LA-ICP-MS U-Pb dating of detrital zircon from low-grade metamorphic rocks and its tectonic significances in the Bulangshan area, Menghai Country, Southern Sanjiang Tectonic Zone

Bulangshan is located in the Menghai area, southern Sanjiang area of southwestern China, where Lancang Group is exposed. Lancang Group is mainly composed of sandy and muddy rocks with low-grade metamorphism and a small amount of intermediate and basic volcanic rocks. A series of issues about the Lancang Group such as the formation age, material sources and geological significance of the strata have been disputed for a long time. Detrital zircons from the low-grade metamorphic rocks without metavolcanic strata in the upper layer of the Lancang Group were carried out cathodoluminescence image analysis and LA-ICP-MS U-Pb chronology analysis. Clear oscillatory zoning and higher Th/U ratios indicate that most detrital zircons are magmatic origin. The chronological results show that the magmatism events recorded by the zircons were related to the supercontinent assembly of Rodinia and Gondwana and the subduction of the Proto- and Palaeo-Tethyan ocean (1182-1104 Ma, 593-560 Ma and 378-328 Ma), the youngest detrital zircon ages (328 Ma) suggesting that the strata didn’t form before the Early Carboniferous, rather than the Neoproterozoic. Combined with the rock assemblage characteristics, it is speculated that the Late Paleozoic low-grade metamorphic rocks may be the Carboniferous Nanduan Formation. In the detrital zircon age spectrum of the Lancang Group, the young peak age is ~550 Ma, which is similar to the Tethys Himalayas. So the materials sources of the Lancang Group maybe mainly come from the northerm margin of Eastern Gondwana, and have relationship with the Proto- and Paleo- Tethys oceans.