Characterization of Impact Melt-Bearing Impactite Dykes from the Central Uplift of the Mistastin Lake Impact Structure, Labrador

Introduction: Asteroid and comet impacts are an important geological process on all solid planetary bodies, including Earth. They generate pressures and temperatures that may reach several hundred GPa and several thousand K over very limited spatial and temporal scales. This results in the vaporization and melting of target rocks [1]. Through this process both impact melt rocks and a variety of impact melt-bearing breccias are created. This study investigates the characteristics of such material located in the central uplift (Horseshoe Island and Bullseye Island) of the Mistastin Lake impact structure. These allochthonous impactites are particularly interesting as they represent intrusive melt-bearing bodies within the central uplift. Geological Setting: The Mistastin Lake impact structure is located in northern Labrador, Canada (55°53!N 63°18!W). This structure is a complex impact structure that has a diameter of approximately 28 km [2]. The age of the Mistastin event is 36.4 ± 4 Ma [3]. The central uplift of the structure is in the form of a 3 by 4 km island in the middle of Mistastin Lake. This landmass, known as Horseshoe Island, is the main site for this study. This island was mapped in reconnaissance fashion by Currie [3], who suggested that it consisted mainly of mangerite and anorthosite. Most of the rocks on the island are highly weathered and eroded and are primarily found in the form of glaciated outcrops scattered on the island. The remainder of the island is covered with dense brush and moss and some wooded areas. Methodology: Fieldwork was conducted over two weeks during the summer of 2010 on Horseshoe Island in Lake Mistastin. On this island 103 sites were visited and documented and 76 samples were collected. The majority of these sites consist of glaciated outcrops and consist of either mangerite or anorthosite. One site located in the central area of the island also contained impact melt rocks and impact melt-bearing breccias, and 4 sites along the eastern coast contain other impact melt-bearing impactites. These 5 sites and the material from there are the focus of this study. Thin sections were made of samples from these locations which were then analyzed using optical spectroscopy and a scanning electron microscope (SEM). Field Relationships: The way in which the impact melt-bearing impactites interacts with the surrounding material is an important part of understanding the emplacement processes of these materials. (Fig. 1) Site 43 (samples 38 and 39). At this site there is mangerite, melt rock and impact melt-bearing breccia. The melt rock overlies the impact melt-bearing breccia which overlies the mangerite. There is no distinct contact between the melt and impact melt-bearing breccia visible and this seems to be the only occurrence of true melt rock on Horseshoe Island. Site 67 (sample 56). At this site an impact meltbearing breccia dyke is intruding into the mangerite. The dyke is nearly horizontal and there is a fairly sharp contact between the breccia and the host rock. (Note that in this work the term “dyke” is used as a generic term for an intrusive body as the original orientation of the intrusion is not clear and may have been rotated during uplift formation). The textures in the intrusion shows flow patterns and the melt matrix contains clasts that are sorted, with the larger clasts concentrated in the center of the dyke (Fig. 1). Site 70 (sample 58). At this site a sloping impact melt-bearing breccia dyke is intruding into the mangerite with a fairly sharp contact. Flow is not apparent but clasts are less abundant on the edges. Site 71 (sample 59). Like site 67, this site has a nearly horizontal sill of impact melt-bearing breccia intruding into mangerite. The contact seems fairly sharp at many points and more poorly defined at others. There is large variety in the size of the clasts at this site with clasts ranging from a half centimeter to approx. 10 centimeters. Site 73 (sample 60). Again at this site there is horizontal sill intruding into the host mangerite. The contact is fairly sharp and the clasts are small and fairly uniformly distributed in the dyke. Towards the center of the dyke there are pods of melt that have been elongated by flow which is unique to this site.