【摘要】 本文重点讨论的富镁安山岩（MAs）是指 SiO₂>53%,Mg^#≥55的安山岩,也包括富镁英安岩和富镁闪长岩。阿尔泰南的富锾安山岩形成于中泥盆世,东、西天山和阿拉套山的富镁安山岩形成于石炭纪。本区富镁安山岩 SiO₂ 53%～65%,TiO₂含量低于1%（0.21%～1.08%）,平均0.72%;MgO 平均5.90%,与国外玻安岩平均值相比,Mg 含量较低,Ti、K、Na 含量较高。稀土元素总合量低(<100×10^-6,范围15～95×10^-6),相对富重稀土（La/Yb）_N0.98～6.4,多数在4±;Eu 弱亏损到无亏损（Eu/Eu^*0.65～1.15）。相容微量元素 Cr、Ni 含量高,分别为72×10^-6～790×10^-6和29×10^-6～276×10^-6;高场强元素 Nb、Ta、Ti 明显亏损;相对富集流体中富集的大离子亲石元素 Rb、K、Pb;Y 含量较高(>15×10^-6);Sr/Y 比值较低（4.4～6.2）。(¹⁴³Nd/¹⁴⁴Nd)_i 值变化范围较大0.51221～0.51255,ε_Nd（t）+0.28-+7.2,Nd 模式年龄 t_DM485～1220Ma;(⁸⁷Sr/⁸⁶Sr)_i 值变化范围较大0.7029～0.7065,在ε_Nd（t）-(⁸⁷Sr/⁸⁶Sr)_i 图解中位于第一象限。上述特点表明,本区富镁安山岩源区为复合源,包括有蚀变洋壳、弧前增生棱柱的不成熟、新生地壳物质及地幔楔橄榄岩。成岩作用具复合性,既有俯冲洋壳板片的部分熔融,又有俯冲板片熔体对地幔楔橄榄岩的熔体交代作用。形成于特殊构造环境:高地热梯度和富挥发分;导致弧前增生棱柱俯冲的俯冲剥蚀作用;由俯冲板片撕裂、断离所形成的软流圈窗或洋脊俯冲。更多还原

【Abstract】 The magnesian andesites（MAs）discussed in this paper are simply defined as andesitic rocks having SiO₂>53% and Mg^#≥55 and including magnesian dacite and diorite.The MAs are widespread in north Xinjiang.The MAs distributed in south Ahay were mainly generated during middle Devonian.Carboniferous MAs are distributed in Alataw mountain,west and east Tianshan.The MAs span range of SiO₂53%～65% ,with TiO₂<1%（average 0.72% and range of 0.21%～1.08% ）.Relative to the world boninites the MgO contents（average 5.90%）are lower and the contents of Ti,K and Na are high.Flat chondrite-normalized REE patters are characterized for the MAs with weak-no depletion of Eu anomalies（Eu/Eu^*0.65～1.15 ）,low（La/Yb）_N（0.98～6.4, mostly 4）and low total REE contents(15×10^-6～95×10^-6).The MAs are characterized by high contents of compatible Cr and Ni with 72×10^-6～790×10^-6and 29×10^-6～276×10^-6,respectively.Relative depletion of high field strength elements Nb,Ta,Ti and relative enrichment of mobile LIL elements Rb,K,Pb are evident in primitive mantle-normalized spidergram of trace elements for the MAs.In addition,the relative high Y contents(>15×10^-6)and low Sr/Y ratios（4.4～6.2）,together with low（La/Yb）_N and high Mg^#,provide the evidences of interaction of oceanic slab melts with the peridotite in mantle wedge.The MAs have wide range of isotopic compositions of Sr and Nd such as(143^Nd/¹⁴⁴Nd)_I=0.51221～0.51255,ε_Nd（t）+0.28～+7.2 and(⁸⁷Sr/⁸⁶Sr)_i=0.7029 ～0.7065.In the diagram ofε^Nd（t）vs.(⁸⁷St/⁸⁶Sr)_i,the MAs were plotted in the first quadrant.Based on the above the mechanism for explaining the MA petrogenesis is as follows:（1）,Multi-source materials composed of subducted oceanic slab with the juvenile materials from the forearc accretionary prism,and peridotite in mantle.wedge;（2）,Combination of different processes of petrogenesis including partial melting of subducted oceanic slab together with the juvenile materials and the interaction of slab melts with the peridotite in the mantle wedge（metasomatism of slab melt）;（3）,Typical physicochemical conditions of MA magma generation,such as high geothermal gradient resulting in high temperatures（>1000℃）and volatile-rich source region;（4）Special tectonic settings including oblique subduction of oceanic slab,slab tear or break off resulting in asthenosphere upwelling and forming of slab window, and the subduetion erosion resulting in going of the foreare accretion materials into the source region of MA magma.更多还原