Summary
The Gardar province (
c.
1320–1120 Ma old) relates to repetitive rifting within the Mid-Proterozoic continent and the attendant large-scale generation of alkali and transitional olivine basaltic and hawaiitic magmas. Lavas from the earliest episodes are preserved in down-faulted units (the Eriksfjord formation), whereas younger basic magmatism is recorded only as intrusions, mainly dykes.
The province shows a compositional continuum from basaltic to salic alkaline compositions. The latter are represented among the early lavas, as dykes and, most prominently, as central-type complexes commonly involving layered (cumulate) syenitic intrusions. Most of the central complexes display either a silica-oversaturated trend (augite syenite-quartz syenite-(A-type) alkali granite) or a silica-undersaturated trend (augite syenite-pulaskite-foyaite). Strongly peralkaline products are present in each association with agpaitic rocks occurring as differentiates at Ilímaussaq and Motzfeldt. The oversaturated and undersaturated complexes, irrespective of age, define southerly and northerly zones respectively in the province. ‘Giant-dyke complexes’, produced in late-Gardar rifting events (contemporaneous with Keweenawan activity in N America) are important in providing a link between simple dilatational dykes and the central-type complexes.
An ultramafic lamprophyre-carbonatite association plays a subordinate role in the province, and carbonatites are also associated with some of the salic complexes. The carbonatites at Grønnedal-Ika are thought to be residual from CO
2
-rich phonolitic magmas, whereas carbonate (siderite)-fluoride rocks at Ivigtut crystallized from volatile-rich residues associated with an alkali granite.
The salic alkaline magmas were produced from alkali or transitional olivine basaltic parent magmas, with crystal fractionation being the dominant mechanism. Generally low
87
Sr/
86
Sr
(i)
ratios in the alkaline complexes suggest little assimilation of old radiogenic crustal materials. The basaltic magmas were characterized by high Al
2
O
3
/CaO and FeO/MgO ratios, and it is inferred that they themselves were residues from more magnesian parent magmas that had undergone extended olivine and clinopyroxene fractionation at sub-crustal levels. The primary magmas may have been derived from garnet lherzolite source rocks with high garnet to clinopyroxene ratios. Late-Gardar basic magmas of the Tugtutôq-Ilímaussaq-Nunataq zone are geochemically distinct from all other Gardar basic magmas in being enriched in P, Ba, Nb and light rare earth elements. This feature may be due to metasomatic enrichment of source rocks along a lineament sub-parallel to the Archaean craton margin.
Gardar alkaline magmas were rich in F, Cl and C compounds (oxides and/or hydrocarbons) while being relatively anhydrous. Many crystallized under low
f
O
2
conditions. The fundamental cause of the repetitive alkaline magma generation may have been the ascent of F-, Cl- and C-rich fluids from the deep mantle, leading to partial melting of the lithospheric mantle close to the boundary between Archaean and Proterozoic crustal units.
