The new mapping method for geological survey of 1:50,000 in ophiolite mélange areas： A case study on ophiolite mélange zones in Yunnan, China

A new mapping method for geological survey of 1∶50, 000 in ophiolite mélange areas has been proposed in this paper, based on the detailed studies of 10 ophiolite mélange zones in regional geological survey of 1∶50, 000 in Yunnan and the authors' work experiences of the second-edition revision of Regional Geology of Yunnan in recent years. According to the structural paleo-geographic environments, ophiolite mélange zones can be divided into four categories of rock combinations: mantle rocks, mid-ocean ridge magmatic intrusion rocks, ocean basin and inner ocean arc volcanic deposition rocks, and foreign rock sheets. Based on the typical rock combination, the specific material composition and rock genesis, the four categories of rock combinations are subdivided into 13 sub-combinations of rocks, furthermore. Through the detailed study of the evolution from expansion to subduction of oceanic basins in study areas, it is proposed that the widespread distribution of pelagic sediments, the first appearance of the anterior arc basalts, and the transformation of Azores ocean island basalt to Hawaiian ocean island basalt are the three key discrimination marks of the expansion peak of an ocean basin.