Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume

摘要

Mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the Neoproterozoic supercontinent Rodinia, with supportive evidence including radiating dike swarms, globally synchronous anorogenic igneous activity, large-scale lithospheric doming and unroofing, and geochemical signatures similar to recent plume-related magmatism. However, high-temperature magmas such as picrite or komatiite, a requisite product of mantle plume activities, have not previously been identified for those events. We present here geochronological and geochemical data from a suite of pillow lavas in central South China. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 ± 6 Ma. All but a few highly evolved, crust-contaminated basaltic rocks are characteristically high in MgO (10.2%–17.5%), Ni (183–661 ppm), and Cr (677–1672 ppm), but low in TiO2 (0.5%–0.7%), Al2O3 (10.6%–12.7%), and FeOT (total Fe as FeO) (7.4%–10.5%), typical of komatiitic basalts. Our geochemical modeling, which removes the effect of olivine crystallization, suggests that their primary magma has typical komatiitic compositions with MgO 20%, FeOT 11%, SiO2 47%, TiO2 0.48%, Al2O3 10%, Ni 860 ppm, and Cr 1780 ppm. Such a high MgO content in the primary melts implies a melt temperature of >1500 °C, suggesting that the Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature (Tp) 260 ± 50 °C higher than the ambient mid-oceanic-ridge basalt (MORB)–source mantle, similar to that of modern mantle plumes. The Yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca. 825 Ma mantle plume that played a key role in the breakup of the supercontinent Rodinia.