新疆阿吾拉勒地区群吉沟超球粒陨石Nb/Ta埃达克岩的成因

The geochemical behavior of Nb and Ta is one of fundamental issue in the geosciences. Petrogenesis of the adakites is related to crust-mantle reactions, but their role in fractionate Nb/Ta is still poor understood. In this proposal, we are focus on the Qunjigou superchondritic Nb/Ta (21.4-23.3) adakites in the Awulale Mountains，Xinjiang province, in order to reveal the petrogenesis of the the Qunjigou Adakites and its contributions on Nb-Ta fractionation. Systematic analyses on major and trace elemental composition and isotopic compositions, combine with in-situ analyses on melt inclusion and minerals, including zircon, apatite, pyroxene, amphibole and plagioclase, will be used to constrain the source nature and petrogenesis of the Qunjigou adakites, and then reveal the behavior of Nb and Ta during formation the adakites. In addition, comprehensively investigation on the Qunjigou adakites and the nearby contemporary low Nb/Ta (12.6-20.0) adakites can be used to constrain the regional geodynamic setting. Furthermore, based on detailed study on the amphibole reaction rims in the Qunjigou adakites, we try to reveal geochemical behavior of Nb and Ta during amphibole break-off.

Nb和Ta的地球化学行为一直是地质学界关注的热点问题，而埃达克岩作为典型的壳-幔相互作用的产物，其对Nb和Ta地球化学行为的影响还不清楚。本项目拟以新疆阿吾拉勒地区群吉沟具有超球粒陨石Nb/Ta(21.4-23.3)的埃达克岩为研究对象，探讨埃达克岩形成过程中Nb-Ta的分馏情况。通过系统的全岩地球化学和辉石、角闪石、斜长石、磷灰石和锆石等矿物的原位地球化学分析，结合熔体包裹体显微测温和成分分析，限定群吉沟高Nb/Ta埃达克岩的源区性质和成因机制，揭示在其形成过程中Nb和Ta的地球化学行为；通过对与该类埃达克岩与具有时空耦合关系的相对低Nb/Ta埃达克岩(Nb/Ta=12.6–20.0)进行综合研究，约束阿吾拉勒地区出现两类埃达克岩的地球动力学背景；通过对群吉沟高Nb/Ta埃达克岩中罕见的角闪石反应边进行矿物学和元素地球化学综合研究，揭示角闪石分解过程对Nb-Ta地球化学行为的影响。

地壳拆沉是造山带演化过程中的重要过程，但是由于其缺少有效的岩浆岩记录，使得很难示踪地壳拆沉的时限和速率，另外拆沉的地壳对后续的幔源岩浆会产生何种影响也没有得到很好的限制。本研究通过的西天山阿吾拉勒地区两类不同类型的埃达克岩（高Nb/Ta埃达克岩和低Nb/Ta埃达克岩）和高Nb/Ta比值的玄武岩进行详细的年代学、岩石学、矿物学和地球化学研究，揭示埃达克岩的Nb/Ta和Dy/Yb比值能够很好的示踪埃达克岩是否具有榴辉岩相源区，进而限定造山带地壳拆沉时限是速率。而该地区具有较高Nb/Ta比值的玄武岩的形成可能与拆沉的榴辉岩化下地壳的参与有关，相关结论对揭示岛弧体系下壳幔Nb和Ta的分异有一定的参考价值。