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 we...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.展开更多
In 1997 and 1998, hundreds of specimens of megascopic carbonaceous compressions or algal fossils were found from - 1800-million-year old Changzhougou Formation, which is the lowermost unit of the latest Palaeoproteroz...In 1997 and 1998, hundreds of specimens of megascopic carbonaceous compressions or algal fossils were found from - 1800-million-year old Changzhougou Formation, which is the lowermost unit of the latest Palaeoproterozoic Changcheng Group (~1 600-1 800 Ma) in the Xinglong-Kuancheng areas at the middle Yanshan Range, North China. They are discoid, ellipsoid and sausage-like, namely shaped like Chuaria Shouhsiennia (Ellipsophyta) and Tawuia. By adopting HF acid-resistant maceration coupled with scanning electron microscope and petrologic section, the authors made a preliminary research on the histology for some circular and ellipsoid carbonaceous compressions, namely Chuaria- and Shouhsienia-llke forms, in addition to their morphology. The following three types of multicellular tissues have been found in the fragments of them: colony-like, pseudoparenchyma-like and parenchyma-like. All of the new data about multicelluar tissues not only supply a very important basis in histology to determine the展开更多
基金supported by the National Basic Research Programof China(2009CB219305)the National Natural Science Foundation of China(41072106)
文摘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.
文摘In 1997 and 1998, hundreds of specimens of megascopic carbonaceous compressions or algal fossils were found from - 1800-million-year old Changzhougou Formation, which is the lowermost unit of the latest Palaeoproterozoic Changcheng Group (~1 600-1 800 Ma) in the Xinglong-Kuancheng areas at the middle Yanshan Range, North China. They are discoid, ellipsoid and sausage-like, namely shaped like Chuaria Shouhsiennia (Ellipsophyta) and Tawuia. By adopting HF acid-resistant maceration coupled with scanning electron microscope and petrologic section, the authors made a preliminary research on the histology for some circular and ellipsoid carbonaceous compressions, namely Chuaria- and Shouhsienia-llke forms, in addition to their morphology. The following three types of multicellular tissues have been found in the fragments of them: colony-like, pseudoparenchyma-like and parenchyma-like. All of the new data about multicelluar tissues not only supply a very important basis in histology to determine the
