Anchimetamorphism and diagenesis of the Meso-Neoproterozoic and the Lower Paleozoic along profile Yangjiaping in North Hunan Province, China

作者：Wang HeJin^*; Zhou Zhao; Wang Ling; Yuan Lei

来源：Acta Petrologica Sinica, 2014, 30(10): 3013-3020.

• Illite crystallinity
• Polytypism
• Cell dimension
• Lattice strain
• Anchimetamorphism
• Neoproterozoic
• Lower Paleozoic
• North Hunan Province

Low temperature metamorphic indicators, such as illite crystallinity, cell dimension of K-white micas, polytype, lattice strain, crystal size distribution and mineral assemblage were employed in the study of diagenesis and metamorphism for the Meso-Neoproterozoic and the Lower Paleozoic along profile Yangjiaping in North Hunan Province, China. Mite crystallinity (IC) of the <2 mu m fractions ranges from 0.21 similar to 0.24 degrees Delta 2 theta for the Meso-Neoproterozoic to 0.28 similar to 0.67 degrees Delta 2 theta for the Lower Paleozoic (calibrated with Kisch IC set 1991). This indicates that the Meso-Neoproterozoic was overprinted by high anchimetamorphism whilst the Lower Paleozoic overprinted by geological processes from diagenesis to low anchimetamorphism. According to anchizonal boundaries (0.21 similar to 0.38 degrees Delta 2 theta, CuK alpha), the profile was classified into a northern anchizone and a southern diagenetic zone. The peak anchimetamorphic temperature of the Meso-Neoproterozoic and the Lower Paleozoic was roughly evaluated by chemical geothermometer of chlorite at ca. 260 degrees C. The illites (K-mica) b(0) values in anchizone range from 0.9000 to 0.9045 nm and, on average as 0. 9017nm, implying roughly a low to intermediate pressure condition. Mites occur as the 2M(1) polytype in anchizone (most of the Meso-Neoproterozoic) and as a mixture of 2M(1) + 1M types in the Lower Paleozoic. The product of logarithms mean particle size and logarithms frequency of illite (here we called "general logarithms ripening" abbreviated as GLR) is well separated into two groups, in which these GLRs less than 1. 88 belong to diagenetic while those larger than 1.88 belong to anchimetamorphic. This is a new observation. Post-anchimetamorphic structural stress not only forms a series of faults but also induces the lattice strain in minerals and hence impacts the illite crystallinity and causes the Ktibler Index to increase in width and result in an unusual value. Comparing with the cases in the near south (Changsha-Liling-Liuyang, Huangtudian-Xianxi and Yuanguping), the rocks of the Meso-Neoproterozoic and the Lower Paleozoic in the profile underwent diagenesis and anchimetamorphism with a low to intermediate pressure condition. This result is different from the greenschist or subgreenschist facies of the Banxi Group or from the lower-greenschist facies or sedimentary cover of the Sinian to the Lower Paleozoic, of which most previous researchers thought. Geological implication is discussed as well.

• 出版日期2014-10
• 单位北京大学