Geologic field studies of the Miki Fjord Area, East Greenland

Abstract

Field relations between Tertiary intrusions and the lithologic units near Miki Fjord provide a record of physical and chemical processes associated with crustal extension and the formation of the East Greenland continental margin. At several localities the Precambrian basement has been remobilized and partly incorporated in the large macrodikes northeast of the Skaergaard intrusion. Blocks of vesicular basalts sank into the macro­dike magma and were metamorphosed to equigranular aggregates of olivine and ortho- and clinopyrox­enes with concentrations of plagioclase filling amygdale-like structures. We suggest that contact metamor­phic dehydration of hydrous alteration minerals in the basaltic xenoliths and diffusion of this water into the macrodike magma were responsible for the abundant rhythmic layering that occurs near swarms of the xenoliths in a manner similar to that proposed by Taylor & Forester (1979) and Mc Birney & Noyes (1979) for the Skaergaard intrusion. Concentrations of xenoliths of leucogabbro and migmatitic basement gneis­ses are found near the top of one macrodike and in six smaller mafic dikes and sills that intrude the lower lavas. In contrast to the basalt blocks, the leucocratic xenoliths appear to have floated to the top of these mafic intrusions, thus providing an effective mechanism for transport of basement material during the early stages of crustal extension. Basement rocks and Tertiary basalts of the area have undergone hydrothermal alteration that is con­centrated in fracture systems near the intrusive bodies. Alteration mineralogy in the basalts consists of calc-silicate assemblages similar to those found in active geothermal systems in Iceland. Extensive calcium metasomatism is evident in a number of coast parallel dolerite dikes that are mineralized by iron-rich prehnite. Thermodynamic analysis of the dehydration of prehnite to form epidote and garnet indicates that the fluids responsible for the formation of prehnite mineralized dikes were at temperatures less than about 250°C. Mineralized shear zones and mafic dikes of presumed Proterozoic age define a pronounced structural trend in the basement of the area. This east - west trend is parallel with the coastal dike swarm north of 68°N and with topographical trends within the basalts. We therefore suggest that the pre-Tertiary struc­tures may in part control the deflection of the coastline at 68°N and other aspects of the Tertiary structural and intrusive development.