Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China

Abstract

The Mesozoic volcanic sequence from Lishui basin in the Lower Yangtze region is subdivided into four formations which constitute the associated K-rich calc-alkaline series and shoshonite series. The mafic rocks in the Lishui shoshonite series are nearly saturated in silica. Various rocks of the whole shoshonite series are significantly rich in K 2O and have high Al 2O 3 and low TiO 2. Fe content is not high, but has a high Fe 2O 3/FeO ratio. Moreover, the Lishui shoshonite series are notably enriched in highly incompatible lithophile elements, such as Rb, Ba, Th, U, Zr, LREE, etc. which are generally observed in typical shoshonite association. Incompatible elements patterns show that the shoshonite series from Lishui and other volcanic basins in the Lower Yangtze region are, geochemically, similar to those of the East Africa rift and distinctly different from those of a typical island arc. It is considered that the formation of the high-K volcanic series from Lishui and other volcanic basins in the Lower Yangtze region have no direct relationship to the subduction of the Pacific plate but are connected with crustal thinning, mantle upwelling and the development of the Yangtze River deep fracture zone in the area. RbSr and SmNd isotope data and model calculations provide constraints on the petrogenesis and evolutional relationship of the Lishui shoshonite series.