新疆东天山觉罗塔格带阿奇山南部雅满苏组火山岩的年代学、地球化学特征及其成因

Geochronology,geochemistry and genesis of Yamansu Formation volcanic rocks of southern Aqi Mountain in Jueluotage tectonic belt,eastern Tianshan of Xinjiang

东天山觉罗塔格带阿奇山南部雅满苏组火山岩的岩石学、锆石LA-ICP-MS U-Pb年代学、全岩地球化学和Sr-Nd同位素分析结果表明阿奇山南部雅满苏组火山岩主要由安山岩、英安岩、流纹岩和相应成分的火山碎屑岩组成,夹少量玄武岩。流纹岩和安山岩中的锆石多呈自形-半自形晶,振荡环带发育,Th/U比值为0.44~1.53,指示岩浆成因。定年结果表明它们形成于早石炭末—晚石炭初(318.6~324.4 Ma)。阿奇山南部雅满苏组火山岩的地球化学特征主要为:酸性火山岩为中钾钙碱性系列,中基性火山岩主要为高钾钙碱性系列;稀土配分模式均呈右倾型,轻重稀土分馏明显(LREE/HREE为2.91~9.92);强烈富集Rb、Ba、K、La、Ce等大离子亲石元素,明显亏损Nb、Ta、P、Ti等高场强元素;(^(87)Sr/^(86)Sr)_i值为0.704 01~0.706 36,ε_(Nd)(t)值主要为4.14~7.21。研究结果表明,中基性火山岩的岩浆源区主要为受俯冲流体交代的亏损地幔楔,而酸性火山岩则源于年轻地壳物质。结合前人研究成果,认为阿奇山南部雅满苏组早石炭世末—晚石炭初火山岩形成于大陆边缘环境,其地球动力学机制与古亚洲洋板块向中天山地块之下的俯冲作用有关。

The petrology,zircon LA--ICP--MS U--Pb dating,whole rock geochemistry and Sr--Nd isotope analysis of the volcanic rocks from Yamansu Formation within the southern Aqi Mountain in Jueluotage tectonic belt,eastern Tianshan,suggest that the volcanic rocks in the study area are mainly composed of andesite,decite and rhyolite as well as corresponding volcaniclastic rocks,with a small amount of basalt.Zircons from the main rocks(andesite and rhyolite)are euhedral-subhedral in sharp,displaying fine-scale oscillatory growing zoning with high Th/U ratios(0.44~1.53),indicating a magmatic origin.The geochronological results indicate that the volcanic rocks formed in the Early Carboniferous to Late Carboniferous(318.6~324.4 Ma).Geochemical data indicates that all of the volcanic rocks have the following characteristics:acidic volcanic rocks fall in the medium-potassic calc-alkaline series,andesite and basalt fall in the series of high-potassic calc-alkaline;the REE distribution patterns are right deviated with significant fractionation of light and heavy rare earth elements(LREE/HREE=2.91~9.92);the rocks are all enriched in large ion lithophile elements(Rb,Ba,K,La,Ce)and relatively depleted in high field strength elements(Nb,Ta,P,Ti);the(87 Sr/86 Sr)i values andεNd(t)values range from 0.704 01 to 0.706 36 and 4.14 to 7.21,respectively.In conclusion,all the evidence indicates that the primary magma of the mafic rocks are generated by the partial melting of the depleted mantle wedge metasomatized by subduction fluids,whereas the primary magma of the acidic volcanic rocks are generated by partial melting of the juvenile crustal material.Combined with the previous research results,it is suggested that the primary magma of the late Early Carboniferous to early Late Carboniferous volcanic rocks from the Yamansu Formation within the southern Aqi Mountain area formed in continental margin environment during the subduction of the Paleo--Asian Ocean plate beneath the middle Tianshan plate.