西藏冈底斯东段扎雪地区林子宗群帕那组火山岩锆石U-Pb年龄、地球化学特征及其构造意义

Zircon U-Pb ages and geochemical characteristics of the Pana Formation volcanic rocks from the Linzizong Group in Zhaxue area, eastern Gangdese belt, Xizang(Tibet), and its tectonic significance

本文对扎雪地区林子宗群帕那组火山岩进行主微量地球化学和锆石U—Pb—Hf同位素研究,以探讨其形成时代、岩石成因及其构造意义。锆石U-Pb年龄表明,扎雪地区帕那组火山岩形成于~52 Ma,与冈底斯东段帕那组火山岩形成时代相似,暗示了帕那组在时空上具有相对一致的活动时限。研究区火山岩高硅、富钠、富碱,A/CNK值为1.30~1.73,属于过铝质钾玄岩系列。同时,这些火山岩富集大离子亲石元素和轻稀土元素,亏损高场强元素,有岛弧岩浆的亲属性,且其低Mg#(19.77~38.96)及低含量的Cr(3.00~34.3μg/g)、Ni(2.90~15.6μg/g)、Co(2.20~9.58μg/g),La/Sm值随La含量升高而无明显变化,表明这些火山岩在成岩过程中经历了明显的分离结晶作用。此外,扎雪地区帕那组火山岩锆石εHf(t)平均值-4.4^-3.6,表明该火山岩起源于新生地壳物质的部分熔融,有古老壳源物质加入源区中,形成了相对富集的Hf同位素特征。结合前人在林周盆地及邻区的研究结果,表明冈底斯东段林子宗群火山岩母岩浆随时间由岛弧环境的亏损地幔源区向碰撞环境壳源物质加入相对富集型源区的转变,指示其形成环境经历了由岛弧向陆陆碰撞的转变;帕那组火山岩形成于印度板块与欧亚板块碰撞的初期阶段,是记录这种体制转换的主要载体。

Objectives:The Pana Formation volcanic rocks of the Linzizong Group are widely distributed in Zhaxue area,eastern Gangdese belt,which is an important object to understand the geological evolution of the Gangdese belt.Methods:In this study,we report the results of whole-rock geochemistry,zircon U-Pb age and Lu—Hf isotopic composition date for the Pana Formation volcanic rocks in the Zhaxue area to constrain their emplacement ages,petrogenesis and tectonic significance.Results:The LA-ICP-MS zircon U-Pb age reveals that the Pana Formation volcanic rocks were erupted at 52 Ma,which is similar to the geochronology of the Pana Formation volcanic successions in the eastern Gangdese arc,indicating that there has a relatively consistent time of the Pana Formation in the Gangdese arc.Geochemical data display that these volcanic rocks have high content of Si O2 content,high sodium and alkali with A/CNK values ranging from 1.30 to 1.73,suggesting that it belongs to the peraluminous shoshonite series.Meanwhile,they present obvious enrichment in LILEs(such as Rb,Th,U and K)and LREE,depletion in HFSEs(such as Nb,P and Ti),showing the arc magma affinity.These samples have low Mg#(19.77~38.96),Cr(3.00~34.3μg/g),Ni(2.90~15.6μg/g),Co(2.20~9.58μg/g),and the La/Sm ratios show no significant change with the increasing content of La,suggesting that the Pana Formation volcanic rocks in this study experienced crystal fractionation in the petrogenesis.Additionally,in combination with their negativeεHf(t)values(-4.4 to-3.6),the Pana Formation volcanic rocks in Zhaxue area mainly derived from partial melting of juvenile crust with some continental crust material involvement.Conclusions:Combined with the published isotopic data around the Linzhou region,the Linzizong group volcanic rocks changed from a depleted mantle in island arc environment to a relatively enriched source formed by involvement of continental crust material in collision environment over time,indicating that the formation environment underwent a transition from island arc to continental collision.What’s more,the Pana Formation volcanic rocks were considered as a magma response caused by the transfer from subduction to collision at initial stage of the Indian—Asia continental collision.