The effects of selective contamination on the early Paleozoic intracontinental mafic rocks in the South China Block: New insights from high-δ18O zircon

Abstract

The petrogenesis of continental mafic rocks that are enriched in large-ion lithophile elements (LILE) and that have enriched Sr–Nd–Hf isotope compositions is debated, with some models suggesting an enriched mantle source and others arguing for crustal contamination. Crustal contamination has generally not been favored as numerical models indicate that this would require the addition of significant amounts of crustal materials. Here we present the first detailed study of the oxygen isotope composition of zircons from representative early Paleozoic mafic, intermediate, and felsic magmatic rocks in the South China Block (SCB), and combine this information with available geochemical data to re-examine whether crustal contamination or an enriched mantle source is the principal cause of the crustal geochemical signatures in these rocks. Zircon grains in the high-MgO mafic rocks (MgO > 8 wt%) with low εNd(t) values (< −6) have similar, or slightly lower, mean δ18Ozrn values (e.g., 8.3 ± 0.1‰ and 8.5 ± 0.2‰ for zircons in the Fuxi monzonorite and Xinchuan hornblendite, respectively) than those in coeval granitoids (9.2 ± 0.2‰ and 8.6 ± 0.3‰ for zircons in the Fuxi granite and Xinchuan quartz monzonite, respectively). Zircon grains in the Yonghe low-MgO olivine-bearing gabbro (εNd(t) = −2.9 to −1.5) and the Guantian quartz diorite (εNd(t) = −3.5 to −3.0) are characterized by relatively low mean δ18Ozrn values (7.2 ± 0.1‰ and 7.7 ± 0.1‰, respectively). Considering that the early Paleozoic mafic rocks in the SCB have heterogeneous Sr–Nd–Hf isotope signatures (whole-rock initial 87Sr/86Sr = 0.7041–0.7114; εNd(t) = −8.4 to +1.8; weighted mean zircon εHf(t) = −8.7 to +5.2) and crustal Ce/Pb, Nb/U, Rb/Sr, and Th/La ratios (0.59–13.1, 3.3–20.9, 0.05–0.68, and 0.11–0.51, respectively), it is suggested that crustal contamination, rather than an ancient subduction-modified enriched mantle source, was the principal cause of the crustal geochemical signatures in these rocks. A refined model is proposed in which water played a key role in the preferential transfer of fluid-mobile elements from crust-derived hydrous felsic magmas to mantle-derived mafic magmas, leading to the enrichment of fluid-mobile elements (e.g., LILE), the fractionation of these elements (e.g., elevated La/Sm, La/Ta, Rb/Sr, and Th/La, and depressed Ce/Pb, Nb/U, and Sm/Nd), and the enrichment in 18O in the selectively contaminated mafic magmas.