Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust
Thorsten J. Nagel et al., Steinmann-Institute, Universitt Bonn, 53115 Bonn, Germany. Published online 28 Feb. 2012; doi: 10.1130/G32729.1.
Around four billion years ago, the first continents emerged through partial melting of hydrated basaltic rocks. The tectonic setting of this event is a matter of intense debate and is connected to the problem of how plate tectonics worked on the early Earth. The trace element composition of the oldest well-preserved continental fragments found in western Greenland has widely been interpreted to have originated from melting of basaltic crust at great depths in a subduction zone. Thorsten J. Nagel of Universitat Bonn, Germany, and colleagues combine thermodynamic calculations of partial melting and subsequent trace element modeling to simulate the formation of continental crust in different tectonic environments. They find the best coincidence between observed and calculated trace element compositions in model runs assuming melting of early Archean metamorphic basalts from western Greenland at much shallower depths than expected for melting in a subduction zone. They also find that these local basalts show similarities to modern arc volcanics rather than to mid-ocean ridge basalts, which would be expected to enter a subduction zone. The results presented here by Nagel and colleagues challenge the traditional view of continental crust formation via melting of normal oceanic crust in a down-going slab and support scenarios of melting within tectonically thickened, hot crust.
Drought drove forest decline and dune building in eastern upper Michigan, USA, as the upper Great Lakes became cl
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