THE IRON KING VOLCANICS: A REMNANT OF AN OCEANIC PLATEAU ACCRETED TO SOUTHWESTERN LAURENTIA 1.7 GA

The 1.75-Ga Iron King volcanics is part of the largely volcanic Big Bug Group of west-central Arizona. The Big Bug Group is a terrane bounded by the Chaparral and the Shylock shear zones on the west and east, respectively. The Iron King volcanics include chiefly submarine basalts and associated sills, dykes and hyaloclastic breccia, with minor amounts of hydrothermal chert, and variable amounts of andesite and felsic volcanics.

The distribution of relatively immobile incompatible elements (viz., Nb, Ta, Th, REE, Ti, Zr, Hf) on primitive-mantle normalized diagrams indicates four mantle sources for the mafic components of the Iron King, of which only one component has negative Nb-Ta anomalies characteristic of arcs. Intermediate and felsic components also carry the arc signature. The other three volcanic components lack Nb-Ta anomalies and appear to reflect two enriched and one non-enriched mantle source, probably in a mantle plume. These rocks are entirely submarine volcanics with compositions much like the mafic components in oceanic plateaus such as Ontong Java and Caribbean. Geochemical modeling shows that the three non-arc sources cannot be related by partial melting or fractional crystallization, and thus they must represent distinct geochemical mantle reservoirs, perhaps all in the same mantle plume as in Phanerozoic plumes. This confirms that long-lived mantle heterogeneities were established by the Paleoproterozoic.

Although most of the accreted Paleoproterozoic terranes in the southwestern United States represent remnants of arcs or back-arc basins, the Iron King volcanics appear to be part of an accreted oceanic plateau. Arc components within the Iron King may be remnants of an arc that developed along the margin of the plateau prior to collision and accretion to Southwest Laurentia at about 1.7 Ga. The Iron King volcanics is one of the few known examples of Paleoproterozoic oceanic plateaus that were at least partly accreted to the continental crust.