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% SiOn.), 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 graniticgranodioritic 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% SiOn.) 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% AI•.O3 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.

A model of the Chicxulub impact basin based on evaluation of geophysical data, well logs, and drill core samples

“…These results are supported by Sr, Nd, O, and Os-Re isotopic studies on the C1-N10 samples with regard to both the continental affinity of the target rocks (Blum et al, 1993) and the heterogeneous distribution of up to 3% meteoritic contamination (Sharpton et al, 1992;Koeberl et al, 1994). Considering current constraints on excavation depth (15 to 25 km) of the Chicxulub impact event (Sharpton et al, 1994b) and the potential lithologic diversity within this volume, the observed chemical variability (andesitic to dacitic) is rather small and in keeping with the gross compositional homogeneity of melt rocks from other terrestrial impact structures, such as Manicouagan (Grieve and Floran, 1978) and West Clearwater . However, given that Chicxulub melt compositions were derived from very small samples compared to those available from other well-exposed melt sheets, some heterogeneity might be expected.…”

Composition and heterogeneity of anorthositic impact melt at Mistastin Lake crater, Labrador

“…It has long been recognized that bulk chemical compositions of impact melt rocks at Mistastin (Grieve, 1975) and in many other impact structures such as Manicouagan , Clearwater West and Boltysh (Grieve, 1978) are remarkably homogeneous at the crater scale. This implies that impact melts are well mixed by the highly dynamic impact process (Grieve et al, 1977;Grieve and Floran, 1978). However, as reviewed by Dressler and Reimold (2001), some compositions of impact melt rocks are heterogeneous on the macroscopic, hand-sample scale, as are many impact glasses on the microscopic scale.…”

Vitric Compositions in the Onaping Formation and Their Relationship to the Sudbury Igneous Complex, Sudbury Structure

“…Generally, investigations of terrestrial melt sheets have shown both the clast populations and the impact melts to be homogeneous and well mixed (Grieve and Floran, 1978;French, 1998). Similarly, the melt composition of the vitric Onaping Formation, including the intraformational dikes, is homogeneous.…”