Geochemical studies of Pacific oceanic plateaus: The Ontong Java Plateau and Shatsky Rise.

摘要

Oceanic plateaus are enigmatic features of the seafloor. Little is known about the cause of their large volumes, or the compositions and locations of their mantle source regions. This study focuses on the largest (Ontong Java) and the oldest (Shatsky Rise) oceanic plateaus.; Up to 4 kin of igneous upper crust is exposed subaerially along the Ontong Java Plateau's southern margins, in the islands of Malaita, Ramos, and Santa Isabel (Solomon Islands, southwest Pacific Ocean). Reconnaissance geochemical and 40Ar-39Ar studies of the basements of these islands reveal two identical ages, 121.3 ± ID.9 Ma and 92.0 ± 1.6 Ma, virtually indistinguishable Pb, Sr, and Nd isotopic compositions, and very similar trace element characteristics to those of submarine lavas previously drilled on the plateau, confirming that the islands contain upthrusted Ontong Java Plateau crust. In detail, the geochemical and 40Ar- 39Ar results for the ∼3.5-km-thick basement section in Malaita indicate a much expanded version of the plateau's upper crust drilled in a 149-ni section 1600 km to the north, consisting of two isotopic groups previously termed Units A and C–G: an ∼750-m-thick upper volcanic sequence, designated here as the Singgalo Fm. (equivalent to Unit A); and a >2800-m-thick lower volcanic sequence, designated here as the Kwaimbaita Fm. (equivalent to Units C–G). The bimodal isotopic grouping probably reflects a two-component, vertically zoned, plume mantle source. The ∼90 Ma lavas are geochemically similar to the ∼121 Ma Kwaimbaita Fin. lavas, suggesting that they tapped similar mantle sources. Major and trace element modeling indicates large extents (17–29%) and up to 4 GPa initial depths of partial melting, suggesting that Ontong Java Plateau lavas may have evolved from picritic primary melts and experienced up to 55% polybaric crystal fractionation.; The latest Jurassic-earliest Cretaceous Shatsky Rise in the northwest Pacific is one of the least-studied oceanic plateaus. The Pb, Nd, and Sr isotopic compositions of dredged lavas range from MORB-like in the southwest to ocean-island-like toward the northeast. A northeastward age progression and decrease in crustal volume can be explained via a plume initiation origin, but the isotopic data pose some complications to conventional models of mantle plumes.