Geochemistry, zircon Uonology and Luic 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 U and LA-ICP-MS geochronology and the Luic 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 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 Und Hf isotopic compositions of the inherited magmatic zircon cores of an eclogite sample (ε _Hf (800)=2.6–9.2, T_DM1 =1.0–1.3Ga, T_DM2 =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 (ε_Hf (900)=−7.3 to −0.2; T_DM2 =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 (ε_Hf(t)=−19.4 to +10.6) suggest three crust formation and reworking events: (1) Archean (T_DM2 =2.7–2.9Ga) juvenile crust reworked at ~2.5Ga; (2) early Paleoproterozoic (T_DM2 =2.3–2.5Ga) juvenile crust reworked at ~1.8Ga; and (3) late Paleoproterozoic (T_DM2 =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 U and the Luic 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. [Copyright &y& Elsevier]