祁连拉脊山中段中奥陶世石英二长斑岩成因及其动力学意义

Petrogenesis and geodynamic implication of Middle Ordovician quartzmonzoporphyry at central Lajishan,Qilian Mountains

花岗岩类广泛分布于地球地壳中,主要为壳源物质岩浆作用的产物,其地球化学组成、岩石组合特征及时空分布规律等信息记录了源区性质、岩浆演化过程及岩浆形成的动力学机制。以祁连拉脊山中段克前垭口石英二长斑岩为研究对象,通过对其开展岩相学、锆石U-Pb年代学、全岩地球化学及Sr-Nd同位素研究,并结合区域内已有的同时期岩浆作用的研究,探讨石英二长斑岩的岩石成因及其形成的深部动力学背景。研究结果显示,石英二长斑岩的年龄为(466±5)Ma。在地球化学组成上,其具有相对较高的SiO 2(63.57%~64.04%)和Na_(2)O(7.08%~7.36%)以及相对较低的CaO(1.19%~2.52%)和K_(2)O(1.53%~1.96%)含量;在微量元素蛛网图上,其表现为富集Ba、U及Pb,亏损Zr、Hf及Sr;此外,其具有轻微的Eu负异常(Eu*=0.80~0.91)以及较为明显轻重稀土元素分异((La/Yb)N=13.0~15.7);其Sr、Nd同位素组成较为均一,初始Sr同位素和εNd(t)分别介于0.7054~0.7062和0.87~1.03。综合研究表明,克前垭口石英二长斑岩形成于加厚新生玄武质地壳物质的部分熔融;其与研究区同时期岩浆岩具有向缝合带逐渐变年轻的趋势,指示了俯冲板片回转的深部动力学过程;拉脊山构造带内470~460 Ma的花岗岩类的(La/Yb)N值整体具有持续降低的趋势,记录了由板片回转引发的岩石圈持续拉张过程。

Granitoids are widespread over the Earth s crust.As products of magmatism from crustal materials,their geochemical compositions,features of rock association and spatial and temporal distribution collectively recorded properties of their magmatic sources,magmatic evolutional processes,as well as geodynamic mechanism for their formation.Based on petrography,zircon U-Pb geochronology,and whole-rock geochemistry and Sr-Nd isotopes on Keqianyakou quartz monzoporphyry at central Lajishan,Qilian Orogen Belt,together with other available researches on coeval magmatism at this region,the authors are able to make discussions on petrogenesis of the quartz monzoporphyry and deep geodynamic setting for their generation.The result turns out that the age of quartz monzonite porphyry is(466±5)Ma.it has relatively high SiO_(2)(63.57%-64.04%)and Na_(2)O(7.08%-7.36%)and low CaO(1.19%-2.52%)and K_(2)O(1.53%-1.96%)content.In trace elemental spider diagram,they are characterized by enrichment of Ba,U and Pb and depletion of Zr,Hf and Sr.Additionally,they have slightly negative Eu anomaly(Eu*=0.80-0.91)and obviously fractionated rare earth elemental distribution((La/Yb)N=13.00-15.72).They have uniform Sr and Nd isotopic compositions with(87 Sr/86 Sr)i=0.7054-0.7062 andεNd(t)=0.87-1.03.Synthetic study manifests they are derived from partial melting of thickened basaltic crustal materials.The quartz monzoporphyry,together with other coeval magmatic rocks in this region,shows a decreasing trend in magmatic crystallization age towards the suture,which indicates their generation at a tectonic setting of rollback of the subducted oceanic slab.The decreasing(La/Yb)N value with ages for the 470-460 Ma granitoids in Lajishan tectonic belt implies persisting extension of lithosphere during that period.