A sedimentary archive of tectonic switching from Emeishan Plume to Indosinian orogenic sources in SW China

摘要

Upper Permian to Middle Triassic sedimentary rocks in the Youjiang Basin, SW China, record a change from a within-plate mafic-dominated source to the NW, to a mixed source involving magmatic arc and recycled orogenic detritus that lay to the west and east. Upper Permian and some Lower Triassic sedimentary rocks are characterized by relatively high contents of TiO_2 and Nb, and low ratios of Al_2O_3/TiO_2 and Th/Nb. Detrital zircons yield U-Pb ages of c. 260 Ma and have geochemical affinities to those crystallized from the within-plate type magmas. These features, combined with extensive basaltic lithic fragments and plagioclase grains in the sandstones, imply a sedimentary source dominated by the Emeishan Large Igneous Province exposed to the NW of the basin. The Early and Middle Triassic marked a significant change in sediment provenance, with rocks displaying geochemical affinity to upper continental crust. U-Pb ages and trace elements of detrital zircons from this material show input from multiple sedimentary sources including the subduction-collision rocks related to the Indosinian Orogeny to the west of the basin and the recycled Precambrian-early Palaeozoic sedimentary and granitic rocks in the South China hinterland to the east of the basin. The Early Triassic change in the nature of the source supplying detritus to the Youjiang Basin, together with regional sedimentary and tectonothermal events, suggests a tectonic transition from the Emeishan Plume to the Indosinian Orogeny in SW China. Regional stratigraphic correlation and provenance data suggest that the Emeishan basalts provided a large sediment influx for the riverine-littoral-marine systems in the Late Permian. This basaltic particulate and dissolved input may have contributed significant unradiogenic Sr to the Late Permian seawater.