越南西北部莱州地区新生代煌斑岩地球化学特征及其成因

摘要

越南西北部菜州地区出露的新生代煌斑岩岩脉对理解特提斯造山带东段的深部岩石圈特征和演化具有重要的地质意义.本文报道莱州地区煌斑岩的元素地球化学和Sr-Nd-Pb同位素组成特征,探讨其岩石成因.该地区煌斑岩属于钙碱性,钾质-超钾质煌斑岩特征.地球化学特征对比表明,菜州地区煌斑岩与哀牢山断裂带碱性岩具有相似的地球化学特征,但与海南和越南南部火山岩存在明显差异.分析结果表明,煌斑岩具有高的87Sr/86Sr比值、低143Nd/144Nd比值和高放射性成因Pb同位素组成特征.岩石的微量元素组成特征指示,形成煌斑岩的地幔源区可能经历过流体交代作用或沉积物组分的加入.低208Pb*/206Pb*比值暗示地幔源区富集事件是近期发生的,可能与晚古生代-早中生代印支地块向扬子地块俯冲事件有关.%Cenozoic lamprophyre dykes exposed in the Laichau area of northwestern Vietnam hold geological significances forrnunderstanding geochemical features and evolution of deep lithosphere in the eastern part of the Tethyan orogenic belt. This study reports geochemical and Sr-Nd-Pb radiogenic isotopic composition of lamprophyre dykes from the Laichau area and discuss their petrogenesis and geological implications. Lamprophyre samples reported here are of calc-alkaline and potassic to ultra-potassic characteristics in major element contents. When compared with relating rocks in the neighbouring areas, lamprophyre dykes from the Laichau area are similar in geochemicai composition with alkalic rocks in the Ailaoshan fault zone, but considerable different from volcanic rocks exposed in Hainan and southern Vietnam. The Laichau lamprophyre dykes show enriched isotopic feature of high 87Sr/86Sr ratios, low 143Nd/ Nd ratios and high radiogenic Pb isotopic composition. Evidence of trace element contents from these lamprophyre samples indicates that the magma source of the lamprophyre dykes likely suffered metasomatic processes or input of sedimentary components before partial melting of the mantle. Low 20S Pb */206 Pb * ratios of lamprophyre samples imply that enrichment of the mantle source producing the Laichau lamprophyre dykes took place recently, probably related to amalgamation of the Indochina block to the Yangtze block during the Late Paleozoic to Early Mesozoic.