Volcanic successions in Marquesas eruptive centers: A departure from the Hawaiian model

The temporal evolution and geochemical evolution of Marquesas hotspot volcanoes have often been interpreted with reference to the Hawaiian model, where a tholeiitic shield-building stage is followed by an alkali basaltic post-shield stage, followed after a 0.4 to 2.5 Myr long quiescence period, by a rejuvenated basanitic/nephelinitic stage. Here we discuss geochemical data on 110 Marquesas lavas also dated using the unspiked 40K-40Ar method on separated groundmass (including 45 new ages measured on the southern islands of Hiva Oa, Motane, Tahuata and Fatu Hiva). Sample locations were positioned on detailed geological maps to determine their shield or post-shield position with respect to the caldera collapse event(s), without taking into account their geochemical features. A rather regular decrease of the ages towards SE, consistent with the Pacific plate motion, is observed from Eiao (5.52 Ma) to Fatu Hiva (1.11 Ma), and rejuvenated basanitic volcanism occurs only in Ua Huka (1.15-0.76 Ma). The occurrence of intermediate and evolved lavas is restricted to the post-caldera stage, with the exception of Eiao island. However, many other features of the Marquesas chain are rather atypical with respect to those of Hawaii. Although Marquesas shields are tholeiitic, several of them (Eiao, Tahuata) contain interbedded alkali basaltic and basanitic flows. Moreover, post-shield volcanoes are either alkali basalts (Ua Huka), tholeiites (Hiva Oa, Tahuata, Fatu Hiva) or both (Nuku Hiva). This feature is consistent with the temporal continuity of the two stages and the usually short length of the post-shield period (< 0.2 Myr). In a given island, the trace element and isotopic compositions of shield and post-shield lavas overlap, although both display large variations. The sources of alkali basalts and basanites are more enriched than those of the contemporaneous tholeiites. These specific features support the hypothesis of an extremely heterogeneous Marquesas plume. The “weak” character of this plume led to low partial melting degrees, which in turn resulted in the preservation in the basaltic magmas of geochemical features inherited from small-size source heterogeneities.