Zircon and titanite behaviors during partial melting of metabasite in the post-collisional stage: Constraints from garnet pyroxenite in the Dabie orogen, China

Abstract

Abstract An integrated study of petrology, mineral inclusions, U-Pb ages and trace elements in zircon and titanite was carried out for garnet pyroxenites from the Dabie orogen, China. The results provide not only geochemical constraints on zircon and titanite behaviors during partial melting but also petrological insights into anatectic reworking of ultrahigh-pressure metamorphic eclogites in the collisional orogen. The target garnet pyroxenites were transformed from ultrahigh-pressure eclogites produced by the continental deep subduction in the Triassic, and underwent granulite facies metamorphic superimposition in the early Cretaceous under P-T conditions of 746–837 °C and 0.8–1.2 GPa, followed by amphibolite facies retrogression at 737–791 °C and 0.7 GPa. Both zircon and titanite contain two types of metamorphic domains at ca. 131 Ma based on CL images, mineral inclusions, REE patterns and trace element variations. Peritectic zircon domains (D3) contain abundant multiphase crystal inclusions of quartz and K-feldspar and exhibit variably high Nb, Ta, Th, U, P, REE and Y contents. In contrast, anatectic zircon domains (D4) are relatively free of crystal inclusion and display variably low Nb, Ta, Th, U, P, REE and Y contents. Metamorphic and peritectic titanite domains are clearly distinct by a series of differences in petrological and geochemical compositions. There are reduced 176Hf/177Hf ratios for both peritectic and anatectic zircon domains, indicating involvement of Hf-rich minerals in the anatexis of the ancient crust. Therefore, the discrimination between the polygenetic zircon and titanite domains in the metabasites not only provides a powerful means to decipher the nature of crustal anataxis in the collisional orogen, but also places constraints on the formation mechanism of metamorphic, peritectic and anatectic minerals.