(I didn’t mean to start this, but we are already in the middle of it).
Tim, Brett, and all,
Yes, perfectly clear it is. I must have missed an important
publication as well as some lousy little crenulation. What is the
reference? I must admit I am hopelessly conservative in that I believe
that mineral growth is primarily controlled by changes of P, T and
compositional parameters, while a rock is getting deformed, going to
be deformed, or has been deformed already. Nucleation and growth
characteristics, on the other hand, may well be controlled by
deformation patterns and other parameters.
There are minerals that may grow over extended periods of P-T change
from a single continuous reaction, minerals that may grow in multiple
generations from different reactions (continuous or discontinuous),
and minerals that are only produced once over a short P-T interval on
a P-T path. This is reflected in the way deformation structures are
preserved as foliation patterns in minerals, depending on what the
state of rock deformation was at any one point on the P-T path where
mineral growth (or dissolution) took place. Thus, the interplay
between the mechanical behaviour of the crust (or parts of it) and the
migration of isotherms before, during or after any deformation episode
essentially determines what we see in thin section. All this offers a
rather large range of possible “interference patterns” between
deformation and mineral growth. For example, from P-T modeling, it
seems that in many (though not necessarily all) collisional orogens
much of the T rise followed the main thickening process. If I hear
that porphs ALWAYS grow early in a deformation event, it makes me
shiver (“always” and “never” are amongst my favourite words in
geology, anyway). So much for my simple-minded philosophy.
Do you want to tell me that the reaction history is controlled by
deformation such that the appearance of a differentiated crenulation
cleavage brings prograde metamorphism to a screeching halt, waiting
for the infamous orthogonal event to come along?
Cheers, Juergen
J. Reinhardt
Dept. of Geology
University of Natal
Durban, 4041
South Africa
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