Archean versus Phanerozoic oceanic crust formation and tectonics: Ophiolites through time
Harald Furnes, Yıldırım Dilek
Abstract
A global compilation of structural, lithological, and geochemical data on a selection of Archean, Proterozoic and Phanerozoic magmatic complexes, interpreted as ophiolites, is presented. Ophiolites, based on Phanerozoic examples, can be classified into subduction-related and subduction unrelated categories. These categories can be further subdivided into several subtypes depending on the proximity to subduction zones, and on sequential development from rifting – drifting to seafloor spreading for the subduction-unrelated category. From bottom to top ophiolites exhibit a magmatic sequence of ultramafic rocks (upper mantle units), gabbros (layered and isotropic), basaltic dikes and lavas, as well as boninites and felsic dikes and lavas in the subduction-related types. Archean greenstone belts show large variations in their construction, but Eoarchean examples display identical structure and lithology to the Phanerozoic ophiolites, attesting to the operation of seafloor spreading and subduction zone processes in the early Earth's history. Lithological differences between the Archean and Proterozoic/Phanerozoic ophiolites are demonstrated in the common occurrence of komatiites and felsic rocks, and scarcity of sheeted dike complexes in the former, compared to the opposite situation in those ophiolites that are younger than ca. 2 Ga. Geochemically there is a concomitant decrease in the content of incompatible elements (e.g., Sr, Zr, Y, Nb) and an increase in the content of compatible elements (e.g., Mg, Cr, Ni). In terms of tectonic environment analyzes, the Archean ophiolites are more abundantly subduction-related than those of Proterozoic and Phanerozoic ages. The subduction-dominant Archean oceanic crust (ophiolites) was characterized by accretionary cycle plate tectonics, whereas for those of Proterozoic and Phanerozoic ages, a combination of both accretionary cycle and Wilson cycles plate tectonic processes was operative.