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The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher 176Hf/177Hf ratios than their protolith zircons. More interestingly, the 176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated 176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution. • Peritectic zircon have higher Lu/Hf and Hf isotopic ratios than protolith zircons. • Peritectic zircon also have higher HFSE contents than protolith zircons. • Minerals with high Lu/Hf ratios than zircon were decomposed during crustal anatexis. [ABSTRACT FROM AUTHOR]

• Triassic rocks in northern Qiangtang Block include basalts-rhyolites (237–235 Ma). • Basalts were originated from metasomatized mantle sources. • Rhyolites were generated from newly formed basaltic lower crust. • The formation of studied suite was related to Paleo-Tethys oceanic slab subduction. Triassic magmatic rocks are widely exposed in the northern Qiangtang Block in Tibet. However, their petrogenesis and relationship to the evolution of Tethys Ocean are matters of debate. Here we report zircon U-Pb ages, mineral compositions, whole-rock major and trace elements, and Nd isotope for newly reported Triassic volcanics in order to provide constraints on the formation of these rocks in Tethys ocean's evolution. The volcanic suite consists of grey-porphyritic basalts (B1), black-aphyric basalts (B2), and rhyolites. SIMS, LA-ICPMS zircon U-Pb age dating shows that they were erupted at 237–235 Ma. All rocks show no continuous trends in major-element compositions, and are enriched in Light Rare Earth Elements (LREE) relative to heavy REE (HREE) with various Eu-anomalies. Basalts have narrow εNd(t) range (−0.90 to −0.01), slightly higher than those of rhyolites (−4.36 to −0.66). Given similar Nd-isotope and REE patterns, B1 and B2 basalts were probably produced from a common parental magma by mineral accumulation and fractional crystallization during late- and early-stages of magma evolution, respectively. Based on indicative element-ratios of slab-derived materials, we suggest that parental magmas of these basalts were derived by partial melting of metasomatized mantle source with subducted oceanic slab component. Rhyolites were generated by partial melting of basaltic lower-crust as a result of underplating by magma that corresponds to the studied basalts. Given spatial–temporal distribution of Triassic magmatic-metamorphic rocks, and geological background, we suggest that magmatism of studied suite was triggered by northward subduction of the Paleo-Tethys ocean, and recorded late-stage of Paleo-Tethys Wilson cycle. [ABSTRACT FROM AUTHOR]