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

The Geological Society of America
1994 Annual Meeting
Seattle, WA, October 24-27, 1994
1994 Program with Abstracts, v. 26, no. 7, p. A-403


The M-1 core was drilled on the flank of the Manson Impact Structure’s Central Peak. The lower 107.9 m (106.4 to 214.3 m below ground surface) of the core consists of breccias composed mainly of clasts, grains, and matrix derived predominately from crystalline basement lithologies but including rare siltstones and shales from the thick sedimentary cover that was present at the time of impact. The breccias are divided into two units (unit 1 from 106.4 to 147 m and unit 3 from 161 to 214.3 m) based upon size and composition of components. Unit 2 represents a zone of heterogeneous mixing between units 1 and 3 (147 to 161 m). Petrographic and microprobe data suggest that unit 1 and unit 3 are lithologically different reflecting different suites of constituents from the basement. The crystalline clasts present in unit 1 are biotite gneises and leucogranites. Biotites in the gneisses are aluminous and have high titanium contents. These are two feldspar meta-granites containing K-feldspar and albitic plagioclase. Leucogranites are predominately quartz and K-feldspar. Mineral clasts in unit 1 include quartz, K-feldspar, plagioclase, aluminous high and low titanium biotite, zircon, and sphene. The lower part of unit 1 contains the most calcic plagioclase (An 35) found in free grains of the crystalline breccias. In addition to biotite gneisses and leucogranites, unit 3 has abundant amphibolites and biotite amphibolite clasts, which are not present in unit 1. Biotites in the amphibolites are aluminous but have lower titanium content than those in the biotite gneiss. Plagioclase is albitic and hornblende has low Ti contents. Diorite is also present in unit 3 and contains sub-calcic augite and albitic plagioclase. Mineral clasts in unit 3 include K-feldspar, plagioclase, quartz, aluminous high and low titanium biotite, magnetic, chalcopyrite, and sub-calcic augite. Blue-green to brown hornblende is abundant and grains with the highest Ti content are present in the upper part of unit 3. Within individual samples, mineral grains <1 mm in length and minerals within large clasts have similar compositions indicating that they are derived from the same target rocks. However, variations in compositions between samples from different portions of the crystalline-clast bearing breccias indicates that they consist of different mixtures of basement lithologies.