Contrasting topography of Rodinia and Gondwana recorded by continental-arc basalts

Widespread orogenesis associated with supercontinent assembly generates extensive collisional mountain chains with high elevations that are crucial for modulating the interaction between the atmosphere and lithosphere. However, not all supercontinents formed over Earth's history shared the same topography. This study investigates the temporal variation over the last 1.5 Ga in La/Zr ratio of continental-arc mafic volcanic rocks. We show that 0.6–0.5 Ga rocks related to Gondwana assembly are characterized by much higher La/Zr ratios compared with 1.2–0.9 Ga rocks formed during Rodinia assembly. Thermodynamic modeling reveals that the La/Zr ratios of continental-arc magmas are predominantly controlled by melting depth, which is directly tied to the thickness of the overlying arc crust. Thick continental arc decreases the extent of melting in the mantle wedge, leading to an increase in La/Zr ratios. Higher La/Zr ratios of continental-arc rocks during Gondwana vs. Rodinia assembly thus imply thicker continental arcs for the younger supercontinent. We suggest that the secular La/Zr ratio of continental-arc mafic magmas is an effective geochemical indicator of topography difference between the Rodinia and Gondwana supercontinents, with implications for coupling between the evolution of the lithosphere and atmosphere.