冀东曹庄群钙长石-钙榴石岩的年代学研究及其地质意义

Geochronology of anorthite-garnetite in Caozhuang Group of eastern Hebei and its geological significance

位于华北克拉通的冀东曹庄群是中国最古老的变质火山-沉积岩系,由于后期多次强烈的岩浆-构造事件影响,使得曹庄群的原始产状和原岩特征被强烈改造,导致对曹庄群的认识不清。精细刻画后期地质事件、评估其对原岩改造程度,是解决争议的基础。前人报导曹庄群发育有钙长石-钙榴石岩。本工作继续对该岩性开展地质学、岩相学、地球化学和U-Pb年代学研究,查明斜长石中的钙长石端元比例≥90%、石榴子石属铁铝榴石(14~31)-钙铝榴石(23~47)-钙铁榴石(23~61)固溶体、绿帘石将石榴子石和斜长石两种矿物分隔而开。钙铝-钙铁榴石为均质、全消光、无成分环带、无异常光性特征,亏损轻稀土(LREEs)、富集中稀土(MREEs)、略亏损重稀土(HREEs)、具Eu的负异常,地球化学特征显示钙铝-钙铁榴石是在稳定、封闭,由一期/阶段热液流体,在W/R(水/岩)比值较低的情况下,通过扩散交代作用缓慢结晶而成,并非前人所认为的区域变质作用成因。利用钙质榴石的地球化学特征,进一步反演交代流体的物化性质为弱氧化、中性-弱碱性、富Ca^(2+)、富Cl^(-)、温度在250~500℃。通过钙铝-钙铁榴石原位LA-ICP-MS U-Pb年代学工作,得到不一致曲线的上交点年龄为2529±35Ma(MSWD=1.4,n=35),作为其形成年龄,并与共生热液锆石的SIMS U-Pb年龄在误差范围内一致,推测该期流体交代作用发生于~2530Ma,与研究区内新太古代末(~2.5Ga)经历的区域性麻粒岩相-角闪岩相变质作用时代相近。综合前人工作,初步推断此次交代作用对曹庄群部分岩石Sm-Nd体系的改造程度大于近同时发生的区域变质作用。

Located in eastern Hebei area of the North China Craton,the Caozhuang Group has been considered as the oldest metamorphic volcanic-sedimentary rock series in China.However,multi-period strong magmatic-tectonic events have greatly modified the primary occurrences and protolith characteristics of the Caozhuang Group,leading to the controversial understandings about it.The basement for resolving disputes is to carefully depict the geological events in the later stage and evaluate the degree of the protolith transformation.Anorthite-garnetites were found in the Caozhuang Group by previous researcher.This study has been continued to carry out the field geology,petrography,geochemistry and geochronology researches on the rocks,and identified that the molecular weight of anorthite in the plagioclase is more than 90%,the garnet belonging to the solid solution of almandine(14~31)-grossular(23~47)-andradite(23~61),and the epidote separating the aforementioned two kinds of minerals.The grossular-andradite solution is homogeneous,full extinction,no composition ring,no abnormal optical characteristics,LREEs-depleted,MREEs-enriched,HREEs-slightly depleted,and has a negative anomaly of Eu.Its geochemical characteristics show that grossular-andradite is formed in the stable and closed environment by one phase/stage hydrothermal fluid and slowly crystallized by diffusion metasomatism under the condition of low W/R(water/rock),and not the cause of regional metamorphism as previously thought.Based on the geochemical characteristics of grossular-andradite,the physical and chemical properties of metasomatic fluid are weakly oxidic,neutrally to weakly alkaline,rich in Ca^(2+),Cl^(-)and its temperature between 250~500℃.Through the in-situ LA-ICP-MS U-Pb geochronology of grossular-andradite,we obtain an upper intercept age in the concordance curve of 2529±35Ma(MSWD=1.4,n=35)as its formation age.Since it is consistent with the SIMS U-Pb age of coexistent hydrothermal zircons,it is estimated that the hydrothermal metasomatism occurred at~2530Ma,which is close to the regional granulite-amphibolite facies metamorphism experienced at the end of Neoarchean(~2.5Ga)in the study area.Based on previous work,it is preliminarily inferred that this metasomatism event has transformed the Sm-Nd system of some rocks in the Caozhuang Group to a greater extent than the regional metamorphism that occurred almost simultaneously.