Lithological and age structure of the lower crust beneath the northern edge of the North China Craton: XENOLITH evidence

Deep-seated xenoliths in volcanic rocks offer direct glimpses into the nature and evolution of the lower continental crust. In this contribution, new data on the U-Pb ages and Hf isotopes of zircons in six felsic granulite xenoliths and one pyroxenite xenolith from the Hannuoba Cenozoic basalts, combined with published data from mafic to felsic xenoliths, are used to constrain the lithological and age structure of the lower crust beneath the northern edge of the North China Craton. Two newly-reported felsic granulites contain Precambrian zircons with positive (+7.5-+10.6) and negative ε_Hf values (-10.1 to -3.7) corresponding to upper intercept ages of 2449±62Ma and 1880±54Ma, respectively, indicating crustal accretion in the late Archean and reworking in Paleoproterozoic time. Zircons in another four felsic xenoliths give Phanerozoic ages from 142Ma to 73Ma and zircons from one pyroxenite xenolith give a concordant age of 158Ma. The zircon ε_Hf values of these four felsic xenoliths range between -23.3 and -19.1, reflecting re-melting of the pre-existing lower crust. Integration of geothermobarometric, and geochronological data on the Hannuoba xenoliths with seismic refraction studies shows that the lower crust beneath the northern edge of the North China Craton is temporally and compositionally zoned: the upper lower crust (24-33km) consists dominantly of Archean (~2.5Ga with minor 2.7Ga) felsic granulites with subordinate felsic granulites that reworked at 140-120Ma; both Precambrian and late Mesozoic mafic granulites are important constituents of the middle lower crust (33-38km); major late Mesozoic (140-120Ma) and less Cenozoic (45-47Ma) granulites and pyroxenites are presented in the lowermost crust (38-42km). The zoned architecture of the lower crust beneath Hannuoba suggests a complex evolution beneath the northern margin of the craton, including late Neoarchean (~2.5Ga) accretion and subsequent episodic accretion and/or reworking during Paleoproterozoic (~1.8-1.9Ga) and Phanerozoic (~220Ma, 120-140Ma, 45-47Ma) times, with possible links to circum-craton subduction/collision events and the destruction of the North China Craton.