云南思茅大平掌矿区火山岩的地球化学特征及构造意义

Geochemistry of the volcanic rocks in the Dapingzhang ore district, Simao, Yunnan province and its tectonic significance

大平掌矿区细碧岩-石英角斑岩建造为典型的双峰式火山岩组合,缺失SiO_2为52％～61％之间的中性火山岩。火山岩的TiO_2含量低及其它岩石化学特征、稀土元素地球化学特征均表明这套火山岩很可能形成于岛弧环境。不相容元素N-MORB标准化型式以Ba、Nd、Sm相对富集和Nb、Sr、Ti的相对亏损为特征,其中Ba 富集和Nb亏损更是岛弧火山作用的主要特征之一。大平掌矿区细碧岩具非常低的 Nb/Y(<0.15)和Zr/TiO_2(<0.01)比值,石英角斑岩的低Zr/TiO_2和Nb/Y特征以及Zr/Y比值(3.34～4.23)、(La/Yb)_N比值(0.47～2.50)变化范围都可以反映出火山岩形成于岛弧环境。大平掌火山岩的Sr、Nd同位素特征与世界上典型岛弧火山岩的Sr、Nd特征相似,其岩浆来源于亏损地幔,且经受地壳混染和(或)海水热液蚀变的影响。岩石中富含的放射成因铅也与岛弧环境密切相关。因此,大平掌铜多金属矿床的形成很可能受岛弧环境下的双峰式火山作用控制。

The spilite-quartz keratophyre formations in the Dapingzhang ore district are typical bimodal rocks lack of intermediate rocks with 52% similar to 61% SiO2, which are most probably formed under island-are environment indicated by their low TiO2 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/TiO2 ratios of spilites, low Nb/Y and Zr/TiO2 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-are-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.