>We are working on a suite of calc-silicate granulites at the contact of an
>anorthosite body. We have got mesoperthite like intergrowth between garndite
>garnet (Grs65-75 Adr25-35) and clinopyroxene (Al2O3~ 4-6 wt%). The other
>minerals are wollastonite,scapolite with or without quartz. The Cpx grains
>devoid of garnet exsolution are highly aluminous (up to 11 wt% Al2O3).
>Please enlighten me about the natural occurrences where such an intergrowth
>between grandite and Cpx has been observed.
In response to your message and Eric's reply, we indeed found similar
textures in rocks of the Cascade Slide in the Adirondacks. You
should definitely check out the references he supplied. I also found
an interesting symplectic texure in the same Cascade Slide marble
involving cpx and vesuvianite, which is crystallochemically and
compositionally similar to grossular. The symplectite host was cpx,
with about 5% of intergrown vesuvianite, probably resulting from a
breakdwn of CaTs, as Eric suggested for the grt exsolution. In this
case, however, some H2O is involved so the reaction may have occurred
later in the cooling sequence from the contact-metamorphic thermal
maxiumum, or even during later granulite-facies remetamorphism.
I have also seen oriented, lamellar Mg-rich grt in large
(many-cm-long) opx megacrysts from Keene Valley in the Adirondacks.
These large opx crystals also contain apparent exsolution lamellae of
Mg-rich olivine and Ca-rich plagioclase. This texture occurs in
igneous rocks and is likely related to subsolidus exsolution
following development of metastably Al-rich opx grown very rapidly on
the opx-plg (+/-cpx) liquidus from anorthositic or gabbroic
anorthositic liquids. Ron Emslie has described similar phenomena
from rocks in Labrador (involving cpx megacrysts) and Tony Lasaga has
provided a kinetic explanation (in his "Geospeedometry" paper, as I
Dr. Robert J. Tracy
Professor of Geological Sciences
Blacksburg VA 24061-0420
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"We can't solve problems by using
the same kind of thinking we used
when we created them."
-- Albert Einstein