燕山地区中生代中基性火山岩地球化学对岩石圈性质的约束

燕山地区中牛代中基性火山岩包括钙碱性-高钾钙碱性-钾玄岩系列，岩石类型包括粗叫玄武岩、玄武质安山岩、玄武质粗面安山岩、粗安岩，具有LREE富集的右倾平滑稀土配分模式，富集LILE（如Ba，K），亏损Nb-Ta和Th—U的微址元素特征，Sr-Nd同位素组成中等富集，与华北内部和兴蒙带中生代基性火山岩具有不同的特征，其Nd同位素组成和（Nb／La）s，（Hf／Sm）s比值都介于华北陆块内部和兴蒙造山带之间，反映了其熔融地幔源区继承了华北陆块内部的EMI型地幔特征外，还很可能受到俯冲板片交代作用的影响，暗示了古亚洲洋板块消减过程对华北陆缘岩石圈地幔的改造作用。类似于华北陆块南缘岩石圈减薄和置换机制，古亚洲洋俯冲板块对燕山地区古老岩石圈的交代作用在岩石圈减薄和置换过程中起了很重要的作用，而不同于华北板块内部以软流圈沿地幔薄弱带或剪切带上升熔蚀和置换岩石圈的机制

Geochemistry of Mesozoic Mafic-Intermediate Volcanic Rocks and Its Constraints on Lithospheric Characteristics

The Mesozoic volcanic rocks in Yanshan area comprise a wide spectrum of rock types including calc-alkaline, high-K calc-alkaline and shoshonitic lava series. All these rocks show strong LREE, LILE enrichment and HFSE depletion as well as moderately enriched Sr-Nd isotopic compositions, which are different from the mafic volcanic rocks within North China craton and from Xingmeng Orogen. The Yanshan volcanic rocks show transitional geochemical and isotopic signatures when compared to the Mesozoic counterparts in the adjacent cratonic area and orogenic belts, e. g., La/Nb, （Nb/La）N, （Hf/Sm）N ratios and end values, indicating that the melting mantle source of these rocks was modified by the subducted Paleo-Asian Ocean Plate on the basis of inheriting characters of EMI mantle within North China craton. Similar to lithospheric thinning and replacement mechanism, subductionrelated metasomatism of Paleo-Asian Ocean Plate played an important role during lithospheric thinning and evolution beneath Yanshan area, which is different from lithospheric thinning and replacement mechanism within North China craton