Very low-grade metamorphism of the Lengjiaxi and Banxi Groups around the area of Changsha-Liling-Liuyang, Hunan province, China.

Abstract

The Lengjiaxi and Banxi Groups of the Middle Upper Proterozoic around the area of Changsha Liling Liuyang of the eastern Hunan Province, China underwent very low grade metamorphism. Illite crystallinity (IC) data of the 2μm fractions vary from 0.18 to 0.26°△2θ in the Lengjiaxi Group and from 0.21 to 0.28°△2θ in the Banxi Group (Kubler index, corrected with Kisch IC set, 1991). This indicates that the metamorphic grade of the Lengjiaxi Group is in the epizone and upper anchizone whilst the Banxi Group in the upper anchizone around the area investigated. The evaluated metamorphic temperature of the Lengjiaxi Group is 280~400℃ and that of the Banxi Group is 250~300℃. This result does not agree with the greenschist or subgreenschist facies of which most previous researchers thought. The range of b 0 values of illites varies from 0.8999 to 0.9042 (nm) in the Lengjiaxi Group and from 0.9018 to 0.9042 (nm) in the Banxi Group. Based on the cumulative frequency curves of illite (muscovite) b 0, the metamorphic pressure of the Lengjiaxi Group in the area was in a typical intermediate type.