Age And Provenance Of The Late Paleozoic Fuxingtun Formation In Zhangguangcai Range:Implications For Regional Tectonic Evolution

In this paper,petrography,zircon LA-ICP-MS U-Pb chronology and petrogeochemistry of the late Paleozoic Fuxingtun Formation in Zhangguangcai Range have been studied.The sedimentary time of the Fuxingtun Formation is redetermined,and the provenance analysis and tectonic setting discrimination are carried out,which provid new evidences for the combination of Songnen-Zhangguangcai Range Massif and Jiamusi Massif in the late early Paleozoic.Studies on U-Pb chronology of zircons from the Fuxingtun Formation show that most of the rhyolitic crystalline tuff zircons collected in the middle and lower part of the standard section are subhedral-euhedral crystal,with clear internal structure and typical magmatic oscillating growth rings.The weighted average age of the 30 test points is391.6±3.2Ma.The detrital zircons of the fine-grained arkose collected in the middle of the standard section are mostly stumpy or rounded,with subhedral-euhedral crystal,typical oscillatory growth rings are developed at the edges,reflecting the characteristics of magmatic zircons.The 42 detrital zircons yield age populations of392.7,422,510.9,789,1711,1768,1833,1912,1964 and 2422Ma,the minimum age represents the lower limit of arkose sedimentary age,implying that the sedimentation of arkose could take place after 392.7 Ma.In conclusion,the age results of tuff and arkose are consistent with each other,it can be concluded that the formation age of the Fuxingtun Formation is the middle Devonian.The petrogeochemistry of fresh fine-grained arkose samples from the Fuxingtun Formation are studied.The geochemical indexes of CIA,ICV and PIA are 53.72⁵6.62,1.29¹.50 and 54.40⁵7.46,respectively,the Th/U and Rb/Sr average ratios were 3.83and 0.18.The characteristics of major elements and trace elements indicate that the maturity of the fine-grained arkose from the Fuxingtun Formation is low,the materials in the source area experience weak weathering and relatively simple sedimentary cycle.The Al₂O₃/TiO₂ values of fine-grained arkose samples from the Fuxingtun Formation are 11.73²2.91,with characteristics of light rare earth enrichment,heavy rare earth stability and negative Eu anomaly.The characteristics of major elements and rare earth elements as well as the discriminant diagram of source area indicate that the sediments of the Fuxingtun Formation are mainly derived from the source area of felsic or intermediate-acid igneous rocks.Based on the age distribution of detrital zircons and characteristics of petrogeochemistry of fine-grained arkose from the Fuxingtun Formation,it is concluded that the sediments of the Fuxingtun Formation in Zhangguangcai Range have characteristics of mixture source area.Combining with the regional geological bodies and volcanic events,to determine that the sedimentary provenance of the Fuxingtun Formation is mainly derived from felsic or intermediate-acid igneous rocks of the early late Paleozoic around the sedimentary basin and the early Paleozoic granitic rocks distributedinthesouth-northdirectionoftheeasternmarginofthe Songnen-Zhangguangcai Range Massif,as well as the Paleoproterozoic geologic bodies from the surface or surface shallower.Secondary provenance is provided by the Neoproterozoic felsic or intermediate-acid igneous rocks around the sedimentary basin.The tectonic setting of the Fuxingtun Formation in the Middle Devonian is discriminated.The petrogeochemistry of the fine-grained arkose from the Fuxingtun Formation indicates that the sediments of the Fuxingtun Formation are formed in continental island arc tectonic environment.The type A rhyolite from the Baoquan Formation of the Middle Devonian in Lesser Xing'an Range suggests that the Songnen-Zhangguangcai Range Massif was an intracontinental extensional environment at that time.In combination with Songnen-Zhangguangcai Range Massif collided with Jiamusi Massif in the late early Paleozoic,and the Songnen-Zhangguangcai Range Massif is extensional environment after land-land collision in the middle Devonian,it can be determined that the sediments of the Fuxingtun Formation were formed in backarc extensional environment of the continental island arc.The U-Pb chronology of detrital zircons from the Fuxingtun Formation in Zhangguangcai Range also provides new evidences for the merging of the Songnen-Zhangguangcai Range Massif and the Jiamusi Massif in the late early Paleozoic.The sediments of the early Devonian Heilonggong Formation,the middle Devonian Fuxingtun Formation in Zhangguangcai Range,the early-middle Devonian Baoquan Formation in Lesser Xing'an Range and the Heitai Formation from Baoqing Basin in the eastern margin of the Jiamusi Massif are all rich in 489-551Ma detrital zircons.The detrital zircons are consistent with the ages of the Mashan Complex in Jiamusi Massif and the early Paleozoic granite which widely distributed.It shows that the Jiamusi Massif provides part of provenance for the formation of the early late Paleozoic strata in the Songnen-Zhangguangcai Range Massif,and suggests that the Jiamusi Massif is integrated with the Songnen-Zhangguangcai Range Massif before the deposition of the Heilonggong Formation,the Fuxingtun Formation and the Baoquan Formation,which is completed in the early Devonian.