Isotopic re-equilibration of Rb-Sr whole rock systems during reworking of Archaean gneisses in the Nagssugtoqidian mobile belt, East Greenland

Abstract

Samples from small vertical shear zones cutting retrogressed granulites near the northem margin of the Nagssugtoqidian mobile belt give an Rb-Sr whole rock age of 2660 ± 180 m.y. (I.R. 0.7016) indistinguishable from that of their country rocks in spite of massive alkali metasomatism among the shears. Regional samples of garnet gneisses formed during partial remelting of Archaean quartzo-feldspathic gneisses and pelites during Proterozoic contact metamorphism yielded points which scatter about a 1950 m.y. reference isochron (I.R. 0.7054). The scatter shows that re-equilibration has not occurred on a scale of kilometres when the gneisses were partiaIly remelted. Samples from a single 10 x 20 m outcrop of highly deformed tonalitic gneiss with basic inclusions near the centre of the mobile belt yielded points which scatter about a 1875 m.y. reference isochron (I.R. 0.7039). Samples less than 500 g in weight yield points close to the reference isochron, while those from samples over 1000 g Iie above the 1875 m.y. isochron suggesting re-equilibration has been on a scale of 5-10 cm. Samples from a post-tectonic pluton emplaced near the centre of the mobile belt give a whole-rock age of 1583 ± 27 m.y. (I.R: 0.7035). Regional low Rb-Sr ratios in the country gneisses do not perrnit use of the initial ratio to distinguish mantle from a lower crost source.