TECTONOSTRATIGRAPHIES OF THE 1.7 GA PAYSON OPHIOLITE AND THE 3.5 GA KOMATIITE-BEARING JAMESTOWN OPHIOLITIC COMPLEX: ACCRETION OF MAGMATIC ARCS

Ophiolites record seafloor spreading, and the tectonostratigraphies of host terranes record processes leading up to orogenesis and incorporation of the seafloor into continental crust. Comparing tectonostratigraphies may reveal trends due to the evolution of plate tectonics, but only if we can compensate for selective preservation of sections from different original tectonic settings. The preservation problem is highlighted by the 3.5 Ga Jamestown Ophiolitic Complex (JOC), Barberton Greenstone Belt, South Africa, one of only two well preserved, Early Archean, mafic-ultramafic sequences in the world. New mapping, experimental petrology on komatiites, and geochemistry support a suprasubduction zone (SSZ) origin, possibly in a forearc. The 6-km thick section overlies a major shear zone and consists entirely of submarine lavas with no evidence for sheeted dikes. In contrast, the Payson Ophiolite (PO) is one of only 3 or 4 Early Proterozoic ophiolites with well developed sheeted dike complexes, the hallmark of crust formed by seafloor spreading. Tectonic evolution within an arc setting is indicated by local roof pendants and screens of older arc crust (ca. 1.75 Ga), SSZ composition, overlying dacitic breccias and turbidites with airfall ash beds (ca. 1.72 Ga), and intrusion by younger arc granitoids (ca. 1.71 Ga). Both the PO and JOC were first deformed pre- to syn-magmatically with intrusion and eruption of granitic magma within 30 m.y. of their formation. Both are 5-6 km thick sections sandwiched between batholiths and coeval felsic volcanics. Unlike the PO, the JOC remained the floor of a marginal basin for 200 m.y. prior to siliciclastic sedimentation and fold and thrust deformation.