内蒙古北山造山带北部早二叠世末期高镁辉长岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Implications of the End Early Permian High Magnesium Gabbro from Northern Beishan Orogenic Belt,Inner Mongolia

在北山造山带北部百合山蛇绿混杂岩带南清河沟‒红柳峡地区新识别出一套辉长岩.通过LA-ICP-MS锆石U-Pb定年,全岩主微量及Sr-Nd同位素工作,显示辉长岩侵位于278.2±1.3 Ma,具有高镁(Mg^(#)=55~66)、低钾拉斑‒钙碱性的地球化学特征,富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素.辉长岩ε_(Nd)(t)介于5.64~6.77,具有较高的Sm/Yb比值,暗示岩浆起源于亏损地幔的部分熔融.较高的Mg^(#)及Cr、Ni含量及Zr、Hf负异常,表明母岩浆经历了有限的分离结晶与地壳混染.结合区域地质资料,认为北山造山带北部增生造山作用结束时限为早‒中二叠世,北山造山带古亚洲洋最终闭合可能在中二叠世之后.

A suite of high-magnesium gabbro is located in the Qinghegou-Hongliuxia area in the southern part of the Baiheshan ophillite mélange belt in the northern Beishan orogenic belt.Based on LA-ICP-MS zircon U-Pb dating,whole-rock geochemical and Sr-Nd isotopic analyses,the gabbro was intruded at 278.2±1.3 Ma and they show high magnesium geochemical characteristic,belonging to low-tholeiite to calc-alkaline series.They are enriched in large-ion lithophile elements of Rb,Ba,Sr and depleted in high field strength elements of Nb,Ta,Ti.The values of ε_(Nd)(t)of the gabbro vary from 5.64 to 6.77,with high Sm/Yb,implying magma originates from depleted mantle.Relatively high Mg^(#),Cr,Ni and negative Zr and Hf indicate that the primary magma has undergone limited fractional crystallization and crustal contamination.Combined with regional geological material,it is suggested that the end of the accretionary orogeny is Early to Middle Permian in the northern Beishan orogenic belt.The Paleo-Asian Ocean in the Beishan orogenic belt may have been lasted until or after Middle Permian.