Epilogue: What if Anything Have We Learned About Precambrian Ophiolites and Early Earth Processes?

Boninites of Phanerozoic age occur in ophiolites or intraoceanic island arcs such as the Izu-Bonin-Mariana arc system. These primary liquids are interpreted as second-stage high-temperature, low-pressure melting of a depleted refractory mantle wedge fertilized by fluids and/or melts, above a subduction zone. Sedimentary basins may form in the graben, and as the extension continues magmatism sometimes affects the rifted margin, either forming volcanic rifted margins, or migrating to a spreading center forming an oceanic spreading center. New asthenospheric mantle upwells along the new ridge, and may intrude beneath the extended continental crust. In some cases, wedges of extended mid-to-lower continental crust overlying mylonitic lherzolitic subcontinental mantle become intruded by numerous dikes and magmas from this new asthenospheric mantle. The formation of the Gondwanan supercontinent at the end of the Precambrian and the dawn of the Phanerozoic represents one of the most fundamental problems being studied in Earth Sciences today. It links many different fields, and there are currently numerous and rapid changes in our understanding of events related to the assembly of Gondwana.