青藏高原西北缘第四纪火山岩的年龄、地球化学特征及意义

Age and geochemical characteristics of Quaternary volcanic rocks in the northwestern margin of Tibetan Plateau and their significance

对青藏高原西北缘普鲁北钾质火山岩进行40Ar-39Ar定年得到的最新年龄为0.81Ma±0.21Ma。火山岩的SiO2含量为53.08%～55.12%,全碱含量为7.36%～8.04%,K2O/Na2O比值为1.17～1.2,属橄榄安粗岩系列。岩石以高度富集LREE、LILE(K、Rb、Sr、Ba、Th等)和高度亏损HREE、HFSE(Nb、Ta、Ti等)元素为特征。岩石具高的(87Sr/86Sr)i比值(0.7088～0.7089)和低的εNd值(-6.05～-5.54),表明火山岩源区为EMⅡ型的经古俯冲作用改造的大陆岩石圈富集地幔。同时,其Ti/Y、Zr/Y、La/Rb、Zr/Rb、K/La、Pb/La比值类似于板内洋岛玄武岩(OIB),可能暗示岩浆源区经历过软流圈流体或者深部流体对古俯冲地幔楔的叠加交代作用。普鲁北钾质火山岩为青藏高原西北缘晚新生代陆-陆碰撞造山深部岩石圈拆沉作用在地表响应的岩浆产物。

A whole-rock ^40Ar-^39Ar age of 0.81±0.21Ma was obtained for the North Pulu potassic volcanic rocks in the northwestern margin of Tibetan Plateau.The rocks belong to shoshonite series and are mainly composed of shoshonite and trachyandesite.Geochemically,they have coherent content of SiO2 53.08%～55.12%,total alkali 7.36%～8.04% and K2O/Na2O ratios 1.17～1.2.They are significantly enriched in LREE and LILE（K,Rb,Sr,Ba,Th etc.） and remarkably depleted in HREE and HFSE（Nb,Ta,Ti etc.）.Elemental geochemistry,combined with high（^87Sr/^86Sr）i ratios（0.7088～0.7089） and low ｜εNd ratios（-6.05～-5.54）,indicates that the potassic volcanic rocks were most likely derived from EM type enriched continental lithospheric mantle that had been modified by ancient oceanic subduction.Nevertheless,the ratios of Ti/Y,Zr/Y,La/Rb,Zr/Rb,K/La and Pb/La of the potassic volcanic rocks are comparable with those of OIB（ocean island basalt）,suggesting that the magma source might have undergone the process of metasomatism of asthenosphere fluid or the deep fluid overlapped the ancient subduction mantle wedge.The North Pulu potassic volcanic rocks might have been the product that responded to the delamination of the mantle lithosphere beneath the northwestern margin of the Tibetan Plateau during the Late Cenozoic continent-continent collision orogeny.