The metamorphic evolution of the Isua supracrustal belt: Implications for Archean tectonics

Knowledge on the metamorphic evolution of the Eoarchean Isua supracrustal belt (ISB) remains fragmentary and results presented to date are contradictory. This translates into an ambiguity on the interpretations on the tectonic evolution of the belt as the three main competing tectonic models predict different metamorphic signatures. The distinction between the three possibilities has larger implications for early Archean geodynamics, since two argue for an early onset of plate tectonics while the other proposes a non-uniformitarian alternative. This work presents a detail P-T-X-d-t evolution of the ISB to provide further constraints in its tectonic evolution. Detail characterization and quantification of the metamorphic fabrics reveal a syn-tectonic metamorphic event (M1) that evolved along a near isothermal burial path followed by an increase in T and decrease in P that culminated in the peak metamorphic conditions 550-600 °C and 0.5-0.7 GPa. M1 homogeneously affected the entire belt. This also suggested by the detailed garnet study showing the diversity in porphyroblasts is a consequence of (local) chemistry and previous microstructures, and not the product of different tectonometamorphic histories as previously interpreted. Preliminary analysis suggest that M1 occurred at c. 3.3 Ga or c. 2.9-2.7 Ga. Post-tectonic Neoarchean (c. 2.6 Ga) lower amphibolite facies (540°C, ≤ 0.5 GPa) metamorphism (M2) later affected the ISB, recorded by the inclusion-poor rims of garnets and the appearance of titanite overgrowing the foliation. Proterozoic (<2220 Ma) low-grade retrogression (M3) features such as late quartz-calcite veins, garnet pseudomorphs and chloritization overprinted M1 and M2. Moreover, pre-M1 (3.60-3.68 Ga) local contact metamorphism is recorded in the enclaves, while magmatic garnet dating expands the intrusion of the granites from 3.66-6.60 to 3.66-3.55 Ga. The data and analysis provided in this thesis show that non-uniformitarian tectonics are a plausible scenario to explain the tectonic evolution of the ISB during the Archean.