Manicouagan Impact Melt, Quebec 2. Chemical interrelations with basement and formational processes

Abstract

The basement rocks of Manicouagan exhibit a wide compositional range (40–72% SiO 2 ), and factor analysis techniques have been employed to determine the average composition of chemically distinct lithologies. Least squares mixing calculations using some of these lithologies as melt components indicate that the average composition of the Manicouagan melt sheet can be modeled for 9 major and 11 trace elements by a mixture of 4.5% anorthosite, 55.5% mafic gneiss, and 40.0% tan gneiss with a granitic‐granodioritic composition. These lithologies have a large areal extent close to the center of the structure, and the mixing model is considered compatible with the genesis of the melt rocks during a hypervelocity impact event. The underrepresentation of anorthosite relative to its present distribution is in keeping with evidence that the bulk of the presently exposed anorthosite is a horst which was uplifted to form a central peak after melt formation. The homogeneous composition of the melt (57.75 ± 1.21% SiO 2 ) relative to that of the postulated target is explained in a model outlining the dynamic conditions existing during the formation of the melt and its accompanying movement into the excavated cavity. Gross differences in target composition may be reflected in a minor variation of less than 1% Al 2 O 3 between the average melt in the northern and southern areas of the structure. A relatively rare mafic composition occurs sporadically within the melt sheet and may have resulted from the incomplete homogenization of local concentrations of anorthosite with a high proportion of mafic bands.