Rock Specimens on the Lawn, Garnet Amphibolite, 1100 North University Building, 1100 N University Ave, University of Michigan, September 22, 2020

https://lsa.umich.edu/earth/about-us/about-our-building/rock-specimens-on-the-lawn.html

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.'