Dear Koushik,
Sounds interesting. It is definitely worth exercising a little caution if your AMS fabrics do not correspond obviously to a visible fabric. You should have a go at trying to get to the main magnetic carrier with thermomagnetic analyses to start with. Is the AMS fabric consistent in orientation? Or does the lineation correspond to the intersection of the fabrics (field and AMS)? Where and when the magnetic carrier grew could be key here.
Check out:
Tomezzoli et al. (2003) Composite magnetic fabrics and S-C structure in granitic gneiss of Cerro de los Viejos, La Pampa province, Argentina. Journal of Structural Geology, 25, 159-169
Aranguren et al. (1996) Composite magnetic fabrics from S-C mylonites. Journal of Structural Geology, 18, 863-869
Perhaps it might be worth getting hold of one of the new MFK kappabridges (see agico.com) to look for field dependent versus field independent AMS:
Hrouda (2009) Determination of field-independent and field-dependent components of anisotropy of susceptibility through standard AMS measurement in variable low fields I: Theory. Tectonophysics, 466, 114-122
Hrouda et al (2009) Determination of field-independent and field-dependent components of anisotropy of susceptibility through standard AMS measurement in variable low fields II: An example from the ultramafic body and host granulitic rocks at Bory in the Moldanubian Zone of Western Moravia, Czech Republic. Tectonophysics, 466, 123-134
Hope this helps, if anything else comes to mind I'll let you know.
Regards
Carl Stevenson
University of Birmingham, UK
________________________________
From: Tectonics & structural geology discussion list on behalf of koushik sen
Sent: Sun 05/04/2009 13:46
To: [log in to unmask]
Subject: on composite fabric in granitoids
Can anyone provide me some references regarding development of
composite fabric in granitic rocks i.e. where the mesoscopic
fabric(magmatic/gneissosity) and magnetic fabric (AMS) fabric are
discordant? I am presently working on some Proterozoic and Paleozoic
granitoids of Himalaya and there is discrepancy in their field and AMS
fabric. This must have been caused by the Tertiary orogeny. Can anyone
provide information about similar work from other parts of the world?
Best Regards
Koushik
--
Dr. Koushik Sen
Scientist 'B'
Wadia Institute of Himalayan Geology
Dehra Dun- 248001
India
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