Paleomagnetism and geochronology of Upper Triassic volcanic rocks of the Lhasa Block: Implications for the extent of the Neo-Tethys

The opening and subsequent evolution of the Neo-Tethys Ocean played a critical role in shaping Mesozoic paleogeography and biogeography. The Lhasa Block, located in the southern Tibetan Plateau and bounded by the Bangong Co–Nujiang and Yarlung–Zangbo suture zones to the north and south, respectively, represents a key to unraveling the kinematic history of the Neo-Tethys. However, the absence of reliable early Mesozoic paleomagnetic data significantly limits our ability to resolve the rifting and drift history of the Lhasa Block. Here we report a systematic paleomagnetic and geochronologic investigation on newly identified Upper Triassic volcanic rocks from the Lhasa Block. We isolated a high-temperature remanent magnetization that passes both fold and reversal tests, and the corresponding paleomagnetic pole ( Plat. = 31.7°N, Plong. = 320.3°E, N = 27 sites, A95 = 8.8°) averages paleosecular variation. We interpret these paleomagnetic data to reflect a primary magnetization, acquired during emplacement of the volcanic rocks, which we date to 218.9 ± 5.2 Ma. These results imply that the paleolatitude of the Lhasa Block was about 12.6 ± 8.8°S at ∼219 Ma. Integration of these new results with existing paleomagnetic and geologic data suggests that the Lhasa Block remained at approximately this latitude from the middle Permian to the Late Triassic, but began to drift northward after the mid- Late Triassic. Placed in the context of the drift history of other Tethyan blocks, this suggests that while the Yarlung–Zangbo Ocean and the Bangong–Nujiang Ocean (as distinct branches of the Neo-Tethys) may have opened together before the middle Permian, they subsequently experienced contrasting spreading histories, with the Bangong–Nujiang Ocean undergoing sustained rapid expansion, while the Yarlung–Zangbo Ocean shrunk until the mid-Late Triassic, whereafter it began to re-expand.