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

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• Ti content and Hf isotope ratios in Tengchong zircons are negatively correlated. • Hf isotope and oxygen isotope ratios in Tengchong zircons are negatively correlated. • Ti content, O-Hf isotope correlations indicate assimilation fractional crystallization. • Temperature and isotope correlations suggest monotonic cooling without recharge. • Assimilation of supracrustal materials is significant in post-collisional lavas. 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 (ε Hf values) increases with decreasing zircon Ti content and falling temperature ( T ), suggesting gradually increasing incorporation of relatively high ε Hf materials in the crystallizing zircons during magma evolution. In addition, negative correlations between zircon ε Hf and zircon δ 18 O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high ε Hf and low δ 18 O 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 (126.5 Ma) 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.