Allochemical metamorphism is an old idea recently revived by studies
of metamorphic fluid flow in several diverse metamorphic settings.
Ague [1994a, 1994b] examined mass transfer attending
Barrovian
style regional metamorphism of the Wepawaug Schist, Connecticut. He
documented correlations among metamorphic grade, quartz vein density, and
vein accessory-phase mineralogy. Amphibolite-facies veins were found
to be surrounded by aluminous selvages, and staurolite and kyanite
were seen to occur preferentially within the selvages. Ague calculated
that the selvages are the result of a 35
reduction in mass during
metamorphism, and he concluded that growth of staurolite and
kyanite was the direct result of metasomatism born of regional-scale
fluid flow. The implication is that the positions of staurolite and
kyanite isograds in Barrovian metamorphic terranes delimit the
locations of fossil fluid-filled fracture networks as well as
pertinent isotherms and isobars.
Selverstone et al. [1991] found that fluid flow that
accompanied ductile shearing in the Tauern Window of the eastern Alps,
Austria, transformed metagranodiorite protolith into aluminous schist
along a shear zone. Weight loss in this process was as much as 60
. The
aluminous composition caused by metasomatism of the shear zone resulted in
formation of garnet-chlorite-staurolite schist during metamorphism to
amphibolite facies conditions soon after shearing.
The process described by Selverstone et al. is essentially an
extreme example of the phenomenon described by Ague brought about by
comparatively large fluxes of fluid. Ague attributed metasomatism in the
Barrovian terrane to a time-integrated fluid flux of 10
m
/m
.
Selverstone et al. calculated a larger flux of 
10
m
/m
for shear-zone metasomatism.