The Generation of Archaean TTG: insights from Lake Inari migmatites

Tonalite-trondhjemite-granodiorites (TTGs) are a dominant lithology in many Archaean terrains and they retain a pivotal position in discussions of crust generation in the Archaean. Their major element compositions are consistent with partial melting of hydrated mafic source rocks, and yet their juvenile radiogenic isotope ratios indicate that they represent new continental crust.

This study links field evidence from the Lake Inari migmatite-granitoid terrain in northern Finland to petrogenetic models applicable to Archaean terranes. Lake Inari is part of the Meso- to Neoarchean TTG (tonalite-trondhjemite-granodiorite)-amphibolite terrains of Arctic Fennoscandia that form an extensive network of amphibolite metatexite-diatexite transitions controlled by melt proportions and syn-anatectic strain. The Lake Inari migmatite-granitoid terrain therefore provides a natural laboratory in which the bimodal association of felsic TTGs and their basaltic precursors are spatially and genetically linked, encouraging models in which the TTGs form directly through partial melting of the basalt. The zircons ages range over 300 Ma from 2.9-2.6 Ga (Joshi et al. 2024) and the geochemical data (Halla et al., 2024) confirm systematic trends supporting partial melting as the dominant TTG formation process. La/Sm increases from mafic rocks to TTGs, indicating progressive differentiation, but decreases at higher degrees of melting, defining a specific melting range. Th/Nb increases with La/Sm suggesting that negative Nb anomalies result from partial melting and differentiation. On average, Th/Nb increases from 0.17 in basalt to 0.96 in TTG (K₂O/Na₂O < 0.5). Co covaries with Ti in the TTG trending towards the mean Ti and Co values in the basalts, highlighting the role of ilmenite rather than rutile, and the REE variations indicate residual ampbibole rather than garnet. The average TTG was modelled as an 18% partial melt of basalt, assuming a bulk D-value of 0.01 for highly incompatible Th. The source mineralogy follows the thermodynamic model of Palin et al. (2016) for 20% melting at relatively shallow depths. While 20% represents an upper estimate, an 18% melting estimate yields bulk D-values of 0.4–0.5 for Rb, Sr, U, and Th; 1.37 for Nb and Ta; and 3.4–2.7 for Lu, Yb, and Y. Th/Nb increases with La/Sm in TTGs worldwide, highlighting its sensitivity to partial melting processes. The Lake Inari model is applied to other TTGs, allowing the distinction between TTGs derived from relatively high Th/Nb subduction-related sources and those formed in non-subduction settings, offering new insights into early continental growth. By linking field evidence with geochemical modelling, this study offers alternative insights into Archaean crustal evolution.

Halla et al (2024) Precam. Res. doi.org/10.1016/j.precamres.107407

Joshi et al (2024) Precam. Res. doi.org/10.1016/j.precamres.107418

Palin et al (2016) Precam. Res. doi.org/10.1016/j.precamres.11.001