One or Two Early Cretaceous Arc Systems in the Lhasa Terrane, Southern Tibet

Spatial and temporal variations of arc-related magmatism are key to determining the subduction polarity of incompletely preserved arc systems. Petrographic, geochronological, geochemical, and isotope data of Early Cretaceous volcanic rocks from the northern Lhasa Terrane around Yanhu indicate south dipping subduction of the Bangong Tethys. Two distinct calc-alkaline magmatic successions are recognized: older medium-K basalts and Mg-rich andesites (131-116Ma), and younger high-K basalts and trachyandesites (116-110Ma). The medium-K basalts exhibit a typical arc signature, whereas the medium-K andesites show higher MgO contents relative to arc andesites. The medium-K series are interpreted as partial melting of a metasomatized mantle wedge source at lower pressure and greater water activity in generating the Mg-rich andesites. The high-K series are characterized by enrichments in highly incompatible elements and are considered as low-degree partial melting of asthenosphere mantle source that was previously metasomatized. All samples show arc-related signatures, which indicate the development of what we term the Baingoin-Yanhu arc in the northern Lhasa Terrane. This observation, in combination with the distribution of Early Cretaceous arc magmatism across the Lhasa Terrane, which prior to subsequent deformation had a width of at least 600km, requires the existence of two arc systems flanking the Lhasa Terrane and related to opposed oceanic plate subduction: north dipping subduction of the Neo-Tethys and south dipping subduction of the Bangong Tethys. Compositional change from medium-K to high-K calc-alkaline volcanism around Yanhu records changing mantle geodynamics, which we infer to reflect rollback and breakoff of the south dipping Bangong Tethyan slab.