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

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 low Sr-87/Sr-86(i) ratios (0.703538-0.705698) and high epsilon(Nd)(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 similar to 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.

Automated summary

Southern Tibet is a key region for studying the magmatism during the collision between India and Asia. The Lianxiang gabbros provide an ideal opportunity to investigate the initial slab break-off, with geochemical characteristics suggesting a shallow asthenospheric mantle source. N-MORB-type rocks are proposed to reflect the tearing of the Neo-Tethyan slab, with variations in the process of slab break-off between western and eastern southern Tibet possibly due to different lithospheric structures and compositions.