Hello to everyone --
I would like to ask a general question about what is your experience with
the record of progressive metamorphism in an environment of contact
metamorphism.
I'm working in the Bushveld complex and I'm having a problem with
andalusites that are partially replaced either by prismatic sillimanite or
by a spinel-cordierite symplectite. This indicates heating at LP (from
about 600 to probably about 700-750 or even more) and I thought that
moreover it could indicate that there were _two_ episodes of contact
metamorphism. The rocks are quite close to the contact with the magmatic
body (let's say less than 400m - there is a serious outcrop problem, so
it's difficult to say where exactly the contact is and most of the samples
are just loose blocks). I would suppose therefore that an intrusion of a
very hot body would cause directly the crystallisation of the
high-temperature paragenesis (sil, spl-crd etc.), because the temperature
increase would be too quick to give the necessary time for the
crystallisation of And. I would even expect that this would be the case in
most contact aureoles (i.e. that there will be no major evidences for
progressive metamorphism).
These textures are for example often observed in xenoliths in magmatic
rocks, where they probably do reflect a "two-stage event" (crystallisation
of And outside the magmatic body, then its recrystallisation due to higher
T when it is incorporated in the magma). However, during the discussions
with other people I frequently met the opinion that progressive
metamorphism features (replacement, reaction textures etc.) may easily be
present in contact environments, and that my textures may therefore simply
reflect a continuous increse of temperature due to a single intrusion. If I
understand their conclusions well, Grant & Frost (1990 - AJS 290, p. 425ff)
for example seem to attribute these textures to the 'peritectic' reaction
(i.e. retrograde in character) of andalusite with the magma resulting from
the partial melting (And+L = Crd+Spl+Kfs+...).
Any comments are very much appreciated!
Cheers, Pavel
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Pavel PITRA e-mail: [log in to unmask]
Departement Geologie tel: + 27 - 12 - 420 2242
Universiteit van Pretoria fax: + 27 - 12 - 362 5219
Pretoria 0002 http://www.up.ac.za/science/geology
Suid-Afrika/South Africa http://www.up.ac.za/science/geology/pers/pavel
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