Geochemical and chronological constraints on the mantle plume origin of the Caroline Plateau

Abstract

The genesis of an oceanic plateau provides important information on the composition and melting dynamics of the deep mantle. The Caroline Rise, consisting of the West and the East Caroline Ridges, has played an important role in the evolution of the subduction system of the western Pacific; however, the nature and origin of the Caroline Rise are poorly known. Here, on the basis of seafloor sampling, we confirm for the first time that the Caroline Rise represents an oceanic plateau (the Caroline Plateau) that formed as a large igneous province. The Caroline Plateau is connected with the Caroline Seamount Chain to the east, and the seamount chain formed above a deep-rooted hotspot. To investigate the nature of the mantle source of the Caroline Plateau and its genetic relationship with the Caroline Seamount/Island Chain, we age-dated basalt samples from the Caroline Plateau and analyzed their major and trace elements and Sr–Nd–Pb–Hf isotopes. The samples can be classified as alkali or tholeiitic basalt. The tholeiitic basalts (15.62 ± 3.16 to 19.26 ± 0.35 Ma) are older than the alkali basalts (8.13 ± 0.81 Ma) and the volcanics of the Caroline Seamounts/Islands to the east. The tholeiitic basalts have trace element patterns similar to those of basalts of the Ontong Java Plateau. We suggest that the tholeiitic basalts represent the main stage of volcanism on the Caroline Plateau and the alkali basalts the late stage. The alkali basalts of the plateau have isotopic compositions similar to those of the basalts of the Caroline Islands, indicating an origin from the same enriched mantle end-member. The tholeiitic basalts with a MORB-like depleted geochemistry indicate the involvement of depleted components in their mantle source, and the Sr–Nd–Pb–Hf isotope compositions of the tholeiitic basalts can be reproduced by mixing alkali basalt with a depleted MORB mantle component. Based on the relatively low Sm/Yb ratios of the two groups of basalts, the Caroline Plateau is inferred to have formed on a young Caroline Plate with a thin lithosphere. We suggest that the geochemistry and ages of formation of volcanic rocks in the Caroline Plateau/Seamount system can be explained by the activities of the Caroline hotspot. The volcanic rocks of the Caroline Plateau and Islands show increasing alkalinity and Sm/Yb ratios with time, which reflect thickening of the lithosphere and decreasing activity of the Caroline mantle plume.