Early Cretaceous Felsic Magma Chamber Process In Central Great Xing'an Range

Based on the new magma chamber model,this paper described the early Cretaceous magma chamber process of Baiyintaohai basin in Suolun area,central Great Xing'an Range by means of petrology,geochemistry,zirconology,single mineral geochemistry,numerical simulation and thermodynamic simulation,and presented a universal magma chamber evolution model.Baiyintaohai basin is located in the middle of Great Xing'an Range and the southern margin of Xing'an block.Baiyintaohai basin is a typical late Mesozoic volcanic basin,which is widely distributed with early Cretaceous intermediate-acid volcanic rocks and contemporaneous intermediate acid intrusive rocks,including Manketouebo formation,Manitu formation,Baiyingaolao formation,monzonitic porphyry,granite,granodiorite,and so on.With the help of traditional analysis methods,combined with thermodynamic simulation and numerical simulation,this paper draws the following conclusions.(1)The acid volcanic rocks in this study are mainly rhyolite and a small amount of dacite,belonging to high K(calc-alkaline)series.The REE is 83?245 ppm,LREE/HREE is 5.79?12.19,Eu/Eu*is 0.20?0.87,showing strong to weak Eu negative anomaly.The rocks are depleted in Ba,Nb,Ta,Sr,P,Ti and enriched in Rb and K.It was formed in 140?123 Ma,which is the product of Early Cretaceous magmatism.The?_Hf(t)ranges from+1.7 to+9.9,which is mainly derived from the partial melting of the young crust,and the mantle material may have a small contribution.Acid volcanic rocks are highly fractionated I-type volcanic rocks.They have experienced strong fractional crystallization.The main fractionation minerals are plagioclase,alkali feldspar and zircon.(2)The intermediate volcanic rocks include basaltic andesite,basaltic trachyandesite,andesite,trachyandesite and trachyte,most of which belong to trachytic volcanic rocks.It is mainly high K(calc-alkaline)series.The REE is 95?252ppm,LREE/HREE is 6.18?13.74,and Eu has no obvious negative anomaly(Eu/Eu*=0.8?1.02).The rocks are depleted in Nb,Ta,P,Ti and enriched in Ba,K.It was formed in 138?126 Ma,which is the product of Early Cretaceous magmatism.?_Hf(t)is+4.0?+13.67,which mainly comes from the mantle and experiences the mixing of crust materials.The plagioclase phenocrysts are mainly labradorite,with a small amount of andesine and bytownite.The composition of the same plagioclase shows a regular change of An from the center to the edge,indicating that the magmatic properties in the magma chamber have undergone periodic changes.(3)Both monzonite porphyry and its enclaves monzodiorite belong to high K(calc-alkaline)series.The REE of monzonite is 131?157ppm,and Eu has no obvious negative anomaly(Eu/Eu*=0.78?0.93).The REE of enclaves is 137?203ppm,and Eu shows a medium to weak negative anomaly(Eu/Eu*=0.65?0.83).Compared with the enclaves,the REE of monzonite is relatively low,and the enclaves show a stronger negative Eu anomaly.Both of them are depleted in Nb,Ta,Ti,P and enriched in Rb,Ba,K.The?_Hf(t)of monzonite porphyry are+4.3?+11.1,and the?_Hf(t)of enclaves are+2?+8.4,which all originated from mantle,but is contributed by crust material.Plagioclases in monzonite porphyry and its enclaves is mainly andesine.The change of plagioclases composition shows that the magmatic properties changes periodically.Amphiboles in monzonite porphyry can be divided into high Si O₂,TFe₂O₃,K₂O,low Al₂O₃,Ti O₂,Mg O,Na₂O and low Si O₂,TFe₂O₃,K₂O,high Al₂O₃,Ti O₂,Mg O,Na₂O.Amphiboles in inclusion are similar to high Si O₂,TFe₂O₃,K₂O,low Al₂O₃,Ti O₂,Mg O,Na₂O in monzonite porphyry.Amphiboles in monzonite porphyry include high Mg/(Mg+Fe²⁺),low Si and low Mg/(Mg+Fe²⁺),high Si amphiboles.There are only low Mg/(Mg+Fe²⁺)and high Si amphiboles in the enclaves.(4)The zircon Ti saturation temperature of the acid volcanic rocks indicate that they experienced a reheating process in the magma chamber,and the heating range may be as high as?150?.This process promoted the continuous evolution of the high viscosity acid magma.The intermediate volcanic rocks experienced fractional crystallization of plagioclase and zircon,but the results of numerical simulation and thermodynamic simulation show that their composition evolution is controlled by fractional crystallization of plagioclase and accessory minerals,as well as the reabsorption of alkali feldspar.The monzonite porphyry and its enclaves underwent fractional crystallization of amphibole,plagioclase.Due to buoyancy and heat flux,enclaves which were deep high-temperature magmas entered the monzonite porphyry when the host rock is in mush state.(5)The process of Early Cretaceous magma chamber in Baiyintaohai basin is that high temperature magma from deep crust pulsates into the bottom of shallow magma chamber,which makes the early mush in the magma chamber undergo reheating process,resulting in fractional crystallization of reinjected high temperature magma and reactivation of minerals in the early mush.The accumulation of melt pressure at the bottom of magma chamber promotes the gradual accumulation of melt at the top of mush layer,which eventually leads to volcanism and intrusion.The large-scale magmatic activity of the Early Cretaceous in this area was triggered by the dehydration of the Mongol-Okhotsk oceanic crust under the extensional environment,which subducted to the depth of the mantle.The multi-stage pulsating activity of the deep high-temperature magma provided power and material for the volcanic eruption and the emplacement of intrusive rocks.