Can you explain why some muscovite-garnet bearing granites display highly magnetic features?
Apparently these granites contains and magnetite together with almandine and muscovite.:-)
In muscovite, formed after replaced by it biotite, laths of magnetite often are located along cleavage fissures (e.g. http://www.mindat.org/photo-199867.html ).
Probably replacement of biotite by muscovite accompanied with magnetite formation able to elevate magnetic properties of a rock/
Pavel has given a possible explanation. I think you should check the occurrences of the magnetite grains. Sometimes the magnetite grains are formed by alteration under subsolidus. Are the muscovite and garnet magmatic in origin?. There are two papers mentioning the oxidized S-type granite in Canada :(http://bulletin.geoscienceworld.org/content/112/7/1119.full.pdf. )(http://econgeol.geoscienceworld.org/content/79/5/826.full.pdf).
Best wishes,
Rongqing
There may be another explanation: accessory pyrrhotite (strongly magnetic variety) in microscopic grains inside the garnet or on grain boundaries. Did you check it?
Presence of magmatic (?) almandine may indicate relatively high Fe content of an aluminous magma.
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