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.

最早的长英质地壳的出现可以通过锆石和辉长岩-长闪长岩-花岗闪长岩 (TTG) 成分的岩石测年来估计。然而，形成变质作用和分异基础的必要地球动力学过程以及新兴的 TTG 地壳对演化地壳动力学的作用仍然知之甚少。为了研究具有 TTG 成分的长英质地壳的形成，我们对一系列先前发表的 3D 高分辨率岩浆热力学模型在对应于太古代条件的升高地幔温度下进行了详细分析。

在这些模型中，我们在耦合循环构造岩浆壳幔演化过程中观察到两个不同的阶段：一个长期的安静生长阶段，然后是一个短暂的快速翻转阶段。此处提供的详细模型分析结果表明，

(1) 地壳底部在生长和翻转阶段均形成低压和中压TTG。低压和中压 TTG 的形成与莫霍面深度有关。中低压 TTG 的比率随着地壳的生长或变薄而变化，并给出了地壳厚度的近似值。(2) 要形成高压 TTG，需要一种完全不同的机制，因为水合玄武岩需要埋在地壳下。由于其低温和快速沉入深部地幔，冷榴辉岩滴的直接部分熔化可以排除为有效机制。相反，我们建议将分层（剥离）或俯冲作为某些高压 TTG 生产的主要过程。