胶-辽-吉带北段集安群中古元古代变质辉长岩侵入体成因研究及其地球动力学演化意义

Petrogenesis of Late Paleoproterozoic metamorphic gabbroic intrusions of Ji an Group in the northern segment of Jiao-Liao-Ji Belt and their geodynamic evolution significance

胶-辽-吉造山带北段辽河群和集安群中的古元古代镁铁质侵入体成因及其指示的地球动力学背景一直存在较大争议。原因之一在于研究资料多集中于辽河群中出露的镁铁质侵入体,而对集安群中的镁铁质侵入体报道较少,制约了对胶-辽-吉造山带地球动力学演化的整体性认识。因此,本文对集安群中镁铁质侵入体进行详细的野外实地调查、地球化学、年代学和同位素研究,讨论这些岩石成因,揭示地幔源区性质并制约地球动力学背景。锆石LA-ICP-MS U-Pb定年结果显示这些变质辉长岩侵入体变质时代介于1866~1871Ma,为古元古代晚期的产物;一粒锆石核部207 Pb/206 Pb年龄为2139±14Ma,可能代表了原岩的结晶时代。样品属于亚碱性系列,并具有向富铁成分演化的拉斑系列特征。岩石富集大离子亲石元素(如Ba、Rb、Sr和K),亏损高场强元素(如Nb、Ta和Ti);这些特征与典型岛弧玄武岩相似,指示了它们来自俯冲流体交代的地幔源区。结合样品2.07~2.45Ga的一阶段Hf同位素模式年龄,认为交代作用发生在古元古代早期。本次研究的集安群中变质辉长岩侵入体与前人研究的辽河群中镁铁质岩石在结晶变质时代和地球化学特点方面具有很多相似性:(1)同属于亚碱性系列并具有向富铁成分演化的趋势;(2)相似的微量和稀土元素配分曲线;(3)具有高场强元素亏损和大离子亲石元素富集这一“俯冲流体交代信号”;(4)轻度富集的Sr-Nd-Hf同位素和古元古代早期的一阶段Hf同位素模式年龄;(5)一致的结晶年龄(2139~2099Ma)与变质时代(1879~1852Ma)。这些相似性指示了集安群和辽河群在古元古代早期可能经历了相同的岩浆-构造-变质事件。镁铁质岩石应来源于古元古代早期经历了俯冲流体交代的地幔源区。在构造环境判别图解上,集安群和辽河群中镁铁质岩石大部分样品都落入弧后盆地环境中。结合同期火成岩组合和演化特点,认为胶-辽-吉造山带东北段在古元古代早期处于弧后盆地背景中,古元古代晚期则处于造山后伸展的地球动力学背景下。

There has been considerable controversy over the genesis of the Late Paleoproterozoic mafic intrusions in the northern segment of Jiao-Liao-Ji Belt and the geodynamic background they indicate.One reason for this is that most research has focused on mafic intrusions exposed in the Liaohe Group,with less attention given to those in the Ji an Group,making it difficult to form a holistic understanding of the geodynamic evolution of the Jiao-Liao-Ji Belt.Therefore,this study carried out detailed field investigations,geochemistry,geochronology,and isotope research on the mafic intrusios in the Ji an Group,discussing their origin,revealing the nature of the mantle source region,and constraining the geodynamic background.Zircon LA-ICP-MS U-Pb dating results show that the metamorphic age of these metamorphic gabbroic intrusions ranges from 1866Ma to 1871Ma,representing products of the late Paleoproterozoic;a single zircon core 207 Pb/206 Pb age of 2139±14Ma probably represent the crystallization age of the protolith.The samples belong to the subalkaline series and have the characteristics of a tholeiitic series evolving towards iron enrichment.The rocks are enriched in large-ion lithophile elements(e.g.,Ba,Rb,Sr,and K)and depleted in high-field-strength elements(e.g.,Nb,Ta,and Ti);these features are similar to typical island arc basalts,indicating that they originated from a mantle source region metasomatized by subduction fluids.Combined with the 2.07~2.45Ga first-stage Hf isotope model ages of the samples,it is believed that the metasomatic process occurred in the Early Paleoproterozoic.The metamorphic gabbroic intrusions in the Ji an Group studied here have many similarities with the mafic rocks in the Liaohe Group:(1)both belong to the subalkaline series and have a trend of evolving towards iron enrichment;(2)similar trace and rare earth element distribution curves;(3)depletion of high-field-strength elements and enrichment of large-ion lithophile elements,a“subduction fluid metasomatism signal”;(4)slightly enriched Sr-Nd-Hf isotopes and Early Paleoproterozoic first-stage Hf isotope model ages;(5)consistent crystallization ages(2139~2099Ma)and metamorphic ages(1879~1852Ma).These similarities suggest that the Ji an Group and Liaohe Group may have experienced the same magmatic-tectonic-metamorphic event in the Early Paleoproterozoic.The mafic rocks should be derived from the same mantle source region that experienced subduction metasomatism in the Early Paleoproterozoic.On tectonic discrimination diagrams,most samples of mafic rocks from the Ji an Group and Liaohe Group fall into the arc-back basin environment.Considering the igneous rock assemblages and their evolution during the same period,it is suggested that the northeastern segment of the Jiao-Liao-Ji Belt was in an arc-back basin setting in the Early Paleoproterozoic,and in a post-orogenic extensional geodynamic background in the Late Paleoproterozoic.