Rejuvenation of magma reservoir:A case study of the Matou quartz monzonite in western Zhejiang
Previous studies indicate that significant amount alkaline rocks,primarily consisting of quartz syenite and quartz monzonite,were formed during the late Early Cretaceous to early Late Cretaceous in the coastal area of southeastern China.However,their origin is still controversy.To determine the genesis of these rocks,this study focused on the quartz monzonite in the Matou pluton in western Zhejiang and conducted detailed investigations of whole-rock major and trace element analysis,mineralogical scanning,zircon U-Pb geochronology,zircon Hf isotopes,zircon trace elements,apatite trace elements and plagioclase electron probe analysis on it.The zircon U-Pb dating reveals that the Matou pluton formed at 100±1Ma,representing a product of magmatic activity during the late Early Cretaceous.Field observations found abundant mafic enclaves within the Matou pluton.Combining the zircon Hf isotopic data(εHf(t)=-10.4～-0.4),it is suggested that the Matou pluton resulted from the magma mixing of felsic magmas derived from ancient continental crust and mafic magmas derived from the mantle.Zircons in the Matou monzonite show complex textures,commonly displaying a black core,a white mantle,and black rims in CL(cathodoluminescence)images.The earliest black zircon cores exhibit relatively high Th,U,Hf contents,varying Eu/Eu*(0.50～0),Zr/Hf(40～70)and Th/U ratios,as well as Ti content.This suggests that they crystallized from a high-degree,low-temperature,and highly crystalline silicic magma reservoir in the crust.The white mantle zircons have lower Th,U,Hf contents,and relatively higher Eu/Eu*ratios(0.15～0.37)which indicates that injection of mafic magma caused reactivation and melting of pre-existing magma reservoirs,leading to an increase in temperature and melt proportion within the reservoir.As minerals continued to crystallize and felsic melt extraction occurred,the remaining felsic melt within the magma reservoir underwent significant evolution.This led to the crystallization of the latest-stage black zircon rims characterized by high U and Hf contents and low Eu/Eu*and Zr/Hf ratios.Electron probe data on plagioclase suggests,as plagioclase phenocrysts crystallized,the magma reservoir underwent a crystal-melt separation process,causing the evolution of melt from being rich in calcium to being rich in sodium.Ultimately,this led to the crystallization of fine-grained quartz and sodium feldspar.Apatite in the quartz monzonite has low Sr content(113×10-6～417×10-6),high U,Y and light rare earth element,and low heavy rare earth element contents.This implies that,after reactivation,the pre-existing magma reservoir also underwent crystal fractionation of minerals such as plagioclase,titanite and apatite.Zircon records the reactivation and melting processes of the magma reservoir,while minerals like plagioclase and apatite only record the crystal-melt separation and melt extraction processes after the reactivation of the reservoir.Therefore,the Matou quartz monzonite suggests that it represents a residual magma reservoir that underwent melt extraction after reactivation,with contemporaneous high-silica rhyolite(or high-silica granite)representing silica-rich melt extracted from the magma reservoir.Finally,our study indicates that the magma reservoir predominantly existed in a low-temperature,low-melt proportion state for most of the time.The injection of mafic magma led to the reactivation and melting of the pre-existing magma reservoir,which is a crucial factor enabling shallow crustal magma reservoirs to undergo crystal-melt separation and volcanic eruptions once again.
Magma rechargeMagma evolutionCrystal mushAlkali rocksMagma reservoir
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