Very Low‐Grade Metamorphism of Clastic Rocks from the Meso‐Neoproterozoic and the Paleozoic along the Profile Yueyang‐Linxiang in Northeastern Hunan Province and Its Geological Implications

Abstract

AbstractThis study uses illite crystallinity, chlorite crystallinity, illite polytypes, the b0 cell‐dimension of K‐white mica, clay mineral assemblages and mineral geothermo‐geobarometers to investigate the overprint of diagenesis and metamorphism on the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang in northeastern Hunan Province, China. Illite crystallinity Kübler index (KI) of the <2µm fractions ranges from 0.225 to 0.485°Δ2θ while chlorite crystallinity Árkai index (AI) ranges from 0.244 to 1.500°Δ2θ. This indicates that the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang were overprinted with diagenesis and anchi‐ to epimetamorphism. Peak metamorphic temperature is estimated with the IV site chlorite geothermometer roughly at 360°C. The b0 cell dimension values of illites (K‐mica) range from 0.9002 to 0.9054 nm and, on average, at 0.9030 nm for the Meso‐Neoproterozoic. Based on cumulative frequency curves of illite (K‐mica) b0 cell dimension, the peak metamorphic pressure of the Meso‐Neoproterozoic along the profile Yueyang‐Linxiang is derived of an intermediate pressure type. Most illites occur in the 2M1 polytype and some of them in a mixture of 2M1+1M types especially those in the Paleozoic. This result partly agrees with the conclusion of the lower greenschist and greenschist facies of the Lengjiaxi and Banxi Groups. However, it is not agreed with the sedimentary cover from the Sinian to the Lower Paleozoic or from the Banxi Group to the Lower Paleozoic. Crustal thickening due to “collision” between the Yangtze and Cathaysia blocks led to an increase in the thickness of the Meso‐Neoproterozoic to ca. 14 km and resulted in a temperature increase in those rocks due to burial. The very low grade to low grade metamorphism overprinting the Meso‐Neoproterozoic implies that the so called “Chiangnania or Jiangnan orogen” was no relative with the “Grenvillian orogeney; instead, it might be a continuous amalgamation product between the Yangtze and Cathaysia blocks.