EARLY PROTEROZOIC OPHIOLITE, CENTRAL ARIZONA

The 1.73 Ga Payson ophiolite is a pseudostratigraphic sequence of mafic plutons, dike swarms, sheeted dikes, and submarine basalts that intruded and erupted upon a 1.75-1.76 Ga magmatic-arc complex. The composition of the sheeted dikes is tholeiitic basalt (minor andesite) with island-arc affinites. The submarine basalts are overlain by dacitic breccias and a thick section of turbidites with ca. 1.72 Ga ash beds. The entire sequence was deformed, intruded by ca. 1.70 Ga granites, and unconformably overlain by fluvial to shallow-shelf sediments. Although most of the 1.8-1.6 Ga juvenile crust of Arizona consists of magmatic-arc rocks, the Payson ophiolite is unique and is interpreted to have formed the floor of an intra-arc basin. The ophiolite developed in situ on the older arc basement, as opposed to being thrust over it. The basin was accreted to the continent by ca. 1.70 Ga.