TTG |
tonalite–trondhjemite–granodiorite; a plutonic rock series that is characterized by high Na2O/K2O; |
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towards the Mid- to Late Archaean elevated Sr/Y and heavy rare-earth element depletion through retention in residual garnet and/or amphibole during partial melting become increasingly common |
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proposed to be derived from melting of (hydrous) mafic successions |
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trondhjemites versus tonalites may be a function of pressure of melting |
transitional TTG |
similar to TTG, but with a spectrum of K content intermediate between TTG and ACP granites |
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within Archaean Domes typically offset from TTG melts by 100–300 Ma, in the North Kaapvaal craton of South Africa often dispersed within TTG gneisses |
sanukitoids |
potassic diorites with elevated Mg, Cr and Ni; |
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Proposed to have formed by interactions of TTG melt with peridotite. The sanukitoids s.s. are the root of a series of more differentiated rocks (sanukitoid suite) |
ACP granites or high-K granites |
Archaean crustal progeny granites, formed by remelting of earlier, silicic crust. Plutonic rocks with a markedly elevated K content compared to TTG. Plausible petrogenetic models to create elevated K contents granitic melts towards the Mid- to Late Archaean argue for a re-melting of earlier TTG succession. For this, petrologic phase equilibria require either excess heat in TTG-bearing crust, or elevated pressure–temperature through subsidence |
stagnant lid |
the entire lithosphere of the planet that covers the mantle as a lid without active tectonics. Sporadic subduction is possible; the lid is characterized by felsic intrusions into a stationary lithosphere |
(down- or lithospheric) drips |
drip of the lowermost lithosphere into the underlying mantle, initiated by converging convection cells. These drips have a limited lifespan of several hundreds of millions of years and occur randomly between rather small convection cells |
downwellings |
similar to drips, but with a substantially longer lifespan and stationary at the interface between thick and thin plates, i.e. a proto-craton and a proto-plate |
lithosphere |
the outermost shell of the planet, including the crust and its underlying, non-convecting mantle |
lithospheric mantle |
the rigid, non-convecting part of the lithosphere, underpinning the crust. In post-Archaean time, this is often termed sub-continental lithospheric mantle |
proto-craton |
former part of a stagnant lid regime that thickens due to melt injection and intra-crustal flow. This part is today preserved as cratons and can date back to the early Archaean, even with Hadean components |
proto-oceanic crust |
former part of the stagnant lid that thins out though thermal erosion, e.g. via plumes, or through intra-crustal flow. Proto-oceanic crust have been destroyed through subduction, with modern oceanic crust as analogues |