Stratigraphic Sequences of the Changning-Menglian Belt, Western Yunnan, China

Abstract

The South Qiangtang terrane (SQT), the Tibetan representative of the Cimmerian continent, is bounded in the south by the Bangong-Nujiang suture zone (BNSZ). The BNSZ is geologically manifested by the Mugagangri Group (MG) accretionary complexes developed during northward Meso-Tethyan subduction, whose timing and processes are, however, contentious. Herein, we conducted provenance analysis on the MG trench deposits, via petrography, whole-rock geochemistry and detrital-zircon geochronology. The results reveal a complete picture of provenance evolution during passive- to active-margin transition (from subduction initiation through fringing oceanic-arc, to continental-arc developments) at the SQT margin. In particular, the earliest trench deposits (subunit Trma; ~222 Ma), which contain olistostromes with blocks of oolitic limestone shed from the destructing SQT passive margin, mark the subduction initiation. A second subunit (Trmb; ~210 Ma) has periodically elevated volcanogenic components. Conglomerates of subunit Trmb have abundant basaltic clasts of intra-oceanic arc geochemical affinities, which were likely derived from an intra-oceanic arc fringing the SQT. The youngest subunit (Jmd; ~190 Ma) exhibits enriched ferromagnesian elements and a major youngest detrital-zircon age peak, indicating a continental-arc provenance. Moreover, detrital-zircon age spectra suggest that, at ~210 Ma, the central Qiangtang metamorphic belt became an important source area, and the North Qiangtang terrane (NQT) replaced SQT as the main provenance. This is best explained by that the SQT underthrusted beneath the NQT and was downwarped to form a foreland basin. Our findings confirm that the initial subduction of Meso-Tethys beneath SQT was coeval to other Cimmerian terranes, likely a uniform response to the Paleo-Tethys closure and collision between Eurasia (NQT) and Cimmeria (SQT).