Dear Eric,
Although, we do not have Spl composition, Spl in different textural domains 
have a difference in colour. Spl enclosed within An is dark brown, compared 
to its green colour in its association with Opx,Cpx and Olv. The field 
relation and the rock associations do not suggest that these rocks had an 
early history in the Grt peridotite field. The rocks occur in association 
with norites, which show an early stability of Opx-Pl assemblage. In 
intermediate bulk composition of XMg~0.50, coarse Opx is distinctly 
aluminous (Al(VI)=0.11) and reacted with Pl to produce porphroblastic 
Grt+Qtz assemblage. The norite-orthopyroxenite association occurs as thin 
layers or lenses within garnet-cordierite gneiss, often containing 
hercynite. Sillimanite is the stable aluminosilicate in different stages of 
mineralogical evolution of the Crd gn.

Santanu

At 08:00 02/11/2006 -0500, you wrote:
>Santanu,
>    Olivine intergrowths are interesting and certainly sound odd.
>Anorthite-spinel symplectites could represent progress of the reaction
>olivine-spinel-pyroxene(s) = anorthite-pyroxene(s) during
>decompression.  This reaction can occur as long as the spinel is not
>too Cr-rich.  Is there any possibility that the rocks were once in the
>garnet peridotite field?
>eric
>
>Quoting Santanu Kumar Bhowmik <[log in to unmask]>:
>
>>Dear Eric,
>>Thanks for your immediate response. The reaction suggested by you is
>>a distinct possibility. My textural description was incomplete. Olv
>>occurs as detached layers within coarse Opx. Spl also occurs as
>>lamellae or spindle-shaped grains within Olv, giving possible
>>textural support in favour of the suggested reaction, in between
>>included Olv and host aluminous Opx. However, I do not have mineral
>>compositional data at present to look for alumina depletion in Opx in
>>contact with Spl.
>>What about An+Spl+/-Cpx symplectite blebs with Opx ? Olv in this
>>microdomain is absent. Are we looking for micro-scale, original
>>compositional heterogeneity in Opx/Ol and Opx/Pl ratios, coupled with
>>variation in Ca content in aluminous Opx?
>>
>>Santanu
>>
>>At 06:51 02/11/2006 -0500, you wrote:
>>>Santanu,
>>>    We interpreted spinel-orthopyroxene symplectites to represent
>>>progress of the reaction olivine + high Al pyroxene = spinel + low-Al
>>>pyroxene during cooling (Kuo & Essene, 1986).  That could also produce
>>>lamellar spinel in orthopyroxene but requires communication with matrix
>>>olivine.  Is there any evidence of Al depletion in the pyroxene around
>>>the spinel? eric
>>>
>>>Kuo, Lung-Chuan & Essene, E.J. (1986)  Petrology of spinel harzburgite
>>>xenoliths from the Kishb Plateau, Saudi Arabia.  Contr. Mineral.
>>>Petrol. 93, 335-346.
>>>
>>>Quoting Santanu Kumar Bhowmik <[log in to unmask]>:
>>>
>>>>Dear Friends,
>>>>I am looking for literature, which has dealt with exsolution textures
>>>>in orthopyroxene in ultramafic rocks. In olivine-bearing
>>>>orthopyroxenite in the Central Indian Tectonic Zone, cumulus Opx
>>>>contains abundant exsolution lamellae of green Spl, moderate Cpx of
>>>>equant to elongate habit. Pl in addition to its occurrence as
>>>>intercumulus phase, also occurs as thin curvilinear grains within
>>>>Opx. These grains are optically continuous and often associated with
>>>>fine grains of green Spl and Cpx. Is there a possibility that
>>>>Opx-Spl-Anorthitic Pl and Cpx association be part of a  complex
>>>>symplectite, developing from breakdown of an alumious, low-Ca
>>>>pyroxene phase? Are there reports of aluminous pigeonite in
>>>>literature?
>>>>
>>>>Thanking you in advance.
>>>>
>>>>With warmest regards,
>>>>
>>>>Santanu Kumar Bhowmik
>>>>Associate Professor
>>>>Dept of Geology & Geophysics
>>>>Indian Institute of Technology
>>>>Kharagpur-721302
>>>>India
>>>>
>>>
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>>
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