Hi Tim,
Just a couple of comments:
1. Re: lawsonite: if lawsonite grew in plagioclase from the breakdown
of anorthite (as it happened in other high-P terrains as well), then
it cannot be isolated, as more than a trace of water is required,
presumably supplied through microfractures. This means that you
cannot exclude any contact to other phases.
2. Re: early porph growth: There is a good example of a porphyroblast
that has overgrown a differentiated crenulation cleavage in Bell et
al., J. Struct. Geol. 14, p.800. I'm sure you will explain to us
what's wrong here. I'm getting worried about all these samples around
me with late and post-deformational minerals, perhaps I'll put them
on the dump, so they won't confuse anybody.
But back to tectonic overpressures; it's good to see the fundamental
things being questioned now and then, whatever the outcome. I just
find it difficult to accept the argument that there is as yet no good
evidence for substantial non-lithostatic pressures. For example,
fifteen years ago, there was no evidence for ultrahigh pressures of
crustal rocks, and hence the understanding of crustal metamorphism
did not include ultrahigh pressures (for most petrologists anyway).
Quartz everywhere, but no coesite, as easy as that. Nowadays we
wonder just how much quartz around us has been coesite at some stage.
Times change (sometimes). I have the feeling that most petrologists
(I won't exclude myself) don't like the idea of non-lithostatic
pressures because it questions a very simple concept, and thus makes
life less comfortable.
Cheers, Juergen
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