Grenvillian and early Paleozoic polyphase metamorphism recorded by eclogite and host garnet mica schist in the North Qaidam orogenic belt

The North Qaidam orogenic belt (NQOB) is generally considered to be an early Paleozoic ultrahigh pressure metamorphic belt, but increasing reports of the Neoproterozoic magmatic and metamorphic events indicate that the NQOB probably also experienced the assembly of the Rodinia. However, the Neoproterozoic evolution of the NQOB is not well constrained due to the sparse records and ambiguous nature of the Neoproterozoic metamorphism. In order to reveal the multi-orogenic history of the NQOB, an integrated study of petrology, phase equilibrium modelling and geochronology was conducted on an epidote eclogite and host garnet mica schist from the Yuka-Luofengpo terrane. New zircon and monazite U-Pb ages show that the protolith of the garnet mica schist was deposited during 994-920 Ma and experienced Neoproterozoic (920-915 Ma) and early Paleozoic (451-447 Ma) polyphase metamorphism together with the enclosed eclogite. Relic omphacite inclusions were first identified in garnet and early Paleozoic zircon domains from the garnet mica schist, which provide solid evidence for the early Paleozoic eclogite facies metamorphism of the mica schist. Similar early Paleozoic peak P-T conditions of >27.4 kbar/613-670℃ and 30.2-30.8 kbar/646-655℃ were obtained for the garnet mica schist and enclosed eclogite, respectively, indicating that eclogites and their host paragneisses in this region underwent continental deep subduction as a coherent metamorphic terrane in early Paleozoic. The peak P-T conditions of the Neoproterozoic metamorphism were roughly constrained at 7.7-12.0 kbar and 634-680℃ for the garnet mica schist, based on stability field of mineral inclusions in Neoproterozoic zircons domains in P-T pseudosection, the relic garnet core composition and Ti-in-zircon thermometer. The high thermal gradients (16-37℃/km) defined by presently our and previously reported P-T conditions indicate that the Neoproterozoic metamorphism likely occurred in continental collision setting at >945-890 Ma. Since the Grenvillian syn-orogenic granitic magmatism and metamorphism (ca. 1.0-0.9 Ga) in the NQOB are much younger than the Grenvillian orogeny in the central part of Rodinia, the Qaidam Block was probably located at the north margin of Rodinia in Neoproterozoic.