Research About Genesis And Geochemistry Of Zhangjiakou Volcanic Rocks In The Yu County Area,Hebei

The volcanic rocks of the Zhangjiakou Formation are located in the middle of the Mesozoic magmatic complex zone in the Daxinganling-Taihang Mountains.It is a component of the volcanic belt in the eastern part of the North China Block.The study area is located in the Yuxian area of Zhangjiakou,and the tectonic unit of the study area is located in the Xuanhua-Yixian Basin in the Yanshan-Liaoxi rift zone.The volcanic rocks of the Zhangjiakou Formation are an important part of the late Mesozoic volcanic activity of the Yanshan fold thrust nappe belt.It is an important magmatic event representing the Early Cretaceous Yanshan Belt.Based on petrological and geochemical studies of the volcanic rocks of the Zhangjiakou Formation,this paper discusses the formation age,rock series,genesis,differential evolution characteristics,and tectonic environment of the set of rocks.The results show that the Zhangjiakou Formation volcanic rocks are a set of acid rhyolite tuffs.The main elements indicate that the volcanic rocks of the Zhangjiakou Formation belong to the peraluminous high-potassium calcium-alkali rock combination series,with a higher degree of differentiation and a higher degree of acidity.The volcanic rocks of the Zhangjiakou Formation are moderately alkaline to calcareous rocks.Its low alkalinity index and small change interval indicate that it is an island arc and an active continental marginal environment.It can be seen from the trace diagram of the trace element after the normalization of the original mantle that the 7 samples of the volcanic rocks of the Zhangjiakou Formation in the study area have basically the same curve trend,and the curve shape is right-inclined.In general,it is characterized by the enrichment of large ion lithophilic elements(Rb,Th,U,K,La,Nd)and the strong loss of high field strength elements(Nb,Ta,P,Ti),reflecting the island arc magmatism.Features.The volcanic rocks of the Zhangjiakou Formation are low in rare earth elements,rich in light rare earth elements,and depleted in heavy rare earth elements.Light rare earth fractionation is stronger than heavy rare earth fractionation,and heavy rare earth fractionation is not obvious.Has strong negative negative characteristics.In the standardized illustration of rare earth element chondrites,it is a right-inclined seagull-type division.The above analysis results ofgeochemical data all indicate that the tectonic setting of the volcanic rocks of the Zhangjiakou Formation is continental marginal volcanic rocks and co-collisional volcanic rocks.In this study,U-Pb isotopic samples were collected from the upper and middle volcanic rocks(ryolithic tuff)of the Zhangjiakou Formation.The weighted average age is 140.7 ± 1.1Ma(95% confidence,MSWD = 2.4).Therefore,the formation age of the Zhangjiakou Formation 140.7 ± 1.1Ma is the Early Cretaceous.This is consistent with the formation of the volcanic rocks of the Zhangjiakou Formation in the region.The volcanic rocks of the Zhangjiakou Formation have the characteristics of aluminous rhyolite,suggesting that their provenance is crust.Various geochemical data and geochemical data analysis results show that their magma cannot be directly derived from the enriched mantle,and the formation of Zhangjiakou volcanic rocks is inferred In the lower and middle crust.By analyzing the petrological and geochemical characteristics of the volcanic rocks of the Zhangjiakou Formation,it can be inferred that the volcanic rocks of the Zhangjiakou Formation are a set of volcanic rocks that subducted below the Eurasian continent(plate)during the early chalk Yanshan orogeny and formed a large area in Zhangjiakou Volcanic rocks.