Hannuoba intermediate-mafic granulite xenoliths revisited: Assessment of a Mesozoic underplating model
The intermediate-mafic granulite xenoliths from Tertiary Hannuoba alkali basalts at the northern margin of the North China Craton have been commonly considered to be formed by Mesozoic basaltic underplating due to their abundant Mesozoic zircons and compositionally higher Mg# (molecular Mg / (Mg + Fe)) than rocks of the adjourning Archean granulite terrain. However, such a Mesozoic underplating model cannot account for the presence of Precambrian zircons in the xenoliths and their evolved Sr-Nd isotopic compositions which are dissimilar to expected basaltic melts from both the lithospheric and asthenospheric mantle. Instead, their Sr-Nd isotopic compositions resemble those of the granulite terrain and of the Mesozoic low Mg# granitoids. Here newly acquired data and published data are combined to assess the Mesozoic basaltic underplating model and to discuss the origin of the Hannuoba intermediate-mafic granulite xenoliths in terms of their relationships with the Mesozoic magmatism and the granulite terrain. Zircons from a mafic granulite xenolith (JN0724), which has geochemical and Sr-Nd isotopic compositions indistinguishable from other mafic granulite xenoliths, yield ∼ 2.5 Ga U-Pb ages, providing unambiguous evidence that the mafic xenoliths represent portions of ancient lower crust. Combined data reveals that the pyroxene-rich granulite xenoliths can be best explained as restites left after partial melting of the ancient lower crust to produce the Mesozoic low Mg# granitoids. Hf isotope analysis on Mesozoic zircons from the Hannuoba feldspar-rich granulite xenoliths demonstrates that the zircons crystallized from Mesozoic granitic magmas that were derived from ancient lower crust rather than from Mesozoic underplated magmas. The proposed scenarios successfully explain the age and compositional differences between granulite xenoliths and the granulite terrain and also provide new insights into derivation and evolution of granitic magmas. It is concluded that most of the Hannuoba intermediate-mafic granulite xenoliths and the Mesozoic granitoids reflect reworking of ancient lower crust rather than juvenile crust addition. It provides strong evidence that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust. Our data imply that whether younger zircon ages in granulite xenoliths from many other Archean cratons register post-Archean basaltic underplating could be tested by integrated analysis of U-Pb age and Hf isotopes.