Geochemistry, zircon U[sbnd]Pb geochronology and Lu[sbnd]Hf isotopic composition of eclogites and their host gneisses in the Dulan area, North Qaidam UHP terrane: New evidence for deep continental subduction

Abstract

In this study, we link zircon UPb SHRIMP and LA-ICP-MS geochronology and the LuHf 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. UPb dating suggests that the protolith of one of the eclogites was a Neoproterozoic mafic intrusive rock (828±58Ma) and the protolith of enclosing orthogneiss was an early-Neoproterozoic granitoid (923±12Ma). Detrital zircons from Grt-bearing mica-schists yield ages of 0.9–2.5Ga, with a dominant range of 1.0–1.8Ga, indicating sedimentary sources from Neoproterozoic to Neoarchean crust and a depositional age ≤0.9Ga. The matching metamorphic ages of eclogites (438±5Ma, 436±4Ma) and their country rocks (Grt-bearing mica-schists: 438±4Ma, 439±8Ma; orthogneiss: 427±8Ma) indicate that all studied samples experienced coeval Early Paleozoic HP/UHP metamorphism. The UPb ages and Hf isotopic compositions of the inherited magmatic zircon cores of an eclogite sample (εHf (800)=2.6–9.2, TDM1=1.0–1.3Ga, TDM2=1.1–1.4Ga) 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=1.8–2.1Ga) 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=2.7–2.9Ga) juvenile crust reworked at ~2.5Ga; (2) early Paleoproterozoic (TDM2=2.3–2.5Ga) juvenile crust reworked at ~1.8Ga; and (3) late Paleoproterozoic (TDM2=1.5–1.9Ga) 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 UPb dating and the LuHf 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.