Extreme differentiation in the proterozoic Gjerstad-Morkeheia Complex of South Norway

The Gjerstad-Morkeheia Complex contains a series of highly differentiated, iron and alkali enriched intrusions. The rocks range from basic to acid compositions and were variably mylonitised (under anhydrous conditions) before being metamorphosed (and hydrated) at upper amphibolite facies grade. They now form a series of ironenriched metagabbros, metadiorites and granodioritic gneisses, together with granitic and charnockitic augen gneisses. The augen formed by shearing of original megacrysts in the igneous rocks. A major mylonite zone formed at the same time.The rocks are enriched in Fe, Mn, K, Na, Ti, Zr, Ba, P, La, Ce and impoverished in Mg, V, Ni, Cr and are considered to result from extreme differentiation of an alkaline magma. They are interpreted in terms of a two-stage model, with the intrusion of several fractionated batches of magma, each of which differentiated in situ.The rocks show some affinities with other Proterozoic complexes, particularly some anorthosite-monzonite-charnockite suites (e.g. in SW Norway and in the Grenville Province) and differentiated alkaline suites (e.g. in the Gardar Province of South Greenland).The intrusions were anorogenic and emplaced in rocks which had been metamorphosed (between 1,600 and 1,500 Ma). Subsequent thrusting and mylonitisation was followed by crustal thinning and attempted rifting with widespread intrusion of basic rocks (around 1,200 Ma). The rifting was aborted by the onset of the Sveconorwegian (Grenvillian) orogeny and probably relates to similar basic intrusive activity elsewhere in the Proterozoic supercontinent which was ultimately, successfully broken up after the Grenville orogeny.