Abstract

Late Mesozoic granitoids, widely distributed in the Yanshan Belt of the North China Craton (NCC), are important markers of cratonic destruction and lithospheric thinning. We report new petrological, geochemical, zircon U–Pb and Lu–Hf isotopic data on two late Mesozoic granitic plutons from the central NCC near the Trans-North China Orogen. We report zircon U–Pb emplacement ages of 162 – 156 Ma and 132 Ma from the Siganding and Fangshan plutons respectively, indicating Late Jurassic–Early Cretaceous magmatism. The presence of a dusty zone, and discontinuous zoning, in plagioclase phenocrysts and the occurrence of mafic microgranular enclaves in both granitoid plutons suggest mingling and mixing of felsic and basic magmas. The rocks investigated in this study show high Na2O + K2O and high Mg# (Mg# = Mg/(Fe + Mg)) and are metaluminous, with enrichment in light rare earth elements and large ion lithophile elements (Rb, Ba, U, and Sr) and depletion in high strength field elements (Th, U, Nb, Ta, P, and Ti). They also display negative εHf(t) values and high Sr/Y ratios, comparable with adakitic rocks, suggesting that the felsic magmas of these intrusions were probably derived from the partial melting of the Palaeoproterozoic (two-stage zircon Hf-depleted model ages between 2.5 and 1.9 Ga) thickened lower crust of the NCC, whereas the mafic magma was probably derived from subcontinental lithospheric mantle. We correlate the magmatism with lithospheric thinning beneath the NCC and associated crust–mantle interaction. It is inferred that lithospheric thinning was of limited extent in Late Jurassic (162 – 156 Ma) times. The magma flare-up at around 130 Ma was produced by large-scale melting of the enriched subcontinental lithospheric mantle with development of extensive intracontinental adakitic magmatic rocks derived from thickened ancient lower continental crust with variable involvement of subcontinental lithospheric mantle.

Supplementary material: Mineral assemblages and abundance, representative electron microprobe analyses, LA-ICP-MS zircon U–Pb age data and LA-MC-ICP-MS Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3815308

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