Tim and all,
Lawsonite only requires 3 kbar to form, that is with quartz and pure
water at equilibrium from laumontite. If one is metastably reacting
disordered plagioclase, it is difficult to say what pressures might be
involved. Survival of apparently unaltered adjacent rocks is not uncommon
in cold blueschist facies rocks.
How good is the 5 Ma age for these rocks? Does it apply to the
metamorphism or to an age of Ar resetting?
The obvious way to get to higher pressures in New Caledonia is by
offsets across faults.
Eric
>Geoff & All
>One observation that intrigued me from New Caledonia of interest to this
>discussion. Going Across a part of the North Western end of NC from S to N
>one can go from effectively zero - or very low grade to eclogite. In the
>effectively very low grade rocks, shear zones develop that are well spaced.
>Within these shear zones, feldspar gets highly deformed brittlely but with
>some degree of plastic strain and very fine scale strain shadowing visible
>at very high magnification. Lawsonite nucleates within the highly strain
>shadowed portions of these feldspars! It does not show up in the country
>rocks until some considerable distance further north. This suggests to me
>that Lawsonite could nucleate using the stored strain energy that would be
>present in the highly strain shadowed feldspar in the form of a high
>dislocation density - ah la Wintsch & Coesite in the 80's. Perhaps we have
>strain energy controlled nucleation as a possibility rather than the old
>stress mineral concept that could give us anomalous pressures. Nick
>Brothers - till the day he died - was convinced the NC rock could never
>have reached the 45 km the experimental work would suggest. They had 5
>million years to go from the surface to 45 km and back up again. That
>always worried him.
>Cheers
>Tim
>
>Prof Tim Bell
>School of Earth Sciences
>James Cook University
>Townsville
>QLD 4811
>Australia
>ph: +61 7 47814766
>fax: +61 7 47251501
>email: [log in to unmask]
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