Geochronology and geochemistry of deep-seated crustal xenoliths in the northern North China Craton: Implications for the evolution and structure of the lower crust

Abstract

The age and composition of the lower crust are critical in understanding the processes of continental formation and evolution, and deep-seated granulite xenoliths can offer direct information on the lower crust. Here, we report mineral chemistry, whole-rock major and trace elements, Sr–Nd isotopes and zircon U–Pb–Hf results for a suite of deep-seated crustal xenoliths, recently discovered in the Cenozoic basalts of the Nangaoya area in the northern part of the North China Craton (NCC). Based on the P–T estimates, these xenoliths including mafic, intermediate and felsic granulites and hornblendites were sampled from different levels of the lower crust. While a hornblendite has a flat REE pattern, all other xenoliths display LREE enrichment and depletion of Nb, Ta, Th and Ti. The mafic granulite xenolith has relatively high whole-rock εNd(t) value of −13.37, and yields Mesozoic (188–59Ma) zircons ages with high εHf(t) values from −15.3 to −9.2. The garnet-bearing intermediate granulite-facies rocks show low εNd(t) values from −16.92 to −17.48, and reveal both Paleoproterozoic (1948Ma) and Mesozoic (222–63Ma) zircon U–Pb ages. Their Mesozoic zircons have lower εHf(t) values (from −18.4 to −13.8) than those from the mafic xenolith. The remaining intermediate to felsic xenoliths show Paleoproterozoic zircon ages, and the lowest εNd(t) values (from −20.78 to −24.03).The mafic–intermediate granulites with Mesozoic zircons originated from the interaction of lower crust-derived magmas with mantle melts, with higher proportions of mantle magmas involved in the generation of mafic granulite, whereas intermediate to felsic xenoliths without Mesozoic zircons represent ancient Paleoproterozoic to Neoarchean deep crust. These deep-seated xenoliths reveal complicated crustal evolution processes, including crustal growth during Neoarchean (2.5–2.7Ga), middle Paleoproterozoic (2.2–2.1Ga) and Mesozoic, and reworking during early Paleoproterozoic, late Paleoproterozoic and Mesozoic related to magmatic underplating. The integrated analyses of lithological, geochemical and age data for a suite of deep-seated xenoliths show that the lower crust in the Nangaoya area is temporally and compositionally zoned. The upper part of the lower crust mainly comprises Neoarchean to Paleoproterozoic intermediate–felsic rocks with intercalated hornblendites, the majority of which record ~1950 and ~1850Ma metamorphism; the middle part is dominated by a Paleoproterozoic and Mesozoic intermediate garnet-bearing granulite-facies hybrid layer; and the lowermost crust is represented by a Mesozoic mafic granulite layer, which was significantly modified by episodic magmatic underplating. Such a modification induced by crust–mantle interaction can result in Mesozoic ages and more mafic components for xenolith granulites, and thus is an effective mechanism to explain the differences between exposed and xenolithic granulites.