胶东青山群基性火山岩的Ar-Ar年代学和地球化学特征:对华北克拉通破坏过程的启示

^(40)Ar-^(39)Ar geochronology and geochemistry of mafic rocks from Qingshan Group,Jiaodong area:Implications for the destruction of the North China Craton

青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。

The volcanic rocks of the Qingshan Group formed during the destruction of the North China Craton,therefore retaining important information as to the deep processes in this region,40Ar/39Ar dating and geochemical analyses have been carried out on mafic volcanic rocks of the Qingshan Group collected from Jiaodong Peninsula,In combination with previously published data on the silicic rocks of the Qingshan Group,the newly acquired data show that,（1） the mafic volcanic rocks erupted during 122～113Ma,predating the silicic volcanism（110～98Ma）;（2） the mafic and silicic rocks display distinct elemental and isotopic compositions,Based on these data,it is shown that the mafic rocks may have resulted from melting of metasomatically enriched lithosphere,whereas the silicic magmas represent partial melts of ancient lower crust and late Mesozoic magmas underplated at the crust-mantle boundary（Ling et al.,2009）,The volcanic rocks of the Qingshan Group in Jiaodong Peninsula therefore records a shift in magma source from the upper mantle to the lower crust,Such a temporal transition in magmatic composition mirrors the thermal transfer from mantle to crust during the protracted lithospheric thinning processes,but is at odds with the magmatic evolution trends predicted by the crustal delamination model.