Abstract
Recently discovered volcaniclastic rocks of andesitic composition form major parts of the mid-Archaean, amphibolite facies supracrustal belts at Qussuk, on Bjørneøen and on part of Storø in western Godthåbsfjord (Fig. 1). These rocks are interpreted as an island arc that represents the onset of the magmatic accretion of the Akia terrane 3070 Ma ago; this terrane is the north-westernmost of several Archaean tectono-stratigraphic terranes in the Nuuk region, which were all amalgamated by 2720 Ma (cf. Hollis et al. 2006). The presence of the arc in the Akia terrane points to similarities between high-grade orthogneiss-amphibolite associations in West Greenland and lower-grade granite-greenstone terrains of other Archaean cratons e.g. in Canada and Western Australia. Volcaniclastic rocks belonging to the ancient arc have been subject to intense synvolcanic, hydrothermal alteration associated with gold-copper mineralisation especially in parts of the Qussuk area. Another important gold prospect occurs on central Storø, which is currently being explored by NunaMinerals A/S (Knudsen et al. 2007 – this volume). This contribution presents new field observations from some of the best preserved parts of the ancient arc at Qussuk and on Bjørneøen, while it remains unclear if the volcano-sedimentary associations and their gold mineralisation at Qussuk, Bjørneøen and the nearby Storø share a common mid-Archaean geological history.
Highlights
The age and setting of the ancient arcThe eastern Akia terrane comprises c. 3060–3000 Ma tonalitic to trondhjemitic orthogneisses, isoclinally folded panels derived from the older andesitic arc, and granites mobilised from the orthogneisses during a late-kinematic thermal event at c. 2980 Ma which led to granulite facies P-T conditions west of Qussuk and Bjørneøen (Garde 1997, 2007; Garde et al 2000)
Discovered volcaniclastic rocks of andesitic composition form major parts of the mid-Archaean, amphibolite facies supracrustal belts at Qussuk, on Bjørneøen and on part of Storø in western Godthåbsfjord (Fig. 1)
The eastern Akia terrane comprises c. 3060–3000 Ma tonalitic to trondhjemitic orthogneisses, isoclinally folded panels derived from the older andesitic arc, and granites mobilised from the orthogneisses during a late-kinematic thermal event at c. 2980 Ma which led to granulite facies P-T conditions west of Qussuk and Bjørneøen (Garde 1997, 2007; Garde et al 2000)
Summary
The eastern Akia terrane comprises c. 3060–3000 Ma tonalitic to trondhjemitic orthogneisses, isoclinally folded panels derived from the older andesitic arc, and granites mobilised from the orthogneisses during a late-kinematic thermal event at c. 2980 Ma which led to granulite facies P-T conditions west of Qussuk and Bjørneøen (Garde 1997, 2007; Garde et al 2000). Andesitic metavolcanic and volcano-sedimentary rocks were first reported from the eastern Akia terrane by Garde (1997) and Smith (1998). Recent age determinations of volcanic zircon grains from central Bjørneøen show that the age of the andesitic arc is 3071 ± 1 Ma (sample 479827, Garde 2007). Zircon grains from yet another volcano-sedimentary rock on central Bjørneøen (Fig. 3A) yielded a range of 2908–2742 Ma ages with a cluster around 2825 Ma The 2825 Ma cluster was interpreted as the depositional age, and a complex tectonic model for central Bjørneøen was proposed with thrust-stacking of supracrustal rocks and orthogneisses of different ages and origins at the eastern margin of the Akia terrane. The zircon grains allegedly extracted from sample 479745 seem to have been incorrectly labelled during the sample preparation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Geological Survey of Denmark and Greenland Bulletin
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
