Tracing Early Earth Processes: Geochemical and Metamorphic Evolution of felsic veins in TTG-Associated Rocks of the Isua Supracrustal Belt

The Isua supracrustal belt (ISB) represents a key example of Archean crustal development and is renowned for containing some of Earth's oldest supracrustal rock sequences. However, the precise conditions and timing of its metamorphic history remain subjects of ongoing debate. Crucially, this understanding forms the foundation for competing hypotheses regarding Eoarchean vs. Neoarchean tectono-metamorphic evolution.

This study investigates felsic (Qz + Pl) veins associated with Grt-Bt-Pl-Qz schists located near a large tonalite-trondhjemite-granodiorite (TTG) body with an intrusion age estimated at 3.7 Ga. These felsic veins may represent either in-situ melts of the host rock or TTG-derived melts and subsequent interaction with surrounding rock.

Special focus is placed on observed mineral textures within the host rock, the felsic veins, and their interfaces. Garnet porphyroblasts in the host rock exhibit rounded grains and complex zoning pattern, particularly in grossular and spessartine components, along with various mineral inclusions. In contrast, garnet grains in the veins are larger, irregular and more homogeneous in composition and host polyphase melt inclusions, suggesting distinct growth mechanisms between the host rock and the veins.

Zircon and monazite grains in felsic veins, garnet porphyroblasts and in the host rock will be dated and rare earth element (REE) patterns will be analyzed to distinguish detrital and crystallization ages to constrain the timing and nature of melting and/or melt-rock interaction events and to compare these data with the known history of the ISB. This integrated approach will clarify the formation of felsic veins and contribute to models of Archean crustal evolution.