A swarm of thin NW-SE lamprophyric carbonatite dykes of Mesozoic age occurs south of Frederikshåb associated with a contemporaneous, parallel swarm of thick dolerites. Apart from local country rock material, inclusions in the lamprophyric carbonatites are mainly of the following types: 1) Single crystals of olivine which were probably mainly derived from the upper mantle. 2) Relatively unmodified garnet- and pyroxene-granulite nodules brought up from a lower crustal level. 3) Nodules, and single crystals, consisting mainly of hornblende and salite which are considered to have formed by metasomatic reaction between the carbonatite magma and mainly acid to intermediate lower crustal rocks, possibly at relatively low levels in the dykes. Hornblende shows various stages of growth from initially small, iron-rich crystals to larger, iron-poor crystals which have commonly replaced pyroxenes. The pyroxenes show a similar but less pronounced development. 4) Alkaline nodules which are again thought to have developed by metasomatic reaction between the magma and country rock inclusions, but possibly at higher levels in the dykes. 5) Phlogopite megacrysts which may be partly xenocrystal but which are thought to have mainly crystallised from the contaminated magma. Complete chemical analyses of lamprophyric carbonatites and partial analyses of individual minerals are presented.
Associated with the TD dolerites on Igaussaq island and the adjacent mainland south of Frederikshåb there occurs a parallel (NW-SE) swarm of carbonatite-lamprophyre dykes. These range in thickness from veins of a few millimetres to dykes of 1.5 m. The carbonatites are most abundant in the extreme west of the area, as are the TD dolerites, and die out towards the east. They cut all generations of dykes earlier than TD, and in one case can be seen to be intruded into a TD parallel to its margin. This field evidence suggests that they are probably contemporaneous with the TD dolerites. An isotopic age date for one of these carbonatites is given in the article by Ole Larsen at the end of this volume.
Precambrian (Ketilidian) foliated granites and gneisses are intruded by Kuanitic basic dykes and large bodies of noritic gabbro, diorite and hypersthenemonzonite of Sanerutian age. The latter constitute an appinitic suite, the recognition of which is new to South Greenland. Younger granites are developed around the basic masses and from the occurrence of relict basic dykes are shown to be at least partly due to recrystallization of earlier granitic rocks, though in places the granite has been clearly mobile. It is suggested that the Sanerutian plutonic episode may represent the epeirogenic uplift phase of a root-zone of the orogeny which developed in Ketilidian time rather than the expression of a separate orogenic development as has been thought previously. The appinitic rocks are considered to belong to the normal orogenic calc-alkali suite. Hypersthene monzonite and locally syenite are thought to have developed by diffusive potassium enrichment in basic to intermediate magma. The Gardar alkali province, represented here by swarms of trachydolerites and microsyenites, together with carbonatites, is shown to occupy a characteristic post-orogenic position. Its development is related to continental disruption during the later phases of the Svecofennid chelogenic cycle. The rocks of this region are interpreted as belonging to a unified tectono-magmatic cycle.
Amphibolite bands within the pre-Ketilidian gneisses on the island of Igaussaq and the adjacent mainland contain in places clearly recognizable epidotic relic pillow structures, together with probable meta-keratophyre layers and a meta-agglomerate horizon. Zoned calc-silicate skarn bodies have developed from the pillow structures in areas of more intense folding and granitization. In these areas the amphibolites have become agmatitic, and the pillows have suffered more thorough recrystallization and metamorphic differentiation to give zoned bodies consisting of calcite, garnet, diopside, epidote and hornblende.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.