Geochronology,geochemistry And Petrogenesis Of Early Paleozoic Magmatic Rocks In Faku Area,Northern Liaoning

Laterally,the Faku area in the study area is located between the Yilan-Yitong fault and the Hongshan-Balihan fault,which is just in the middle of the traditional Bainaimiao arc belt and the continental margin accretionary belt in Northeast China.Longitudinally,it is located on the southernmost edge of the continental margin accretionary belt of the eastern segment of the northern margin of the North China Plate,and is close to the Chifeng-Kaiyuan fault,which divides the North China Craton and the Central Asian orogenic belt.Therefore,the study of the Early Paleozoic magmatic and tectonic activities in the study area can provide important evidence for the tectonic evolution of the Early Paleozoic in the area.Based on the chronology,geochemistry and Lu-Hf isotopic analysis of the Early Paleozoic magmatic rocks in the northwest of Faku area,this paper discusses the tectonic evolution of the continental margin accretionary belt in the eastern segment of the northern margin of the North China Plate in the Early Paleozoic.(1)The Bachagou granodiorite was formed in the Early Silurian with a weighted mean age of 433±4Ma.The content of Si O_2.is 56.66%-63.66%.The content of Al₂O₃is 15.35%-17.30%.Na₂O/K₂O>1.The Rocks are enriched in Sr and depleted in Yb and Y.The samples have a weak positive Eu anomaly(?Eu=0.98-1.21),with high Sr/Y=4.62-55.13 and La/Yb=11.9-20.2,which are typical adakitic rocks formed by partial melting of subducted oceanic plates.(2)The weighted average age of the Laolingshan granodiorite is 425±3Ma,and it formed in the Late Silurian.The rock is S-type granite,the content of SiO₂is65.65%-68.66%,A/CNK=1.35-1.71,belonging to peraluminous series.As a whole,The REE of the sample is right-leaning,enriched in light rare earth elements(LREE)and depleted in heavy rare earth elements(HREE).It is characterized by weak positive Eu anomaly(?Eu=0.98-1.21),enrichment of large ion lithophile elements(LILE)Cs,Ba,K and Sr,and depletion of high field strength elements(HFSE)P,Nb and Ti.The?_Hf(t)values are positive(+0.5-+8.1),and the two-stage model age T_DM2=1096-1763Ma,indicating that it derived from the partial melting of the Paleo-Mesoproterozoic juvenile crust materials.(3)The weighted average age of Donggou monzogranite is 423±4Ma.The content of SiO₂was 75.11%-77.57%.Na₂O/K₂O=3.03-4.58,A/CNK=1.07-1.15,belonging to A-type granite,formed in the extensional environment induced by slap breakoff.The high negative Eu anomalies(?Eu=0.37-0.44)and the depletion of HFSE(Th,Nb,P and Ti)indicate the fractional crystallization of plagioclase,apatite and Fe-Ti oxides.The?_Hf(t)values range from+0.1 to+3.1,and the two-stage model age T_DM2=1551-1801Ma,indicating that it originated from the partial melting of the Paleo-Mesoproterozoic juvenile crust,and was accompanied by variety degrees of differentiation during the formation process.(4)The Early Silurian Bachagou granodiorite has the characteristics of Andean-type active continental margin,the Late Silurian Laolingshan granodiorite was formed in the arc-continental collision stage,and the Late Silurian Donggou monzonite was formed under the extensional environment generated by slab breakoff.Combined with the regional magmatic-tectonic events of the Early Paleozoic,we believe that the eastern segment of the northern margin of the North China Plate was influenced by the southward subduction of the Paleo-Asian Ocean in the Early Cambrian,and it turned into an Andean-type active continental margin.Meanwhile there was an intra-oceanic arc in the north of the North China Plate(500?430Ma).In the Late Silurian(430?410Ma),the intra-oceanic arc began to collage on the northern margin of the North China Plate in the arc-continent collision stage.The orogenic event in the Early Paleozoic entered the late stage.