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Identity HelpMigmatite?

29th Aug 2014 21:54 UTCSteve Federico

Dear Sir's this is a glacial erratic found in Northwest,Ohio,USA.Thank you Canada.At first I thought it was Gneiss but not now.It look's like pictures of Migmatite in books etc.The rounded bands seem right for it.Please let me know what you think.Thank's for any replies.......Steve

30th Aug 2014 00:55 UTCPaul Brandes 🌟 Manager

To me, it doesn't look metamorphosed enough to call it a migmatite. In a migmatite, one would expect to see a very sharp and definite contrast between alternating bands of light coloured (leucosome) and dark coloured (melanosome) rocks; I just don't see it here. I believe it's still a gneiss that was well on its way to becoming a migmatite, but didn't quite make it.

30th Aug 2014 03:41 UTCBill Cordua 🌟 Manager

I agree with Paul, but there is I think somewhat of a gradation between the two. Migmatites are intimate intermixtures of igneous-looking and metamorphic material. (The "looking" refers to an old debate about the origin of granite rocks in these high grade metamorphic rocks) Some would be ok with this as a migmatite.

30th Aug 2014 13:41 UTCHarold Moritz 🌟 Expert

We have plenty of migmatite here in Connecticut and based on outcrops I've seen during a field trip lead by USGS geologist Greg Walsh, yours seems to qualify. But these and other complex metamorphic rock types are best characterized at an outcrop scale due to heterogeneities much bigger than your sample, which of course you don't get in erratics.

Your nice find reminds me of my college days at CWRU in Cleveland, where we would visit stream valleys cutting down to Lake Erie level. Here we'd find all kinds of cool Canadian Shield rocks eroded from till that we would have to characterize in petrology class.