PREFERENTIAL FELDSPAR COMMINUTION IN IMPACT MELT BRECCIAS FROM THE M-1 CORE OF THE MANSON IMPACT STRUCTURE

by
M.K. Reagan, C.T. Foster Jr., M.S. Bell, and R.R. Anderson

The Geological Society of America
Special Paper 302, 1996, p. 235-244
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.

ABSTRACT


The M-1 core from the Manson Impact Structure contains two types of impact melt breccia with matrix compositions that differ dramatically from clast compositions. The uppermost few meters of the impact melt breccia section have a matrix composition similar to a potassic phonolite. The underlying breccias have matrix compositions similar to a benmoreite. Normative feldspar contents of both types of matrix exceed 80% with less than 10% quartz. Both matrix compositions differ from their igneous counterparts by having normative corundum.

Although quartz clasts are abundant in the Manson impact melt breccias, quartz instability within the impact melt breccia matrices is indicated by the presence of cpx-bearing reaction coronas around quartz clasts. These reaction coronas are similar to those reported at a number of impact structures with basements of crystalline lithologies. The presence of quartz clasts in silica-undersaturated, feldspar-rich matrix is attributed to finer fragmentation of feldspar with respect to quartz by impact-generated shock waves. This produces differing distributions of grain sizes of quartz and feldspar, with the larger sizes being enriched in quartz and the smaller in feldspars. The enrichment of feldspar in the fine fraction of Manson impact melt breccias and perhaps other melt-rich terrestrial impact breccias is akin to the enrichment of feldspar documented in the fine fractions of lunar regolith (e.g. Papike et al., 1982). This implies that differential comminution of mineral constituents may be an important chemical fractionation process during major planetary impacts.


REFERENCE:

Papike, J.J., Simon, S.B., and Laul, J.C., 1982, The lunar regolith: Chemistry, mineralogy, and petrology: Reviews of Geophysics and Space Physics, v. 20, p. 761-826.