Cyclic tectono-magmatic evolution of TTG source regions in plume-lid tectonics

Abstract

The appearance of the earliest felsic crust can be estimated by dating zircons and rocks of tonalite-trondhjemite-granodiorite (TTG) composition. However, the necessary geodynamic processes that form the basis for metamorphism and differentiation as well as the role of emerging TTG crust on evolving crustal dynamics is still poorly understood. To investigate the formation of felsic crust with TTG composition, we conduct a detailed analysis of a series of previously published 3D high-resolution magmatic-thermomechanical models at elevated mantle temperature corresponding to Archean conditions. In these models we observed two distinct phases during coupled cyclic tectonomagmatic crust-mantle evolution: a long quiet growth phase followed by a short rapid overturn phase. Results of the detailed model analysis presented here suggest that (1) Low- and medium-pressure TTGs are formed at the base of the crust during both growth and overturn phase. The formation of low- and medium-pressure TTGs is linked with Moho depth. The ratio of low- to medium-pressure TTGs changes with crustal growth or thinning and gives an approximation for crustal thickness. (2) To form high-pressure TTGs an entirely different mechanism is required, as hydrated basaltic rocks need to be buried below the crust. Direct partial melting of cold eclogitic drips can be excluded as a valid mechanism due to their low temperatures and rapid sinking into the deep mantle. Rather we suggest delamination (peeling-off) or subduction as the main process for some high-pressure TTG production.