U-Pb Dating And Hf Isotopic Compositions Of Detrital Zircons In The Heihe Rivers' Sediments From The Northern Qilian Orogen And Their Geological Implications

River sediments can record the formation and evolution histories of their provenances' regional crust,and are ideal for uniform sampling of the crust in river drainage.Compared with the studies of the outcropped geological bodies,river detrital zircons can reflect the peak time and the intensity of the magmatic events within catchment.Thus,the research on the river detrital zircons is significant to explore the formation and evolution of the crust in a catchment.In this paper,through systematic study of geochemistry,zircon U-Pb dating and Hf isotopic compositions on the Heihe rivers' sediments in the Northern Qilian Orogen,the conclusions have been drawn as follows:(1)Sediments in the Heihe river show that the average contents of major elements,REE and other trace element patterns are consistent with those of upper continental crust.(2)Archean ancient continental components are very scarce in the Northern Qilian Orogen,which may exist in the Paleoproterzoic metamorphic basement complexs,such as the Beidahe Complex and Yema Nanshan Complex.(3)Zircons with ages between 2700 Ma and 1700 Ma account for 35% totally,which recorded the main formation stage of basement in the Northern Qilian Orogen.Zircons between 1700 Ma and 710 Ma occupy 14% of the total zircons,they recorded formation history of the Qilian oceanic crust.(4)Zircons with ages between 630 Ma and 360 Ma account for 33% of all measured ages,which mainly recorded magmatism for the processes of oceanic crustal subduction,continental-continental collision and post-collision.Zircon with ages of 360-220 Ma indicated that there is slight magmatism of Hercynian and Indosinian in the Northern Qilian Orogen.(5)Detrital Zircon U-Pb ages indicate that Sala river is the larger contributor to the Heihe river,but tributary of 15HH2 is the minor contributor,contribution of 15HH5 is more difficult to determine.(6)Detrital zircons with Archean age deposited during downstream migration but without further supply,and that the afflux of other age composition of zircons,which leads to the zircons with Archean age disappeared in the lower reaches.In contrast,zircons represent the main magmatic events in drainage,will be supplied by surrounding source ceaselessly,and keep their proportions with a certain range,which means that various sampling locations distributing in the drainage are all representative for the characteristic magmatic events within the drainage.(7)The crustal accretion of the Northern Qilian Orogen occurred at two stages of 3.2~2.0 Ga and 2.0~1.0 Ga through contraints of the Heihe river detrital zircons Hf isotopic compositions.The crustal accretion almost stopped since 630 Ma in the Northern Qilian Orogen.