The geology and tectonic setting of the Zaxoi beschtauite massifs in the Gangdise zone, Xizang

The Zaxoi beschtauite massifs occur in the central part of the Gangdise magmatic belt, and intruded into the pre-Ordovician Songduo Group Complex and Carboniferous-Permian Laigu Formation overlain unconformably by the Pana Formation volcanic rocks. The biotite in the beschtauite may be traced back to the Eocene, with a K-Ar age of 54.42 Ma. w(SiO₂)=67.04%-69.75%, w(Al₂O₃)=14.85%-16.75%, w(TiO₂)=0.45%-0.53%, w(K₂O+Na₂O)=9.64%-10.28%, w(K₂O/Na₂O)=1.8-1.97. The above-mentioned values indicate that the Zaxoi beschtauite massifs should be assigned to the acidic shonshonitic hypabyssal intrusive rocks. These massifs originated from post-collision stages of the Indian plate and Eurasian plate are believed to be coeval and allophased with the widespread volcanic rocks of the Linzizong Formation also in the Gangdise belt and recorded the transition from the oceanic subduction regimes to the continental collision regimes.