Sig Cruciani,

On the mailing list GRANITE-RESEARCH
there has been some discussion of how
much country rock can be assimilated by
igneous granites. Its owner, Barrie
Clarke

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is one of three geologists who should
like to organize a symposium on this
subject at the May, 2005 GAC & MAC
meetings in Halifax, Nova Scotia.

Assimilation processes are commonly
thought of as melting, metamorphism, or
metasomatism; but Norman Bowen, in his
1928 treatise, argued reasonably (using
other words) that the reaction on a
p,T-diagram that Schreinemakers would
label (S), the isoentropic reaction,
divides some processes into two kinds:
those that produce liquid (dissolution)
and those that consume liquid. The
former are endothermic, the latter
exothermic.

If your amphibolite crossed the curve
(S) twice during its path (because of
compositional heterogeneities or other
concomitant processes), the
decomposition of amphibole would be
expected to consume heat and possibly
produce a solution that should
crystallize like your lighter leucosome,
to hypidiomorphic granular minerals; and
it should also be expected to produce
heat and possibly consume some of this
liquid to convert other amphibole to the
same mesosome, but convert minerals of
the darker leucosome to granoblastic
assemblages like those you describe.

There are many ways of doing this: the
lighter leucosome need not be prograde
and the darker retrograde, for example.
I'm also not aware of any name, other
than crystallization, assigned to a
petrological process that consumes
liquid to produce a solid.

The proof of this qualitative theorem of
Bowen adds some assumptions to the Le
Chatelier-Braun principle, and I have
not reviewed all the assumptions
carefully. However, Bowen and
researchers on the GEO-METAMORPHISM and
GRANITE-RESEARCH mailing lists may give
you some ideas to pursue. 


Bruce Bathurst, PhD

> I am a PhD student working on high-grade metamorphic rocks from the variscan
> chain of Sardinia, Italy. Amphibole migmatites from Sardinia consist of
> homogeneous mesosome and two  types of leucosomes: tonalitic and
> granodioritic-granitic leucosomes.
> 
> Mesosomes are tonalitic, medium-grained, foliated rocks, made up of quartz,
> plagioclase, biotite, ± amphibole, ± garnet.
> 
> The tonalitic leucosomes are granoblastic, poorly foliated,
> amphibole-bearing patchy leucosomes. They are made up of quartz,
> plagioclase, biotite, ± amphibole and ± garnet.
> 
> The granodioritic-granitic leucosomes consist of quartz, plagioclase,
> K-feldspar, biotite, ± garnet. No amphibole is present in these leucosomes
> and quartz, plagioclase, and K-feldspar are the most abundant minerals.
> 
> No crosscutting relations have been observed between the two leucosome types.
> 
> Can somebody suggest a possible genesis for the different types of
> leucosomes or some recent papers about similar topics ?
> 
> Thanks in advance
> 
> Gabriele Cruciani