Lithofacies, geochemistry, and sequences of basalt and carbonate rocks of a Middle Permian composite seamount (central Yarlung Zangbo Suture Zone, Tibet): Implications to the incipient opening of the Neo-Tethys Ocean

Neo-Tethyan paleo-seamounts remain poorly understood due to the scarcity of key holistic evidence although some cases have been reported. This work focuses on a Middle Permian (Guadalupian) composite seamount in the central Yarlung Zangbo Suture Zone, using textures and sequences of its basaltic basement and carbonate cap to characterize the development of the oceanic crust. We conducted field investigations, lithological studies, and geochemical analyses of a huge composite rock block (the Buma paleo-seamount) north of Lang Co, Ngamring, Tibet. Results show that the basement rocks are mainly subaqueously erupted, tholeiitic basalts, which are very similar to the Hawaiian oceanic island basalts (OIB-types) in terms of geochemistry and tectonic setting; and the cap rocks are characterized by four general sedimentary facies and ten limestone microfacies, all of which are primarily of shallow marine nature. Four different contact relationships between basement and cap rocks are further recognized as “regular”, “scouring”, “brecciation”, and “karstification” types. Together, they demonstrate that multiple complete sequences of basalt basement and sedimentary cap, rich in bryozoans (reef/atoll) and crinoids exist, representing the composite seamount in the central Yarlung Zangbo Suture Zone. At least eight sets of seamount sub-sequences are superimposed as submerged and subaerial types, recording the oceanic crustal evolution of the Neo-Tethys Ocean during the Guadalupian epoch. Particularly, residual basaltic and karst breccia indicate subaerial exposure and, thus, prove an island stage. Taking into account the average mature seamount lifespan of ∼26 Ma and the complete sequence of the newly recognized Buma seamount in an intraplate setting, we propose that the incipient opening of the eastern Neo-Tethys could have taken place much earlier than the Guadalupian (Middle Permian), likely during the Cisuralian (Early Permian) or even earlier.