地质学报2016，Vol.90Issue(2)：267-283,17.
北祁连的形成与演化历史：来自河流沉积物地球化学及其碎屑锆石 U-Pb 年龄、Hf 同位素组成的证据
Formation and Evolution History on the Northern Qilian Orogen:the Evidences from Compositions of Rivers’Sediments and Their Zircon U-Pb Ages,Hf Isotopic Compositions
摘要
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 1.89Ga and 1.12Ga,the average age of which is 1.68Ga.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 3.2~2.0 Ga and 2.0~1.0Ga 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 1 43 Nd/1 44 Nd isotopic ratios and 1/Nd as well as between 1 43 Nd/1 44 Nd isotopic ratios and Na2 O/K2 O 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.关键词
北祁连/河流沉积物/地球化学/锆石/U-Pb 年龄/Hf 同位素组成/地壳形成与演化Key words
the Northern Qilian Orogen/rivers ’ sediment/geochemistry/zircon/U-Pb dating/Hf isotopic composition/formation and evolution of crust引用本文复制引用
李兆,陈岳龙,刘长征,古远,李大鹏..北祁连的形成与演化历史：来自河流沉积物地球化学及其碎屑锆石 U-Pb 年龄、Hf 同位素组成的证据[J].地质学报,2016,90(2):267-283,17.基金项目
本文为国家自然科学基金项目（编号41473015）资助。 （）
