Geochronology And Geochemistry Features Of Mafic-ultramafic Rocks From Yongji Area In Jilin Province

This paper mainly discussed the mafic-ultramafic rocks in Yongji district at Jilin province, by the use of zircon LA-ICP-MS U-Pb dating technique, identified its formation age; Through the study of the geochemistry characteristics of the major elements and the trace element of the mafic-ultramafic rocks, discussed its magma source, clarified its tectonic setting .CL images of zircons show that the zircons studied in this paper was mostly subhedral, a small number of to break column, have clear internal structure, typical developed magma concussion growth zoning or with a streak-like absorption features, combined with its high Th/U ratios (0.1?1.8) , suggesting they are magmatic zircons. Combined with the feature of rock under the Microphotographs and zircon LA-ICP-MS U-Pb dating results indicate that Toudaogou metadiorite's formation age was 474.4±2.9Ma,Xiaochengzi metadiabase's formation of age was 469.4±5.4Ma, Toudaogou ultromafic complexes's formation age was 468.7±4.0Ma, Heishizui ultromafic complexes's formation age was 469.3±2.5Ma. Indicating that the mafic-ultramafic complexes in the study area may be formed in Early-Middle Ordovician, proved that there are Early-Middle Ordovician magmatic events in central jilin Province.Metamorphic volcanic rocks in this paper having relatively enrichedLILE and LREE,deficit Nb, Ta, Ti etc. HSFE features, and geochemical characteristics of Eu anomaly weak.Ultromafic rocks having relatively enriched LILE,such as Th,U,Pb etc. deficit Sr, K, P, Ti etc. HSFE features and geochemical characteristics of Eu anomaly weak.Based on the mafic-ultramafic complexes geochemical analysis,combined with various tectonic environment and provenance analysis diagram, metadiorite and metadiabase from Toudaogou area should be the product of partial melting on the progress of subducting plate melt metasomatic mantle wedge, and there are some crustal contamination.The mafic-ultramafic complexes may be derived from the depleted mantle. Combined with the tectonic environment analysis,mafic-ultramafic rocks formed in the active continental margin environment of back-arc basin.The early Paleozoic in the study area under Paleo-Asian Ocean subduction,has the characteristics of the active continental margin.