John D. Winter wrote:
>
> Thanks to all who responded. The responses fall into three camps on this
> issue. One group prefers the temporal sense of "progressive," a second
> prefers the spatial sense, and a third is comfortable with the ambiguity.
Oh dear, it looks as if we've lost any useful distinctions there may
once have been.
Progressive metamorphism describes a view of prograde metamorphism in
which the mineral assemblage recrystallises continuously to keep pace
with increasing temperature and pressure (see Nockolds, Knox and
Chinner, Petrology for Students, 1978), i.e. passing through a
succession of equilibrium states. Harker (1932, 1950, page 12) clearly
uses progressive in the temporal sense, with much the same meaning.
Prograde metamorphism refers to the replacement of a lower grade
assemblage by a higher grade one, without necessarily implying the
continuity of intermediate states. A few decades ago there was an
important debate (which I hope still exists in some quarters!) about
whether metamorphism was truly progressive. It's nicely summarised in
the introduction to Dugald Carmichael's 1969 paper on metamorphic
reaction mechanisms (Contrib. Min. Pet. 20, 244-267), Carmichael
advocating the progressive view.
I wouldn't use either term in the spatial sense. A metamorphic field
gradient has a high grade and a low grade end, and you can move upgrade
or downgrade along it, but there's no process involved here. Isograds
move during metamorphism, but that's only because there's prograde (or
even progressive) metamorphism going on in individual rocks.
We take our terms and concepts too much for granted, and use their names
in vain. Progressive metamorphism, as defined above, is an idealised
concept. Prograde metamorphism describes a real process which may or may
not be truly progressive. Earlier in this discussion, Bruce Yardley
pointed out that in the real world we have reactionary metamorphism. I
hope you didn't think he was only joking.
yours,
Dave
--
Dave Waters - Lecturer in Metamorphic Petrology, Oxford University
Dept of Earth Sciences, Parks Rd, Oxford OX1 3PR, UK
Tel: +44 1865 272000
Direct: +44 1865 272058 Email: [log in to unmask]
Fax: +44 1865 272072 http://www.earth.ox.ac.uk/~davewa/
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