Malcolm,
Sounds like an interesting project.
I have a couple of questions that were not made clear in your message and
might have a bearing on an interpretation.
1) is the Alpine Fault extensional in the sense that it is exhuming the
Southern Alps in the footwall? (excuse my regional geological ignorance!)
2) is the fault ductile or brittle? if it is brittle is it simply a
reactivation of an earlier ductile shear zone? what is the grade of the
fault (shear zone)?
Clearly faults and shear zones are a focus for fluids and, as such, it is
usual to expect to find zones of retrogression (hydration related) in the
vicinity of shear zones. I know from my own work on eclogites that you can
walk from well-preserved eclogite towards a ductile extensional detachment
and within 500m the eclogite has been transformed into biotite bearing
ultramylonite.
It certainly sounds like there is a bulk compositional effect as well where
you have garnets in graphite rich lithologies.
I would think very carefully about your samples in relation to structures
when considering P-T determination and geochemistry. Presumably the goal is
to estimate peak metamorphic conditions as well as retrograde conditions
related to exhumation.
Hope this might be of some little help. I would recommend general reading on
exhumation processes (I think there is a new geol soc of london book on this
related to low angle normal faulting etc. edited by U.Ring) and reaction
processes in shear zones. I'm afraid I'm away from my office now so I don't
have any more specific ref's to hand, but please get in touch if you would
like some.
Hope I haven't misunderstood your problem.
Best regards,
Craig
Hi all
I see everyone is introducing themselves and I feel somewhat obliged to do
the
same.
My name is Malcolm Dixon and I am a Metamorphic Geologist / Geochemist
studying
towards a M.Sc. at Victoria University in New Zealand.
The basic aim of my research is to work out the metamorphic history for a
section of the Southern Alps, South Island New Zealand. The Southern Alps
lie
to the east of and have been exhumed by the Alpine Fault, the boundary
between
the Pacific and the Australian plates. At the highest grade the exposed
schist
is garnet-oligoclase bearing.
I am using a mix of thermometry and barometry techniques, with garnet growth
dates and a fluid inclusion study to build a P-T-t path for the region. In
addition, constraints on the overall geological and metamorphic history of
the
wider study area are being obtained through a study of a thin suite (500m)
of
garnet-kyanite-sillimanite gneiss collected from the western side of the
Alpine
Fault.
In places against the fault, the schist is absent of garnet, and sometimes
even
biotite, yet rocks a few kilometers along strike, identical in appearance
can
have up to thumbnail sized garnets. However rocks that lie further from the
fault (~4-5 km) do have garnets, (microscopic), these garnets seem to occur
in
bands of higher graphite content, or in regions were the schist has a
slightly
greener coloration. My initial feeling is that the garnet growth is being
controlled by the bulk rock composition of the original protolith, and/or
some
sort of fluid interaction. I am awaiting isotope data to identify the types
of
fluids that have been present, and have only just started examining my bulk
rock and trace element data from the XRF.
If anyone had any ideas or references on this I would be extremely grateful.
Many thanks
Malcolm
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Malcolm Dixon
School of Earth Sciences
Victoria University of Wellington
New Zealand
Ph 00 64 04 4635233 x8408
Fx. 00 64 04 4635186
[log in to unmask]
http://www.geo.vuw.ac.nz/
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