Reappraisal of Crustal Evolution at Kangimut Sammisoq, Ameralik Fjord, Southern West Greenland: Fluid Movement and Interpretation of Pb/Pb Isotopic Data

Abstract

Gneisses affected by ca. 2800 Ma granulite facies metamorphism outcrop at Kangimut sammisoq, Ameralik fjord, southern West Greenland. At ca. 2700 Ma these granulite facies rocks were thrust over a terrane dominated by >3600 Ma AmÎtsoq gneisses and a terrane dominated by 2800–2750 Ma Ikkattoq gneisses, which had not undergone the granulite facies metamorphism. Following the thrusting event, further deformation gave rise to folds and subvertical shear zones formed between ca. 2700 and 2550 Ma. Thrusting and subsequent folding and shearing were accompanied by retrogression of the granulite facies rocks under amphibolite facies conditions. From Nd, Sr, Hf and U-Pb isotopic systematics the rocks at Kangimut sammisoq have an age of no more than 3000 Ma. However, they contain variable amounts of unradiogenic Pb derived from early Archaean rocks such as >3600 Ma AmÎtsoq gneisses. Retrogression involved an increase in the Pb content of the gneisses at Kangimut sammisoq. Some or all of this Pb was introduced at the time of retrogression via hydrous fluids that were derived from, or passed through, the underlying tectonic units of AmÎtsoq and Ikkattoq gneisses.