Mantle source of Late Paleozoic mafic-ultramafic magmatic rocks in the Baoshan block of Sanjiang Tethys: Constraints from Mg isotopes

In the Baoshan Block of the Sanjiang Tethys, southwestern China, intense Late Paleozoic (~ca. 320~280Ma) mafic-ultramafic magmatism developed (~1.2×104km2), producing widespread continental flood basalts, mafic-ultramafic intrusions, and diabase dikes. However, the mantle source of this magmatism remains to be constrained. In this study, we investigate the mantle metasomatic features of the Baoshan block through whole-rock Mg isotope analyses of continental flood basalts and mafic-ultramafic intrusions, integrated with whole-rock major and trace element geochemistry as well as Sr-Nd-Hf isotopes. The continental flood basalts from the Baoshan block display highly variable δ26Mg values (-0.36‰ to +0.02‰, average -0.22‰), which show no correlation with geochemical indicators of magmatic processes such as (La/Sm)N, Nb/Zr, MgO, or CaO/Al2O3. This suggests that partial melting and fractional crystallization had only minor effects on the Mg isotopic compositions of the basalts. These basalts exhibit slightly depleted mantle-like Sr-Nd isotopic signatures, with no correlation to SiO2 and MgO, together with their Nb/Ta and Nb/La ratios, indicating negligible crustal contamination. Therefore, the Mg isotopic variations of the Baoshan continental flood basalts primarily reflect mantle source heterogeneity. By comparison with typical mantle reservoirs, we infer that the Baoshan continental flood basalts were derived from a deep-seated mantle plume carrying recycled altered oceanic crust. Both altered oceanic crust and clastic sediment melts contributed to the wide Mg isotopic range (~0.38‰) and relatively heavy values (-0.19‰ to +0.02‰), whereas the light Mg isotopic signatures (-0.36‰ to -0.30‰) observed in a few samples together with low CaO/Al2O3 (<0.7) and high Hf/Hf* (1.1~1.4) indicate weak contributions from carbonate melt metasomatism. In contrast, the Daxueshan mafic-ultramafic intrusions, which host typical magmatic Ni-Cu sulfide mineralization, exhibit relatively homogeneous δ26Mg values (-0.30‰ to -0.19‰, average -0.25‰), mostly within the mantle range, suggesting a mantle source distinct from those of the Baoshan flood basalts. The Daxueshan intrusions are characterized by enriched Sr-Nd isotopic compositions, together with widely variable whole-rock γOs(t) and zircon εHf(t) values, implying derivation from a metasomatized, enriched lithospheric mantle that experienced significant crustal contamination. Compared with giant Ni-Cu sulfide deposits hosted in the Jinchuan and Xiarihamu intrusions, the Daxueshan mafic-ultramafic intrusions may not have experienced pronounced metasomatism by carbonate sediments. Instead, a pyroxenite mantle source metasomatized by silicate melts/fluids may have played a more critical role in Ni metallogenesis at Daxueshan.