Dear Alfredo and others,
This is an intriguing proposal - because a granite pluton, almost by
definition, cannot be rapidly cooled (we would have chilled rhyolites).
So, determination of the geometry of the pluton, depth of emplacement,
related fluid flow etc. and their influence on the thermal history are
likely to introduce uncertainties greater than the closure temperature
itself. These come in addition to problems in defining the boundary
conditions for which one should be determining the closure temperatures.
In this connection, note that absence of hydrous alteration is not
necessarily proof of no heat transport by fluid flow.
In contrast, it is not true anymore that laboratory diffusion rates of
REE in garnets at metamorphic temperatures are too slow to determine -
see for example, Ganguly et al., 1998, Science, 281:805-807 and
subsequent abstracts; recent advances in experimental techniques should
make more data forthcoming. I think a more relevant approach may be to
calculate cooling rates based on experimental data, and test it for
consistency against other field and model observables, to check the
applicability of the data to natural systems.
Having said that, a scenario which may interest you is the reverse of
the one you are seeking (regional metamorphism and related overgrowth on
contact metamorphic garnets) may be found in: Rumble and Finnerty, 1974,
Am Min 59:558-562 and has been modelled by Ganguly et al., Am Min. 1996,
81:1208-1216.
Regards,
Sumit
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***************** Sumit Chakraborty
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http://www.mineralogie.ruhr-uni-bochum.de
Institut fuer Mineralogie; Ruhr-Universität Bochum; D-44780 Bochum;
Germany
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Tel: +49-(0)234-322 4395 / 8521
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