Cenozoic magmatism in Diancanshan Range, southwestern Yunnan, China, and its tectonic implications

The Diancangshan (DCS) Range is an important part of the Ailoashan-Jinshajiang strike-slip shearing zone. New LA-ICP-MS zircon U-Pb analyses showed that the granites in the DCS intruded at a time span of 27. 9 +/- 0. 7Ma to 34. 8 +/- 0. 4Ma. Those granites are characterized by: (1) the mineral assemblages containing biotite and muscovite; (2) high SiO, (>70. 95%), Na2O + K2O (7.16% similar to 8.44%), Al2O3 (>13.48%) and A/CNK ratios (most of them >1.1), and low Na2O/K2O ratios (<1.0); (3) enrichment in LREE, depletion in HREE, and moderate to strong negative Eu anomalies (delta Eu = 0.13 similar to 0.75); (4) strongly enrichment in LICE (Rb, K, Th and U), hygromagmatophile elements (La, Ce, Zr and Hf), and strongly negative anomalies of Nb, P, Ti, Sr and Ba in primitive mantle-normalized spider grams; (5) plots close to the partial melting field of the psammite and shale in Rb/Ba vs. Rb/Sr diagram and along the trendline of sediment melts in Th/Yb vs. Ba/La and Th/Yb vs. Sr/Nd diagrams. These characteristics imply that the Cenozoic granites in the DCS were derived from partial melting of metasedimentaiy rocks, possibly triggered by the isothermally decompression during exhumation of the DCS. The large scale ductile strike-slip shearing in the DCS was initiated at 28Ma, which is marked by the extensively mylonitization to the Cenozoic granite intrusions, the diverse ductile deformation degrees at different locations for the same intrusion, and the higher zircon crystallized temperature.