Geochemistry, Zircon U-Pb Geochronology and Lu-Hf Isotopic Composition of Eclogites and Their Host Gneisses in the Dulan Area, North Qaidam UHP terrane: New Evidence for Deep Continental Subduction

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In this study, we link zircon U-Pb SHRIMP and LA-ICP-MS geochronology and the Lu-Hf isotopic composition of eclogites and their host gneisses/schists with whole-rock geochemistry of eclogites in the Dulan area to constrain their protoliths and metamorphic relationships. U-Pb dating suggests that the protolith of one of the eclogites was a Neoproterozoic mafic intrusive rock (828 ± 58 Ma) and the protolith of enclosing orthogneiss was an early-Neoproterozoic granitoid (923 ± 12 Ma). Detrital zircons from Grt-bearing mica-schists yield ages of 0.9–2.5 Ga, with a dominant range of 1.0–1.8 Ga, indicating sedimentary sources from Neoproterozoic to Neoarchean crust and a depositional age ≤ 0.9 Ga. The matching metamorphic ages of eclogites (438 ± 5 Ma, 436 ± 4 Ma) and their country rocks (Grt-bearing mica-schists: 438 ± 4 Ma, 439 ± 8 Ma; orthogneiss: 427 ± 8 Ma) indicate that all studied samples experienced coeval Early Paleozoic HP/UHP metamorphism. The U-Pb ages and Hf isotopic compositions of the inherited magmatic zircon cores of an eclogite sample (εHf (800) = 2.6–9.2, TDM1 = 1.0–1.3 Ga, TDM2 = 1.1–1.4 Ga) suggest that the protolith may be derived from Neoproterozoic depleted mantle with variable proportions of an older crustal component. The magmatic zircon cores of the orthogneiss (εHf (900) = − 7.3 to − 0.2; TDM2 = 1.8–2.1 Ga) suggest that the parental magma was derived from a Paleoproterozoic crustal source. Hf isotopic compositions of the detrital zircons from the metasediments (εHf(t) = − 19.4 to + 10.6) suggest three crust formation and reworking events: (1) Archean (TDM2 = 2.7–2.9 Ga) juvenile crust reworked at ~ 2.5 Ga; (2) early Paleoproterozoic (TDM2 = 2.3–2.5 Ga) juvenile crust reworked at ~ 1.8 Ga; and (3) late Paleoproterozoic (TDM2 = 1.5–1.9 Ga) juvenile crust reworked in the Neoproterozoic.

Whole-rock geochemical data suggest that the protoliths of the Dulan eclogites were probably derived from a continental rift or an incipient oceanic basin rather than a large, long-lived ocean basin. Thus, combined with field relationships, petrology, geochemistry, zircon U-Pb dating and the Lu-Hf isotopic analysis presented in this paper and reported from previous studies, we suggest that the Dulan eclogites and their country rocks experienced a common UHP metamorphism during Late Ordovician deep continental subduction.


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