Determination and oceanic crust subduction of Early Palaeozoic accretionary complexes in the western Yunnnan Province——New cognition based on the geological survey of Wendong sheet (1∶50000)

This paper is the result of geological survey engineering.

Objection The purpose of this study is to reveal the material composition, tectonic nature, age, metamorphism, and deformation of the Lancang Group, which are controversial but of great significance.The results will provide fundamental data for further reconstruction of the spatiotemporal framework of the Tethys.

Methods Systematically syntheticstudies were performed based on 1: 50000 detailed field investigation and Laser Ablash Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) U-Pb dating of zircons.

Results The Lancang Group is characterized by a tectonic mélange with matrix and blocks. It underwent three stages of metamorphism and deformation, overall as athrust nappe accretionary orogen. The formation age of the Lancang group is determined as the Early Paleozoic based on the zircon U-Pb ages from detrital zircons and meta acid rocks and previous research data. The protolith of the high-pressure metamorphic rocks in the Lancang Group has the characteristics of magmatic arc, OIB and E-MORB, indicating that the Lancang Group experienced deep subduction and accretionary orogeny.

Conclusions Combined with the available research data in this region and the results obtained in this paper, the Lancang Group is not a basement but an Early Paleozoic accretionary complex formed by the subduction of the Changning-Menglian Proto-Paleo-Tethys Ocean.