U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth's history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.