Petrogenesis of mafic dykes and high-Mg adakitic enclaves in the Late Mesozoic Fangshan low-Mg adakitic pluton, North China Craton

摘要

Late Mesozoic mafic dykes, high-Mg adakitic enclaves and the low-Mg adakitic host from the Fangshan plu-ton provide important insights into the process of crust-mantle interaction. The mafic dykes (134 ± 2 Ma) have a wide range of total alkali (Na_2O + K_2O: 3.41-7.11 wt.%), MgO (2.6-6.5 wt.%), Ni (60.1-541.7 ppm), and Cr (138.2-749.5 ppm) contents and high Mg~# values (57.6-72.0). They are enriched in LREEs and LILEs (Rb, Ba, K) and depleted in HREEs and HFSEs (Nb, Ta, Zr, Hf), suggesting a garnet-bearing enriched mantle source. The host granodiorites (136 ± 2 Ma) have low-Mg adakitic geochemical compositions. They belong to the of high-K calc-alkaline series with low MgO contents (1.4-3.0 wt.%) and Mg~# values (41.9-48.1). They exhibit extremely high Sr and low Y and Yb contents, and are strongly enriched in LREEs resulting in high Sr/Y and (La/Yb)_N ratios. The low-Mg adakitic granodoirites were probably generated by partial melting of thickened eclogitic lower continental crust (LCC). The enclaves (136 ± 2 Ma) with a strong affin-ity to the mafic dyke show high-Mg adakitic geochemical characteristics. They are alkaline series with rel-atively low SiO_2 (52.2-59.6 wt.%) and high MgO (Mg~# = 44.7-57.2) contents. They contain high Sr, Ni and Cr, low Y and Yb, and have very high Sr/Y and (La/Yb)_N ratios, with depletion in HFSEs (Nb, Ta, Zr, Hf, Ti) but without Eu and Sr negative anomalies. Numerical simulation indicates that the high-Mg adakitic enclaves were likely produced by mixing of low-Mg adakitic melt with mantle- derived magma. We propose that the underplating of the mantle-derived magma might have triggered partial melting of the thickened LCC and resulted in the mixing between them in the Early Cretaceous.