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GEO-METAMORPHISM  2001

GEO-METAMORPHISM 2001

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Subject:

Shocking pelites

From:

"Gibson, Roger, Dr" <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Tue, 16 Jan 2001 12:47:09 +0000

Content-Type:

text/plain

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Parts/Attachments

text/plain (61 lines)

Hi all,

In the past few years I have been involved in a study of the
Vredefort Dome in South Africa, which has been interpreted as the
deeply exhumed (ca. 7-10 km of erosion) central uplift of a large
impact structure. The centre of the dome comprises Archaean
granitoid gneisses with rafts of amphibolite to granulite facies
pelites, BIFs and mafic gneisses. This metamorphism occurred in
2 stages - the first event at ~3.1 Ga was a regional amphibolite to
granulite event. This was then overprinted at 2.02 Ga by the impact
event and its effects. Owing to their central position in the impact
structure, the granulites experienced the highest shock pressure
(>40 GPa) during the impact event and, because of their great
depths of burial following the impact, they were then statically
metamorphosed at temperatures of between 550 and 1000-1300 oC
by the shock heat released into the rocks following decay of the
impact shock wave. This has created a range of unique features
that bridge the gap between shock metamorphism and
'conventional' metamorphism. For instance, individual minerals
show internal evidence of shock deformation and shock melting,
but there is also evidence of partial melting and reaction between
adjacent minerals. I would now like to try to replicate some of
these textures in the pelitic granulites by performing a series of
shock experiments on a suitable pelitic starting material. The
Vredefort pelites comprise
qtz, plag (oligoclase-andesine), Kfeldspar, biotite, garnet,
cordierite, sillimanite +- opx and spl.
For the shock experiments, we will need to cut between 10 and 15
disks with a 1.5 cm diameter and 1-2 mm thick from a natural
sample WITH A GRAIN SIZE OF LESS THAN 1-2 mm.
I would, thus, like to know if anyone has any samples with a
similar assemblage (the opx and spl are optional) that they might
be prepared to donate for this purpose. Obviously, hornfelses are
likely to be the only suitable candidates, and an additional proviso
is that the disks will have to be free of fractures and retrograde
alteration which would influence the shock wave propagation. An
assemblage like that shown in Fig. 9 of Pattison & Tracy (1991)
from the New Galloway Granite aureole looks like a good
candidate, although it does contain muscovite. I will, of course,
cover any mailing costs.

As this is not really a mailbase discussion item, other members
might appreciate it if replies came direct to me rather than through
the mailbase!

Regards,

Roger Gibson.


Dr. Roger Gibson
Department of Geology
University of the Witwatersrand
Private Bag 3
P O WITS
Johannesburg 2050
REPUBLIC OF SOUTH AFRICA

Tel. + 11 717 6553 (Sec. + 11 717 6547)
Fax + 11 339 1697

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