Early Jurassic adakitic rocks in the southern Lhasa sub-terrane, southern Tibet: petrogenesis and geodynamic implications

Cretaceous-Miocene adakitic rocks in the southern Lhasa sub-terrane have been intensively investigated, while possible Early Jurassic adakitic rocks in this area have been largely neglected. Petrological and geochemical studies revealed adakitic affinities of an Early Jurassic quartz diorite intrusion with mafic enclaves and three tonalite bodies from the Jiacha area in the southern Lhasa sub-terrane. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating suggests crystallization ages of 199-179 Ma for these rocks. Both quartz diorites and tonalites have typical adakitic geochemical characteristics such as high Al2O3 (15.14-18.22 wt.%) and Sr (363-530 ppm) contents, low Y (4.46-15.9 ppm) and Yb (0.51-1.74 ppm) contents and high Sr/Y ratios of 27-106. The adakitic quartz diorites are further characterized by high MgO (2.63-3.46 wt.%), Mg-# (48-54) and epsilon(Hf)(t) (6.6-13.4) values, which were probably produced by partial melting of a subducted oceanic slab with a mantle contribution. The adakitic tonalites have very low abundances of compatible elements and relatively low epsilon(Hf)(t) values (3.5-10.3), and are interpreted to have formed by partial melting of Neoproterozoic mafic lower crust. Upwelling asthenosphere, triggered by rollback of the subducting Bangong-Nujiang (Meso-Tethys) oceanic plate, provided the necessary heat for slab and lower crust melting, resulting in the geochemical diversity of the coexisting felsic intrusive rocks. Contrary to other models, this study further demonstrates that the Bangong-Nujiang oceanic plate was subducted southward beneath the Lhasa terrane during the Early Jurassic.