Abstract
The Isukasia area, southern West Greenland, is dominated by early Archaean rocks. These rocks are cut by major Proterozoic faults (mylonites), associated with which are greenschist facies retrograde metamorphism and metasomatism. Tonalitic gneisses are the dominant lithology of the area, and are used in this study to characterise the retrograde metamorphism and metasomatism. Retrogression took place between 350 and 450°c and involved a low δ- 18O, H2O-rich, CO2-bearing fluid of meteoric origin. Fluid access to the tonalitic gneisses occurred along the faults, and movement may have been by seismic pumping. Retrograde reactions adjacent to the fault zones resulted in leaching of SiO2, mobility of alkalis on a scale of less than 100m, and some disturbance of trace element chemistry (e.g. Rb, Pb, Cl). The degree of SiO2 leaching from retrogressed gneisses close to the faults is compatible with fluid:rock ratios of the order of 5:1. On the other hand, SiO2 (as vein quartz) is commonly enriched in the faults, indicating fluid:rock ratios of at least 100:1, but probably much greater. These results, together with isotopie data (Rosing, 1983; Baadsgaard et al., 1986) clearly indicate that rocks affected by Proterozoic retrogression are too altered to provide reliable information on early Archaean crustal genesis. This rules out at least half of the ca. 3810 Ma Isua supracrustal belt.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call