Titanite evidence for Triassic thickened lower crust along southeastern margin of North China Craton

Titanite U–Pb isotopic and major and trace element compositions of one mafic garnet granulite from a rare suite of lower crustal xenoliths (e.g., eclogite, garnet pyroxenite, and mafic garnet granulite) hosted in Early Cretaceous dioritic porphyries in the Xu–Huai area along the southeastern margin of the North China Craton (NCC) were analyzed by laser ablation ICP-MS. Titanite occurs as granular grains or coronary rims on rutile. The coronary titanite is clearly a secondary product of rutile decomposition. The granular titanite exhibits zonation in U–Pb age and chemical composition. Petrographic and geochemical evidence suggests that the zonation was formed by thermal diffusion and later fluid-assisted recrystallization. Occurrences of granular titanite between garnet grains point to a pressure of >10kbar, while inclusions of rutile inside granular titanite rims imply that the pressure might have reached 15kbar. Granular titanite cores give U–Pb ages of 237–241Ma and Zr-temperatures of 794–831°C at 10kbar and 850–892°C at 15kbar, indicating high-pressure granulite-facies metamorphism. Together with previous P–T estimates of coeval eclogite-facies xenoliths, a geotherm of above 60mWm−2 is implied. The geotherm plots below the temperature field of amphibole dehydration melting, consistent with presence of abundant amphibole. This geotherm is similar to that of the Kohistan arc, which has preserved a 12-km-thick dense lower crust, but significantly cooler than the geotherm of the Talkeetna arc, where most of the dense lower crust has been foundered.