DEFORMATION AND METAMORPHIC HISTORY OF THE PRESINIAN ROCK GROUPS IN THE SONGSHAN AREA, HENAN PROVINCE, CHINA

Abstract

A systematic discussion has been made on the deformation and metamorphic history of two Presinian rock groups (the lower Proterozoic Songshan Group and the Archean Dengfeng Group) in the Songshan area, Henan Province, southern part of the North China platform. Structures of the two Groups provide clear evidence for at least two phases of deformation in the early Proterozoic Zhongyue tectonic cycle and three in the Archean Songyang tectonic cycle. By structural analysis, two important aspects of superimposition relationships in polyphase deformation are illustrated: the control of earlier structures upon later ones and the reform of the former by the latter, and their geometrical regularities are also dealt with respectively. Structural sequences of two tectonic cycles developed under the general tendency that the heat flow field of regional metamorphism was continuously weakening and correspondingly the plasticity of the rock media was continuously decreasing. Together with the regional metamorphism, magmatic activity and other important geological events, they reflect the evolution regularity of the tectono-thermal cycle during the early period of crustal development.