The differentiation of a continental nucleus: Implications from Mesoarchean garnet-bearing granite in the Kongling Complex of the Yangtze Craton

The compositional maturation of continental crust is marked by increasing diversity in granitic rocks. However, whether fractional crystallization of felsic magmas contributes to crustal maturation remains contentious, primarily due to the scarcity of well-documented highly fractionated Archean granites. To address this gap, we present an integrated petrological and geochemical study of newly identified Mesoarchean garnet-bearing granites (highly fractionation granite), in conjunction with coeval sodic tonalite-trondhjemite-granodiorite (TTG) suites and potassic granites in the Kongling Complex of the Yangtze Craton.

Zircon U‒Pb dating results show that the studied TTGs, potassic granites, and garnet-bearing granites in the Kongling Complex were formed at 3.0–2.9 Ga. The TTGs have low K₂O/Na₂O ratios, high Sr/Y and (La/Yb)_N ratios with negative zircon ε_Hf(t) values and mantle-like zircon δ¹⁸O values, indicating they were originated from partial melting of thickened lower crust. Potassic granites have higher K content and K₂O/Na₂O ratios with negative zircon ε_Hf(t) values and mantle-like zircon δ¹⁸O values, suggesting they were generated from anatexis of ancient felsic crust. Garnets in the garnet-bearing granite are euhedral and most of them are inclusion-free. These garnets are mainly composed of almandine and spessartine with homogeneous major elemental compositions, which are consistent with the characteristics of magmatic garnets. The garnet grains show decreasing trends of HREE and Y content from core to rim, indicating the fractional crystallization of garnet and zircon. The garnet-bearing granitic plutons show a blurred contact interface with the contemporaneous potassic granites and their zircon ε_Hf(t) and δ¹⁸O values are similar to those of potassic granites, implying a congenic process between them. The Mesoarchean garnet-bearing granites have moderate whole-rock A/CNK values, high MnO content, MnO/FeO_T ratios and 10000×Ga/Al ratios, but lower Zr content with lower zircon saturation temperature. These features of garnet-bearing granites suggest that they were formed from highly evolved K-rich granitic melts. The occurrence of highly fractionated granite in the Mesoarchean may imply that a mature continental nucleus was formed in the Yangtze Craton at that time. Furthermore, global detrital zircon records document a decreasing trend of Zr/Hf ratios during the Mesoarchean, with ultra-low zircon Zr/Hf values (<25) first appearing at the same time. This shift highlights the intra-crustal felsic magma fractionation as a significant mechanism driving crustal maturation since the Mesoarchean, coincident with global geodynamic transitions.