摘要
尼勒克二叠纪火山岩位于伊犁盆地北缘阿吾拉勒山西段，该地区发育一套完整的陆相二叠纪地层。本文对该区二叠纪中下统哈米斯特组和乌郎组火山岩进行了系统的岩相学、岩石地球化学和Sr-Nd同位素研究。尼勒克火山岩岩石类型多样，有玄武岩、安山岩、粗面岩、流纹岩等，地球化学特征复杂。上火山岩哈米斯特组火山岩具有典型的钾玄岩特征，高K2O（2.81％-3．91％）、sr（〉1000×10^-6）、稀土（∑REE〉200×10^-6）和（La／／Yb）N比值（9．7～11．7），低Nb／La比值（大多〈0．2）。下火山岩乌郎组可分为二组，一组具最低SiO2，最高MgO，较低SK〈500×10^-6）、稀土（50×10^-6〈∑REE〈80×10^-6）和（La／Yb）N比值（1．6～2．2），相对较高的Nb／La比值（〉0．35），类似富集型洋中脊（E-MORB）特征，暗示其岩浆是浅部源区高程度部分熔融的产物；另一组火山岩有着相对较大的变化，总体特征介于哈米斯特组与第一组火山岩两者之间。尼勒克火山岩有高度亏损的Sr—Nd同位素组成，类似MORB，表明它们源自长期亏损的地慢源区。然而，火山岩高度亏损的Nb—Ta、分异且富集大离子亲石元素的特征暗示，地幔源区受到近期俯冲带流体的交代作用，并受部分熔融程度及结晶分异作用的控制，是不同批次岩浆作用在不同演化阶段的产物。尼勒克大规模玄武质火山岩的形成可能和塔里木二叠纪地幔柱活动有关。
The terrestrial Nileke Permian volcanic rocks outcrop at the most western section of the Awulale Late Paleozoic volcanic belt. In this contribution, we reported petrography, elemental and Sr-Nd isotope compositions of the Nileke Permian basaltic rocks in aiming to have a better understanding its geodynamic implications. The Nileke volcanic strata could be divided into two series, i. e. , the Wulang series （lower） and the Hamisite seires （upper） and the diverse rock types include basalts, andesites, traehytes and rbyolites. In geochemistry, the upper Hamisite seires exhibit shoshonitic signatures such as having high K20（2. 81% ～ 3.91% ） , Sr （〉1000×10^-6）, totalREE （〉200×10^-6） contents, high （La/Yb）N（9.7～11.7） but low Nb/La ratios （most 〈0.2） and shows. The lower Wulang series could be divided into two sub-groups： sub-group one contains the lowest SiO2 , low Sr （ 〈 500 × 10^-6） , total REE（50 × 10^-6 〈EREE 〈80 × 10^-6）and low （La/Yb）N （1.6 ～2. 2） but the highest MgO and relatively higher Nb/La ratios （ 〉 0. 35 ）. These features are comparable with those of E-MORB, suggesting that they could be derived from high-degree partial melting in a decompressing process. Sub-group two has a wide range of geochemical compositions straddling between the Hamisite seires and the sub-group one. The Nileke Permian volcanic rocks have intensive depleted Sr-Nd isotopic compositions similar with those of MORB, indicating that they were derived from the time-integrated depleted mantle sources. However, their significant depletion in Nb-Ta, intensive differentiation and enrichment in LILE argue that the Permian basaltic rocks in Nileke area were derived from a recently metasomatized depleted lithospheric mantle sources. Additionally, our study reveals that their chemical signatures were constrained by partial melting degree and crystal fractionation process. In combination with regional geology and previous studies, the Nileke Permian volcanic rocks could
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2013年第10期3389-3401,共13页
Acta Petrologica Sinica
基金
973项目(2011CB808903)资助
关键词
西天山尼勒克
二叠纪火山岩
地球化学
构造意义
Nileke, Western Tianshan
Permian volcanic rocks
Geochemistry
Tectonic implications
