Geochemistry of Meso- and neoproterozoic mafic-ultramafic rocks from northern Guangxi, China: Arc or plume magmatism?

摘要

Meso- and Neoproterozoic mafic-ultramafic rocks from northern Guangxi show mainly calc-alkaline features. The mafic rocks have (Nb/La)(pm)(1)) = 0.13similar to0.51, (Th/La)(pm) = 0.85-3.3, Ti/Ti*(2)) = 0.29-0.79 and show negative Nb and Ti anomalies in primitive mantle normalized diagrams. The Mesoproterozoic mafic rocks have lower epsilon(Nd)(T) than those of Neoproterozoic, -1.99 similar to -5.13 vs. -0.74 similar to 2.4, respectively. They plot in the field of volcanic arc basalt in tectonic discrimination diagrams and display geochemical signatures of island arc volcanics. The mafic-ultramafic rocks are thought to be the products of magmatism of convergent plate boundary rather than derived from mantle plume. Therefore, they can not be regarded as an indicator of breakup of Rodinia supercontinent.