To Dougald Carmichael,
It sounds like the bio cleavage in the high temp rocks resulted from a room
problem - is that so? Have you come across high temp rocks with a decent
cleavage where the fold geometry does not suggest a room problem to be the
cause of the cleavage? Also, was the biotite in these rocks
crystallographically oriented, or was the alignment (some at least) due to
orthogonal dissolution of haphazardly-oriented bio crystals?
I understand pressure solution to be a low temperature phenomena, and had an
inkling that cleavage in higher temp rocks would only form if there was a
severe room problem. I am not convinced that cleavage occurs in low
temperature rocks only because of a room problem - rheology and size of beds
must play a part, amongst other considerations. I am also curious as to
under what circumstances cleavage forms before folding - intuitively I had
thought only in low temp or massively bedded (schistose?) rocks?
Another question: does cleavage always result from material loss (I know
shortened markers show that this is often the case). Is cleavage ever
simply an exchange of material between the P and Q domain without a
significant bulk material loss from the system?
Thank you for your interest - don't seem to be many of us!
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