Geotectonic Identity of Cretaceous-Paleogene Granitoids in the Tsukuba Igneous Complex, Japan: A New Multi-Proxy Reassessment

Abstract

To reassess the geotectonic identity of the Tsukuba Igneous Complex (TIC) granitoids, we compiled a comprehensive data set of the granitoids and associated microgranular enclaves (MGEs) using zircon U–Pb geochronology, magnetic susceptibility (MS) analysis, and whole-rock geochemistry. The TIC granitoids comprised high-K, calcic to calc-alkaline, and peraluminous I-type granite. SiO₂ values were relatively high, and enrichment in Pb was observed while Ti and Nb were both depleted. The trace element signatures are diagnostic indicators for rocks formed in subduction-related settings. The MGEs in the Kabasan granitoid body were almost coeval with the host granitoids at ca. 79–76 Ma. Taking into consideration the mineral and geochemical compositions between the MGEs and the host granitoids, it was concluded that the MGEs originated from diorite xenoliths. Moreover, our new zircon U–Pb dating of TIC granitoids and MGEs showed that the TIC emplacement ages are divided into two groups; that is, ca. 80–76 Ma and ca. 70–61 Ma. This fact clearly demonstrates that TIC magmatism occurred two times during the Late Cretaceous to Early Paleogene. On the other hand, the data of TIC MS is two or three orders of magnitude lower than that of San-in granitoids MS, and the TIC granitoids belong to the ilmenite-series. The MS differences between the TIC and San-in granitoids can be explained by the amount of involved sediment, indicating that the zircon U–Pb dating is the most appropriate proxy for the reassessment of the geological identity of TIC granitoids. Accordingly, we propose that the TIC granitoids have two separate origins based on the emplacement age: that is, the Late Cretaceous TIC granitoids belong to those of the Ryoke or San-yo belts, whereas the Early Paleogene TIC granitoids are considered as the eastern extension of the San-in belt.