阿拉善地块北缘雅布赖地区基性岩墙地球化学、年代学和Sr-Nd同位素特征

Geochemistry,Geochronology and Sr-Nd Isotope of Mafic Dykes in the Yabulai Area,Northern Margin of the Alxa Massif

雅布赖地区位于阿拉善地块北缘,雅布赖-诺日公-红谷尔玉林构造带西端,该地区晚古生代岩浆岩内广泛出露基性岩墙。锆石LA-ICP-MS U-Pb定年结果显示,雅布赖地区基性岩墙成岩年龄为268±1.6 Ma,时代归属为中二叠世早期,岩石地球化学分析表明,该地区基性岩墙∑REE=143.01×10^(-6)~278.8×10^(-6),LREE/HREE=7.43~10.16,(La/Yb)_(N)=7.27~12.31,轻、重稀土分馏明显,具不明显的负铕异常(δEu=0.84~1.05),在稀土元素球粒陨石标准化配分模式图上,配分曲线右倾,重稀土较平坦;在微量元素原始地幔标准化蛛网图中,表现出相对富集LREE、Rb、Ba、K、Sr等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素。基性岩墙初始^(87)Sr/^(86)Sr比值变化于0.707403~0.707986之间,ε_(Nd)(t)值介于-2.9~-4.7之间。分析认为,雅布赖地区基性岩墙岩浆来源为板片俯冲作用改造的富集岩石圈地幔的部分熔融,形成于碰撞后伸展的构造背景。

In the Yabulai area,the western end of the Yabulai-Nuorigong-Honggueryulin tectonic zone,northern margin of the Alxa massif,mafic dykes are widely exposed in the Late Paleozoic magmatic rocks.Zircon LA-ICP-MS U-Pb dating shows that the ages of the mafic dykes in this area are 268±1.6 Ma,belonging to early Middle Permian.Geochemical analysis suggests that the mafic dykes have high content of ∑REE(143.01×10^(-6)~278.8×10^(-6)),exhibiting significant fractionation between LREE and HREE(LREE/HREE=7.43~10.16,(La/Yb)_(N)=7.27~12.31),depleted in HREE with slight Eu anomaly(0.84<δEu<1.05).The REE patterns show a negative slope.The rocks in these dykes are featured by enrichment of large ion lithophile elements(such as LREE,Rb,Ba,K,and Sr),depletion of high field-strength elements(such as Nb,Ta,P,and Ti),with ^(87)Sr/^(86)Sr rations ranging 0.707403~0.707986 and ε_(Nd)(t)values-2.9~-4.7.Research suggests that the magma source area of the mafic dykes in the Yabulai area was the partial melting of enriched lithospheric mantle transformed by plate subduction,which formed in a post-collision extensional environment.