漠河地区黑云母花岗闪长岩地球化学、Hf同位素特征及其成因

Geochemistry,Hf Isotopes and Petrogenesis of Biotite Granodiorites in the Mohe Area

以往学者的研究多集中在印支期出露于漠河县城南的黑云母花岗闪长岩,而对该地区燕山早期构造演化的研究相对薄弱.运用LA-ICP-MS锆石U-Pb年代学、全岩地球化学与Hf同位素分析的方法确定其形成时代、岩浆源区性质及成岩构造背景.结果表明:该岩石的加权平均年龄分别为185±2Ma和182±2Ma,表明其形成于早侏罗世;岩石属于高钾钙碱性系列,A/CNK介于0.90~1.03,Mg#值为42~48,具有高Sr(489×10-6~653×10-6)低Yb(1.33×10-6~1.99×10-6)的特征,判定其属于埃达克岩类;岩石具有弧岩浆的微量元素特征,轻重稀土元素分馏明显((La/Yb)N=8.36~15.6),较弱的Eu负异常(Eu/Eu*=0.79~0.95),富集Rb、K等大离子亲石元素,明显亏损Nb、Ta、Ti等高场强元素;岩石的εHf(t)值为-3.26^-1.46,二阶段模式年龄介于1.25~1.59Ga,结合该时期的地幔特征认为该岩石岩浆起源于中元古代亏损地幔新增生的下地壳部分熔融.综合认为岩石形成于蒙古-鄂霍茨克洋板块向南俯冲的活动大陆边缘环境.

Previous studies are mostly concentrated on the biotite granodiorites exposed during the Indosinian period in southern Mohe county,while the research on the Early Yanshanian tectonic evolution in this area is relatively weak.This paper presents new zircon U-Pb dating,Hf isotope,major and trace elements for biotite granodiorites in southern Mohe County for the discussion in their geochronology,magma sources and tectonic setting.Zircon dating results demonstrate that biotite granodiorites were formed in Early Jurassic（185±2 Ma and 182±2 Ma）.Geochemically,these rocks have A/CNK=0.90~1.03,Mg#=42~48,belonging to the high-K calc-alkaline series.The pluton has features similar to adakitic rock with high Sr（489×10-6~653×10-6）and low Yb（1.33×10-6~1.99×10-6）.They have the geochemistry of arc magmatic rocks,which are enriched LREE and LILE（e.g.,Rb and K）,depleted HREE and HFSE（e.g.,Nb,Ta and Ti）,as well as very weak negative Eu anomalies（Eu/Eu＊=0.79~0.95）,indicating that the magma source was derived from the partial melting of lower crust.ZirconεHf（t）values and two-stage Hf model ages（tDM2）of doitite granodoirites range from-3.26 to-1.46 and 1.25 Ga to 1.59 Ga,respectively,indicative of formation from primary magmas generated by partial melting of thickened low crust that formedfrom the depleted mantle during the Meso-to Neoproterozoic.Considering the regional tectonic revolution,we suggest that the diotite granodiorite was formed in the setting of continuous southward subduction of Mongol-Okhotsk oceanic plate during the Early Mesozoic.