胶莱盆地晚白垩世玄武岩的年代学和地球化学特征及其对华北岩石圈减薄-增生的制约

Geochronology and Geochemistry of the Late Cretaceous Basalts in the Jiaolai Basin:Constraints on Lithospheric Thinning and Accretion Beneath North China Craton

已有研究认为华北岩石圈减薄的时空不均一是显著的,主要表现在软流圈来源的岩浆最早出现的时间呈现南北差异。本文通过40Ar-39Ar法获得胶莱盆地西官庄拉斑玄武岩的年龄为96±3Ma,属于青山群火山岩的一部分,早于王氏群大西庄碱性玄武岩(73Ma)。两套岩石均缺乏Nb、Ta负异常,分别具有与E-MORB和OIB类似的微量元素配分特征。西官庄拉斑玄武岩((87Sr/86Sr)t=0.7058,εNd(t)=0.37～0.57)和大西庄碱性玄武岩((87Sr/86Sr)t=0.7040,εNd(t)=5.5～5.7)的同位素组成特征均显示软流圈的印记。前者为软流圈熔体与古老富集岩石圈地幔相互反应的产物,而后者是软流圈部分熔融的产物。研究区岩浆的εNd(t)值随年龄的降低而逐渐增大,暗示古老岩石圈减薄过程中软流圈的上涌,说明研究区晚白垩世-早新生代岩石圈仍在经历减薄作用。类似的岩浆演化趋势在辽西和辽东地区也有发现,不过演化的时间分别为120～100Ma和80～60Ma,再次证明华北岩石圈减薄作用的时空不均一性。

Previous studies showed a heterogeneous temporal and spatial pattern of lithospheric thinning beneath North China Craton（NCC）.The idea is mainly built on the diachronous eruption of the earliest asthenosphere-derived magmas in northern and southern NCC.In this study,we carried out Ar-Ar dating on the Xiguanzhuang tholeiitic basalt（the Jiaolai basin）.40 Ar-39 Ar data show that it erupted at 96±3 Ma,thus belonging to the Qingshan Group,earlier than the Daxizhuang alkaline basalt（73 Ma） of the Wangshi Group.Both rocks show no negative Nb and Ta anomalies,but display E-MORB-like and OIB-like trace element distribution patterns,respectively.However,the Xiguanzhuang tholeiitic basalts have higher（87 Sr/86 Sr） and lower ε Nd（t）（0.7058,0.37～0.57） than the Daxizhuang alkaline basalts（0.7040,5.5～5.7）.It is suggested that the former was formed as a result of interaction between asthenospheric melts and old-enriched lithosphere,whereas the latter represents asthenosphere-derived melts.The ε Nd（t） values in the studied magmas increase with decreasing age,consistent with upwelling of asthenosphere subsequent to the thinning of ancient lithosphere.This marks lithospheric thinning during Late Cretaceous-Early Cenozoic.Similar temporal variations in ε Nd（t） values are also observed for magmas in the western（120～100 Ma） and eastern（80～60 Ma） Liaoning province.This again attests to diachroneity of lithospheric thinning processes beneath the North China Craton.