Zircon In High-silica Rhyolite Recording Remelting Of Crystal-rich Mush

The formation of high–silica rhyolite may have undergone complex magmatic processes and is not just the result of high–degree crystallization or low–degree partial melting.This paper reports the data of zircon U–Pb age,zircon trace elements and Hf–O isotopic data from Nuocang rhyolite in Cuoqin area,Lhasa Terrane,Tibet.The zircons in the rhyolite which has high–silica content(SiO2 = 74.5–79.3 wt.%)can be divided into two types according to the different internal structures from the cathode luminescence(CL)images.Type A is reverse–zoning zircons which have dark–CL cores and light–CL rims;Type B is light–CL grains with no obvious dark–CL areas inside,mostly of this kind of zircons have oscillatory zoning.Data and Mapping images obtained by LA–ICP–MS show that there is no significant difference in U–Pb ages and Hf isotopic data between the two types of zircon,but noteworthy difference in trace element characteristics,and systematic difference in O isotopic data between dark–CL and light–CL regions(~0.4‰).Compared with the light–CL rims of reverse–zoning zircon,the dark–CL cores have higher content of U,Th,Hf,Y,HREE and higher Nb/Ta ratio,as well as significantly negative Eu anomalies,showing a highly evolved feature,which means that the two regions crystallized in magma with different compositions.Type B zircon is similar to the light–CL rims of the reverse–zoning zircon.It has the lowest trace element content and the highest zircon Ti temperature,which confirms the addition of new and hotter melt.Both zircon and quartz phenocryst have resorbed features and quartz Ti thermometers also indicate the dissolution and subsequently regrowth of crystals due to reheating events.We suggest that the crystallization of the two types of zircon with similar U–Pb age and different internal structures,characteristic of trace elements and O isotopic data was the result of three–stages magma processes in the upper crust.The dark–CL zircon and quartz came from a highly–evolved magma;the subsequently reheating of crystal–rich MUSH formed resorbed crystals;after the self–mixing of magmas with different concentration of incompatible elements,the light–CL crystals formed in new hotter magma chamber.During the whole process,mafic magma provided heat of remelting but with no or little contribution of mass.Our study shows that,in terms of exploring the formation and storage state of high–silica rhyolites that have undergone varying degrees of alteration,zircon is a reliable tool to record the multi–stages magma processes compared with the whole–rock geochemical data and other minerals.