周口店官地杂岩中斜长角闪岩的年代学、地球化学及其地质意义

Geochronology and Geochemical Characteristics of Amphibolite in Guandi Complex,Zhoukoudian Area and Its Geological Significance

周口店房山岩体南北两侧的官地杂岩主要由长英质片麻岩、斜长角闪岩和混合岩组成。笔者首次对东岭子官地杂岩中斜长角闪岩进行研究,斜长角闪岩LA-ICP-MS锆石U-Pb测年上交点年龄为（2 551±37）Ma,代表岩浆结晶年龄。其地球化学显示具有低TiO_2,高MgO的特点,Mg#平均值为70,LREE与HREE分异度高,具有Eu的负异常。地球化学特征和岩相学揭示其岩浆经历分离结晶作用,原岩矿物主要由辉石、斜长石组成。锆石Hf同位素ε_（Hf）（t）处于亏损地幔演化线和球粒陨石之间示踪源区为岩石圈富集地幔,微量元素方面富集LILE（Rb,Ba,K）,亏损HFSE（Nb,Ta,Zr）的特点显示岛弧有关的富集地幔源区。初步认为大洋板片向东俯冲于增生岛弧和东部陆块之下,镁铁质成岩物质来源于大洋板片重熔流体交代的岩石圈地幔楔,同时,对于华北地块新太古宙末期大陆地壳生长的时间提供约束。

The Guandi complex outcrops on both the north and south sides of the Fangshan pluton,being composed offelsic gneiss,plagioclase amphibolite and migmatite.The amphibolites are dispersed throughout the gneiss in Guandi complex.Metamorphic zircons from the amphibolite dike yield a LA-ICP-MS U-Pb upper intercept age of（2551±37）Ma,which have been interpreted as the crystallization age.According to the geochemical results,these amphibolites are characterized by low TiO_2 and high Mg~# contents,with Mg-number of 70.These amphibolites display the enriched LREE,depleted HREE pattern with pronounced negative Eu anomaly.Element variation diagrams and petrography characteristics indicate that the protolith consists of clinopyroxene and plagioclase_（Hf） isotopic compositions indicate that they are sourced from an enriched lithospheric mantle.Data of trace elements illustrates that these amphibolitic dikes arecharacterized by enriched LILE（Rb,Ba,K）,and have depleted HFSE patterns with pronounced negative Nb,Ta and Zr anomalies,which are consistent withan arc-related enriched mantle source region.Therefore,it＇s proposed that the amphiboliticdikes were formed during the eastdipping subduction.The enriched mantle signatures are consistent with the metasomatization of melts and fluids derived from the subducted slab.Combining previous research results,these results help elucidate important components about the crustal growth of the North China Block during the Neoarchean.