Hello Tomas and Bob

Thank you for your kind messages. First I have to appologize to give  
the idea that the picture I sent is from the rocks I am studing. They  
are no.! I take the picture from the site in the internet, just to  
show the texture since I was the field and didn´t had my own picture  
ready to send.I´m still organizing the pictures from the field,  
because we were with a group of students and each one toke pictures of  
different things.

The rocks I´m studing are from south Brazil, almost in the border with  
Uruguay.

The question that puzzled me about this texture is that it was seen in  
two differents close outcrops in context slightly different from each  
other.

In the first one, the texture is developed in a granitic banded rock  
(most of the time considered to be a igneous structure, although in  
the oldest time it used to be mapped as migmatite). In this rock the  
mafic mineral is sometimes biotite, other times horblend. In some  
parts of these outcrop, the texture looks like classic leucossome  
nucleation.

In a nearby outcrop, this texture is only observed in granitic veins,  
that clealy cut the banded rock.

I was wondering if the rection indicated by thet texture might have a   
"dehydration micro-melting accompanying oxidation as an origin" (as  
Bob suggested), but I was more used to associated that reactions to  
garnet;cordierita and pyroxene production.


We are jsuta strating to study this area, so I hope I was not to confuse.

Many thanks to your help.

Carla


Citando Robert Tracy <[log in to unmask]>:

> Mon ami Tomas,
>
> I didn't mean to suggest that the haloes were devoid of quartz, only  
> that using the standard model one might expect them to be unless  
> some metasomatic mischief had occurred. There are clearly lots of  
> issues here regarding the energetics both of the reaction itself and  
> of the textural development that results. Maybe that's why no one  
> seems to have tackeld the study of them (or at least reported such a  
> study). I have seen very similar looking haloes in outcrop  
> (magnetite surrounded by some combination of quartz and feldspar)  
> from quite a few locations and quartz is always present, typically  
> with either Kfs or Plag (not both), but sometimes with both. Given  
> the quartz problem with pure dehydration, one might even suggest  
> dehydration micro-melting accompanying oxidation as an origin, given  
> that virtually all the examples I have seen are from uppermost  
> amphibolite-facies or granulite-facies locations. Shear might also  
> play an essential role, given that these occurrences could typically  
> be called "augen gneisses".
>
> Bob
>
> Dr. Robert Tracy
> Professor of Geosciences
> Associate Department Head
> Director, Museum of Geosciences
> Virginia Tech
> Blacksburg VA 24061-0420
> 540-231-5980
> 540-231-3386 (F)
>
>
>
>
> On Feb 28, 2013, at 11:55 AM, Tomas Feininger  
> <[log in to unmask]> wrote:
>
>>
>>                                                      28.ii.'13
>>
>> Hello Carla,
>>
>>         I replied to your initial inquiry at your personal courriel  
>> address.  I hope that you received it.  Now I amplify my response  
>> and put it on the List because it may interest other geologists.   
>> Yes, in May 1958, while mapping for the USGS I found and sampled a  
>> gneiss megascopically identical to the one that you pictured.   
>> Yesterday, to my great surprise, I found not only the sample, but  
>> its corresponding thin section as well(!).  Oddly, I had written  
>> two labels, and they differ from one another slightly, although  
>> they refer to the same outcrop.  The labels are: #1. Biotite  
>> gneiss.  New route 3 and Cemetery Street, Wyoming, Rhode Island.   
>> May 1958.  Exsolved(?) magnetite crystals surrounded by felsic  
>> (Fe-depleted) haloes.  No. 125.   #2.  
>> Feldspar-quartz-biotite-magnetite gneiss.  Route 95 at Cemetery  
>> St., Richmond, R.I.  May 1958.  Magnetite "exsolution" has left  
>> felsic haloes.  Cambrian(?) Plainfield Formation.  The first label  
>> was written on the heels of my taking the sample.  The second one a  
>> few years later, after I'd studied the thin section, and "New Route  
>> 3" had become Route 95 of the then nascent Interstate Highway  
>> System in the U.S.
>>
>>         Last night I studied the thin section in a summary fashion.  
>>  The section transects two haloes, each made up of quartz (sorry,  
>> Prof. Tracy) and two feldspars (microcline and oligoclase).  One of  
>> the haloes is ringed chiefly by intensely pleochroic amphibole (Z =  
>> opaque) with dispersion so strong as to render it impossible to  
>> obtain decipherable interference figures, even with a pinhole  
>> occular.  The other halo is ringed chiefly by brown biotite, also  
>> with extreme pleochroism (Y = Z = opaque).  Sphene is a major  
>> accessory, and constitutes 3 to 5 percent modally exterior to the  
>> haloes.  Note that the thin section is of normal thickness (0.03 mm).
>>
>>         The thin section rests patiently on the stage of my  
>> microscope, and this weekend I shall try to glean more information.
>>
>>         Bon week-end!
>>
>>                                                      Tomas Feininger
>>                                                      (Professeur de  
>> minéralogie, Université Laval)
>>
>> P.S.  "BR" must mean that you are in Brazil.  No?
>>
>
>



-- 
Profa. Carla Porcher
Departamento de Geologia -IG/UFRGS