Tonalite-Trondhjemite-Granodiorite

Tonalite - granodiorite complexes Trondhjemit, short TTG complexes or TTG suites are typical of the archaic cratons ( oldest TTG suite 3.8 Ga in southern West Greenland ), where they slightly changed also occur later in the Phanerozoic. They probably represent the first and oldest continental crust at all.

Chemical composition

TTG complexes consist of tonalite, granodiorite Trondhjemiten and are characterized by a particularly high sodium content, otherwise the term covers a wide range of chemical compositions.

Formation

The Origin of TTG - complexes is controversial and took place under conditions which plate tectonics as we know it today do not know needed, probably oceanic lithosphere was then only pushed subducted but not deep. What is certain is that TTG complexes are the result of partial melting of greatly thickened oceanic crust. The best match of the geochemical source rock was achieved with a eoarchaischen Tholeiit the Isua Greenstone Belt. Probably it was at that time due to the higher geothermal gradient to rapid dehydration of the oceanic crust, which is why they are not as fast as the now common metamorphosis learned the essential dense rock eclogite. This in turn had the deferred over time being less dense oceanic crust (as today ) and could not be subducted deeper. Over time, the deferred over crust experienced a strong warming causing a metamorphosis to amphibolite and eclogite and partial melting took place. The melting has this in the pressure / temperature stability field of garnet ( ie, in at least 40 km depth ) have taken place, this corresponds to (starting from today thickness of oceanic crust ) were approximately 4 times the normal thickness, are lacking in the classical archaic TTG complexes evidence of interaction ( such as foreign mineral inclusions or hydrothermal alteration ) of the magmas with Erdmantelgestein ( peridotite ), in contrast to post- archaic TTG complexes or Adakiten. The partial melting caused by these and any fractionation acidic magmas rose due to their lower density and intruded or erupted.