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 similar to 22,000 km(2). In contrast, exposures of mafic rocks total only similar to 45 km(2). These mafic rocks have extremely heterogeneous isotopic signatures that range from depleted to enriched (whole rock initial Sr-87/Sr-86 = 0.7041-0.7102; epsilon(Nd)(t) = -8.4 to + 1.8; weighted mean zircon epsilon(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 epsilon(Nd)(t) values (<-4) and Sm/Nd ratios (<0.255), while the majority of the low-MgO mafic rocks (MgO <10 wt.%) have higher epsilon(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 epsilon(Nd)(t) and initial Sr-87/Sr-86 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 SO during the early Paleozoic was moderately depleted than enriched by ancient subduction processes. (C) 2017 Elsevier B.V. All rights reserved.