Early Cretaceous potassic volcanic rocks in the Jiangnan Orogenic Belt, East China: Crustal melting in response to subduction of the Pacific-Izanagi ridge?

Eastern China is characterized by widespread Mesozoic magmatism that was closely related to subduction of the Paleo-Pacific Plate, but the genetic relationship between magmatism and subduction is not always clear. The Jiangnan Orogenic Belt (JOB) is a Neoproterozoic collisional belt between the Yangtze and Cathaysian blocks. Early Cretaceous felsic magmas from the Tianmushan basin in the eastern JOB form a > 3000 m thick volcanic sequence erupted within an extremely short period of time (132-130 Ma). These rocks are potassic (K2O = 3.86-6.05%) and peraluminous (A/CNK = 1.0-1.23), and are compositionally similar to experimental melts derived from predominantly clay-poor and K-rich metasediments or granodiorites at crustal conditions. They are LILE- and LREE-enriched ((La/Yb)CN = 9.0-14.6) and display variable Eu anomalies and Nb-Ta depletion. The weakly variable and negative whole-rock εNd(t) (− 5.4 to − 2.5) and heterogeneous zircon εHf(t) values (− 6.7 to + 3.5) of these rocks are similar to those of basement rocks beneath the JOB, suggesting a genetic relationship with basement. No magmas with clear subduction-related composition occurred at this time in the area and the compositions of the potassic rocks more closely resemble high-temperature melts of extensively thinned crust in continental rift environments. Our data favors a model that the Early Cretaceous potassic magmas formed as subduction of the Pacific-Izanagi ridge resulted in upwelling asthenosphere, providing the high temperatures required to extensively melt the lower-middle crust in the eastern JOB.