Tholeiitic magmatic record on an extensional passive continental margin: Constraints from Late Paleozoic Woniusi rock associations in the Baoshan block, SE Tibetan Plateau

The magmatic records on passive continental margins are mostly alkaline-tholeiitic series or bimodal volcanic suites commonly induced by active or passive processes. However, the origin of the relatively rare pure tholeiitic lavas remains poorly understood. In this paper, we document pure tholeiitic records, the Woniusi rock associations, on the passive northern Gondwana margin. During the Late Paleozoic, accompanied with opening of the Meso-Tethyan Ocean, the Woniusi rock associations made up a unique tholeiitic magmatic flare-up on the Baoshan block, the north tip of Sibumasu. Therefore, the Woniusi rock associations can provide essential insights into the origin of tholeiitic magmatism on passive continental margins and the geodynamics for opening the Meso-Tethyan Ocean. This study reports new whole-rock geochemical, SrNd, and zircon UPb and LuHf isotopic data for the Woniusi rock associations. Combined with previously published data, 234 mafic-ultramafic rocks were used to document a complete record of the Late Paleozoic tholeiitic magmatic activities on the Baoshan block. Our new zircon UPb ages denote the basalts of the Woniusi rock associations emplaced at ~290 Ma. They are tholeiitic in composition with moderately fractionated rare earth elements, obvious NbTa depletion, and variable εNd(t) values. The high-εNd basaltic rocks were generated by lower degrees of partial melting of a less enriched deeper lithospheric mantle followed by fractional crystallization of olivine, orthopyroxene, and clinopyroxene. In contrast, the low-εNd basaltic rocks were derived from moderate-degree partial melts of the regional more enriched shallower lithospheric mantle, which experienced olivine and orthopyroxene fractional crystallization. Detailed field observations and geochemical studies on the Woniusi rock associations found no solid evidence to support the existence of a mantle plume or oceanic subduction in this region. The pure tholeiitic magmatic flare-up on the passive northern Gondwana margin should be generated in an extensional environment, as evidenced by the sublinear orientated mafic dikes, synchronous development of rift system and half-graben structure, and typical extensional basin depositions. Consequently, the passive extension of the northern Gondwana margin facilitated the opening of the Meso-Tethyan Ocean during the Early Permian. Therefore, the Woniusi rock associations provide direct evidence for the critical role of passive extension in the contribution of tholeiitic magmatic flare-up on the passive continental margin and regional opening of the Meso-Tethyan Ocean.