Age of the Miyun dyke swarm:Constraints on the maximum depositional age of the Changcheng System  

作　　者：PENG Peng LIU Fu ZHAI MingGuo GUO JingHui 

机构地区：[1]State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China

出　　处：《Chinese Science Bulletin》2012年第1期105-110,共6页

基　　金：supported by the National Basic Research Programof China(2009CB219305);the National Natural Science Foundation of China(41072106)

摘　　要：A group of NE-trending(～30°)mafic dykes,termed Miyun swarm herein,are distributed around the Miyun Reservoir,northe China,and have individual widths of over 40 m,and lengths of up to 8000 m.Baddeleyite grains were extracted from a～40 wide dyke.Using an isotope dilution thermal ionization mass spectrometry method,these grains yield an average 207Pb/206Pb a of 1731±4 Ma(n=4;or 1731±1 Ma,n=3).Because this dyke is overlain unconformably by conglomerates of the Changzhoug Formation,the lowermost formation of the Changcheng Group,the maximum depositional age of this group is constrained to no earlier than 1731 Ma.This result also suggests that the Changcheng System,with the Changcheng Group as its standard stra graphic section,is younger than 1731 Ma.The Miyun dykes possibly have distinct petrogenesis from the subsequent anorth site-rapakivi intrusions(1730-1680 Ma),which are thought to be responsible for the opening of the Yan-Liao rift,where Changcheng Group was deposited.Thus it is reasonable to set the initial boundary of the Changchengian Period at 1730 Ma slightly younger.A group of NE-trending （-30°） mafic dykes, termed Miyun swarm herein, are distributed around the Miyun Reservoir, northem China, and have individual widths of over 40 m, and lengths of up to 8000 m. Baddeleyite grains were extracted from a -40 m wide dyke. Using an isotope dilution thermal ionization mass spectrometry method, these grains yield an average ^207Tpb/^206pb age of 1731± Ma （n = 4; or 1731±1 Ma, n = 3）. Because this dyke is overlain unconformably by conglomerates of the Changzhougou Formation, the lowermost formation of the Changcheng Group, the maximum depositional age of this group is constrained to be no earlier than 1731 Ma. This result also suggests that the Changcheng System, with the Changcheng Group as its standard stratigraphic section, is younger than 1731 Ma. The Miyun dykes possibly have distinct petrogenesis from the subsequent anortho- site-rapakivi intrusions （1730-1680 Ma）, which are thought to be responsible for the opening of the Yan-Liao rift, where the Changcheng Group was deposited. Thus it is reasonable to set the initial boundary of the Changchengian Period at 1730 Ma or slightly younger.

关 键 词：密云水库 长城系 年龄 岩墙 沉积 热电离质谱法 长城集团 谷物产量 

分 类 号：P588.3[天文地球—岩石学] X524[天文地球—地质学]