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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, northe China, and have individual widths of over 40 m, and lengths of up to 8000 m. 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 Changzhou 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. Miyun dykes may 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.