Geochemistry of the volcanic rocks in the Dapingzhang ore district,Simao,Yunnan province and its tectonic significance
云南思茅大平掌矿区火山岩的地球化学特征及构造意义

The spilite-quartz keratophyre formations in the Dapingzhang ore district are typical bimodal rocks lack of intermediate rocks with 52% - 61% SiO_2, which are most probably formed under island-arc environment indicated by their low TiO_2 concentrations and other petrochemical characteristics and REE geochemistry. Furthermore, the N-MORB normalized spider diagrams of incompatible elements of the rocks are characterized by enrichments in Ba, Nd, Sm, and depletions in Nb, Sr, Ti. The enrichment in Ba and depletion in Nb are regarded as major indicative characteristics of island-arc volcanism. The very low Nb/Y and Zr/TiO_2 ratios of spilites, low Nb/Y and Zr/TiO_2 ratios and range of Zr/Y, (La/Yb)_N ratios of quartz keratophyres in the Dapingzhang ore district suggest that they are formed under island-arc environment. The Sr, Nd isotopic characteristics of the volcanic rocks are similar to those of the typical island-arc-type volcanic rocks in the world. It is suggested that the parent magma was derived from a depleted mantle, which was influenced by crustal contamination and/or seawater hydrothermal alteration. The high concentrations of radiogenic lead in the rocks are also related to the island-arc environment. As a result, the formation of Dapingzhang copper-polymetallic deposit might be controlled by bimodal volcanism under island-arc environment.