Hi Tom
The arguments were not just about preventing or allowing equilibration but
also about our ability to recover P-T-path information. The factors that
play a major role in deciding whether reaction takes place or not are fluid
presence/availability and deformation. If the 'dunk' into the mantle of
crustal rocks (as postulated for most UHP terranes) is only short-lived
then the amount of energy and material that can be transported at peak P-T
is quite small. Rocks sitting waiting for a drink may refuse to react.
Also, eclogites in a subducting slab that are taken to even deeper eclogite
facies conditions may only need to show a minor modification of
garnet-pyroxene ratio to remain in grain-boundary equilibrium. This is
sometimes reflected as a minor overgrowth rim of garnet on previously
existing garnet. During later retrogression this rim is the first part to
be consumed as kelyphites form and so the rock shows no apparent sign of
the UHP stage. Thus two problems hinder the reconstruction of the true P-T
path in HP/UHP terranes. 1) rocks didn't react 2) if they did react,
critical parts of this history were erased during subsequent lower pressure
overprint(s).
Paddy
>Hi Paddy:
>
>I clearly understand the factors you propose to control what students
>take away form a lecture but I'm unclear about what the controlling
>factors are that prevent/allow equilibration in UHP metamorphism. Can
>you elaborate?
>
>Tom
>
>p.s. I just jumped into this discussion a few minutes ago and haven't
>read the preceding messages yet, so maybe I've missed something.
>
>
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>C. T. Foster, Professor and Department Chair
>Department. of Geoscience
>University of Iowa
>Iowa City, IA 52242 USA
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