PETROGRAPHY OF THE CRYSTALLINE BRECCIAS FROM THE MANSON M-1 CORE: COMPOSITIONAL CORRELATION TO TARGET LITHOLOGIES

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

Lunar and Planetary Science Conference XXIV, Lunar and Planetary Institute, Houston
Abstracts of Papers, 1993, p. 87-88

ABSTRACT


The Manson M-1 core was drilled on the eastern edge of the Central Peak of the Manson Impact Structure in Iowa. The basal 107.9 m of the core consists of crystalline breccias that we have divided into three units, defined by matrix size and abundance. Petrographic and microprobe data suggest that the major lithologic differences between unit 1 (106.4 to 147 m) and unit 3 (161 to 214.3 m) are controlled by pre-impact target properties. Unit 3 has abundant amphibolite clasts (not present in unit 1), coarse mineral matrix grains, and ubiquitous shock deformational features (including multiple, intersecting planar features in quartz). Mineral grains include blue-green to brown hornblende, K-feldspar, plagioclase, quartz, biotite, magnetite and clinopyroxene. There is no evidence that unit 3 experienced high temperatures, except for rare melt fragments. Shock metamorphic features are common in unit 1, but not ubiquitous. Mineral grains include quartz, K-feldspar, plagioclase, biotite and zircon. The matrix in the upper portion of unit 1 is exceedingly fine-grained and nearly isotropic. The lower portion of the unit was hot enough to generate a finely crystalline groundmass and reaction rims on quartz. Unit 2 (147-161 m) is a mixture of units 1 and 3. Our preliminary interpretation is that unit 3 represents in-situ basement that was shock metamorphosed, brecciated, and mixed during crater formation. Rare shale and glass fragments present in unit 3 were likely injected during impact. Unit 1 consists of select basement lithologies that were transported to the site. The evidence of high temperatures toward the base of the unit and the underlying mixed zone suggest that it originated as a transient crater lining and was emplaced by flow during the uplift of the Central Peak.