延边地区中生代火山岩的岩浆源区:来自Sr-Nd同位素和深源捕虏体(晶)的证据

延边地区中侏罗世和早白垩世火山岩中包含有较多的辉石岩捕虏体和角闪石捕虏晶,前者为普通辉石变种,其化学成分类似于中国东部新生代玄武岩中单斜辉石巨晶的成分,具有岩浆堆积成因特征;后者为韭闪石和镁质绿钠闪石变种,其成分类似于中生代晚期玄武岩中角闪二辉石岩包体里的角闪石和新生代玄武岩中的角闪石巨晶.矿物温压计算结果显示,它们形成深度介于25～37km.延边地区中生代火山岩的(87Sr/86Sr)i值介于0.704 3～0.705 0,εNd(t)值介于2.33～4.71,表明岩浆源区应是一套具有亏损性质的新增生的地幔物质.综合上述结果,可以判定延边地区中生代火山岩的原始岩浆应来源于新增生的壳幔过渡带物质的部分熔融,地壳增生事件的时间为新元古代.

The Magma Source of Mesozoic Volcanic Rocks in Yanbian Area:Evidence from Sr- Nd Isotopes and Deep-Seated Xenoliths(Xenocrysts)

The pyroxenite xenoliths and amphibole xenocrysts occur in Middle Jurassic and Early Cretaceous volcanic rocks from Yanbian area. The pyroxenite xenoliths, showing cumulate texture, are composed of augites. The augites are similar to those of clinopyroxene (Cpx) megacrysts in Cenozoic basalts from Eastern China in chemical composition. The amphibole xenocrysts are pargasite and magnesium-hastingsite which are similar to those in the amphibole-websterite xenoliths from Late Mesozoic basalt. The PT estimated results show that the pyroxenite xenoliths and amphibole xenocrysts could be from the depth of 25~37 km. The (~(87)Sr/~(86)Sr)_(i) ratios and _(Nd)(t) values of Mesozoic volcanic rocks in Yanbian area are 0.7043~0.7050 and 2.33~4.71, respectively, which implies the magma source could be composed of a suite of the depleted and newly accreted mantle material. On the basis of above-mentioned characteristics, it is suggested that the primary magma of Mesozoic volcanic rocks in Yanbian area could be derived from the partial melting of newly accreted material in the crust-mantle transitional zone. The accretion event could take place in the Neoproterozoic.