Geochronological,geochemical,and Nd-Hf isotopic studies of the Qinling Complex,central China:Implications for the evolutionary history of the North Qinling Orogenic Belt

The Qinling Complex of central China is thought to be the oldest rock unit and the inner core of the North Qinling Orogenic Belt(NQOB).Therefore,the Qinling Complex is the key to understanding the prePaleozoic evolution of the NQOB.The complex,which consists of metagraywackes and marbles,underwent regional amphibolite-facies metamorphism.In this study,we constrained the formation age of the Qinling Complex to the period between the late Mesoproterozoic and the early Neoproterozoic(ca.1062-962 Ma),rather than the Paleoproterozoic as previously thought.The LA-ICP-MS data show two major metamorphic ages(ca.499 and ca.420-400 Ma) for the Qinling Complex.The former age is consistent with the peak metamorphic age of the high- and ultra-high pressure(HP-UHP) rocks in the Qinling Complex,indicating that both the HP-UHP rocks and their country rocks experienced intensive regional metamorphism during the Ordovician.The latter age may constrain the time of partial melting in the NQOB between the late Silurian and early Devonian.The Qinling Complex is mostly affiliated with subduction-accretion processes along an active continental margin,and should contain detritus deposited in a forearc basin.The available data indicate that the NQOB was an independent micro-continent at least prior to the Neoproterozoic,and included a portion of the Grenville orogenic belt during the period of 1.2-0.8 Ga.The NQOB has experienced a unique geological history,which is obviously different from that of the North China Craton(NCC) and the Yangtze Craton during the Precambrian.The Neoproterozoic granitoids that intruded the Qinling Complex can be interpreted as the products of assembly of the supercontinent Rodinia.The NQOB was separated from Rodinia at ca.830-740 Ma.Subsequently,the NQOB moved closer to the northern margin of the NCC,and the initial accretion or collision with the NCC occurred from the late Cambrian to the early Ordovician.