义敦地块中-晚三叠世中酸性侵入岩起源及其地质意义探讨

摘要

地处青藏高原东部的义敦地块不仅发育特征显著的中生代构造-岩浆活动,而且也是重要的多金属成矿区.然而,对于区域上的构造-岩浆-成矿的时空格架及其深部动力学机制目前还未得到很好的约束.综合分析了近年来发表的该区中-晚三叠世中酸性侵入岩的研究资料和数据,进一步探讨了它们的起源、成因和构造意义.高精度的年代学结果显示,区内闪长斑岩和花岗岩的形成年龄为225～215Ma,其峰值为～216Ma.其中出露于义敦南部的闪长质岩石具有高的Mg#值、Cr、Ni、Sr含量和Sr/Y比值,低Y、Yb含量及极弱的负Eu异常和正Sr异常等特征,显示出埃达克质岩的地球化学特征.结合其高K2O/Na2O比值和壳源型的Sr-Nd-Hf同位素组成特征,以及与缝合带东侧九龙地区同时代相同特征的钾质埃达克质花岗岩的对比,认为这些闪长质岩石并非前人认为的起源于俯冲洋壳板片或者交代富集的岩石圈地幔楔,而是下地壳部分熔融的产物.而巨大规模出露于该区中北部地区的花岗岩则比闪长质岩石具有更高的SiO2 (67.33％～78.01％)和K2O(2.38％～6.41％)含量,以及明显的Eu和Sr负异常,略微弱的轻重稀土元素分馏以及更富集的全岩Sr-Nd和锆石Hf同位素组成的特征,表明这些花岗质岩石并非来自闪长质岩石的分离结晶作用,而是来源于更浅的中上地壳的部分熔融,为中上地壳不均一的中元古代地壳物质重熔的产物.结合甘孜-理塘缝合带两侧岩浆-沉积作用的特征,认为义敦地块中-晚三叠世岩浆作用形成于后碰撞的构造环境,是造山作用晚期岩石圈地幔拆沉作用的结果.因此,区域上中-晚三叠世中酸性侵入岩的综合分析表明,弧/陆-陆碰撞(义敦地块和松潘-甘孜地块),即甘孜-理塘洋盆的闭合在中-晚三叠世已经完成;义敦南部与铜多金属矿床密切相关的闪长质斑岩的源区并不具有成矿所需的高氧逸度和高水分的特征.%The Yidun terrane,located in the eastern part of the Tibetan Plateau,is not only a characteristic intrusive magmatic tectonic belt,but also a very important poly-metallic metallogenic belt.In this paper,we have integrated all published geochronological,element geochemical,Sr-Nd-Hf isotope data for the Middle to Late Triassic intermediate-felsic intrusive rocks in the Yidun terrane,along with the reported data from the Songpan-Ganzi terrane,in order to further decipher their origin,petrogenesis and rock-forming tectonic setting.All precisely dating results of zircons from the Middle to Late Triassic diotitic and granitic rocks in the Yidun terrane show that the magmatism developed during 225-215 Ma,with a peak age of～216 Ma.Dioritic porphyries from southern Yidun terrane characterized by high Mg#,Cr,Ni and Sr contents and Sr/Y ratios,but low Y and Yb contents,together with weak Eu anomalies and positive Sr anomalies,are geochemically similar to adakities.In consideration of their high K2O/Na2O ratios,and crustal-type Sr-Nd-Hf isotopic compositions,and in comparison of the chemical compositions of coeval potassic adakitic rocks in the Jiulong area (on the eastern side of the suture zone),it is clearly seen that these dioritic intrusive rocks were derived from remelting of the thickened lower crust rather than from the subducting of oceanic slab or sub-arc metasomatized lithospheric mantle as suggested before.Different from the coeval dioritic rocks,the granites show higher S iO2 (67.33％-78.01％) and K2O (2.38％-6.41％) contents,and strongly negative Sr-Eu anomalies,along with their more enriched whole-rock Sr-Nd and zircon Hf isotopic compositions,indicating the granites are not a product of crystallization differentiation of dioritic magma.In combination with the unpublished data for the basalts in this region,we propose that these granites should be derived from partial melting of the mid-Proterozoic rocks at a middle to upper crustal level.In comparison with the elemental and geochemical natures coupled with the pace-time patterns of coeval magmatism in the Songpan-Ganzi terrane to the east of the Ganzi-Litang suture zone,a most feasible explanantion for the coeval magmatism is that they were generated in a common post-collisional extentional setting associated with the partial lithospheric delamination.Therefore,the occurrence of the Late Triassic post-collisional magmatism clearly indicates that the arc/continent-continent collision (between the Yidun and Songpan-Ganzi terranes) or the final closure of the Ganzi-Litang ocean had been completed at least by the Middle-Late Triassic.The source region of the dioritic porphyries related to the copper polymetallic deposits from southern Yidun terrane may not have the characteristics of high oxygen fugacity and rich fluid.