Distinction between peritectic and anatectic zircon during crustal anatexis

Zircon records of migmatites in the eastern Himalayan syntaxis reveal two generations of crustal anatexis， corresponding to India-Asia continental collision and subsequent post-collisional reworking. Based on cathodoluminescence (CL) imaging， inclusion data and trace element characteristics， zircon domains from the migmatites can be divided into three types. Type I domains are inherited zircons with variable U-Pb ages (>60 Ma). Type II and III domains represent newly grown zircons， yielding concordant U-Pb ages of ～49 Ma or ～25 Ma. Type II domains show low brightness in CL images with weak oscillatory zoning， unzoning or spongy texture， and contain multiphase solid and melt inclusions. They are enriched in high field-strength elements (HFSE) and heavy rare earth elements (HREE)， alongside high Th and U contents and variable Th/U ratios. Type II domains are attributed to peritectic zircons produced during crustal anatexis through incongruent melting of Ti-rich minerals such as ilmenite， rutile， titanite and biotite， together with HREE-rich minerals such as garnet and amphibole. By contrast， Type III domains show high brightness with oscillatory zoning， planar zoning， or fir-tree zoning. They are characterized by relatively elevated U contents， moderate Th， HFSE and HREE contents， and low Th/U ratios. These features suggest crystallization of anatectic zircons from in-situ partial melts， distinct from magmatic zircons crystallized from evolved， spatially separated magmas that underwent substantial crystal fractionation. The geochemical variations among these zircon domains arise from the chemical diversity of the evolving anatectic melts in a fractional melting model. Furthermore， the overlapping age ranges of peritectic and anatectic zircons suggest a short temporal gap between partial melting and melt migration， pointing to rapid progression from crustal anatexis and melt crystallization in the eastern Himalayan syntaxis. Therefore， features of Type II (peritectic) and Type III (anatectic) domains in the migmatites may serve as diagnostic indicators for differentiating the origins of zircons and related melting regimes.