The tectonic evolution of the Garze-Litang ophiolite mélange zone in the Late Triassic: Constraints from geochronology and geochemistry of the Yongjie batholith in the Garze-Litang area

The Garze-Litang ophiolite mélange zone is an important part of the 'Three-River' orogenic belt in Southwest China, which experienced the evolution of paleo-Tethys and formed a complete trench-arc-basin system. The Yidun arc belongs to "the Garze-Litang arc basin system", located on the west side of the Garze-Litang suture zone. Petrological, geochemical and geochronological studies of the Yongjie batholith in the Yidun arc have provided new evidence for tectonic evolution of the GarzeLitang Ocean basin in the Late Triassic. The zircon U-Pb ages of the biotite adamellite and the granitic aplite are 214.2±1.4Ma and 206.2±1.8Ma, suggesting high-K calc-alkaline metaluminous to peraluminous rocks, with REE patterns displaying enriched LREE, flat HREE and obvious negative Eu anomaly, and trace elements showing relative enrichment of large iron lithophile elements Rb, Ba, Th, U and K and depletion of high field strength elements Nb, Ta, P and Ti. Geochemical characteristics show that the biotite adamellite of the Yongjie batholith has the obvious features of subducting granite, and the granitic aplite has the characteristics of collision granite. All these features suggest that the Garze-Litang Ocean basin had the geodynamic setting from subduction to collision in the late Triassic, and it should be the product of the collisional orogeny in the Garze-Litang ophiolite melange belt.