ZIRCONOLOGICAL CONSTRAINTS ON THE TECTONIC TRANSITION FROM OCEANIC SUBDUCTION TO CONTINENTAL COLLISION IN THE UHP NORTH QAIDAM OROGEN, NORTHERN TIBET

Subduction of low-density continental crust is generally considered to be pulled by sink of dense oceanic slab. Concomitant outcrops of oceanic- and continental-type eclogites in continental collision zones provide us an excellent opportunity to study the tectonic transition from oceanic subduction to continental collision. The North Qaidam orogen in northern Tibet is an ultrahigh-pressure (UHP) metamorphic belt that contains the two different types of eclogites. A combined study of zircon U-Pb ages, trace elements, mineral inclusions and O-Hf isotopes for various eclogites was performed in order to place constraints on their protolith nature and metamorphic evolution. The two types of eclogites are different in zircon U-Pb ages and O-Hf isotopes. Relict magmatic zircon domains revealed that continental-type eclogite has protolith U-Pb ages of 820-830 Ma, Hf model ages of 850-1100 Ma and mantle-like δ¹⁸O values, whereas oceanic-type eclogite has protolith U-Pb ages of 500-520 Ma, Hf model ages of 500-650 Ma and mantle-like δ¹⁸O values. Combined with eclogite geochemistry and local tectonics, the continental-type eclogites were considered to be metamorphosed from Neoproterozoic continental basalts, whereas the oceanic-type eclogites were derived from oceanic basalts that were subducted before the continental subduction. Metamorphic zircon domains have δ¹⁸O values mostly higher or lower than normal mantle for the continental- and oceanic-type eclogites, respectively. This is due to different temperatures of hydrothermal alteration during magma emplacement. Nevertheless, peak UHP eclogite-facies metamorphic ages in the orogen is dated to be uniform around 430-440 Ma, regardless of the eclogite types. There are also eclogite-facies metamorphic ages older and younger than this event, respectively, which are interpreted to date prograde and retrograde stages. The consistent UHP metamorphic ages for the oceanic- and continental-type eclogites indicate that the exhumed oceanic-type eclogite was segments of the oceanic crust that were attached to the subducting continental crust. Therefore, the coexistence of oceanic- and continental-type eclogites in the North Qaidam orogen demonstrates the tectonic transition from oceanic subduction to continental collision in the early Paleozoic.