New zircon ages from theTasiusarsuaq terrane, southern West Greenland

Abstract

The zircon crystallisation age of 2.87-2.85 Ga for samples 499221, 499161 and 514832 and a somewhat speculative metamorphic age at 2.80 Ga for sample 499221 correlate very well with known crystallisation ages and granulite facies events within the Tasiusarsuaq terrane. These ages are significantly younger than ages of rocks from the Kapisilik terrane but older than those of the Tre Brødre terrane. It is thus tempting to ascribe these areas to the Tasiusarsuaq terrane. To include the 'Nunatak 1390' is straightforward, while including Nunataarsuk implies a major northerly extension of the terrane that requires confirmation by further work. Age data alone naturally do not justify the inclusion of these areas into the Tasiusarsuaq terrane, but it appears to be the most straightforward option based on the available information. More importantly, the ages obtained from the rhyolite and granite at 'Nunatak 1390' (2.873 Ga) and Nunataarsuk (2.853 Ga) provide minimum extrusive ages for the associated mafic greenstones and might reflect the onset of crustal growth in this block. Furthermore, the emerging terrane configuration might indicate that the Tasiusarsuaq terrane accreted with the Kapisilik terrane in the north-east and with the Tre Brødre terrane in the south-west. The ages from the schistose tonalite (499227) and the veined gneiss (499228) are too young to readily represent known events within the Tasiusarsuaq terrane. The inferred metamorphic ages rather correlate with the thermal event associated with prograde amphibolite facies metamorphism within the Tre Brødre terrane. However, situated well within the Tasiusarsuaq terrane these rocks would not have experienced a prograde metamorphic path during terrane accretion. Furthermore, there is evidence for ≥2.85 Ga old zircons, which are too old to readily fit with known ages of the Tre Brødre terrane. We speculate that the northern part of the Tasiusarsuaq terrane may represent a nappe complex, and that the investigated rocks either represent a tectonic window exposing footwall rocks that experienced prograde metamorphism and partial melting during overthrusting, or that fluids released from footwall-induced zircon Pb-loss and/or partial melting in the overriding nappe. © GEUS, 2008.