阿尔金山西段巴什库尔干群斜长角闪岩地球化学特征及构造意义

Geochemical Characters and Tectonic Significance of Amphibolite of Baxkorgan Group in Western Segment of Altun Mountains

长城系巴什库尔干群斜长角闪岩主要分布于阿尔金构造带西段北缘,一般呈似层状、透镜体状夹于云母石英片岩、变粒岩、石英岩、黑云二长片麻岩、大理岩等中。通过岩石学、岩石化学、微量元素和稀土元素地球化学研究表明,该斜长角闪岩的原岩为中-基性火山岩类,并形成于两种构造环境:一种斜长角闪岩的球粒陨石标准化配分曲线显示出LREE明显富集,具有类似大陆(板内)裂谷火山岩特有的稀土元素组成特征;另一种斜长角闪岩的球粒陨石标准化配分曲线呈略富集(近平坦)型,与洋中脊玄武岩类非常相似。表明在长城纪时期,阿尔金构造带西段北缘主要处于板内大陆裂谷环境,随着大陆裂谷的进一步扩张,局部地区已形成初始洋盆环境。

Amphibolites of Bashenkuergan Group of Changchengian System mainly distribute in the northern margin of the western segment of Altun tectonic belt. They occur in mica-quratz schist, granulitite, quartzite, biotite-feldspathic gneiss, marble and so on in the layer-like and lense-like shapes. On the basis of detailed researches on petrology, petrochemistry and geochemistry of REE and trace elements, the protolith of amphibolites are neutral-basic volcanic rocks, and two types of amphibolites have been recognized from Bashenkuergan Group in the western segment of Altun mountains. The protolith of type 1 rocks is characterized by the light REE enriched pattern with （La/Yb）N being 3.04-5.68 and δ（Eu） 0.79-0.93. This may indicate that amphibolites are similar to continental（intraplate） rift basalt. The protolith of type 2 rocks is basalt which might have been formed in the mid-ocean ridge. The amphibolites have the flat REE pattern similar to that of Etype MORE with （La/Yb）N being 1.87-1.96 and δ（Eu） 0.89-0.91. Thus, it is considered that the northern margin of the western segment of Altun tectonic belt was mainly in the tectonic setting of continental（intraplate） rift during the Changchengian Period and a primary ocean basin was formed locally in company with the further spreading of continental rift.