The Proterozoic Younger Giant Dyke Complex (YGDC) was emplaced during a period of widespread extensional continental magmatism in southwest Greenland and North America. The complex consists of a series of interconnected branching dyke segments up to 800m wide, which can be traced intermittently over 145km. The dykes were intruded into granitic basement and appear to have been halted at the unconformity between the basement and overlying supracrustal rocks. This project focusses on the Tugtut^oq-Narsaq area of the complex. The magmas producing the dykes were critically-undersaturated alkali olivine basalts to hawaiites with whole-rock MgO = c. 5wt%. They were carrying phenocrysts of olivine, stellate glomerocrysts of plagioclase + olivine, and anorthosite xenoliths up to 100m across. These xenoliths are found in the Narsaq area which represents a roof zone to the dykes. The magmas crystallised to form predominantly troctolitic cumulates. In western Tugtut^oq these are olivine-plagioclase cumulates but in eastern Tugtut^oq and Narsaq, magnetite and ilmenite (± apatite) joined the fractionating assemblage. Olivine and plagioclase core compositions range from Fo_68 to Fo_49 and An_65 to An_30. Salitic to ferrosalitic pyroxene is entirely intercumulus except in two evolved pods of syenogabbro and syenite enclosed by troctolite at the eastern end of Tugtot<SUP>o. These pods are ovoid in shape, elongated along the dyke, and up to 3km long. They are thought to have differentiated <i>in situ</i>. The distinction between syenogabbro and syenite is made on the presence or absence of plagioclase cores to alkali feldspar crystals. The order of appearance of fractionating minerals was ol</SUP>+ pl at c. 5 wt% MgO, Fe-Ti oxides + apatite at c. 4 wt% MgO, cpx at c. 3.5 wt% MgO and alkali feldspar at 3 wt% MgO or less. In ovoid pods at irregular intervals along the dyke segments, modal variation and/or plagioclase lamination define synformal layering. The modal variation takes the form of troctolite alternating with generally narrower layers of mafic cumulates. In the western YGDC these are olivine cumulates (gabbro picrites) but in the east they are olivine + magnetite + ilmenite ± apatite cumulates. Modal layering is best developed in the western YGDC where plagioclase lamination is absent. It may take the form of parallel mafic layers (1-30cm thick) or gabbro picrites filling channels (up to 8m wide and 4m deep) which plunge towards the synform axis. At two localities breccias of gabbro picrite blocks within a troctolite matrix occupy the axial zone. They are inferred to have resulted from the breakup of thick picrite layers. The breccias and channels provide evidence that the synformal structure was primary.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657838 |
| Date | January 1990 |
| Creators | Mingard, Susan C. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/15397 |
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