How do you distinguish the proto-lithology of a metamorphic rock?

Reply Sun 19 Apr, 2015 10:22 pm
I want to know the differences between metamorphic rocks formed from sedimentary

rocks and those formed from igneous rocks. Specifically, I would like to be able

to look at a hand sample and/or thin section of a metamorphic rock then guess its

proto-lithology. What characteristics are commonly found in meta-igneous/meta-

sedimentary rocks (hand sample and thin section)?
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Reply Mon 20 Apr, 2015 12:34 am
Its really hard to describe the issues in hand samples and thin sections.
Stuff like the rising temp mineral assemblages are unique to many metasedimentary rocks, while crystal shapes and mineral assemblages are unique to met igneous rocks .
You need to get a good tech book on petrology and look at the information on field samples.

If you have access to thin sections of samples (at cross section and planar), as they say, a picture worth thousands of words.

Is this a class or a hobby?.

Why not start with something you know the geology of, like schiists or gneisses and look at the "country rock", then look at the structure and mineral assemblages (degree of metamorphism imprts certain minerals behind, like garnet or sillimanite)

Also the style of metamorphism, imprints a unique assemblage and structure on the rocks .

Reply Mon 20 Apr, 2015 12:46 am
thanks that actually helps a lot! I'm currently studying for a petrology class.
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