Petrogenesis of the Wudaogou intermediate-mafic complex, NW China: zircon U-Pb dating, whole-rock and isotope geochemistry, and geological implications

摘要

The Wudaogou plutonic complex is located in the eastern Yanbian area of Jilin Province and consists of hornblende gabbros, gabbroic diorites, and quartz diorites that contain abundant dioritic microgranular xenoliths. Zircon U-Pb dating of gabbroic and quartz diorites yielded weighted mean ~(206)Pb/~(238)Pb ages of 263.5 ± 5.1 Ma (N = 12, mean squared weighted deviation (MSWD) = 0.78, probability = 0.66) and 262.0 ± 5.6 Ma (N = 10, MSWD = 0.50, probability = 0.87), respectively. These units are characterized by high Na_2O/K_2O (0.33-0.77) ratios and Al_2O_3 (15.05-18.91 wt%) concentrations and are large ion lithophile element (LILE) (light rare earth element (LREE), Rb, Ba, K, etc.) enriched and high field strength element (HFSE) (Nb, Ta, P, Ti) depleted. They also have initial ~(87)Sr/~(86)Sr values of 0.70192-0.70420 and ε_(Nd)(t) values of +1.9 to +4.7 with two-stage model ages (T_(DM2)) of 653-878 Ma. These characteristics indicate that these rocks formed from calc-alkaline magmas derived from partial melting of a mixture of juvenile crust formed attending the Neoproterozoic subduction of the Palaeo-Asian oceanic crust and lower crustal material. The dioritic xenoliths have whole-rock compositions that are similar to their host rocks, but with negative ε_(Nd)(t) values (-1.6 to -4.3) and older T_(DM2) ages (1166-1382 Ma), further indicating that this magmatic event involved older crustal material. Combining these data with existing knowledge of the crustal evolution of this area, we conclude that this complex formed in a post-collisional extensional setting during closure of the Palaeo-Asian Ocean.