滇西勐弄乡花岗岩地球化学特征及其构造意义OA

The Tengchong Block is a significant component of the eastern Tethyan tectonic belt.During the tectonic evolution of the Tethys,extensive granitic magmatism occurred in this region.To investigate the petrogenesis and tectonic implications of the Late Cretaceous and Eocene granites in Mengnong Township,western Yunnan,this study presents systematic petrological observations,mineral geochemical analyses,zircon U-Pb dating,and whole-rock major and trace element geochemical data for the two phases of granites in this area.The Mengnong intrusion is primarily composed of Late Cretaceous monzogranite,Paleogene monzogranite,and granodiorite.Zircon U-Pb geochronology yields two distinct emplacement ages:(69.51±0.68) Ma and(55.86±0.52) Ma,corresponding to the Late Cretaceous and Eocene,respectively.Major element and mineral geochemical analyses indicate that the Eocene granite of Mengnong Township is characterized by high silica(SiO2 content=69.52%～71.06%),high alkali contents(total alkali content=7.08%～7.95%),and a weakly peraluminous nature(A/CNK=1.00～1.01).It belongs to the shoshonitic-high-K calc-alkaline series and formed under high temperature(769～810 ℃)and relatively high oxygen fugacity(lg fO2=-13.04～-10.26).In contrast,the Late Cretaceous granite exhibits even higher silica(SiO2 content=71.10%～75.12%),variable alkali contents(total alkali content=6.51%～9.00 %),and strongly peraluminous signatures(A/CNK=1.05～1.33).It has a high-K calc-alkaline affinity and crystallized under high temperature(784～837 ℃)and lower oxygen fugacity conditions(lg fO2=-13.25～-11.35).Trace element compositions reveal that both granite phases are enriched in light rare earth elements(LREEs)and large-ion lithophile elements(Th,K,Pb),while being depleted in Ta,P,and Ti.Based on the integrated geochemical signatures,both the Late Cretaceous and Eocene granites are classified as S-type granites.Combined with regional geological evidence,we propose that the Late Cretaceous granite in Mengnong Township was generated in a thickened continental crust setting related to the subduction of the Neo-Tethyan Ocean,whereas the Eocene granite formed in a post-collisional setting associated with slab rollback.