Petrogenesis of Siletzia: The world's youngest oceanic plateau

Abstract

Siletzia is an accreted Palaeocene-Eocene Large Igneous Province, preserved in the northwest United States and southern Vancouver Island. Although previous workers have suggested that components of Siletzia were formed in tectonic settings including back arc basins, island arcs and ocean islands, more recent work has presented evidence for parts of Siletzia to have formed in response to partial melting of a mantle plume. In this paper, we integrate geochemical and geochronological data to investigate the petrogenetic evolution of the province.

The major element geochemistry of the Siletzia lava flows is used to determine the compositions of the primary magmas of the province, as well as the conditions of mantle melting. These primary magmas are compositionally similar to modern Ocean Island and Mid-Ocean Ridge lavas. Geochemical modelling of these magmas indicates they predominantly evolved through fractional crystallisation of olivine, pyroxenes, plagioclase, spinel and apatite in shallow magma chambers, and experienced limited interaction with crustal components.

Further modelling indicates that Siletzia magmatism was derived from anomalously hot mantle, consistent with an origin in a mantle plume. This plume has been suggested to have been the same as that responsible for magmatism within the Yellowstone Plateau. Trace element compositions of the most primitive Siletzia lavas are similar to suites associated with the Yellowstone Mantle Plume, suggesting that the two provinces were derived from compositionally similar sources. Radiogenic isotope systematics for Siletzia consistently overlap with some of the oldest suites of the Yellowstone Magmatic Province. Therefore, we suggest Siletzia and the Yellowstone Mantle Plume are part of the same, evolving mantle plume system.

Our new geochronological data show the province was emplaced during the time when Eocene sea surface temperatures were their highest. The size of Siletzia makes the province a potential contributing factor to the biospheric perturbation observed in the early Eocene.