Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North China Craton: implications for the interleaving of the Dabie–Sulu orogenic crust
Lamprophyre dikes emplaced in a Jurassic granite at the southeastern margin of the North China Craton (NCC) carry different types of xenoliths. Here, we report a combined study of zircon U–Pb ages and whole-rock geochemistry of the xenoliths, as well as an Ar–Ar age of the lamprophyre, providing constraints on the sources of the magmatism and tectonic evolution in the southeastern margin of NCC. Phlogopite from the lamprophyre dike gave a 40Ar/39Ar plateau age of 116.15 ± 0.33 Ma. The felsic xenoliths can be classified into three groups: monzogranite, banded biotite granitic gneiss, and garnet-bearing gneiss. The internal structures of zircons from the banded biotite granitic gneiss xenolith show complex growth patterns. The mantles of these zircons display low luminance with a concordant SHRIMP U–Pb age of 227 ± 10 Ma. The rims with lighter luminance provide a SHRIMP U–Pb age of 213 ± 10 Ma, both ages are identical within error and significantly different from the ages of the basement rocks in the surrounding Bengbu uplift. However, the age is identical to those of the ultrahigh-pressure metamorphic rocks in the adjacent Dabie–Sulu orogen. In addition, zircon mantle domains of the banded biotite granitic gneiss xenolith have low Th/U ratios (>0.01) and flat HREE patterns ((Yb/Dy)n < 10), which suggest growth in an assemblage with garnet during HP metamorphism. The rim domains show very low Th/U ratios (<0.01) and steep HREE patterns ((Yb/Dy)n > 10), implying growth during exhumation in the absence of a garnet. Our studies show that the banded biotite gneiss represents vestiges of the subducted South China crust injected into or thrust below the North China Craton and provides constraints on the process of underthrusting in a continental collision zone, as well as the Mesozoic tectonic history of the southeastern margin of the NCC.