Zircon U-Pb age and geochemistry of the Guangmashan monzonitic porphyry in Ninglang, Yunnan Province

The Guangmashan monzonitic porphyry in Ninglang is located between Xiaojinhe and Binchuan-Chenhai faults in western Yangtze block. In this paper, the authors carried out LA-ICP-MS zircon U-Pb dating for the monzonite porphyry and obtained a weighted mean age of 33.37±0.26Ma, suggesting Oligocene, which is consistent with the main formation age of Lijiang-Beiya alkali-rich porphyritic belt. The monzonite porphyry shows peraluminous alkaline affinities, with SiO₂ content of 61.55%~69.13%, (Na₂O+K₂O) content of 8.85%~10.92%, N/K ratio of 0.88~1.07, A/NK ratio of 1.54~1.84, and A/CNK ratio of 1.23~1.72. It is also enriched in LREE but depleted in HREE, with high ratio La/Yb, low Y and Yb content, and low values of Mg^#(< 0.5). The porphyry shows continent-type potassium C-type adakite characteristics, and might have been derived by partial melting of the thickened lower crust caused by Jinshajiang-Red River strike-slip fault system, formed in an extensional setting at post-collisional stage, caused by the collision between the India and Eurasian blocks.