Low-pressure metamorphism during Archaean crustal growth: A low-strain zone in the northern Nagssugtoqidian orogen, West Greenland

Greenstones and aluminous metasediments of presumed Archaean age crop out on a few small islands north of Aasiaat and have been excellently preserved in a low-temperature and low-strain window in the northern part of the Nagssugtoqidian orogen in West Greenland. These relatively lowgrade rocks may well represent the oldest component of the region, recording a history of metamorphism and deformation during Archaean crustal growth. They provide a unique opportunity to study the primary lithological components of the Archaean supracrustal belts that are intercalated with the regional grey gneisses. In addition, the state of preservation of the supracrustal rocks provides support for an inhomogeneous Nagssugtoqidian orogenic overprint, where blocks with intense thermal and tectonic reworking seem to alternate with blocks of only weak reworking. Based on hornblende Ar-Ar cooling ages, Willigers et al. (2002) proposed that the Nagssugtoqidian orogeny resulted in uniform heating in most of the orogen (including its northern part), followed by very slow cooling during isostatic uplift. The preliminary observations reported here appear to contradict this, but more work is required to substantiate the new findings.