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

Abstract

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 εHf(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 εHf(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.

Early Jurassic high‐Mg andesites in the Quxu area, southern Lhasa terrane: Implications for magma evolution related to a slab rollback of the Neo‐Tethyan Ocean

“…One model suggests that the subduction of the Neo‐Tethyan oceanic crust developed an active continental margin setting along the southern Lhasa terrane during Late Triassic–Cretaceous (Ji, Wu, Chung, et al, ; Ji, Wu, Liu, et al, ; Meng et al, ). The other model argues that the Gangdese magmatic events were induced due to the southward subduction of the Bangong‐Nujiang Tethyan oceanic crust in a back‐arc extensional region during 220–175 Ma, and the northward subduction of the Neo‐Tethyan seafloor occurred in the very Early Cretaceous (Dong & Zhang, ; Pan et al, ; Shui et al, ; Song et al, ; Wang, Zeng, Gao, Tang, & Hu, ; Zhu et al, ). Generally, the back‐arc model will facilitate the upwelling of asthenospheric mantle and result in the representative bimodal magmatism.…”

“…Second, the dominance of granitoids among the products of the Early Jurassic magmatism in the southern Lhasa terrane is consistent with the modelling of slab rollback where large volumes of granitic magma are likely to be produced by anatexis of the overriding crustal materials (Guo et al, ). Third, the high thermal regime provided by the slab rollback induced the melting of the oceanic crust, which is proved by the presence of Early Jurassic adakitic affinities from the Jiacha area (Shui et al, ). Fourth, from Figure , we can observe that the magmatic front moved further north during 226 Ma to 190 Ma and then moved back to the south during 190 Ma to 160 Ma.…”

“…). Meanwhile, the high thermal regime induced partial melting of the subducted oceanic crust and resulted in the production of Jiacha adakitic rocks (Shui et al, ). Triggered by the upwelling asthenosphere, partial melting of the metasomatized mantle peridotite without significant input of mantle components generated the Yeba mafic magmas and created juvenile lower crust beneath the southern Lhasa terrane.…”

“…Besides, the Yeba Formation (~192-174 Ma) and Sangri Group (~195-180 Ma) volcanic rocks formed during the Early Jurassic (Chen et al, 2009;Dong et al, 2006;Geng, Pan, Wang, Zhu, & Liao, 2006;Kang et al, 2014;Li et al, 2016;Wei et al, 2017;Zhu et al, 2008). Likewise, the recent studies also reported some Early Jurassic adakitic affinities emplaced in the Gangdese Batholith (Shui et al, 2018).…”

“…11). Meanwhile, the high thermal regime induced partial melting of the subducted oceanic crust and resulted in the production of Jiacha adakitic rocks (Shui et al, 2018).…”