苏鲁地体胡家林和梭罗树三叠纪变质超基性岩U-Pb、Lu-Hf和O同位素地球化学研究

Zircon U-Pb Age, Lu-Hf and O Isotope Geochemistry of Triassic Meta-Ultramafic Rocks at Hujialin and Suoluoshu in the Sulu Orogenic Belt

胡家林超基性岩单矿物氧同位素组成基本上接近正常地幔值（石榴石4．7‰～5．4‰）。梭罗树超基性岩及其共生榴辉岩具有氧同位素正异常（超基性岩中石榴石9.8‰～10．4‰；榴辉岩中石榴石9．0‰～12．1‰），指示超基性岩和榴辉岩原岩可能在板块俯冲之前，在浅层或地表环境中经历过蛇绿岩套顶部相似的低温热液蚀变。胡家林超基性岩样品中的变质锆石年龄227．2±2．4Ma，可能代表俯冲板块经峰期超高压变质后，折返初期从金刚石榴辉岩相降压至柯石英榴辉岩相过程中流体活动和锆石结晶的年龄。梭罗树超基性岩样品中变质锆石的一组变质年龄240．4±2．4Ma代表了板块俯冲至超高压峰期变质之前流体活动过程中锆石的结晶年龄，而另一组变质年龄217．1±3．3Ma代表了板块折返过程中高压榴辉岩相重结晶条件下的锆石结晶年龄。从变质锆石到重结晶锆石，其Th／U和^176Lu／^177Hf呈现出相似的变化趋势，可能指示了变质作用对锆石中的U-Th-Pb同位素体系和Lu-Hf同位素体系产生相似的地球化学效应。梭罗树超基性岩可能来源于俯冲地壳内部的“残留地幔”。胡家林超基性岩重结晶锆石中继承的放射成因铅或者来源于俯冲地壳与仰冲盘底部石榴辉石岩相地幔发生的壳幔相互作用而导致碎屑锆石的重结晶；或者来源于超基性岩岩浆在板块俯冲前，通过底侵作用上升侵位与地壳物质发生混染形成岩浆锆石，这种岩浆锆石在超高压变质过程中经历了重结晶。

Zircon U-Pb dating, Lu-Hf and O isotope analyses were carried out for meta-ultramafic rocks at Hujialin and Suoluoshu in the Sulu orogenic belt. The results indicate that despite significant difference in their δ^18O values, the ultramafic rocks experienced Triassic metamorphism during the subduction of the South China plate beneath the North China block. Zircon U-Pb and Lu-Hf isotope systems were reset by crustal contamination during the Triassic subduction, with the fluid activity at the onset of ultrahigh pressure metamorphism and decompressure exhumation from ultrahigh pressure to high pressure phases.