Abstract:Through systematic study of geochemistry, zircon U Pb dating and Hf isotopic compositions on rivers’ sediments in the Northern Qilian Orogen, it shows that REE and other trace element patterns of the sediments are consistent with those of upper crust. Generally, contents of trace elements in sandy sediments are lower than those of silts because of dilution by quartz grains. The higher contents of As, Sb, Cu, and Zn in rivers’ sediments indicate a potential prospect of sulfide mineralization in the Northern Qilian Orogen comparing to the abundances of these elements in averaged upper continental crust. Nd model ages are between 189Ga and 112Ga, the average age of which is 168Ga. Nd model ages of sandy sediments in northern and middle segments, the Northern Qilian Orogen are older than those of silts, which implies that nearby provenances contain more basement components than remote provenances. But, the rivers’ sediments in southern segment behave reversely, which indicates that nearby provenances contain more newly accreted crustal components there. Zircon U Pb ages show the most intensive magmatism during the period of 650 Ma and 400 Ma, which accounts for 31% among whole range of ages and recorded intra crustal magmatism for the processes of oceanic crustal subduction, continental continental collision as well as post collision. Archean basement components are very scarce, which may exist in Paleoproterzoic metamorphic basement complexes, such as the Beidahe Complex. Zircons with ages between 2700 Ma and 1700 Ma occupy ~34% totally, which indicate the main formation stage of basement in the Northern Qilian Orogen. Although zircons between 1700 Ma and 650 Ma hold only 17%, they recorded formation history of the Qilian oceanic crust by sea floor spreading. The crustal accretion of the Northern Qilian Orogen from depleted mantle occurred at two stages of 32~20 Ga and 20~10Ga through constraints of Hf isotopic compositions of representative zircon grains. There is little accreted crustal components from the depleted mantle since 650 Ma in the Northern Qilian Orogen. The populations of zircon U Pb ages show some differences among different segments’ river sediments, the Beidahe river’s sediments of which is characterized by the most significant majority of old zircons, indicating the existence of Paleoproterozoic basement and northwestern end of the Qilian ocean’s rifting starting from the southeastern part and involvement of the basement with greater percentage during processes of subduction and collision. There is slight magmatism of late Late Paleozoic and Triassic period in the Northern Qilian Orogen. Through plots between 143Nd/144Nd isotopic ratios and 1/Nd as well as between 143Nd/144Nd isotopic ratios and Na2O/K2O ratios, it is proven that the rivers’ sediments are mixtures among the oldest felsic basement and its anataxis product, Neoarchean Paleoproterozoic accreted mafic basements and their molten materials, late Mesoproterozoic Neoproterozoic accreted mafic Qilian oceanic crust and its molten materials.