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
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 1871 Ma,representing products of the late Paleoproterozoic;a single zircon core 207Pb/206Pb 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.
万方数据
维普
