大兴安岭敖包查干地区安山岩年代学、地球化学研究

Age and Geochemistry of Andesites in the Qagan Obo Area, Da Hinggan Mountains

在大兴安岭敖包查干地区首次识别出中三叠世安山质岩石，主要岩性为玄武质安山岩、粗面安山岩，应用锆石U—DbLA—ICPM—AS法定年，结果表明安山岩形成于234．8±3．2Ma～231．4±1．2Ma，属中三叠世。地球化学特征显示，研究区安山岩具高SiO2（54．19％～62．30％）、A12O3（15．78％～17．30％）、K20（1．77％～3．92％）、Na2O（2．95％～5．66％）、Na2O＋K20（5．4～8．99）的特征和低MgO（1．23％～3．66％，Mg^#=27．89～50．48）含量，并与SiO2含量呈负相关关系。岩石富Na20（Na2O／K2O=1．01～2．86），铝指数A／CNK=0．81～1．21，属准铝质一弱过铝质钙碱性系列岩石。富集大离子亲石元素（LILE）K、Rb、Th和轻稀土元素，亏损高场强元素（HFSE）№、Ta、P、Ti，轻重稀土元素分馏明显[（LaMb）N=3．31～10．33]．无Eu异常（δEu=0．80～1．03），显示弧火山岩的特征，但所研究样品的Zr、Hf呈弱正异常、Rb弱负异常却不同于正常岛弧安山岩，Zr／Y—Zr图解以及高钾特征显示出板内伸展构造环境的特征。综合研究表明敖包查干地区中三叠世钙碱性安山岩可能为古亚洲洋残余俯冲洋壳部分熔融的产物，与底侵作用有关，形成于大陆内部伸展构造环境。

Middle Triassic andesite rocks, which composed of basaltic andesite and trachyandesite, are fristly identified in Qagan Obo, Da Hinggan Mountains. The LA-MC-ICP-MS zircon U-Pb dating show that the crystal- lization ages of the andesites between 234.8±3.2Ma and 231.4±1.2Ma, which belong to Middle Triassic. Geo- chemical researches indicate that the andesites is high in silica （SiO2 = 54.19% - 62.3%）, A1203（15.78%-17.30%）, K20（1.77%-3.92%）, alkali （K20 ＋ Na20 = 9.59% - 9.90%） and low in MgO（1.23%-3.66%, Mg^#=27.89-50.48）. Andesites is rich in Na20（Na20/K20=1.01-2.86）, which is a typical high-K calc-alkaline series with the character of metaluminous （A/CNK=0.81- 1.21）. The rocks have low contents of rare earth elements with ∑REE= 131.75 × 10^-6- 255.22 × 10^6, enriched in LREE （（La/Yb）N=3.31- 10.33）, LILEs （Rb, Th, K）, and depleted in HFSE, especially Nb, Ta, Sr, P and Ti. Similar REE distribution pattern indicate that the Qagan Obo andesites have the same magma source. The 8Eu values vary from 0.80 to 1.03. The above micorelements indicates that the rocks are similar as island-arc volcanic rocks. But with the characteristics ofZr, Hfpositive anomaly and Rb nega-tive anomaly, are different from island-arc andesites. The Zr/Y-Zr tectonic setting diagram also shows intraplate tectonics. In a word, these characteristics suggest that these high-K calc-alkaline andesites, which related to the underplating of basic magma, were most probably derived from partial melting of the remnant of Paleo-Asian oceanic crust and formed in intraplate tectonics.