鲁西郯城蛇纹岩微量元素和锆石U-Pb年代学:对华北陆块东部岩石圈地幔改造的启示

Trace element and zircon U-Pb geochronology in serpentinites from Tancheng,western Shandong: implication for modification of lithospheric mantle in eastern North China Block

对鲁西郯城蛇纹岩进行锆石LA-ICP-MS U-Pb年代学和全岩微量元素研究。结果为:蛇纹岩中44颗锆石的年龄值可以分为5组,其中岩浆锆石的年龄为古元古代—新太古代(1 815～2 518 Ma)、新元古代(711 Ma、719 Ma)、早白垩世(106～122 Ma)和晚白垩世(78～92 Ma),而变质锆石形成于晚三叠世(225～231 Ma)。郯城蛇纹岩轻重稀土元素分馏不明显,相对富集大离子亲石元素(Ba、Th、U),相对亏损高场强元素(Nb、Zr、Hf),整体上与华北陆块代表古老岩石圈地幔残留的橄榄岩类包体地球化学特征相类似。蛇纹岩的原岩为纯橄岩,是华北陆块东部古老岩石圈地幔的残留。古元古代—新太古代岩浆锆石的存在,代表拆沉的华北陆块基底物质对岩石圈地幔的改造;新元古代岩浆锆石和晚三叠世变质锆石的发育,表明蛇纹岩的原岩受到了由超高压变质作用的俯冲-断离扬子陆壳物质熔融产生的熔体改造;早白垩世和晚白垩世岩浆锆石年龄的出现,与华北陆块东部该时期发育的岩浆作用时间相一致。华北陆块东部岩石圈地幔受到多阶段、不同性质熔体的改造,改造的峰期发生在早白垩世,与华北陆块东部岩石圈发生大规模减薄的峰期时间一致。

Zircon LA-ICP-MS U-Pb geochronology and trace-element geochemistry for serpentinites from Tancheng,western Shandong were carried out in this paper. The results show that ages of 44 zircons in serpentinite are divided into five distinct groups,of which the ages of magmatic zircons are Paleoproterozoic-Neoarchean( 1 815 ~ 2 518 Ma),Neoproterozoic( 711 Ma and 719 Ma),Early Cretaceous( 106 ~ 122 Ma) and Late Cretaceous( 78 ~ 92 Ma),whereas the age of the metamorphic zircons is Late Triassic( 225 ~ 231 Ma). The light rare earth elements and heavy rare earth elements of the Tancheng serpentinites are not well fractionated. The Tancheng serpentinites are relatively rich in large ion lithophilic elements( Ba,Th,U) and relatively deficient in high field strength elements( Nb,Zr,Hf). Generally,the serpentinites are similar to the peridotite xenoliths representing the ancient lithospheric mantle in the North China Block( NCB). Therefore,it is believed that the protolith of serpentinite is dunite and it is the residue of old lithospheric mantel in eastern NCB. The occurrence of Paleoproterozoic-Neoarchean zircons represents the modification of lithospheric mantle from delamination of ancient basement materials in NCB. The development of Neoproterozoic magmatic zircons and Late Triassic metamorphic zircons show that the protolith of serpentinite experienced melt-related modification from the subduction-faulted Yangtze continental crust with ultrahigh pressure metamorphism. The occurrence of Early and Late Cretaceous magmatic zircons is coincident with the coeval magmatism in eastern NCB. The lithospheric mantle in eastern NCB was modified by various melt with multi-stages. The modification peak time is Early Cretaceous,coincident with the peak time for the lithosphere thinning of eastern NCB.