Anchimetamorphism of the Neoproterozoic and the Lower Paleozoic along the Profile of Yuanguping in Western Hunan Province, China

Abstract The Neoproterozoic and Lower Paleozoic along the profile of Yuanguping in western Hunan Province, China underwent anchimetamorphism. The illite crystallinity (IC) of the <2 μm fractions ranges from 0.23–0.34° Δ2θ for the Neoproterozoic to 0.23–0.35° Δ2θ for the Lower Paleozoic (calibrated with the Kisch IC set, Kisch, 1991). This indicates that the metamorphic grade of the Neoproterozoic and Lower Paleozoic is the anchizone. The peak metamorphic temperature is estimated to be 290–210°C. This result does not agree with the greenschist or subgreenschist facies of the Banxi Group, nor with the lower‐greenschist facies or sedimentary cover of the Sinian to Lower Paleozoic, as most previous researchers thought. The illite (K‐mica) b₀ values range from 0.9074 to 0.8963 (nm) for the Neoproterozoic and the Lower Paleozoic. Based on cumulative frequency curves of the illite (K‐mica) b₀, the peak metamorphic pressure of the Banxi Group was derived to be of a type that is slightly higher than that of the N. New Hampshire low‐intermediate pressure type. Most illites occur as the 2M₁ polytype and some in the Neoproterozoic as a mixture of the 2M₁+1M types. The distributions of coherent scanning domains (CSDs) of illites along the profile, measured with the XRD method, display a lognormal model and spread out with decreasing Kübler Index of IC. It indicates that illites underwent an Ostwald ripening. The post‐anchimetamorphic structural movement not only results in a series of faults but also induces the lattice strain in minerals along the faults and hence impacts the illite crystallinity and causes diagenetic samples to occur within the anchizone. Compared with the cases in eastern and central Hunan Province, the Neoproterozoic and Lower Paleozoic rocks in the west underwent a lower‐temperature anchimetamorphism with a pressure lower than that in the east and higher than that in the center of Hunan Province.