Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust

Details Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust

Oct 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.2165b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 10, pp.2165-2172

Mathematics

Logic

6

Scientific paper

Early Archaean gneisses (3400-3700 Ma) in the Isukasia area of West Greenland have been subjected to a series of local and regional metamorphic processes. Metasomatic alteration accompanied the intrusion of the early Archaean white gneisses (~3600 Ma) into the grey gneisses (~3700 Ma), and resulted in local formation of altered rocks. Pb-Pb isotope results on whole rock and feldspar from the gneisses reveal a major rearrangement of Pb isotopes at about 2600 Ma together with some local third stage Pb changes at a later time. This 2600 Ma event is also shown by Rb-Sr data for phengites in the ~3400 Ma Pegmatitic gneiss sheets and can be correlated with metamorphism accompanying the intrusion of a late Archaean pegmatite (~2600 Ma) swarm. Local hydrothermal alteration by fluids emanating from Proterozoic faults and fractures is petrographically recognised in and near major faults and lithological boundaries. This fault-controlled alteration, though only locally significant, may correlate with low grade thermal metamorphism reflected in a 1600 Ma Rb-Sr mineral isochron on biotites from both altered and unaltered gneisses. Oxygen isotope results show that this alteration probably was associated with influx of a low- 18 O fluid, perhaps meteoric water. Selection of the least altered Amitsoq gneisses can be made on the basis of petrographic and field criteria, and furnishes the best material to study original geochemical and isotopic systematics in these early Archaean rocks.

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