南天山南缘下二叠统玄武质岩浆的地球化学特征及其构造意义

Geochemistry,and its tectonic significances ofbasaltic magma of Xiaotikanlike Formation in southern margin of south Tianshan,Baichen County

南天山造山带南缘发育大量的下二叠统火山岩。地球化学指标显示南天山南缘西段拜城县宿相厄肯沟出露的下二叠统小提坎立克组玄武岩为拉斑玄武岩(里特曼指数δ=2.93)。该套岩石低FeO^T、MgO和全碱(Na_2O+K_2O),高CaO和Al_2O_3,相对高钠,低钾;Rb、K、U和Ba等大离子亲石元素富集;稀土配分曲线整体协调一致,为向右缓倾的曲线,轻稀土元素(LREE)相对富集;微量元素配分型式呈"驼峰状";玄武岩的母岩浆经历了以单斜辉石为主的分离结晶作用和强烈的陆壳混染作用。同位素组成^(143)Nd/^(144)Nd=0.512 849±0.000 005,^(87)Sr/^(86)Sr=0.706 845±0.000 014;-10<εNd(t)<0,且ε_(Sr)(t)>0,模式年龄T_(DM)为1.463 279765 Ga。结合区域地质资料表明,小提坎立克组玄武岩形成于板内伸展环境,具有主动裂谷作用性质,说明南天山造山带南缘当时处于碰撞后伸展背景。

Many lower Permian volcano rocks exists along the southern margin of the south Tianshan orogen, basahs attributed to Xiaotikanlike Formation of Lower Permian, outcropping in Suxiangeken in Baicheng County in southern margin of the south Tianshan orogen, are tholeiitic basalts（δ= 2. 93 ）. Basahs show that lower FeO^T, MgO, total alkali（ Na2O＋K2O） and higher CaO, Al2O3, relatively high K. The large ion lithophile elements（LILE）, such as Rb, K, U and Ba are enriched. REE curve is harmonious on the whole and a right inclined curve. The light rare earth elements（LREE）are enriched relatively to heavy rare earth elements （HREE）. The trace element distribution patterns of basalts are ＂humped＂. Geochemical indicators suggest that the tholeiitic basahs are suffered variable crustal contamination and crystallization differentiation. 143Nd/144Nd = 0. 512 849 ± 0. 000 005, 87Sr/S6Sr = 0. 706 845 ±0. 000 014, -10〈εNd（t） 〈0,εsr（t） 〉0. Intraplate extensional environment,with active rifting properties. Further more,in Early Permian,southern margin of the south Tianshan orogen is in the stage of modulation of post-orogenic.