Very low-grade metamorphism of the Meso-Neoproterozoic and the Lower Paleozoic along the profile from Huangtudian to Xianxi in the central-northern part of Hunan Province,China  

作　　者：王河锦 周健 徐庆生 刘楚雄 朱明新 

机构地区：[1]"rogene and Crust Evolution"Key Laboratory of Ministry of Education of China, School of Earth and Space Sciences, Peking University [2]Chinese Academy of Geological Sciences [3]Institute of Mineral Resources, Ministry of National Lands and Resources of China

出　　处：《Science China Earth Sciences》2003年第7期672-682,共11页中国科学（地球科学英文版）

基　　金：This work was suppored by the National Natural Science Foundaition of China(Grant No.49872033).

摘　　要：The Lengjiaxi and Banxi groups of the Meso-Neoproterozoic and the Lower Paleozoic along the profile at Huangtudian-Xianxi of the central-northern Hunan Province, China underwent very low-grade metamorphism. Illite crystallinity (IC) data of the <2 μm fractions range from 0.18-0.21°Δ2θ for the Lengjiaxi Group to 0.19–0.23°Δ2θ for the Banxi Group to 0.20–0.29°Δ2θ for the Sinian to the Lower Paleozoic (Kübler index, corrected with Kisch IC set, 1991). This indicates that the metamorphic grade of the Lengjiaxi Group is in the epizone, the Banxi Group in the epizone-high anchizone, and the Sinian to the Lower Paleozoic mostly in the high anchizone along the profile investigated. The peak metamorphic temperature evaluated is roughly 340–240°C. This result does not agree with the greenschist or subgreenschist facies of the Lengjiaxi and Banxi groups and neither with the lower-greenschist facies or diagenitic zone of the Sinian to the Lower Paleozoic, of which most previous researchers thought. The illites (K-mica) b 0 values range from 0.8989–0.9050 nm for the Lengjiaxi Group to 0.8984–0.9037 nm for the Banxi Group to the Lower Paleozoic. Based on the cumulative frequency curves of illite (K-mica) b 0, the peak metamorphic pressure of the Banxi Group to the Lower Paleozoic along the profile was derived in a low-intermediate pressure type. Illites occur as 2M1 polytype in the Lengjiaxi and Banxi groups and as 2M1+1 M types in some samples of the Lower Paleozoic. This indicates that the transformation of illite polytype from 1M to 2M1 was fulfilled within the high anchizone.The Lengjiaxi and Banxi groups of the Meso-Neoproterozoic and the Lower Paleozoic along the profile at Huangtudian-Xianxi of the central-northern Hunan Province, China underwent very low-grade metamorphism. Illite crystallinity (IC) data of the <2 mm fractions range from 0.18—0.21癉2q for the Lengjiaxi Group to 0.19—0.23癉2q for the Banxi Group to 0.20—0.29癉2q for the Sinian to the Lower Paleozoic (K黚ler index, corrected with Kisch IC set, 1991). This indicates that the metamorphic grade of the Lengjiaxi Group is in the epizone, the Banxi Group in the epizone-high anchizone, and the Sinian to the Lower Paleozoic mostly in the high anchizone along the profile investigated. The peak metamorphic temperature evaluated is roughly 340—240℃. This result does not agree with the greenschist or subgreenschist facies of the Lengjiaxi and Banxi groups and neither with the lower-greenschist facies or diagenitic zone of the Sinian to the Lower Paleozoic, of which most previous researchers thought. The illites (K-mica) b0 values range from 0.8989—0.9050 nm for the Lengjiaxi Group to 0.8984—0.9037 nm for the Banxi Group to the Lower Paleozoic. Based on the cumulative frequency curves of illite (K-mica) b0, the peak metamorphic pressure of the Banxi Group to the Lower Paleozoic along the profile was derived in a low-intermediate pressure type. Illites occur as 2M1 polytype in the Lengjiaxi and Banxi groups and as 2M1+1M types in some samples of the Lower Paleozoic. This indicates that the transformation of illite polytype from 1M to 2M1 was fulfilled within the high anchizone.

关 键 词：ILLITE crystallinity  ILLITE polytype  b0 value  anchi/epimetamorphism  Lengjiaxi Group  Banxi Group  SINIAN to Lower Paleozoic. 

分 类 号：P534[天文地球—古生物学与地层学]