Plumes and hotspots as sources of greenstone belts

Abstract

Greenstone belts that are older than 0.18 Ga formed under circumstances that generate a buoyant, garnet-poor root. Because they do not generate enough depletion in the lithospheric mantle, juvenile arcs involving average temperature Early Precambrian mantle were not a significant source of Early Precambrian greenstone belts. Instead, Early Precambrian greenstone belts originated from mantle plumes and hotspots. We focus on the origin of highly deformed, MORB-like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed, accretionary wedge-like sediments. These greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent, as has happened on the Taitao Peninsula in Chile. The tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot. The MORB-like rocks result from decompressive partial melting within the slab window produced by the subducting ridge. The stratigraphy and tectonic setting of the recently emplaced Taitao ophiolite are a possible Phanerozoic analog of some MORB-like Archean and Early Proterozoic greenstone belts.