Temperature and Hf-O isotope correlations of young erupted zircons from Tengchong(SE Tibet):Assimilation fractional crystallization during monotonic cooling

Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness.We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field,SE Tibet,in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting.As decreasing zircon Ti content and falling temperature,zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease,indicating fractional crystallization of plagioclase and zircon during magma cooling.More importantly,zircon Hf isotope ratio(sHf values)increases with decreasing zircon Ti content and falling temperature(T),suggesting gradually increasing incorporation of relatively high EHf juvenile materials in the crystallizing zircons during magma evolution.Negative correlations between zirconε_(Hf)and zirconδ^(18)O also support open-system magma evolution.Our data suggest fractional crystallization of a magma with simultaneous contamination by highε_(Hf)and lowδ^(18)O juvenile(immature)crustal materials during monotonic cooling after zircon saturation.The low-T,high-ε_(Hf)and low-δ^(18)O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution.Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7±6.8 km,consistent with present crustal thickness(42.5 km)determined by geophysical methods.