延吉地区古新世埃达克岩捕获锆石U-Pb年龄、Hf同位素和微量元素地球化学对区域中酸性岩浆演化的指示

U-Pb ages,Hf isotope and trace element compositions of captured zircons of the Paleocene adakites in the Yanji area,NE China:Implications for magmatic evolution of intermediate-felsic magmas

延吉地区古新世埃达克岩石中出现了大量的捕获锆石,阴极发光图像显示这些锆石主要来源于围岩中酸性侵入岩,其U-Pb年龄变化在87～387Ma之间,ε_(Hf)(t)变化在-2.1～14.7之间,T_(DM)(Hf)为272～895Ma。原位离子探针微量元素分析显示,大部分锆石具有相对含石榴子石变质岩高Y、低Hf,在REE配分模式为LREE亏损、HREE富集、正Ce异常和负Eu异常,说明这些锆石的生长环境为与斜长石共存而缺乏石榴子石的体系;另外两颗锆石的Eu负异常不明显,说明它们结晶于缺少共生斜长石的体系。随着锆石结晶时间的变新,其ε_(Hf)(t)逐渐降低和T_(DM)(Hf)增加,说明年青地壳组分对延吉地区的显生宙中酸性侵入岩熔融源区的贡献减少,中生代以来,尤其是区域岩浆活动峰期时间(早白垩世)地壳增生较弱;此外,锆石的U-Pb年龄与Y、Th和U显示出正相关关系,而与Eu/Eu*和Ce/Yb_(CN)表现出负相关关系,反映区域中酸性岩浆熔融源区或岩浆分异过程中斜长石的贡献减小,而熔融源区残留石榴子石的贡献有增加趋势。

Abstract Zircons captured by Paleocene adakites in the Yanji area, NE China span an apparent ^206rpb/^238U age range of 87 ~ 387Ma, an БHf（t） range of -2. 1~ 14.7, and an TDM （Hf） range of 272 ~ 895Ma. Cathodoluminescence（CL） images suggest a magmatic origin for most of them. In situ ims-5f analytical results of these zircons show that relative to those in garnet-beating metamorphic rocks, they have high Y and low Hf, characterized by considerable LREE depletion, HREE enrichment, and positive Ce and negative Eu anomalies in the chondrite-normalized REE patterns. Such trace element features reflect that the zircons crystallized with coexisting plagioclase in garnet-free magmatic systems. The rest two grains that have negligible Eu anomalies were formed in a plagioclase-out system either crystallizing in a high-pressure crustal magma chamber or overgrowing in a garnet-in metamorphism. Following the youthening of the U-Pb age, these captured zircons show decrease of БHf（t） and increase of TDM （Hf）, suggesting that the contribution of juvenile crust component to the magmatic evolution became less, and thus the coeval crustal growth during the peak （early Cretaceous） magmatic activities was weak in the Yanji area and its surrounding regions. In addition, lowering in Y, Th and U, and increase of Eu/Eu＊ , Ce/YbCN are also followed by decrease of zircon U-Pb ages, indicating that the contribution of plagioclase （either as a fractional phase or as a residual mineral during melting ） to the parent intermediate-felsic magmas decreased, whereas residual garnet in the melting source became more important following the regional tectono-magmatic evolution.