On 20th November Prof. Bill Fitches gave a talk on gneissic rocks, especially in Greenland, in particular those on the western margin of this huge territory, recalling the 1970s working for the Greenland Geological Survey, landed by helicopter to lead isolated mapping teams under canvas for weeks at a time in this beautiful but harsh environment. Greenland is on a craton (ancient continental crust) which extends east beneath the Outer Hebrides and north-west Scotland.
The oldest rocks are 3.75 Ga; even the youngest are 2.3 Ga, but these old rocks are often overprinted by younger events. The gneisses result from extreme metamorphism of mudstones at 600°C after burial up to 30 km deep. The structures at field scale are stunning. The minerals have migrated to large-scale dark and pale bands visible from long distance. In places the gneisses are sheared into puzzling structures, and sometimes extensively cross-cut by igneous dyke swarms. Rocks derived from shale similar to our Silurian black shales, but ten times older, was so metamorphosed that the carbonaceous content has segregated as graphite. Perhaps the organic material came from extremely primitive life 3 Ga ago. We were introduced to a new range of features including en-bayonet dykes, tennis ball gabbros and football anorthosites!
The talk concluded with a look at the Swiss Alps where Bill Fitches, during his time on the academic staff at Aberystwyth University, took students to see similar TTG gneisses (tonalite-trondhjemite-granodiorite) a hundred times younger than those in Greenland, and derived from Permian granites. The Matterhorn, in the alpine pennine nappes, shows rock of the ancient African continent thrust north over the top of European basement gneiss; between the two a band of rock formed from the oceanic crust and sediments of the floor of the long-gone Tethys ocean.