Evidence for a hot spot origin of the Caroline Islands

Abstract

The results of paleomagnetic, petrographic, and radiometric studies of the Eastern Caroline Islands in the western Pacific indicate that the islands were formed by a hot spot located near the paleoequator between 1 and 11 Ma. The islands show a linear progression of mean ages from 1 Ma in the east (Kusaie) to 11 Ma in the west (Truk). The results of volumetric measurements and geochemical studies suggest that the hot spot source is waning and perhaps was slowly dying during the time Truk, Ponape, and Kusaie were being formed. The dominant shield‐building magmas in the Caroline Islands are part of a differentiated alkalic series. The posterosional lavas are highly silica undersaturated and trace element enriched nephelinites. The latter were erupted subsequent to the cessation of the main shield phase of volcanism. The petrography and geochemical evolution of Truk are strongly reminiscent of that of the Hawaiian chain; however, the shield‐building lavas are compositionally similar to the alkalic lavas that typically form only thin, late‐stage caps on many Hawaiian volcanos. No tholeiitic rocks were found despite sampling deep within the eroded volcanic structure of the islands. This absence of tholeiitic lavas and dominance of alkalic lavas stand in contrast with Hawaii, where tholeiitic volcanism dominates and alkalic lavas form only a minor component of the exposed lavas. The absence of tholeiitic lavas in the main shield‐building phase of construction, however, is not unique to the Caroline Islands. Dominant alkalic volcanism appears characteristic of other seamounts in the Pacific, including the Samoan, Austral‐Cook, and Line Islands.