西准噶尔马拉苏早泥盆世火山岩LA-ICP-MS 锆石U-Pb 年龄、地球化学特征及其大地构造意义

马拉苏地区早泥盆世地层为一套滨海—浅海相火山—沉积岩系，对其中的火山岩夹层进行锆石U-Pb同位素定年和岩石地球化学研究，定年结果显示有大量的新太古代和中新元古代锆石，表明该区存在古老的大陆地壳物质。火山岩样品SiO2含量为52.38%~69.6%，Na2O含量为2.80%~4.85%，K2O为0.16%~0.96%，TiO2为0.5%~1.96%，Al2O3为14.62%~18.18%，MgO（1.08%~5.75%）变化范围较大，Mg#值在22.92~38之间，具有高钠、低钾的特征，属于钙碱性、低钾拉斑系列。稀土元素总量∑REE=73×10-6~115×10-6，LREE/HREE值为2.66~3.25，具有明显的Eu负异常（0.83~0.92）。玄武安山岩样品相对富集K、Rb、Ba、Sr等大离子亲石元素，亏损Nb、Ta、Zr、Hf等高场强元素，与典型火山弧玄武岩地球化学特征一致。英安斑岩样品也具有富集大离子亲石元素、亏损Nb、Ta、Ti等高场强元素的特征，反映源区可能有较多壳源物质的加入，其稀土元素配分曲线和微量元素蛛网图与玄武安山岩相似，表明其可能为同源岩浆演化的产物。综合研究认为，这套火山岩具有岛弧火山岩特征，形成于板块俯冲拼贴过程中的岛弧环境。

LA-ICP-MS zircon U-Pb geochronology, geochemical characteristics and tectonic significance of Early Devonian volcanic rocks in Malasu area, western Juggar Basin

The early Devonian strata in Malasu area is a set of littoral-neritic facies volcanic sedimentary rocks. This paper reports the zircon age and geochemical characteristics of the volcanic rocks. A lot of Neo-Archean and middle Neo-Proterozoic zircons indicate that this area contains ancient continental crust material. According to the values of major elements (SiO2 52.38%~69.6%, Na2O 2.80%~4.85%, K2O 0.16%~0.96%, TiO2 0.5%~1.96%, Al2O3 14.62%~18.18%, MgO 1.08%~5.75% ) and the values of Mg# (22.92~38), the volcanic rocks of Malasu area are characterized by high content of sodium and low content of potassium, and thus belong to low-potassium tholeiite series and calc-alkaline series. The volcanic rocks have significant negative Eu anomalies (0.83~0.83), total rare earth element values of 73×10-6~115×10-6, and LREE/HREE values of 2.66~3.25. The geochemical characteristics of basaltic andesite samples are similar to those of typical volcanic arc basalts, being rich in large ion lithophile elements K, Rb, Ba, Sr and depleted in the high field strength elements Nb, Ta, Zr, Hf. The same characteristics of enrichment of LILE and depletion of HFSE for dacite porphyry samples reflect the addition of lots of crust-derived materials. The chondrite-normalized REE and primitive mantle-normalized alteration-resistant trace element patterns for the dacite porphyry are similar to the patterns of basaltic andesite, which suggests that they belong to the comagmatic evolutionary series. Comprehensive research shows that the volcanic rocks have the characteristics of island arc volcanic rocks, and were formed in the volcanic island arc environment in the process of plate subduction.