Petrography of the Crystalline Breccias from the Manson M-1 Core: Implications for Impact Breccia Emplacement

Details Petrography of the Crystalline Breccias from the Manson M-1 Core: Implications for Impact Breccia Emplacement Petrography of the Crystalline Breccias from the Manson M-1 Core: Implications for Impact Breccia Emplacement

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.321b&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 321

Mathematics

Logic

1

Breccia, Breccia Emplacement, Impact Craters, Petrography, Shock

Scientific paper

The M-1 core was drilled on the eastern edge of the central uplift within the Manson Impact Structure in Iowa. The lower 107.9 m (106.4 to 214.3 m below ground surface) of the core consists of crystalline breccias. The breccias are divided into three units by matrix size and abundance. Unit 1 (106.4 to 147 m) has a high volume fraction of matrix (ave. 0.54), a decreasing proportion of matrix with depth, and is very fine grained (<0.005 mm). Unit 2 (147-161 m) is transitional between units 1 and 3. Matrix grain size increases to 0.02 mm and decreases in abundance to 10%. Unit 3 (161 to 214.3 m) has a coarse matrix (>0.02 mm) which changes in abundance from about 10% at the top to about 2% at the base. The top of unit 1 consists of intermixed sedimentary (3%) and crystalline (5%) clasts. The abundance of sedimentary clasts decreases to less than 1% within about 1 m, although they are scattered throughout the rest of the core. Mineral grains in some crystalline clasts appear to have been melted by the impact. Polycrystalline quartz clasts less than 0.8 mm in length are abundant (~20%). Mineral clasts include quartz, plagioclase, K-feldspar, biotite, and zircon. Grain shapes range from angular to subrounded. Some grains have no internal deformation, whereas others have a variety of impact shock deformation features. Deformation in quartz grains ranges from single shock lamellae, to multiple and decorated shock lamellae and multiple lamellae with a pale brown color. Some grains have been converted to diaplectic and thetomorphic glass. Shock deformation features are scarce in mineral grains at 111.8 m where the matrix is crystalline. Quartz grains at this level have halos of clinopyroxene and pholgopite crystals surrounded by a calcium depleted zone. The thermal effects observed in unit 1 disappear in unit 2. Mineral clast size increases with depth in unit 2, as does mineral angularity. Amphibolite clasts associated with unit 3 make their first appearance. Polycrystalline quartz grains decrease in abundance downward through unit 2 and are absent in unit 3. Amphibolite clasts become abundant in unit 3. Gabbroic and diabase clasts are relatively common, and volcanic fragments are rare. Melt fragments, often altered, recrystallized, and sometimes vesicular, are scattered down through the core. Mineral clasts are larger, more abundant, and more angular in unit 3 than in overlying units. Mineral clasts include hornblende, K-feldspar, plagioclase, quartz, biotite, magnetite, and clinopyroxene. With the exception of shale clasts, all clasts within unit 3 exhibit shock metamorphic features. These data suggest that there are major lithologic differences between units 1 and 3. Unit 3 has abundant amphibolite clasts, which are not present in unit 1, coarse mineral matrix grains, and ubiquitous shock deformational features. There is no evidence of high temperatures within unit 3 except for rare melt fragments. Shock metamorphic features are common in unit 1, but not ubiquitous. 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 is a mixture of units 1 and 3. Our preliminary interpretation is that unit 3 represents in situ basement that was brecciated, mixed, and shock metamorphosed by impact. The rare shale and glass fragments present in unit 3 were likely injected during impact. Unit 1 consists of basement lithologies that were transported to the site. The evidence of high temperatures toward the base of the unit and the mixed zone underneath suggest it originated as a transient crater lining emplaced by flow possibly during uplift of the central peak.

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