TECTONIC EVOLUTION OF THE NORTH QILIAN OROGENIC BELT: EVIDENCE FROM DETRITAL ZIRCON RECORD OF RIVERS' SEDIMENTS

Determining detrital zircon ages and Hf isotopic compositions of the clastic sediments can reveal the episodes of secular magmatism and crustal evolution. Several rivers crossing the North Qilian Orogenic Belt (NQOB) are an ideal proxy for the study of crustal evolution of it. More than 800 U–Pb ages and 300 Lu–Hf isotopic composites of detrital zircon grains from rivers' sediments in the NQOB are presented. U–Pb ages obtained from detrital zircons found in the rivers' sediments can be divided into five major age groups, that is, 2800–2200, 2100–1500, 1100–900, 600–400, and 400–200 Ma, and the corresponding peaks occur at ~2500, ~1800, ~1000, ~500, and ~300 Ma. The age distribution patterns indicate that rivers' sediments were mainly sourced from the NQOB. The detrital zircon ages demonstrate that the NQOB may have Archean to late Paleoproterozoic rocks. Furthermore, the Hf isotopic compositions of Neoarchean, Paleo- and Mesoproterozoic, Grenvillian, Caledonian, and Hercynian zircon age groups exhibit a wide range of ε_Hf(t) values, suggesting diverse sources. The existence of strongly negative ε_Hf(t) values among the Grenvillian and Paleoproterozoic zircons indicates that the source magma included reworked Paleoarchean crustal materials. The crustal accretion of the NQOB from depleted mantle occurred at two stages of 3.0 Ga and 2. 0-1.0 Ga through constraints of Hf isotopic compositions of representative zircon grains. New results indicate that continental rifting and the opening of the Qilian Ocean occurred at 775–520 Ma; the subduction and closure of the Qilian Ocean occurred at 520–440 Ma; arc–continent collision and continental subduction occurred at 440–420 Ma; and orogen collapse and extension occurred at 400–360 Ma. Our study indicates that the formation of the NQOB was mainly related to the evolutionary history of the Proto-Tethys Ocean.