内蒙古赤峰北部晚侏罗世中酸性火山岩地球化学特征

GEOCHEMICAL CHARACTERISTICS OF UPPER JURASSIC INTERMEDIATE-GRANITIC VOLCANIC ROCKS FROM NORTHERN CHIFENG,INNER MONGOLIA

内蒙古赤峰北部晚侏罗世中酸性火山位于华北克拉通北缘内蒙古隆起上,熔岩主要有流纹岩、安山岩、辉石安山岩。岩石地球化学研究表明,中性火山岩SiO2含量50.74%～58.82%,Al2O3含量15.32%～17.17%,Mg#值0.45～0.52,全碱(K2O+Na2O)含量4.89～7.05,属于钠质,钙碱性系列;流纹岩SiO2含量74.68%～74.86%,Al2O3含量12.09%～13.13%,Mg#值0.13～0.39,全碱(K2O+Na2O)含量7.83%～7.99%,属于钙碱性—钾玄岩系列。中性火山岩轻重稀土分馏明显,(La/Yb)N值为9.6～16.09,Eu异常不明显,Eu*/Eu值为0.94～1.12,富集Pb等LILE,亏损Ni、Ta等HFSE;流纹岩与中性火山岩具有相似的微量元素分布特征,(La/Yb)N值为7.86～10.05,Eu负异常明显。结合前人研究资料,初步认为研究区中性火山岩形成于板内伸展减薄环境下;安山质岩浆可能是在岩石圈伸展导致软流圈上涌和地温梯度增高背景下富集岩石圈地幔部分熔融的产物。

Upper Jurassic intermediate-granitic volcanic rocks from northern Chifeng lie in Mongolia Paleo-uplift,the northern margin of the North China Craton.The lava mainly consists of rhyolite,andesite and augite andesite.The geochemical study for intermediate-granitic volcanic rocks suggests that the SiO2 is in 50.74% ~58.82%,Al2O3 and K2O＋ Na2O range from 15.32% ~ 17.17% and 4.89%~7.05%,Mg# changes from 0.13 to 0.39,which belong to Sodium and Calc-alkaline series.Geochemical results for rhyolite indicate that the SiO2 are in 74.68%~74.86%,Al2O3 and K2O＋ Na2O range from 12.09%~13.13%,Mg# changes from 0.13 to 0.39 and they belong to Calc-alkaline series or Shoshonitic series.The trace elements analysis for intermediate volcanic rocks demonstrate the insignificantly Eu anomalies（Eu＊/Eu=0.94~1.12） and intense fractionation between LREE and HREE,[（La/Yb）N=9.6~16.09].The primitive mantle-normalized spider diagrams are characterized by LILE enrichment and HFSE depletion,especially an obviously negative Nb-Ta anomalies,and obviously positive Pb anomalies.Rhyolite has the similar characteristics and is characterized by a significantly negative Eu anomalies,[（La/Yb）N=7.86~10.05].Based on the previous studies,we infer that intermediate volcanic rocks formed under the stretching and thinning environment.With geothermal gradient increasing and atmosphere upwelling,andesitic magma resulted from partial melting of enriched lithospheric mantle.