Crustal contamination versus an enriched mantle source for intracontinental mafic rocks: insights from early Paleozoic mafic rocks of the South China Block

Several recent studies have documented that the silicic rocks (SiO₂ > 65 wt.%) comprising Silicic Large Igneous Provinces are derived from partial melting of the crust facilitated by underplating/intraplating of "hidden" large igneous province-scale basaltic magmas. The early Paleozoic intracontinental magmatic rocks in the South China Block (SCB) are dominantly granitoids, which cover a combined area of ~22,000 km². In contrast, exposures of mafic rocks total only ~45 km². These mafic rocks have extremely heterogeneous isotopic signatures that range from depleted to enriched (whole rock initial ⁸⁷Sr/⁸⁶Sr = 0.7041–0.7102; ε_Nd(t) = −8.4 to +1.8; weighted mean zircon ε_Hf(t) = -7.4 to +5.2), show low Ce/Pb and Nb/U ratios (0.59–13.1 and 3.5–20.9, respectively), and variable Th/La ratios (0.11–0.51). The high-MgO mafic rocks (MgO > 10 wt.%) tend to have lower ε_Nd(t) values (<−4) and Sm/Nd ratios (<0.255), while the majority of the low-MgO mafic rocks (MgO < 10 wt.%) have higher ε_Nd(t) values (>−4) and Sm/Nd ratios (>0.255). The differences in geochemistry between the high-MgO and low-MgO mafic rocks indicate greater modification of the compositions of high-MgO mafic magmas by crustal material. In addition, generally good negative correlations between ε_Nd(t) and initial ⁸⁷Sr/⁸⁶Sr ratios, MgO, and K₂O, along with the presence of inherited zircons in some plutons, indicate that the geochemical and isotopic compositions of the mafic rocks reflect significant crustal contamination, rather than an enriched mantle source. The results show that high-MgO mafic rocks with fertile isotopic compositions may be indicative of crustal contamination in addition to an enriched mantle source, and it is more likely that the lithospheric mantle beneath the SCB during the early Paleozoic was moderately depleted than enriched by ancient subduction processes.