Magma Mixing And Tectonic Implications Of Hailin Pluton In The Southeastern Zhangguangcai Range

This thesis presents zircon U-Pb geochronology,whole-rock geochemistry,mineral chemistry and zircon Hf isotopic compositions analytical research of the Hailin pluton from the Mudanjiang region in the southeastern Zhangguangcai Range,which to confirm the formation ages,petrogenesis and related geodynamic mechanisms of the Hailin pluton,further to provide important constraint for the tectonic evolution of the Mudanjiang Ocean between the Songnen-Zhangguangcai Range and the Jiamusi massifs.Through the geological survey and petrographic study,we find that the Hailin pluton is composed of the host granitoids(monzogranites and granodiorites)and the mafic microgranular enclaves(MMEs;diorites).The MMEs are unevenly distributed within the host granitoids,some of which have plastic rheological characteristics.And some MMEs preserve chilled margins and acicular apatite.It can preliminarily determine that the Hailin pluton was the product of the magma mixing.Then mineral chemistry,whole-rock geochemistry and zircon U-Pb-Hf comprehensive research are conducted to further support the role of magma mixing in the petrogenesis of the Hailin pluton.Zircon U-Pb ages data show that the diagenetic ages of the host granitoids in Hailin pluton were 215~219 Ma,and those of the MMEs were 221 Ma,suggesting they were both formed during the Late Triassic and were the product of magma mixing.Here we take the host granitoids as the felsic end-member rocks.The host granitoids are characterized by high silica content(Si O2 = 63.18~70.44 wt.%),high alkali content(Na2O+K2O=6.59~7.95 wt.%),low Mg O(0.69~1.94 wt.%)and low Fe2O3T(2.39~4.68 wt.%).They are enriched in light rare earth elements(LREEs)and depleted in heavy rare earth elements(HREEs),with total rare earth elements of 88.55~116.29 ppm,LREE/HREE ratios of 7.47~13.43 and(La/Yb)N values of 5.80~18.62.They are also enriched in large ion lithophile elements(LILEs;e.g.,Rb,Sr,and Ba)and depleted in high field strength elements(HFSEs;e.g.,Nb,Ta,and Ti).Zircons from the host granitoids yield ?Hf(t)values of-6.6 to +3.0,and two-stage Hf model ages(TDM2)of 1060~1669Ma.Taken together the above geochemical and zircon Hf isotope characteristics,it is shown that the host granitoids belong to I-type granitoids and they were derived from the partial melting of Mesoproterozoic lower crust.The MMEs were formed by the mixing of felsic magma and mafic magma.The mineral compositions of plagioclase and amphibole in MMEs are in disequilibrium.And the plagioclase is composed of oligoclase-andesine,suggesting typical reverse zoning characteristic.The amphibole can be classified as magnesiohornblende and tschermakite and belongs to crust-mantle amphibole.In addition,the whole-rock geochemical characteristics of the Hailin pluton show that they underwent a magma mixing process.The MMEs and the host granitoids show a linear relationship in the Harker diagrams and a hyperbolic relationship in the noncommon denominator ratio diagrams.As for the mafic end-member of the Hailin pluton,previous studies have reported the coeval mafic rocks(basalt and gabbro,diabase;209~228 Ma)with similar geochemical characteristics in the study area,These mafic rocks are characterized by low silica content and high Mg# values,with enrichments in LREEs and depletions in HREEs,Nb,Ta,Zr,Hf and Ti.Zircons of the mafic rocks yield ?Hf(t)values of +2.5 to +9.8,and single-stage Hf model ages(TDM1)of 477~801 Ma,suggesting that the mafic magma were derived from partial melting of mantle wedge that had been metasomatized by slab-derived fluids.Based on related study of the spatial variation characteristics of contemporary igneous assemblages and the history of regional tectonic evolution in the study area,we suggest that the Late Triassic Hailin pluton from the Mudanjiang region in the southeastern Zhangguangcai Range was formed by the mixing of felsic and mafic magmas in an active continental margin setting,closely related to the westward subduction of the Mudanjiang Ocean beneath the Songnen–Zhangguangcai Range Massif.As a result of the westward subduction of the Mudanjiang Ocean beneath the Songnen-Zhangguangcai Range Massif,the mafic magma was derived from partial melting of the mantle wedge that had been metasomatized by slab-derived fluids.The resulting thermal pulse triggered large-scale partial melting of the lower crust,and in turn led to the generation of felsic magma.The mafic magma underwent upward emplacement and magma mixing with the felsic magma.The magmas subsequently continued upward emplacement and crystallized to form the Hailin pluton.