Geochemistry of polygenetic titanite traces metamorphic and anatectic processes during the exhumation of deeply subducted continental crust

Abstract

A systematic study of titanite UPb geochronology and geochemistry was carried out in conjunction with petrographical investigation and zircon UPb geochronology for migmatites from the Weihai region in the Sulu orogen. Petrochronology of titanite and zircon in the migmatites document one metamorphic event at ~227 Ma and subsequent two anatectic events at ~220 Ma and ~210 Ma, respectively, during decompressional exhumation of the deeply subducted continental crust in the Triassic. Different titanite zonations represent changes of metamorphic and anatectic conditions. Metamorphic titanite domains (Ttn-I) have high Al2O3, F but low Fe contents, and are depleted in LREE, Th, Zr, P with low Th/U, LREE/HREE ratios and relatively low εNd(t) values. Two types of peritectic titanite domains (Ttn-II and Ttn-III), both with high Th, Th/U, REE and HFSE but low Al2O3 and F contents, record two stages of crustal anatexis during the exhumation. Compared to Ttn-III domains, Ttn-II domains with brighter BSE imaging have higher Al, Fe, REE, Nb, Ta and P contents but lower Th/U, Nb/Ta and (La/Yb)N ratios. The decreasing pressures, different reaction mechanisms, variable coexisting minerals and changing fluid/melt compositions have significant impacts on titanite geochemistry. Titanite Al2O3 and F contents show synchronous decreases from core to rim, suggesting the effect at lower pressure, consistent with more intense FAl substitution at higher pressure. During the anatectic processes, Ttn-II domains show a tendency to be higher in Al, Fe, REE and HFSE contents than Ttn-III domains. This is attributed to different mineral stabilities and variable accessory minerals that were involved in peritectic reactions at different pressures, which also have influenced the compositions of simultaneously generated anatectic melts. The change of melt compositions (e.g., ASI) at different pressures would drastically modify the partition of trace elements between peritectic titanite and anatectic melt, resulting in highly fractionated trace element ratios, such as high Nb/Ta and (La/Yb)N as recorded in titanite rims. Therefore, accessory minerals such as titanite and zircon are not only important repositories of trace elements that can greatly fractionate the HFSE and REE, but also the excellent recorder of multiple fluid actions during the exhumation in collisional orogens.