Some charnockitic rocks in the Nagssugtoqidian of West Greenland

Abstract

Several charnockite bodies occur in the Precambrian gneiss terrain between Nordre Strømfjord and Agto, West Greenland. The country rocks are mainly enderbitic gneisses, granodioritic gneisses and almandine-sillimanite bearing schists. The largest charnockite body forms the mountain of Kingigtoq between Nordre Strømfjord and Gieseckes Sø. The body has been deformed by at least two fold phases prior to the sinistral simple shear of the Nordre Strømfjord shear zone. Most parts have a gneissose structure, but undisturbed porphyritic textures also occur. The main rock types are charnockite, quartz mangerite and quartz jotunite. The charnockites in the Agto area may be of anorthosite kindred because of chemical similarities: Fe tot./Fe tot. + MgO is about 0.9, average K/Rb is 358, and K2O/SiO2 is high. Average U is 0.52 ppm and average Th is 11.0 ppm. Metamorphism subsequent to intrusion has led to assemblages with ubiquitous gamet and hornblende, commonly accompanied by ortho- and clinopyroxene. Gamet and clinopyroxene stably coexist in felsic charnockites in the Agto archipelago, but on Kingigtoq only in mafic members of the suite. Element distributions indicate an equilibration temperature of 750°C - 850°C, at 8-10 kb solid pressure.