The Late Archaean Qôrqut Granite Complex of southern west Greenland

Abstract

Granites and granite pegmatites composing the ∼ 2550‐Ma Qôrqut granite complex occur in a SSW‐NNE trending linear belt >150 km long extending through the Buksefjorden‐Ameralik‐Godthåbsfjord region of southern West Greenland. The main body of the complex crops out over a distance of 50 km from Ameralik to Kapisigdlit kangerdluat and reaches a maximum outcrop width of 18 km between Storø and Qôrqut. Around Qôrqut the complex comprises three main groups of granites: early leucocratic granites, various grey biotite granites, and late aplogranite‐granite pegmatites. Within the 1500‐m vertical section available in this area the complex has a tripartite structure comprising a lower zone dominantly of polyphase granite, an intermediate zone where country rock occurs as rafts in polyphase granite with a complex sheeted structure, and an upper zone dominantly of country rock sheeted by granite. Fifty‐two specimens of granite have been analyzed for major, minor, and some trace elements. Geochemical variation within the complex is consistent with either fractional crystallization or partial melting, but in both cases, feldspar + biotite must have been involved either as fractionating phases or as residual phases during melting to account for the trace element chemistry. Two possible models for the generation of the complex are either anatexis of granulite facies rocks in the lower crust following an influx of volatiles and heat from the mantle or melting at intermediate depths of amphibolite facies rocks with volatiles supplied by breakdown of hydrous phases.