Downward rejuvenation of the continental lower crust beneath the southeastern North China Craton

Felsic to mafic granulite xenoliths and zircon xenocrysts from the Cenozoic (~1 Ma) Nushan basalts were analyzed to construct a lower-crustal column illustrating the fine structure and evolution of the deep crust beneath the southeastern North China Craton. Felsic granulites, sampled from the lowermost middle to the upper lower crust have a calculated V_p of 6.32 km/s. Relict magmatic zircons from them yield an upper intercept age of 2716 ± 32 Ma and depleted-mantle Hf model ages (T_DM) of 3.1–2.9 Ga. Metamorphic zircons with ages of ca. 2100 Ma have initial ¹⁷⁶Hf/¹⁷⁷Hf ratios similar to the magmatic ones, implying in situ recrystallization of the older zircons. The intermediate granulites with Paleoproterozoic ages (2.1–1.9 Ga; whole-rock Nd T_CHUR = 3.4 Ga and zircon Hf T_DM = 3.1–2.8 Ga; Huang et al., 2004; Zheng et al., 2012) have a calculated V_p of 6.53 km/s, being considered that they underlie the felsic granulites. The lowermost crust is represented by mafic granulites (V_p = 6.86 km/s). With the exception of three inherited zircons from the mafic granulites, which yielded an upper intercept age of 2455 Ma and T_DM of ca. 2.7 Ga, most zircons formed in the late Mesozoic (ca. 115 Ma). Zircon xenocrysts have concordant ages, including the Neoproterozoic, early Paleozoic, late Paleozoic, early Mesozoic and Cenozoic. Our data, integrated with published data, suggest that the lower crust is younger at the bottom and dominantly composed of felsic, intermediate and mafic granulites. This trend reflects the vertical accretion of the crust, including the addition of juvenile material during the Mesoarchean and Neoarchean, and repeated reworking during the Neoarchean, Paleoproterozoic and late Mesozoic. The Neoproterozoic, Paleozoic and early Mesozoic zircon xenocrysts may record the subduction events between the craton and the Yangtze Block, which also affected the nature of the lower crust beneath the southeastern part of the craton.