大别山镁铁质下地壳的Pb同位素成分:来自榴辉岩的制约

Pb Isotopic Compositions of Mafic Lower Continental Crust in Dabie Orogen,China:Constraint from the Eclogites

北大别高温超高压杂岩带中榴辉岩的岩石学和地质年代学研究已经证明它们为华南板块新元古代镁铁质下地壳岩石经三叠纪深俯冲变质成因。基于此,开展了北大别榴辉岩的Pb同位素地球化学研究。结果表明:北大别榴辉岩表现出与北大别片麻岩相似的低放射性成因Pb同位素特征,并具有更低的N(^(208)Pb)/N(^(204)Pb)值;其初始Pb同位素组成分别为N(^(206)Pb)/N(^(204)Pb)i=15.217~17.522,N(^(207)Pb)/N(^(204)Pb)i=15.077~15.540和N(^(208)Pb)/N(^(204)Pb)i=35.219~38.082,表现出目前已知的大别造山带3个含榴辉岩的超高压变质岩片中最低的Pb同位素组成,这进一步证明北大别榴辉岩的原岩来自于俯冲镁铁质下地壳;北大别榴辉岩表现出较宽的Pb同位素组成变化范围,其中最低的Pb同位素组成源自于具有低w(U)/w(Pb)值、低w(Th)/w(U)值演化特征的晚太古代至古元古代镁铁质下地壳,而相对较高的Pb同位素组成则继承于新元古代底侵的幔源岩浆,表明北大别榴辉岩的Pb同位素特征是幔源岩浆混染古老镁铁质下地壳的结果。与大别山中生代埃达克质岩的Pb同位素对比研究表明,早白垩纪拆沉再循环的陆壳物质中不仅含有镁铁质下地壳岩石,还含有长英质下地壳物质,大别山深部的长英质下地壳随镁铁质下地壳一起拆沉并再循环进入地幔。

The eclogites from the North Dabie high-temperature and ultrahigh-pressure complex zone in central China have been previously documented by petrology and geochronology to be transformed from the Neoproterozoic mafic lower continental crust underneath South China block during the Triassic continental deep subduction.An integrated study of Pb isotopic geochemistry on the whole rocks and omphacites of the eclogites from North Dabie was carried out,providing new insight into the Pb isotopic compositions of the mafic lower continental crust.The resultsshow that the eclogites here display low radiogenic Pb isotopic characteristics similar to the surrounding gneisses with even lower N（208Pb）/N（204Pb）;（N（206Pb）/N（204Pb））i,（N（207Pb）/N（204Pb））iand（N（208Pb）/N（204Pb））iof the eclogites are 15.217-17.522,15.077-15.540and35.219-38.082,respectively,showing the lowest Pb isotopic compositions in the three eclogitebearing ultrahigh-pressure metamorphic slices of Dabie orogen,and documenting a mafic lowercrustal origin of their protoliths;the variation range of Pb isotopic compositions of the eclogites is large,and the lowest Pb isotopic compositions are demonstrated to be derived from the Paleoproterozoic to Archean mafic lower continental crust with low w（U）/w（Pb）and w（Th）/w（U）,while the relatively higher Pb isotopic compositions are inherited from the Neoproterozoic mantle-drived magma,suggesting the precursors of the eclogites from North Dabie are formed by mantle-drived magma contaminated with variable degrees of ancient mafic lower continental crustal materials during the Neoproterozoic.Compared to the Pb isotopic compositions of the Mesozoic adakitic rocks derived from partial melting of delaminated lower continental crust in Dabie orogen,it is suggested that the recycled lower crustal materials contain both mafic and felsic lower-crustal rocks,implying the lowermost felsic lower continental crust together with the whole mafic lower continental crust in Dabie orogen may have been delaminated and recycled into the underlying convective mantle.