Continental and Oceanic Crust Recycling-induced Melt–Peridotite Interactions in the Trans-North China Orogen: U–Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

摘要

We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt–lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4–2·5 Ga, 1·6–2·2 Ga and 0·6–1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt–peridotite interaction and subsequent high-pressure metamorphism. The ∼2·5 Ga zircons have positive ε_Hf(t) values (2·9–10·6), whereas the younger Precambrian zircons are dominated by negative ε_Hf(t) values, indicating an ancient continental crustal origin. These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 ± 3 Ma (2σ), 80–170 Ma and 48–64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt–peridotite interactions in the central zone of the NCC. The combination of the positive ε_Hf(t) values (2·91–24·6) of the 315 Ma zircons with the rare occurrence of 302–324 Ma subduction-related diorite–granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt–peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80–170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies. The ε_Hf(t) values of these zircons vary greatly from –47·6 to 24·6. The 170–110 Ma zircons are generally characterized by negative ε_Hf(t) values, whereas the 110–100 Ma zircons have positive ε_Hf(t) values. These observations suggest that melt–peridotite interactions at 80–170 Ma were induced by partial melting of recycled continental crust. The 48–64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive ε_Hf(t) values. These features imply that the melt–peridotite interactions at 48–64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.