Abstract

At the Mistastin Lake structure, Labrador, igneous rocks overlying shock-metamorphosed and brecciated Precambrian anorthosite, mangerite, and granodiorite have a preserved thickness of 80 m and have characteristics believed compatible with an origin by impact melting.

At the base, the impact melt is glassy and very fine grained with abundant shocked and unshocked country-rock inclusions. At higher levels, it becomes medium-grained poikilitic to subophitic with relatively few recognizable inclusions. Plagioclase, pigeonite-ferroaugite, and interstitial glass are the principal melt phases. The volumetric bulk of the melt shows a small compositional range, 53.4 to 58.4 percent SiO2 and 1.1 to 2.3 percent K2O; however, a rare microporphyritic variety occurs locally at the base and contains 65.3 percent SiO2 and 4.6 percent K2O. Minor partial melt with a granitic composition occurs in mangerite inclusions; glass globules with an equivalent composition are found in the immediately surrounding impact melt.

Calculations indicate that the melt can be generated from a compositional mix of the country rocks; the calculations are consistent with a presented impact model for the origin of the Mistastin Lake structure and the formation of the melt as a total melt of the country rocks near the point of impact. The observed compositional range may be the result of the differential assimilation of inclusions and (or) a reflection of original variations in the proportions of the country rocks within the melted volume.

Textural similarities between the Mistastin Lake melt and lunar impact melts lead the writer to infer that the lunar melts may have formed in a similar manner — that is, as total melts of portions of their respective targets and not as partial melts in hot ejecta blankets.

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