Comparative Rock Magnetic Study of Eocene Volcanogenic and Sedimentary Rocks From Yunnan, Southeastern Tibetan Plateau, and Its Geological Implications

Many recent biomagnetostratigraphic studies within and around the Tibetan Plateau have resulted in significant age differences, due to lack of radiometric age constraints. It has hindered the knowledge on the tectonic and paleoclimate evolutions of the Tibetan Plateau. Identifying barely visible tephra-bearing layers is urgent to solve the debate. Here we carry out a comparative rock magnetic study of Eocene volcanogenic and sedimentary rocks from the Jianchuan and Qujing Basins of the Yunnan Plateau. Our aim is to establish an effective method for tephra identification in sedimentary sequences. Petrographic and rare earth element analyses demonstrate that the volcanogenic rocks are of basic-intermediate magmatic origin with no sign of deformation or alteration, and the sedimentary rocks are unaffected by volcanic disturbance. Rock magnetic analyses reveal that the primary ferrimagnetic minerals of the volcanogenic rocks are Ti-poor titanomagnetite and/or magnetite, including pseudo-single-domain, viscous superparamagnetic and ultrafine superparamagnetic (true superparamagnetic, nonviscous around room temperature) grains. Hematite is the dominant magnetic mineral in the sedimentary rocks, and paramagnetic minerals are also abundant; pseudo-single-domain/single-domain magnetite and maghemite are also present. The enrichment of ultrafine superparamagnetic grains in the volcanogenic rocks may induce strikingly peaked signals in low-temperature frequency-dependent susceptibility measurement, in sharp contrast with pedogenically produced viscous superparamagnetic particles that show no peaks. Therefore, we attribute the low-temperature frequency-dependent susceptibility parameters to the effective and sensitive magnetic parameters for identifying tephra-bearing layers within the sedimentary sequences of the Yunnan Plateau.