Zircon geochemistry of two contrasting types of eclogite: Implications for the tectonic evolution of the North Qaidam UHPM belt, northern Tibet

Compared to the extensively documented ultrahigh-pressure metamorphism at North Qaidam, the pre-metamorphic history for both continental crust and oceanic crust is poorly constrained. Trace element compositions, U-Pb ages, O and Lu-Hf isotopes obtained for distinct zircon domains from eclogites metamorphosed from both continental and oceanic mafic rocks are linked to unravel the origin and multi-stage magmatic/metamorphic evolution of eclogites from the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, northern Tibet. For continental crust-derived eclogite, magmatic zircon cores from two samples with U-Pb ages of 875-856 Ma have both very high delta O-18 (10.6 +/- 0.5 parts per thousand) and mantle-like delta O-18 (averaging at 5.2 +/- 0.7 parts per thousand), high Th/U and (LU)-L-176/Hf-177 ratios, and steep MREE-HREE distribution patterns (chondrite-normalized) with negative Eu anomalies. Combined with positive epsilon(Hf) (t) of 3.9-14.3 and T-DM (1.2-0.8 Ga and 1.3-1.0 Ga, respectively), they are interpreted as being crystallized from either subduction-related mantle wedge or recycled material in the mantle. While the metamorphic rims from the eclogites have U-Pb ages of 436-431 Ma, varying (inherited, lower, and elevated) oxygen isotopes compared with cores, low Th/U and Lu-176/Hf-177 ratios, and flat HREE distribution patterns with no Eu anomalies. These reflect both solid-state recrystallization from the inherited zircon and precipitation from external fluids at metamorphic temperatures of 595-622 degrees C (TTi-in-zircon). For oceanic crust-derived eclogite, the magmatic cores (510 +/- 19 Ma) and metamorphic rims (442.0 +/- 3.7 Ma) also show distinction for Th/U and Lu-176/Hf-177 ratios, and the REE patterns and Eu anomalies. Combined with the mantle-like delta O-18 signature of 5.1 +/- 0.3 parts per thousand and two groups of model age (younger T-DM close to the apparent ages and older > 700 Ma), two possible pools, juvenile and inherited, were involved in mixing of mantle-derived magma with crustal components. The relatively high delta O-18 of 6.6 +/- 0.3 parts per thousand for metamorphic zircon rims suggests either the protolith underwent hydrothermal alteration prior to the similar to 440 Ma oceanic crust subduction, or external higher 6180 fluid activities during UHP metamorphism at similar to 440 Ma. Therefore, the North Qaidam UHPM belt witnesses multiple tectonic evolution from Late Mesoproterozoic-Neoproterozoic assembly/breakup of the Rodinia supercontinent with related magmatic emplacement, then Paleozoic oceanic subduction, and finally transition of continental subduction/collision related to UHP metamorphism. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.