Mistastin impact crater, central Labrador.

Mistastin impact crater (centre right).

• Age (ma): 36.4 ± 4
• Diameter: 28 km
• Location: Newfoundland/Labrador, Canada. N 55° 53' W 63° 18'
• Shock Metamorphism: Shatter cones are poorly developed. PDF in quartz and feldspars (Taylor and Dence, 1969). Maskelynite*.
• Dating Method: 40Ar-39Ar by J. Whitehead (pers. comm. 2001)

*Maskelynite: A type of naturally occurring glass having the composition of plagioclase series feldspar, created by the vitrification of plagioclase by shock melting in meteorites and meteorite impacts. Image courtesy of NASA.

RADARSAT radar image of the Mistastin impact crater.

The red dot represents the approximate area of the Mistastin impact 28 million years ago in the Paleogene Period.

The crater was originally thought to be volcanic in origin. In 1968 it was confirmed as a meteorite impact site with the discovery of shock metamorphism features, specifically PDF in quartz, and poorly developed shatter cones (Taylor and Dence, 1969).

Mistastin impact crater - east (looking west).

The impact meteorite type may have been of iron composition (Wolfe et al., 1980). The target rock type is Precambrian, crystalline containing Proterozoic anorthosites, mangerite, granodiorite and quartz monzonite and exhibits a full range of shock features from brecciation to diaplectic glasses. If you ignored the lakes, the area almost looks like a moonscape.

(Courtesy of NASA/LPI) Eastward moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and breccias and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central area of the structure. Isolated patches of fill and sub floor target rocks are preserved (Taylor and Dense, 1969). At the time of the impact 36 million years ago, the continents were approximately in their present positions and a moderate biological extinction had occurred which is associated with a microtektite and an iridium impregnated geological layer. The earliest apes made their appearance 10 million years later.

Mistastin impact crater - west (looking east). Discovery Hill is visible in the centre background.

An outcrop of impact melt rocks known as "Discovery Hill", in the Mistastin impact crater.

The sheet of impact melt (image right - Courtesy of GISP)overlies a thin layer of breccia on the crater floor and has an exposed thickness of 80 m at its base. The melt is fine grained, glassy with numerous country rock inclusions. At higher levels it is medium grained with micro-porphyritic and poikilitic textures similar to many Apollo 16 melt rocks. Reaction, assimilation and local partial melting of the inclusions occur and the partial melts show varying degrees of mixing with the impact melt^#.

Discovery Hill at the Mistastin impact crater. Discovery Hill at the Mistastin impact crater.

^# The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.