Isotopic variations with distance and time in the volcanic islands of the Cameroon line: evidence for a mantle plume origin

Abstract

The oceanic sector of the Cameroon line consists of three volcanic islands: Principe, Sao Tome and Pagalu. New 40Ar---39Ar data for Pagalu basalts, combined with published K---Ar ages for Principe and Sao Tome, indicate that all three islands have been active in the past 5 Ma. They have similar petrogenetic histories, with basements of basaltic flows capped by more evolved rocks. However, the age of the earliest exposed volcanic rocks decreases oceanward from Principe (31 Ma) to Sao Tome (13 Ma) to Pagalu (4.8 Ma). This age progression is consistent with the suggested motion of the African plate over this period of time. The average incompatible trace element compositions of 87Sr/86Sr)t increases from 0.7029 to 0.7037 and (206Pb/204Pb)t decreases from 20.2 to 18.9 from Principe through Sao Tome to Pagalu for all samples younger than 10 Ma. In addition to the overall spatial isotopic variations, Principe and Sao Tome display temporal isotopic variations, with Pb isotopic ratios becoming progressively more radiogenic. Pagalu shows no temporal geochemical or isotopic differences and the island has the least radiogenic Pb but most radiogenic Sr. These distinctive Pb, Sr and Nd isotopic compositions are also found in the early tholeiitic hyaloclastite breccia from Principe (31 Ma). Similarly, the Nd and Sr isotopic compositions are identical to those of the earliest Sao Tome lavas (13 Ma) and the Pb isotopic compositions of early Sao Tome samples are only slightly radiogenic relative to Pagalu. Therefore, it is probable that all these islands were initiated from a common source, similar to that of Pagalu, that migrated relative to the melt zone of each island with time. Since their initiation, the magma conduits at Principe and Sao Tome have been gradually modified by the introduction of a HIMU component. The common source from which the islands were initially derived probably represents ambient upper mantle, entrained with the plume head during ascent. This entrained component is like `PREMA', but the Nd and Sr isotopic data indicate that it represents variably mixed depleted and enriched components, such as DMM and EMI. The HIMU component is probably representative of a lower mantle source from which the plume head was derived. The long-lived episodic magmatism on Principe provides evidence that the initial melt migration paths from the upper mantle form a hot zone that can be re-activated after long periods (107 yr) of apparent quiescence. The progression to HIMU characteristics within each island probably reflects the gradual flattening of the contaminated plume head within this hot zone, near the base of the lithosphere, and the melting of a stem composed of relatively uncontaminated HIMU mantle.