The minor and trace element geochemistry of volcanic rocks from Truk,Ponape and Kusaie,Eastern Caroline Islands; the evolution of a young hot spot trace across Old Pacific Ocean Crust

Recent paleomagnetic and K-Ar age dating studies in the Caroline Islands show that the high volcanic islands of Truk, Panape and Kusaie formed at the same shallow latitude and display a progression of ages increasing westward from 1 m.y. (Kusaie), through 6 m.y. (Ponape) to 12 m.y. (Truk). These data suggest that the islands were formed by a young hotspot or melting anomaly. One hundred and fifty volcanic rocks from these three islands have been analysed for major and 15 trace elements and results indicate that shield-building lavas on each island (1) evolved by low-pressure open-system fractional crystallisation of a small range of partial melts and (2) became more alkaline with time from Truk to Kusaie. The secular geochemical variation along the Caroline chain may represent a hotspot trace whose magma production was slowly declining over the past 14 m.y.