Ca. 2.0 Ga mafic dikes in the Kongling Complex, South China: Implications for the reconstruction of Columbia

Paleoproterozoic mafic dikes from the Kongling Complex in South China provide an ideal opportunity to examine mantle properties and tectonic evolution of the Yangtze Block and thus to understand the paleo-position of South China in the Columbia supercontinent. The 2.0 Ga dikes are the oldest mafic intrusions identified in the Yangtze Block by far. Samples from the dikes consist of fine to medium-grained dolerite and have variable SiO2 (43.48-52.61 wt%) and MgO contents (3.17-7.35 wt%), They are enriched in LREE and show slightly negative to positive Eu anomalies (Eu/Eu* = 0.85-1.06). Their primitive mantle-normalized trace element patterns are characterized by enrichment of large-ion lithophile elements (Rb, Ba, Pb) and depletion of high-field-strength elements (U, Nb, Ta, Ti). Rocks from the dikes have variable initial Sr-87/Sr-86 ratios (0.698874-0.710753), epsilon Nd values (- 3.0 to +4.1) and zircon epsilon Hf values (- 2.7 to + 3.4). These geochemical features indicate that their parental magmas were formed by < 10% partial melting of a spinel-bearing lherzolite mantle in a back-arc basin setting. Paleoproterozoic mafic rocks from South China, Northwest Laurentia, Northwest Australia and Siberia had different trace element ratios and crystallization ages, their mantles had different thermal states. Thus, we argue against the previous model that the four continents were assembled together in the Columbia supercontinent.