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Rock Specimens on the Lawn

Banded Iron Formation (BIF)

Early Proterozoic probably from near Marquette in the Upper Peninsula of Michigan; red quartz layers and black layers of specular hematite and magnetite. The layering is though to be produced biannually in shallow seas. In general, banded iron formations of the oxide-carbonate-silicate- sulphide type occur in Archean greenstone belts and reached their peak development in the early Proterozoic basins about 2000-1800 Ma ago. Mid-late Proterozoic and Phanerozoic basins do not contain BIF of this type. In contrast, iron formations containing chamosite-goethite-siderite are unknown in the Precambrian but are found in Phanerozoic basins. The occurrence of the Precambrian BIF's is thought to be related to anoxic conditions of the early atmosphere. As oxygen content of the atmosphere and ocean waters increased, the solubility of iron decreased. With these changes in solubility, transport of iron in the entire weathering cycle would have decreased. Iron would remain in soils and not be found in solution in ocean waters after the atmosphere became oxygen-rich.

Copper Harbor Conglomerate

Late Precambrian of the Upper Penininsula of Michigan; dominantly rhyolite cobbles (which is curious because the Keeweenawan succession is approx. 99% basalts, and only 1% thin rhyolites); green minerals are copper sulfates and/or copper carbonates formed by the weathering of metallic copper. The original metallic copper was metasomatic, implaced by a convection driven solution related to regional metamorphism. Native copper + hematite were primary. Iron oxidizes before copper. (Rocks lack dellafosasite (CuFe2O4), which indicates copper didn't oxidize ).

Jasper Conglomerate

More correctly called the Lorraine quartzite, 2.3 Ga Huronian rocks of southern Ontario; found associated with the Gowganda tillite, and may in fact be the outwash associated with the Gowganda glaciers. These rocks are the oldest rocks in North America that evidence an early Ice Age. Amount of red chert (jasper) varies considerably as does as the grain size. Quartzitic conglomerates and coarse sandstones are common. The rock is from the Lorraine Fm., which is part of the Paleoproterozoic Huronian Group. This unusual rock crops out exclusively along a 10 km-long ridge about 25 km NE of Sault Ste. Marie, Ontario. The Lorrain overlies the famous Gowganda Fm., which contains debris flows, tillite and varved sediments with ice-rafted debris thought to represent early Proterozoic glaciation. The source of the red pebbles is probably BIF or related siliceous exhalative sediment in adjacent Archean greenstone belt (basement) to the north, although no one has worked that out.

For a little history on this item go here:

http://www.public-art.umich.edu/the_collection/campus/central/52

Garnet Amphibolite

To left as you exit building, garnet amphibolite from Gore Mt., Vermont; dated (U-Pb) 1030 Ma by former post-doc Klaus Metzger. Former professor Abe Heinrich had the garnet mine ship this sample here. Garnets were mined, ground up and then used as abrasives. Note: large garnet crystals with reaction rims of coarse-grained amphibole. Country rock is dominantly amphibole and intermediate composition plagioclase. Also contains orthopyroxene (ï¿óbronziteï¿ó as a field term for color but hypersthene in composition) in the matrix. Interpretation of petrology: formed at 7-8 kb, 750° C, approx. 25-30 km depth which is considered deep crustal level now but may have been mid-crustal level during a major continental collision about 1 billion years ago.

Pegmatite in Granite

Limestone