Neo‐Tethyanslab tearing constrained by PalaeoceneN‐MORB‐like magmatism in southern Tibet

Abstract

Southern Tibet is a key region for investigating the magmatism that occurredduring the collision between India and Asia. The break‐off of the Neo‐Tethyan slab, after the onset of continental collision at ca. 65–60 Ma, affected the lithospheric thermal regime and led to a series of magmatic and tectonic events. Many magmatic rocks generated by slab break‐off have been reported in southern Tibet; however, which of these record the initial slab break‐off (i.e., slab tearing) is still an open question. The Lianxiang gabbros, which intrude an ophiolite in central‐southern Tibet, provide an ideal opportunity to investigate the footprint of the initial slab break‐off. Zircon U–Pb ages show that the Lianxiang gabbros crystallized at 57–53 Ma, coeval with other mafic dikes in southern Tibet. The Lianxiang gabbros are characterized by high Na2O (2.13–5.42 wt%), Al2O3(13.86–17.13 wt%), and V (188–346 ppm) contents and moderate Mg# (40.8–65.9). The gabbrosare slightly depleted in light rare earth elements [LREE; (La/Yb)N= 0.45–0.76], similar to normal mid‐ocean ridge basalt (N‐MORB). They also have low87Sr/86Sriratios (0.703538–0.705698) and highεNd(t) values (9.49–10.68), indicating that they were derived mainly from a shallow asthenospheric mantle source. Combining our results with data from other magmatism at ~57 Ma in the southern Lhasa Terrane, we propose that the N‐MORB‐type rocks reflect the tearing of the Neo‐Tethyan slab. The process of slab break‐off differed between western and eastern southern Tibet, possibly as a result of different lithospheric structures and compositions.

Crustal reworking and growth during India–Asia continental collision: Insights from early Cenozoic granitoids in the central Lhasa Terrane, Tibet

“…During India-Asia continental collision, the subducted oceanic slab would have had a higher density than the passive continental lithosphere, so the subducted oceanic slab is likely to have become delaminated from the continental lithosphere after initial continental collision (Zhu et al, 2015). In such a situation, upwelling of deep mantle would have provided juvenile basaltic magmas to the overlying lower crust (Tian et al, 2021), which is consistent with the geochemical variations we observed in the Chacangka granitoids. The timing of slab breakoff has been well constrained at ca.…”

“…57-50 Ma, on the basis of the presence of extension-related mafic dikes and contemporaneous felsic melts in the Lhasa Terrane (Huang et al, 2016, Huang, Rooney, et al, 2021W.-Q. Ji et al, 2009;Tian et al, 2021;Zhao et al, 2011;Zhu et al, 2015). The Chacangka granitoids that formed before and after slab breakoff have different chemical and isotopic compositions, which is consistent with the proposal that the source of magmas in the central Lhasa Terrane was influenced by a magmatic "flare-up."…”