Origin And Formation Background Of The Volcanic-Sedimentary Rocks Of The Hangaoshan Group In LüLiang Area,Shanxi Province

In recent years,volcanic-sedimentary rocks of the Xiong'er Group in the southern margin of the North China Craton have been identified as the first set of volcanic sedimentary cover after basement consolidation of the North China Craton.However,there are still different opinions about their origin and tectonic setting.The volcanic-sedimentary rocks of the Hangaoshan Group and the Xiaoliangling Formation in the Lüliang area of the North China Craton are considered to be the records of the volcanic sedimentary formations of the Xiong'er Group exposed in the Lüliang area of the North China Craton in the same period.The study of the origin and formation environment of these volcanic sedimentary formations will help us to understand the tectonic setting and regional distribution of the first set of cover after the consolidation of the North China Craton.The Hangaoshan Group is composed of a large number of clastic sedimentary rocks and a amount of volcanic rocks.This paper provides new evidence to discuss their genesis and tectonic setting from the sedimentary environment and the origin of volcanic rocks.According to the rock assemblage and sedimentary characteristics,the volcanic sedimentary structure of the Hangaoshan Group can be divided into three rock groups and seven sedimentary subfacies,representing three sedimentary cycles respectively.The first cycle records the sedimentary evolution from alluvial fan to braided river delta to lakeside.The second cycle is characterized by alluvial fan braided river.Later,the lower part of the third sedimentary cycle was characterized by continental alluvial fan deposits,and the upper part was gradually transformed from continental to Marine deposits with volcanic rocks.,part was coastal sandstone and pebbled sandstone,indicating that the range and water depth of the lake basin increased significantly since the third sedimentary cycle.The youngest age of detrital zircons in the lower part of the Hangaoshan Group is?1783 Ma,which indicates that the volcanic sedimentary formation of the Hanguoshan Group formed since 1.78Ga under the background of transitional extension from continental facies to marine faciesThe volcanic rocks in the third sedimentary cycle of Hangaoshan Group are tholitic andesites with low Si O2,high Ti O2,Mg O,Mg^#,Cr and Ni,relatively enriched in LREES and LILEs such as Ba,U and K,and depleted in HFSEs such as Nb,Ta and Ti,which are similar to the geochemical characteristics of arc magma.Their?_Nd(t)=-4.31?-4.48,T_DM=2509?2520 Ma,and high Ba/Th with low(La/Sm)_N,indicate that they originated from the lithospheric mantle derived from the late Neoarchean oceanic plate subtractive release fluid.Therefore,these volcanic rocks have nature of arc magma is inherited by the new archean arc area transformation lithosphere mantle to add to the characteristics of the crustal material,with other group volcanic rocks and Xiong'er group of volcanic rock also shows arc magma of geochemical and Sr-Nd isotopes reveal new originated in the late archean plate subduction of lithospheric mantle is very consistent.In addition,accompanied by these volcanic activities,the contemporaneous mafic dike swarms associated with continental fragmentation are widely developed in the region,which together prove that the North China Craton began to transform into a continental extensional environment in the early Mesoproterozoic,rather than an arc environment associated with convergenceThe sedimentary sequence and genesis of the volcano-sedimentary formation of the Hangaoshan Group,Xiaolangling Group and the Xiong'er Group in Luliang area are highly consistent,which represents the northernmost volcano-sedimentary formation of the Xiong'er Trigeminal Rift in the same period.Among them,the middle and the lower two depositional cycle is given priority to with terrigenous sedimentary,in the third cycle upper into Marine sedimentation with a small amount of volcanic rock,recorded the rapid filling on early cracking of continental rift sedimentary to middle-late with further stretching rift,gradually from continental to transgressive sedimentary and evolution process of a volcanic eruption,.It represents the result of sedimentary evolution of rift margin facies in the northern branch of Xiong'er rift basin.