Magmatic Evolution Of Intermediate-acidic Complex In Renjiayingzi,Linxi County,Inner Mongolia:Evidence From Mineral Geochemistry

Located in the eastern part of the Inner Mongolia Autonomous Region and to the north of the Xar Moron River,the Linxi county belongs geographically to the southern Great Xing'an Range,and tectonically to the eastern Central Asian Orogenic Belt.Mesozoic intermediate-acidic intrusions are widespread in this area.In recent years,the work of geologists on these intrusions have mainly focused on the study of geochronology,petrogeochemistry,isotopic geochemistry,petrogenesis and tectonic setting.However,little or none analyses has been done on the situ microchemical composition of the rock-forming minerals which can better study the dynamic process of magma evolution.Field geological investigation,petrographic observation and mineral geochemical analysis are carried out in the Early Cretaceous Renjiayingzi pluton which are composed of pyroxene diorite,quartz diorite and granodiorite.Chemical composition of rock-forming minerals,combined with previously published U-Pb zircon ages,Whole-rock geochemistry,and Sr-Nd-Hf isotopic data will be used to constrain the magmatic origin and evolution process of intermediate-acidic complex in Renjiayingzi.The Renjiayingzi complex pluton is composed of three units from northwest to southeast,namely Shuangjianshan,Qianweiliansu and Xingshuwabeishan,with a lithology from the intermediate to acidic evolution.Based on field geological surveys combined with the research data of predecessors,the rocks of each unit of this complex are:the lithology of the Shuangjianshan unit is mainly pyroxene-diorite,the Qianweiliansu unit is mainly quartz-diorite and tonalite,and the Xingshuwabeishan unit is dominated by granodiorite and monzogranite.The results shows that the composition of plagioclases gradually changes from calcium-rich crystals to sodium-rich crystals,with the increasing acidity of the rocks studied.The contents of Al₂O₃,Ca O and Na₂O in the feldspar display obvious linear trend with the increase of Si O₂.The Eu anomaly of plagioclase in the quartz diorite is greater than that of pyroxene diorite,suggesting that plagioclase experienced variable degree of fractional crystallization during magma evolution.Biotite belongs to magnesium biotite and iron biotite,and both of them are the primary magmatic biotites,characterized by enrichment of Rb and Ba and depletion of Th,U and K.Amphibolite can also be divided into two groups:one with high iron and low magnesium(yellow-green),and the other with high magnesium and low iron(dark-green).The yellow-green amphibolites are classified to edenite and magnesium hornblende,while the dark-green amphibolites are magnesium hornblende and actinolite.Amphibolites show unnoticeable fractionation between light rare earth elements(LREEs)and heavy rare earth elements(HREEs),with significant Eu negative anomaly.Pyroxene is divided into salite and augite,which evolves from magnesium-rich to iron-rich ones with an increasing degree of magmatic differentiation.The composition of the biotite in the pyroxene-diorite and quartz-diorite show the characteristics of crust—mantle mixture,implying that the source of the parental magmas is not a single magma source,and they originated dominantly from the partial melting of the mantle material with involvement of crust materials,and have undergone fractional crystallization of pyroxene.The amphibolite in granodiorite has a derivation from a crustal source,indicating that the rocks are products of partial melting of the basal material of the crust induced by underplating of mantle-derived magmas and have experienced the fractional crystallization of plagioclase.The crystallization temperature of pyroxene in the pyroxene-diorite and quartz-diorite is similar,representing the initial temperature of the dioritic magmatic system.The crystallization temperature of biotite is divided into three stages,reflecting the crystallization of biotite with variable components from early to late magma in the dynamic evolution,during which the magma temperature gradually decreases,the oxygen fugacity gradually increases,and the environment becomes more oxidized.The yellow-green amphibolite in granodiorite is formed in two stages,representing the early and late stage of granitic magma evolution,respectively.The temperature and pressure of the dark-green amphibolite are significantly lower,which is the product of the final intrusion of granitic magma at shallow levels,and the oxygen fugacity of the magmatic system increased in this process,but the water content did not change significantly.Combined with the unbalanced structure and chemical composition of plagioclase as well as the presence of mafic microgranular enclaves in the granodiorite,it is believed that magma mixing occurred during the evolution process of intermediate and acidic magma in Renjiayingzi.During the evolution of dioritic magma,it was mixed with acidic magma,and partially mafic magma was added during the evolution of granitic magma.