琼中古-中元古代变质基性火山岩地球化学特征

琼中斜长角闪(片麻)岩为变质的古-中元古代基性火山岩,属于高铝和低钛的高钾钙碱性玄武岩系.其LILE和LREE明显富集,HFSE相对亏损,具有典型岛弧玄武岩特征.微量元素、Sm-Nd同位素组成还揭示其岩浆源区偏离原始地幔,但未受成熟地壳物质的明显污染,是消减带上部地幔楔与俯冲洋壳析出的流体二元混合物部分熔融的产物.古-中元古宙时期,海南地块可能经历了由拉张到挤压的地球动力学转变,早期拉张阶段形成琼西洋脊型和过渡型拉斑玄武岩,中晚期强烈俯冲作用形成琼中岛弧高钾钙碱性玄武岩,该俯冲期与华南统一地块古-中元古宙板块俯冲时期相对应.

Sm and Nd isotopic geochemistry of Paleo-Mesoproterozoic meta-basic volcanics from Middle Hainan, China

The protolith of the plagioamphibolites in Middle Hainan is the metamorphosed Paleo-Mesoproterozoic basic volcanics.It belongs to a suit of high-K calc-alkaline basalts with high Al and low Ti contents.The volcanic rocks have characteristics of typical island-arc basalts.Large-ion elements(LILE) are evidently enriched and high field strength elements(HFSE) relatively depleted the volcanic rocks.The geochemical features of trace elements and Sm/Nd isotopes also indicate that magama source is departed from the composition of primary mantle and not evidently contaminated by mature crust materials.It is the result of patial melting of two-coponent mixture made of the mantle wedge(the MORB type depleted mantle) located over subducted zone and the fluid released from the subducted oceanic crust.During Paleo-Mesoproterozoic Age,the Hainan block might have suffered geodynamic shifting from "extending" to "squeezing",leading to form oceanic-ridge type and oceanic-arc transitional tholeiites in the western Hainan at earlier extending stage and high-K calc-alkaline basalts in the middle Hainan at later strong subduction stage,while the South China unified block was subducting.