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

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 litho-spheric 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 Fe0 (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 approximate to 20%, FeOT approximate to 11 % SiO2 approximate to 47%, TiO2 approximate to 0.48%, Al2O3 approximate to 10%, Ni approximate to 860 ppm, and Cr = 1780 ppm. Such a high MgO content in the primary melts implies a melt temperature of >1500 degrees C, suggesting that the Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature (T.) 260 +/- 50 degrees 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.