Tonalite–trondhjemite–granodiorite (TTG) rocks constitute the majority of preserved Archean crust, but their petrogenesis and geodynamic setting are still matters of great debate. TTGs have been be classified into low-, medium- and high-pressure variants based on their different compositions (e.g., Sr/Y, La/Yb ratios). Among them, the high-pressure type tends to be more trondhjemitic and may indicate the subduction regime. However, the classic model and classification are not consistent with the fact that: (1) for most granitoids, the accessory phases dominate the bulk trace element compositions and have an influence on petrogenetic studies far greater than their abundances might suggest, but have been largely neglected in both geochemical modelling and equilibrium phase modelling; (2) in many terranes, the high-pressure and low-pressure types can be found in the same outcrops and show gradual transitional relationships in mineral assemblages and compositions. Specifically, for the 2.5 Ga TTGs of the eastern North China Craton, geochemical analyses show that dioritic parental magmas may evolve to trondhjemites through a maximum of 27% fractionation of amphibole and apatite. The cumulate apatite-bearing hornblendites, a predicted solid fractionate and chemical complement to the TTGs, are also identified. They are complementary in major and trace elemental compositions to the TTGs. In addition, the Fe isotopes provide an alternative tool that eliminates the disturbances caused by accessory mineral phases. Unlike trace elements (e.g., Sr, REE) that could be disproportionately affected by accessory minerals, the Fe abundances of TTG melts are controlled by mafic rock-forming minerals (e.g., amphibole, pyroxene, and garnet) and Fe-rich oxides. In our analyses, whole-rock Fe3+/∑Fe and δ56Fe data support that the “high-pressure” trondhjemites formed by amphibole-dominated fractionation at low pressure. For the “high-pressure” tonalites, their light Fe isotopes make them unlikely to be the melting products of MORB-like or OIB-like basalts at high pressure within garnet-stability field. In summary, these “high-pressure” trondhjemites and tonalites can form at low pressure with little involvement of residual garnets. Partial melting is not a unique solution to the problem of how to form the Archean TTGs and how to make an Archean continent. Thick crust or lithosphere (within garnet-stability field) is also not a strict requirement. In fact, the heterogeneity observed in the TTG compositions highlights the diversity of their sources and formation processes.

Reference(s)

Liou, P, Wang, Z, Mitchell, RN, Doucet, LS, Li, M, Guo, J and Zhai, M 2022, Fe isotopic evidence that “high pressure” TTGs formed at low pressure. Earth and Planetary Science Letters, v.592, p.117645. Liou, P and Guo, JH 2019, Generation of Archaean TTG Gneisses Through Amphibole-Dominated Fractionation. Journal of Geophysical Research-Solid Earth, v.124(4): p.3605-3619