延吉地区古新世埃达克岩成因的岩浆混合证据

Evidence for Magma Mixing in Generation of Paleocene Adakites from the Yanji Area,NE China

本文通过年代学、矿物学、主量元素、微量元素和同位素的研究,指出延吉地区古新世富镁埃达克岩起源于被俯冲板片熔体交代的地幔楔。单斜辉石和角闪石斑晶所显示的反环带结构为幔源富镁埃达克熔体于壳源中酸性岩浆混合的结果,而非前人普遍认为的熔体—地幔的反应产物;其地球化学特征表现出的岩浆演化趋势可以很好地用地壳混染/AFC过程来解释。延吉古新世埃达克岩形成于俯冲后岩石圈伸展作用背景,与太平洋板块的俯冲后撤作用有关。因此,我们认为富镁埃达克岩的形成不必强调同期洋脊的俯冲作用,只要早先或同时有俯冲板片熔体交代,在合适的p-t条件下交代地幔并发生熔融作用,就能形成这一类岩石。

A comprehensive study（including Ar-Ar,zircon U-Pb dating,mineralogy,major,trace element and Sr-Nd-Pb-Hf isotope analyses） on the Paleocene adakites from the Yanji area,NE China is presented in this paper.These adakites have high MgO,Cr and Ni,and extremely high Sr and low Y and HREE.They were considered as melts derived from a slab melt-metasomatized mantle wedge.During ascent they experienced mixing by crustally derived intermediate-felsic magma to interpret the reversely zoned clinopyroxene and amphibole.Our interpretation is quite different from the previous model-a result of slab melt-mantle interaction.This mixed magma further underwent crustal contamination and/or AFC processes to account for the systematic geochemical variations from the clinopyroxene to amphibole andesites,as well as the inherited zircon.Considering the regional tectonic evolution,the Yanji adakites were formed under a post-subduction extension,possibly related to the late Cretaceous ridge subduction and subsequent rollback of the Pacific Plate.This implies that such rocks could be formed in a variety of settings,where adakitic melt metasomatism occurs and the p-t conditions permit melting of the metasomatized mantle.Generation of such rocks can be either contemporaneous with or after the young and hot ridge subduction.