吉南新太古代高镁安山岩及其地质意义

吉南张三沟岩组和金银别岩组发育一套高镁安山岩类，SHRIMP锆石U-Pb定年为（2 493±12）Ma，形成于新太古代。高镁安山岩类属于钙碱性系列，w(SiO₂)为 53.93%～57.90%，富MgO(w(MgO)为6.54%～8.82%），高Mg^#值(0.68～0.71, 平均0.69)，高铬(w(Cr)为(270.66～1 117.30)×10^－6，平均443.62×10^－6），高镍(w(Ni) 为(141.74～542.98)×10^－6，平均250.50×10^－6），这种高MgO、Cr、Ni的特征表明其成因与地幔部分熔融有关。另一方面，该岩石富集大离子亲石元素（如Sr、Cs、K、Pb、Rb和Ba），亏损高场强元素（如Nb、Ta、Ti、P），富集LREE（w(Ce)为(38.34～59.34)×10^－6），亏损HREE（w(Yb)为(1.38～1.57)×10^－6），显示弧火山岩的特点。上述地球化学特征表明,岩石可能是消减板片脱水流体或者消减板片部分熔融的熔体与地幔橄榄岩相互作用的结果，揭示它们形成于消减带的构造环境,表明新太古代已经存在现代类型的板块构造。

Neoarchean High-Mg Andesites and Its Geological Significance in Southern Jilin Province

The high-Mg andesite occurring in the Zhangsangou and Jinyinbie Formation-Complex in southern Jilin Province is dated at (2 493±12) Ma by the SHRIMP zircon U-Pb method, which indicates that it was generated in the Neoarchean period. The high-Mg andesite belongs to the cal-calkaline series. It is geochemically characterized by SiO₂ content of 53.93%-57.90%, high MgO (6.54%-8.82 %) and high Mg^# (0.68-0.71, avg. 0.69), associated with enrichments of Cr (270.66×10^-6-1 117.30 ×10^-6，avg. 443.62×10^-6) and Ni (141.74×10^-6-542.98×10^-6，avg. 250.50×10^-6). According to its high MgO, Cr and Ni, the high-Mg andesite is relate to the partial melting of mantle peridotites in genesis. On the other hand, the andesite is strongly enriched in light rare earth elements (Ce:38.34×10^-6-59.34×10^-6) and large-ion-lithophile elements (such as Sr, Cs, K, Pb, Rb and Ba), and is also characterized by depletion of high-field-strength elements (such as Nb, Ta, Ti and P) and heavy rare earth elements (Yb:1.38×10^-6-1.57 ×10^-6), which demonstrates geochemical characteristic of the arc volcanic rock. The high-Mg andesite was most likely derived from the products of interaction of slab-derived fluid or melt with mantle peridotites. It can thus be concluded that the high-Mg andesite was derived from the subduction zone and  there occurred the modern plate tectonics in the Neoarchean period.