作　　者： (曾令森）; (高利娥）; (郭春丽）; (侯可军）; (王倩）;

机构地区： 中国地质科学院地质研究所,北京100037

出　　处： 《岩石学报》 2017年第8期2377-2394,共18页

摘　　要： 在现今活动的俯冲带系统中,俯冲板片的后撤或回卷都可能导致弧前伸展作用,但弧前伸展作用是否局限于浅部构造层次,还是涉及整个岩石圈,是理解岛弧构造演化和地球化学效应的重要课题。在西藏冈底斯岩基南缘,保留着世界上典型的弧前盆地——日喀则弧前盆地。在该盆地的北缘,除了发育中新世闪长玢岩脉外,还发育一系列近东西向展布的辉绿岩脉,形成时代为106.6±0.8Ma(锆石U-Pb年龄)。基性岩脉在岩石学、矿物学和地球化学特征上较均一,具体表现为:(1)富集LREE,亏损HREE,但HREE平直;(2)强烈亏损Rb、Ba、Nb和Ta,微弱亏损Ti,但Zr和Hf不具亏损现象;(3)锆石Hf同位素组成(εHf(t)=+11.7^+15.4)较高;和(4)包含捕获的早侏罗世岩浆锆石。上述数据表明:在日喀则弧前盆地形成过程中,伴随着强烈的早白垩纪弧前伸展作用,诱发较亏损地幔发生部分熔融,形成不同于典型岛弧岩浆岩的基性岩,可能是在新特提斯洋北向俯冲过程中,海沟后撤,在弧前形成强烈的地幔角流的结果。同时,日喀则弧前盆地可能构建于早侏罗纪结晶岩系之上。 In modern subduction systems,forearc extension is common and associated with the retreat or roll-back of subducting oceanic slab. Whether such extension is limited to the crustal level or involves the mantle lithosphere is one of the interesting questions poorly resolved. The Xigatze forearc basin developed along the southern margin of the Gangdese batholith is one of the best preserved forearc basins worldwide. Within this basin,except for the Mid-Miocene dioritic dikes,there occur a series of nearly E-W trending dikes of diabasic composition. Zircon U-Pb dating results show that these mafic dikes formed at 106. 6 ± 0. 8Ma,substantially younger than those mafic dikes by - 20 Myr within the Xigatze ophiolite. These mafic dikes are characterized by（ 1） enrichment in LREE,depletion in HREE,and nearly flat HREE distribution patterns;（2） strong depletion in Rb,Ba,Nb,and Ta,weak depletion in Ti,and no anomalies in Zr and Hf;（3） high and positive zircon εHfvalues（ ＋ 11. 7 and up to ＋ 15. 4）; and（4） presence of possibly captured Early Jurassic zircon grains. These observations suggest that the development of the Xigatze forearc basin was accompanied by intensive forearc extension which induced partial melting of relatively more depleted forearc mantle lithosphere and the generation of basaltic magmas different from typical arc basalts. The Early Cretaceous forarc extension and melting could be triggered by enhanced corner flow due to southward retreat of subducting Neo-Tethyan oceanic slab. Zircon grains with ages - 180 Ma from the diabases might suggest the presence of Early Jurassic crystalline rocks beneath the Xigaze forearc basin.

关 键 词： 日喀则弧前盆地 弧前伸展作用 基性岩浆作用 冈底斯岩基 西藏