite-bearing matrix assemblage records peak conditions near or in the coesite stability field (23–28 kbar, 600–730 °C), but no coesite or coesite pseudomorphs have been found. Amphibolitized eclogite in the boudin margin records retrograde conditions of 10.9 ± 1.2 kbar, 630 ± 44 °C. Garnets from the Xitieshan eclogite lack prograde inclusions and compositional zoning. The bimineralic matrix assemblage records minimum eclogite-facies conditions of > 14 kbar, 730–830 °C. Symplectites of plagioclase and clinopyroxene record early retrograde conditions of 10–14 kbar, 750–865 °C. Positive ε Nd values from two Yuka and five Xitieshan eclogites indicate a depleted mantle source, consistent with formation of the eclogite protoliths in a continental rift or incipient oceanic basin setting. A Sm–Nd WR–Grt–Cpx isochron age of 435 ± 49 Ma obtained from one Xitieshan eclogite represents a cooling age. Zircon U–Pb TIMS and SHRIMP geochronology, combined with CL and inclusion analysis indicates a magmatic protolith age of ca. 750–800 Ma, and eclogite–facies zircon growth at 486–488 Ma. For the Yuka eclogite, exhumation to lower-crustal depths occurred before 477 ± 8 Ma (amphibole 40Ar– 39Ar), and a 466 ± 5 Ma phengite 40Ar– 39Ar age indicates an average cooling rate of 13–19 °C/Ma. In contrast, the 407 ± 4 Ma amphibole 40Ar– 39Ar age from the Xitieshan eclogite indicates a much slower cooling rate of 3–4 °C/Ma. HP/UHP rocks of similar age 200 km SE near Dulan, eastern NQD, and in the Altun Mountains on the NW side of the Altyn Tagh fault suggest that the Altun, Yuka, Xitieshan, and Dulan localities are part of the same early Paleozoic HP/UHP metamorphic belt.