Abstract
The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c. 1163 Ma Younger giant dyke complex (YGDC) mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokken) is considered to be coeval and presumably comagmatic with the YGDC. At the unconformity between the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustal assimilation.
 A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik). Three post-extensional complexes (Tugtutôq, Ilímaussaq and late Igdlerfigssalik) along the former rift mark the end of magmatism at c. 1140 Ma. The latter two complexes have oblate plans reflecting ductile, fault-related strain. The Tugtutôq complex comprises quartz syenites and alkali granites. The Ilímaussaq complex mainly consists of nepheline syenite crystallised from highly reduced, Fe-rich phonolitic peralkaline (agpaitic) magma, and resulted in rocks with very high incompatible element concentrations.
 Abundant anorthositic xenoliths in the mafic and intermediate intrusions point to a large anorthosite protolith at depth which is considered of critical importance in the petrogenesis of the salic rocks. Small intrusions of aillikite and carbonatite may represent remobilised mantle metasomites. The petrological similarity between Older and Younger Gardar suites implies strong lithospheric control of their petrogenesis. The parental magmas are inferred to have been derived from restitic Ketilidian lithospheric mantle, metasomatised by melts from subducting Ketilidian oceanic crust and by small-scale melt fractions associated with Gardar rifting.
 There are numerous analogies between the southern Gardar rift and the Palaeogene East African rift.
Highlights
Uniqueness of the southern branch of the Gardar riftThe concept of ‘the Gardar rift’ is sometimes spoken of
In the Mesoproterozoic, which encompasses the Gardar period between about 1300 and 1100 million years ago, Greenland was a component part of the Columbia supercontinent (Rogers & Santosh 2002) that lasted from c. 1800 million years (Ma) until c
Younger Gardar (1180–1140 Ma) magmatic activity was principally manifested in two rift zones developed across the Columbia Supercontinent, viz. the northern (Nunarsuit–Isortoq) and the southern (Tuttutooq–Ilimmaasaq–Narsarsuaq) rifts, in response to lithospheric extension
Summary
The concept of ‘the Gardar rift’ is sometimes spoken of. No such single ‘Gardar rift’ exists, but continental rifting certainly affected part of what is southern Greenland. Up to several hundred metres across, accompanied by inclusions of augite syenite and foyaite, occur at one main horizon (unit +3) and are regarded as resulting from a major roof collapse (Ferguson 1964; Bohse et al 1971; Sørensen 1978) They demonstrate that naujaite was already at or below its solidus by the time the lower layered kakortokites were accumulating (Bohse & Andersen 1981). Whereas in the lower layered kakortokites compositional changes in whole-rocks and minerals ( arfvedsonite and eudialyte) are small, such changes become much more pronounced in the overlying strata This phenomenon had been noted with respect to the upward increase in U in the eudialytes (Bohse et al 1974; Steenfelt & Bohse 1975) and Zr/Y ratios (Andersen et al 1981b).
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