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

Abstract

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 >3000m thick volcanic sequence erupted within an extremely short period of time (132–130Ma). 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.