Dear Jochen and all,
You may get some divergence of view on this. However, I can reliably inform you that S- and I-type are still commonly used. The fact is that there are very clear geochemical and isotopic differences between most examples of the two sorts. There certainly are rocks that cannot clearly be assigned to one or the other type. However, the general scarcity of these transitional rocks probably tells us something very important about the compositional structure of the deep crust and the processes that are going on there during granulite-facies metamorphism.
A lot of problems were caused, in the earlier days, when people tried to use just one or two chemical or isotopic parameters to determine S- or I-type kinships. The only reliable way is a combination of a variety of these parameters, along with mineralogical and mineral chemical observations.
We now think that the S-types are definitely derived through partial melting of metasedimentary rocks (most commonly not pelitic). The I-types seem to be derived more from andesitic to dacitic arc sequences, with intermixed greywackes.
Things are not a simple as the were once thought, but the terms S- and I-type are still very useful.
And then there are the A-types....
Cheers,
JC
John D. Clemens
Professor in Geology & Exec. Head
Dept of Earth Sciences, University of Stellenbosch,
Private Bag X1, 7602 Matieland, South Africa
tel: +27 (0)21 808 3159 fax: +27 (0)21 808 3129
e-mail: [log in to unmask]
web page: http://www.sun.ac.za/geo/people/clemens_e.htm
________________________________________
From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Jochen Mezger [[log in to unmask]]
Sent: 20 September 2011 13:53
To: [log in to unmask]
Subject: I- and S-type magmatism
To the magmatically inclined,
I have a question regarding the useage of the
terms I-type and S-type magmatism (or granite). I
have the vague notion that today one is not
encouraged to apply these terms anymore, since
they are an oversimplification. Am I correct with
this assumption? [I am shamefully aware that as a
crystalline geologist, I should know this!]
Thanks for your help.
Best wishes,
Jochen
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Dr. Jochen Mezger
Lehrbeauftragter - Lecturer
Allgemeine Geologie
Martin-Luther-Universität Halle-Wittenberg Tel.: 49-0-345-5526120
Institut für
Geowissenschaften Fax: 49-0-345-5527220
Von-Seckendorff-Platz 3
06120 Halle Email: [log in to unmask]
GERMANY
http://www.geologie.uni-halle.de/igw/allgeo/staff/mez.html
http://allgeo.geologie.uni-halle.de/mitarbeiter/jmezger/
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