苏鲁超高压变质带仰口蛇纹岩的成因——矿物化学、地球化学和年代学证据

通过矿物化学、全岩主微量元素和铂族元素研究,结合锆石年代学判断苏鲁造山带仰口蛇纹岩的原岩成因和演化历史。蛇纹岩中尖晶石经历了多阶段变质;全岩主量元素具有超基性堆晶岩的性质,代表玄武质组分含量的Ca O+Al_2O_3变化于2.0%~5.83%之间;部分不相容微量元素富集;铂族元素中Ir的含量低(0.64×10~(-9)~1.43×10~(-9)),Pd/Ir值高(1.05~3.42)。蛇纹岩中的锆石一部分为新形成的变质成因锆石(年龄平均为230±3Ma),与高压-超高压变质的年代吻合;另一部分可能是在三叠纪变质阶段古老锆石重结晶形成的。由此认为,仰口蛇纹岩的原岩可能为超基性堆晶岩,三叠纪时随着俯冲的扬子板块发生变质,在发生蛇纹石化作用之前经历了熔体/流体的改造。

The genesis of Yangkou serpentinites in the Sulu ultra-high-pressure terrane: Evidence from mineral chemistry, geochemistry and geochronology

Serpentinites from Yangkou area in the Sulu ultra-high-pressure terrane were analyzed in the aspects of mineral chemistry, whole-rock major and trace elements and platinum-group elements. Zircon geochronologic data of these serpentinites were also investigated with the purpose of better constraining the petrogenesis and the tectonic environment in which serpentinites were formed. The spinels in the examined serpentinites have undergone multi-stage metamorphism. The serpentinites contain highly incompatible trace elements, and have low Ir values (0.64×10^-9~1.43×10^-9) as well as high Pd/Ir ratios (1.05~3.42). Whole-rock major elements show features of ultramafic cumulate, with CaO+Al₂O₃ varying from 2.0% to 5.83%. The serpentinite zircons belong to two groups. The first group represents newly formed metamorphic zircon (average 230±3Ma), whose age coincides with the age of HPUHP metamorphism; the other group contains fuzzy residual magmatic zoning cores, which may have formed through recrystallization of older zircons during Mesozoic metamorphism. The authors hold that the serpentinite protoliths were cumulates, which were subducted and metamorphosed during the subduction of the Yangtze Craton in the Triassic period. The examined serpentinites have undergone melt-rock interactions and fluid enrichment prior to serpentinization.