大兴安岭新生代高镁安山岩的确认

摘要

High-magnesium andesites (HMAs),which refer to the magnesium-rich,intermediate volcanic rocks (SiO2 =54～65 wt％,Mg#≥ 45),are generally distributed in the island-arc setting,related to subduction.The research area of this study is located in the Wuchagou Town,Inner Mongolia Province,which is in the central Greater Khingan Range.The Wuchagou andesites compose a 700 km2 Cenozoic volcanic field.In general,the Cenozoic volcanic rocks in eastern China are mainly basaltic,whereas the andesites are rare in this area.Both of the Cenozoic andesites and basalts are produced in the intraplate setting.However,the chemical compositions of these andesites are similar to these of the arc-related HMAs.Their SiO2and Mg# range from 54.10 wt％ to 57.97 wt％ and from 53 to 60,respectively.Their trace elements are characterized by enrichment of LILEs and depletion of HFSEs.Therefore,we defined those Cenozoic andesites as intraplate HMAs.Additionally,the Wuchagou andesites show negligible variations in major elements.For a given SiO2 content,the Wuchagou andesites have higher contents of MgO (4.95 ～6.64 wt％) and Fe2O3T (7.78～10.28 wt％),lower CaO (5.77～7.55 wt％),Al2O3 (14.33～15.61 wt％) and K2O (0.06～1.45 wt％)than those of Mesozoic volcanic rocks in this area.In the Harker's diagrams,these HMAs show negligible evolution,implying that their compositions are close to the primitive magmas.Compared with the high-pressure experimental partial melts,the chemical compositions of wuchagou HMAs are similar to these of the eclogite-derived melts,except for their high MgO contents.Therefore,we suggest that the Wuchagou HMAs might be derived from the melting of recycled crustal materials in the mantle,and then underwent interactions with the surrounding peridotites during their ascent.%高镁安山岩为一类相对富镁的中性火山岩(SiO2 =54～65 wt％,Mg# ≥45),一般分布于岛弧环境,其形成往往与板块俯冲密切相关.位于大兴安岭中段的内蒙古阿尔山市五岔沟地区分布了面积达700 km2的新生代火山岩,其岩性为安山质熔岩,这在中国东部新生代火山岩(以玄武岩为主)中非常罕见.这些新生代安山岩与新生代玄武岩一样,均是板内岩浆作用的产物,其元素地球化学特征与岛弧高镁安山岩相似:SiO2变化于54.10～57.97 wt％之间,Mg#范围在53～60之间;在微量元素标准化图上,具有富集大离子亲石元素、亏损高场强元素的特征,类似于典型的弧火山岩.因此,笔者定义五岔沟地区的新生代火山岩为板内高镁安山岩.五岔沟新生代高镁安山岩的主量元素变化范围非常小.与该区中生代火山岩相比,在一定的SiO2下,具有偏高的MgO (4.95～6.64 wt％)和Fe2O;(7.78～10.28 wt％)含量,偏低的CaO(5.77～7.55wt％)、Al2O3(14.33～15.61 wt％)和K2O(0.06～ 1.45 wt％)含量.在Harker图解上,这些高镁安山岩没有表现出明显的演化趋势,反映其未经历明显的岩浆演化过程,化学组成接近原始岩浆.与实验熔体对比,五岔沟高镁安山岩的成分大致与榴辉岩来源熔体相吻合,但具有相对偏高的MgO含量.因此,五岔沟高镁安山岩的源区很可能是地幔中再循环的地壳物质(榴辉岩),在熔体上升过程中可能与地幔橄榄岩发生了反应.