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

Abstract

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 θ for the Lengjiaxi Group to 0.19—0.23°Δ2 θ for the Banxi Group to 0.20—0.29°Δ2 θ for the Sinian to the Lower Paleozoic (Kubler 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) 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 2M 1 polytype in the Lengjiaxi and Banxi groups and as 2M 1 +1M types in some samples of the Lower Paleozoic. This indicates that the transformation of illite polytype from 1M to 2M 1 was fulfilled within the high anchizone.