【摘要】 北京西山南大岭组中基性火山岩都表现出LREE富集的右倾平滑稀土配分模式,具有富集LILE（如Ba、K）,亏损Nb-Ta和Th-U的微量元素特征,Sr同位素组成中等富集,Nd同位素组成变化较大。根据其元素地球化学和同位素特征可以划分两组岩石:Ⅰ组火山岩主要为亚碱性玄武岩,高Ti（TiO₂>1.7％）、P（P₂O₅>0.7％）,Ⅱ组火山岩主要由安山岩和亚碱性玄武岩组成,低Ti（TiO₂<1.3％）、P（P₂O₅<0.5％）;Ⅰ组火山岩总体上较Ⅱ组火山岩高相容元素（如Cr、Ni）、REE和HFSE, 二者化学成分上渐变演化趋势不明显;Ⅰ组火山岩Sr-Nd同位素组成(⁸⁷Sr/⁸⁶Sr（t）= 0.705939～0.706057,ε_Nd（t）=-7.4～-7.5)与Ⅱ组火山岩 Sr-Nd同位素组成(⁸⁷Sr/⁸⁶Sr（t）=0.705822～0.706697,ε_Nd（t）=-12.0～-13.5)有明显的差别,以上事实说明两组岩石可能来源于不同的地幔源区。对比于周缘地区中生代基性火山岩特征,西山地区南大岭组两组火山岩的Sr-Nd同位素组成和Nb/La、Hf/Sm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。其相对Ba-La明显的Th-U亏损,可能暗示早期有古老下地壳组分再循环到?更多还原

【Abstract】 Early Mesozoic mafic volcanic rocks of the Nandaling Formation （NFV） occurring in the Xishan area of Beijing all showright-oriented linear REE patterns with LREE enrichment, and are characterized by LILE （e. g., Ba, K）, LREE enrichment andnegative Nb-Ta anomalies as well as moderately enriched Sr isotopic compositions and obviously varied Nd isotopic compositions.According to differences of elemental geochemistry and isotopic component characteristics, the NFV can be divided into two types ofrocks: Group I mainly is composed of subalkaline basalt with high-Ti （TiO₁>1. 75%） and P （P₂O₅>0. 75%）, Group Ⅱ mainly iscomposed of andesites and subalkaline basalts with low-Ti（TiO₂>1. 25%） and P（P₂O₅<0.50%）; and Group Ⅰhas relatively highcontents of comparative elements （Cr, Ni）, REE and HFSE than group Ⅱ, two group volcanic rocks do not exhibit gradient correlationof magma evolution in chemical components. Moreover, GroupⅠ has different Sr-Nd isotopic compositions (⁸⁷Sr/₈₆Sr（t） = 0. 705939-0.706057, ε_Nd（t） = -7.4 - -7. 5) from Group Ⅱ (⁸⁷Sr/⁸⁶Sr（t） = 0.705822 - 0.706697, ε_Nd（t） = -12.0 - -13.5).All these indicate that the two group volcanic rocks of the NFV may have been derived from different mantle sources. Comparing withthe Mesozoic mafic volcanic rocks in adjacent regions, Sr-Nd isotopic compositions and ratios of （Nb/La）_N and （Hf/Sm）_N of the NFVare all located between volcanic rocks within the North China Block （NCB） and volcanic rocks from Hinggan-Mongolian Orogen（HMO）, suggesting that the mantle sources of the NFV had undergone subduction-related metasomatism except inheriting the mantlesource characteristics of the NCB. And this indicates an important role of the subducted slab in the modification of the lithosphericmantle beneath the northern margin of North China Block during the closure of Paleo-Asian Ocean. All volcanic rocks of the NFV havedistinctive characteristics of significant depletion of Th and U relative to Ba and La, suggesting the early ancient lower crust of NorthChina Craton recycling into the mantle source. Combined with the volcanic distribution along major faults and regional early Mesozoictectonic-thermal history, the NFV were formed under extension regime as a consequence of post-collisional stress relaxation, which ledto the reactivation of deep faults and trigged upwelling of asthenosphere and decompressional melting of early enriched continentallithospheric mantle.更多还原