Intra-oceanic arc-type main Palaeo-Tethyan suture in eastern Tibet: evidence from the Biluo Snow Mountain, Yunnan, SW China

The Longmu Co–Shuanghu and Changning–Menglian sutures define the boundary between Gondwana and Laurasia, but their tectonic correlation in eastern Tibet remains uncertain. This study presents new geochronological and geochemical data from the Biluo ophiolite mélange. Zircon U–Pb dating reveals ages of 239 Ma for granites and ~300 Ma for gabbros and monzonites. The S-type granites formed during northeastward subduction of Paleo-Tethyan oceanic lithosphere beneath the North Qiangtang–Simao block, while the MORB-like gabbros represent the Deqin–Biluo back-arc basin. The 239 Ma granitic pluton is interpreted as the southern extension of the North Lancangjiang magmatic belt, and the Biluo ophiolite mélange as the southern extension of the Deqin back-arc suture. These units, together with the Bitu/Longmu Co–Shuanghu suture, constitute an intra-oceanic arc-type orogenic belt in east Tibet. This tectonic regime is comparable to that of the Changning–Menglian suture in the southern region but differs significantly from the architecture of the Longmu Co–Shuanghu suture.The absence of a clear back-arc suture between central and eastern Tibet may result from Mesozoic cover or westtward termination of the back-arc basin.