博格达造山带东段早石炭世火山岩地球化学特征及构造属性

Geochemistry and Tectonic Setting of the Early Carboniferous Volcanic Rocks in the Eastern Section of the Bogda Orogenic Belt in Xinjiang

博格达造山带东段克孜库都克地区七角井组玄武岩和中酸性火山碎屑岩及少量流纹岩在时空上构成双峰式火山岩组合。玄武岩LA-ICP-MS锆石U-Pb年龄为331.0±3.0Ma,属于早石炭世晚期;Si O2含量为47.68%~48.82%,Ti O2含量(1.83%~2.17%)略高于N型大洋中脊玄武岩,高Al(Al2O3含量为15.56%~16.09%),富钠贫钾(Na2O/K2O=5.44~7.76),低Mg(Mg O含量为5.97%~7.17%,Mg#为43~47),表明其原始岩浆发生过明显的橄榄石和辉石的分离结晶作用。玄武岩具有近于平坦的稀土配分模式,轻微负Eu异常(δEu=0.89~0.93),相对富集Rb、Ba、P,亏损Th、Nb、Ta、Sr、Ti等不相容元素。火山岩岩石地球化学特征表明:研究区玄武岩可能是亏损尖晶石相地幔橄榄岩向石榴石相地幔橄榄岩过渡的产物,且在其上升过程中受到较弱程度的地壳物质混染,形成于陆内裂谷环境,其地球动力学体制可能与古亚洲洋壳向先存的准噶尔-吐哈陆块斜向俯冲,产生的侧向撕裂力拉张陆块有关。克孜库都克地区早石炭世玄武岩构造属性的确立进一步证实了博格达造山带在石炭纪时期处于大陆裂谷演化过程的观点,为进一步深入理解博格达地区石炭纪构造岩浆演化过程提供了新的地质与年代学依据。

Basalts,acidic volcaniclastic rocks and intercalated rhyolite of the Qijiaojing formation,as a bimodal combination,occur in the Kezikuduke area of the eastern section of the Bogda orogenic belt. The basalts with LA-ICP-MS zircon U-Pb age of 331. 0 ± 3. 0 Ma should be formed in the end of early Carboniferous,which contain high Al( Al2O3= 15. 56% ~ 16. 09%),Na2 O / K2 O = 5. 44 ~ 7. 76,TiO 2= 1. 83% ~ 2. 17%,slightly higher than N-MORB's,and low Mg( MgO = 5. 97% ~ 7. 17%,Mg#= 43 ~ 47),indicating obvious fractional crystallization of olive and pyroxene during basaltic magmatism,which was the evolution product of original magma's fractional crystallization. These basalts are slightly enriched in trace elements Rb,Ba and P,with almost flat REE distribution patterns,slightly negative Eu anomalies( δEu = 0. 89 ~ 0. 93),but relatively depleted in Nb,Ta,Th,Sr and Ti. These characteristics of volcanic rocks indicate that the basalts have geochemical features indicative of intraplate basalts that formed within a continent and originated from high-degree partial melting of the transition product from depleted spinel phase mantle peridotite to garnet phase mantle peridotite,and suffered from a slight degree of crustal contamination during the rising process. Their geodynamic regime may be related with oblique subduction of the ancient Asian oceanic crust beneath the pre-existing Junggar-Turpan-Hami continental block,which produced a lateral tearing force to stretch the continental block resulting in a rift zone. The study results further confirm the view that the Carboniferous Bogda orogenic belt was situated in a continental rift and provide the geology and chronology evidence for understanding the Carboniferous tectonic magmatic evolution process of the Bogda orogenic belt.