Geochronology and Geological Significance of the Pulang High-K Intermediate-Acid Intrusive Rocks in the Zhongdian Area, Northwest Yunnan Province

The Pulang copper deposit is a super-large porphyry deposit located in Geza island arc in Yunnan Province.Previous studies were mostly concentrated on the ore-bearing quartz-monzonite porphyry, while the research on the diorite and granodiorite porphyry was relatively weak.In this paper, we present geochemistry and zircon chronology analysis on the diorite and granodiorite porphyry in Pulang mine, northwest of Yunnan Province.The diorite is characterized by low SiO₂ (62.46%-62.65%), high K₂O (5.53%-6.27%) and total alkali(K₂O+Na₂O=7.93%-8.72%), K₂O (5.53%-6.27%), MgO (3.58%-3.69%), and K₂O/Na₂O is bigger than 2.Likewise, the granodiorite-porphyry is characterized by medium SiO₂ (66.00%-66.98%) and high total alkali (7.60%-7.81%), with MgO (2.04%-2.17%), and the K₂O/Na₂O is bigger than 1.They are enriched in large-ion lithophile and light earth elements, but depleted in high field-strength and heavy rare earth elements, and with weak to negative Eu anomalies.Furthermore, diorite yields lower (⁸⁷Sr/⁸⁶Sr)_i ratios (0.705 24-0.705 31) than granodiorite-porphyry ((⁸⁷Sr/⁸⁶Sr)_i 0.705 58-0.705 62), and the granodiorite-porphyry yields negative ε_Nd(t) values from -3.34 to -1.01, while the diorite yields variable ε_Nd(t) values from -1.65 to 7.78.New U-Pb dating of zircons of diorite and granodiorite-porphyry are 227.0±2.9 Ma and 211.5±3.7 Ma respectively.Comprehensively, we propose that a transformation from compression to extension process may have occurred in the Pulang area during the long period of westward subduction of Garze-litang ocean (about 25 Ma), which has triggered the mantle source magmatic activity, and has resulted in the emplacement of deep source (ultra)potassium alkaline.And the transformation process is crucial to the formation of super-large porphyry deposit.