Distinct mantle sources of two types of Neoproterozoic diabase in the North China Craton: Implications for mantle plume activity and ancient continent reconstruction

Abstract

Large igneous provinces (LIPs) are often used as powerful tools for constraining continental reconstructions. The configuration of the North China Craton (NCC) in the Rodinia supercontinent remains a subject of controversy due to limited reliable records of Neoproterozoic magmatism. In this study, two types of Neoproterozoic mafic magmatic rocks, known as the Langan diabase (∼913 Ma), have been identified in the southeastern margin of the NCC: (1) Group Ⅰ that exhibits low La/Yb ratios (3.8–6.4), ∑REE contents (51–74 ppm), ε_Nd(t) (-6.9 – -0.3), ε_Hf(t) (1.98–6.26) and relatively high ⁸⁷Sr/⁸⁶Sr_(i) (0.705047 to 0.706353), Mg^# (58–63) and δ¹⁸O values (6.65–7.82‰); (2) Group Ⅱ-1 that has high ∑REE contents (214–222 ppm), ε_Nd(t) (2.3–4.1), ε_Hf(t) (5.03–10.70), and low ⁸⁷Sr/⁸⁶Sr_(i) (0.704479–0.704829), Mg^# (~33), La/Yb ratios (6.1–6.3) and δ¹⁸O values (6.36–7.20‰). Meanwhile, Group Ⅱ-2 mafic rocks can be identified in previous studies that differ from the Group Ⅱ-1 samples in having much higher La/Yb ratios (8.0–19.0). The geochemical differences between Group Ⅰ and Group Ⅱ mafic rocks indicate that they might have originated from distinct mantle sources under varying melting conditions and undergone different differentiation and contamination processes. In the shallow magma chamber, Group Ⅰ samples underwent a process of fractionation involving olivine and clinopyroxene, whereas Group Ⅱ samples exhibited fractional crystallization encompassing olivine, clinopyroxene, plagioclase, and rutile. According to REE inversion calculations, the primary magma of Group Ⅰ samples were derived from the high degrees of partial melting (5–20 %) of a spinel-dominant peridotite mantle source, and the Sr-Nd-O-Hf isotope composition indicates that there are approximately 30 % continental lithospheric mantle components in their mantle source. In contrast, the Group Ⅱ-1 diabase’ parental magmas were generated by 5–20 % partial melting of a garnet-bearing pyroxenite mantle source, while the Group Ⅱ-2 samples were derived from low degrees of partial melting (3–10 %) of a garnet-dominant pyroxenite mantle. All Group Ⅱ sample mantle sources contain approximately 20 % lithospheric components.

Combining all Neoproterozoic mafic samples in the São Francisco Craton and the Congo Craton, this research pointed out that these rocks might have resulted from a mantle plume. In detail, at the beginning of mantle plume activity, the primary magma of Group Ⅱ-1 sample was derived from the mixing between the mantle plume margin and a small amount of overlying lithospheric mantle in the spinel-garnet transition zone. Shortly thereafter, in a brief span of time, the subsequent upwelling of high-temperature magma assimilated more lithospheric components in the plume’s axial area, producing the Group Ⅰ samples’ primary magma in the spinel stability field. Within the periphery of the mantle plume, the thicker lithospheric mantle confines melting activities to occur only in the garnet stability field, resulting in a relatively low degree of melting and producing the primary magma of Group Ⅱ-2 samples. Finally, based on the spatial chemical variation of all Neoproterozoic mafic magmas, a new model for reconstructing ancient continents has been proposed, which illustrated the NCC was adjacent to both the São Francisco and the Congo Craton, and the Xuhuai Basin, Pedro Lessa diabase, Espinhaço mafic rocks and Gangila meta-basalts could represent the mantle plume’s central position.