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
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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
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    摘要:

    通过对发源于北祁连造山带北段、中段、南段河流沉积物地球化学与碎屑锆石U Pb年龄及Hf同位素组成的系统研究,确定河流沙、泥沙质微量元素平均组成与大陆上地壳平均值基本一致。一般沙质成分由于石英的稀释作用使其微量元素含量稍低于大陆上地壳平均值,泥沙质成分的微量元素含量一般稍高于大陆上地壳均值。北祁连造山带河流碎屑沉积物As、Sb、Cu、Zn等元素的平均含量明显高于大陆上地壳的平均含量,可能指示北祁连硫化物矿床成矿具有良好的前景。河流碎屑沉积物Sm Nd同位素组成得到的Nd亏损地幔模式年龄为189~112Ga,平均为168Ga;北段、中段河流沙质部分的Nd同位素亏损地幔模式年龄比相对较细的泥沙质部分的老,说明河流近源古老地壳基底组分更多;而南段的部分河流则相反,说明近源有更多的新生地壳组分。锆石U Pb年龄结果表明,北祁连造山带最强的岩浆作用发生于新元古代—晚古生代早期(650~400Ma),此期间形成的锆石占全部锆石的31%,它们主要记录了北祁连洋壳的俯冲消减、陆 陆碰撞及碰撞后的壳内岩浆作用过程。太古宙古老基底组分在北祁连造山带存在,但所占比例极低,它们可能以碎屑组分存在于古元古代的北大河岩群等变质基底中。新太古代到古元古代(2700~1700Ma)期间形成的锆石占全部锆石的~34%,此期间是北祁连基底的主要形成阶段。古元古代末到新元古代(1700~650Ma)期间形成的锆石占全部锆石的17%,它们可能代表着古祁连洋盆的主要形成阶段。锆石Hf同位素组成确定的北祁连由亏损地幔增生物质到地壳主要发生于32~20Ga、20~10Ga这两个时间段,650Ma以后基本没有增生地壳组分的加入。不同地段河流碎屑锆石U Pb年龄有些差别,北段的北大河中最老基底组分所占比例最高,而南段河流中的古老基底组分比例相对较低,印证了古祁连洋是从东南向西北拉开的过程,西北段在俯冲消减及碰撞过程中有较多古老地壳基底组分的参与。晚古生代晚期—三叠纪期间北祁连仍存在较弱的岩浆作用。由Nd同位素比值与Nd含量、Na2O/K2O比值之间的关系进一步证明北祁连河流沉积物是最古老的基底长英质组分、较老的基底镁铁质组分或其熔融产物、古祁连洋壳镁铁质组分或其熔融产物三者混合的产物。

    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 143Nd/144Nd isotopic ratios and 1/Nd as well as between 143Nd/144Nd isotopic ratios and Na2O/K2O 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.

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陈岳龙,李兆,刘长征,古远,李大鹏.2016.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,90(2):267-283.
CHEN Yuelong, LI Zhao, LIU Changzhe, GU Yuan, LI Dapeng.2016. 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[J]. Acta Geologica Sinica,90(2):267-283.

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  • 收稿日期:2015-06-22
  • 最后修改日期:2015-07-23
  • 录用日期:2015-08-31
  • 在线发布日期: 2016-02-24
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