The origin and evolution of low-δ18O magma recorded by multi-growth zircons in granite

Abstract

We for the first time reported low-δ18O granites within craton in Bengbu uplift, southeast margin of North China Craton. Integrated ion microprobe study of δ18O and U–Pb age on single zircon grains and zircon populations, gives direct evidence for the origin and later magmatic evolution of the low-δ18O granites. Three types of zircon domains are recognized: inherited magmatic core (Type I), inherited metamorphic core/mantle (Type II) and overgrown magmatic rim (Type III). Type I zircons were formed in Neoproterozoic, and have the average δ18O values of 5.3±0.6‰ around mantle value. Triassic Type II zircons have extremely low and highly heterogeneous δ18O values ranging from −9.4‰ to 8.6‰, consistent with the values and variations exhibited by metamorphic zircons from Dabie–Sulu orogen. This feature argues for a Triassic metamorphic origin for the 18O-depletion signature of zircons from ultrahigh pressure metamorphic rocks in Dabie–Sulu orogen. Jurassic Type III zircons have isotopically homogeneous δ18O values of 3.3±0.5‰. Collectively, zircon populations recorded the δ18O variations from source to low-δ18O granite. The primary protolith of Jurassic granites in Bengbu uplift is a Neoproterozoic granitic intrusion in South China Block. Later high-temperature meteoric hydrothermal alteration has lowered the whole-rock δ18O values in various degrees. The low-δ18O protolith was then buried to the middle-lower crust by Triassic continental subduction and Jurassic anatexis produced the low-δ18O granitic magma. Moreover, δ18O values of Type III zircons are isotopically lighter than previously reported values of magmatic garnets in Jurassic granites, indicating later magmatic evolution of low-δ18O magma toward high δ18O values probably as a result of infiltration of high-δ18O fluid from wall-rocks.