Bernardo and Juergen,

These garnet textures are common everywhere, and in a variety of metamorphic conditions, whether they are greenschist, amphibolite or blueschist. I would agree with Juergen that rather than a second garnet growth episode, it must be some more fundamental process that happens during growth of any garnet porphyroblast. I was thinking that initially, garnet growth could have been skeletal around matrix  quartz etc. (possibly controlled by initial nucleation site requirements), and then once the skeletal strands join up, garnet can simply nucleate on itself, rather than on other phases. My thoughts, for whatever they are worth!

Saibal Gupta.

----- Original Message -----
From: "Juergen Reinhardt" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, May 24, 2017 7:19:28 PM
Subject: Re: [geo-metamorphism] Porphyroblasts with Inclusions

Bernardo,

I suppose with ‘single event’ you mean a single progressive-metamorphic
episode, as there are multiple potential reactions garnets can grow from
(or continue to grow from), particularly in metapelites, depending on exact
bulk rock composition. Some may start growing under greenschist facies
conditions, some perhaps only at lower amphibolite facies conditions. The
garnet microstructures you show, with fine-grained inclusions in the centre
and an inclusion-poor to –absent rims/mantles, are very widespread from
what I have seen in many different metamorphic terrains. Therefore, I would
think such a structure does not require a truly poly-episodic P-T history,
but simply reflects a typical prograde growth pattern along a P-T path from
greenschist to mid-amphibolite facies conditions. With variations on the
theme looking at the blueschist you included. The fine-grained nature of
the core inclusions is in line with a former lower-grade state of the rock
matrix (like chlorite-quartz-muscovite) at the time of initial garnet
growth, even taking into account that some quartz would have been been
consumed by that first garnet-forming reaction. The explanation for the
inclusion-poor part is then another matter to discuss, and solutions have
been offered like slower growth rates, consumption of reactants, increased
diffusion rates at higher T, etc. Seeing that the structure is so common,
the conditions for the second garnet growth stage should then be the norm
rather than something exceptional.

Regards,

Juergen Reinhardt



Dept of Earth Sciences

University of the Western Cape

Bellville, 7535

South Africa

On Wed, May 24, 2017 at 2:33 PM, bernardo cesare <[log in to unmask]>
wrote:

> Dear all,
> following an earlier message on a similar topic, I’d greatly appreciate to
> receive your opinion and see if there’s some kind of consensus on the
> microstructural occurrence of inclusions in porphyroblasts.
> In your experience, *in the case one can confidently assume a
> porphyroblast grew in a single event*:
> -do porphyroblasts often show an inhomogeneous distribution of inclusions,
> with core-rim textures?
> -if so, are inclusions more common in the cores or in the rims?
>
> My experience (or my bias) is that the porphyroblasts (garnet in
> particular) commonly show inclusion-rich cores rather than mantles or rims.
> I am attaching a few examples from metapelites and metabasites, supra- and
> sub-solidus. But I cannot be sure garnet formed in a single event in all of
> them.
>
> Your feedback is very appreciated!
>
> Thank you very much in advance and
> Best Regards,
>
> Bernardo
>
> *******************************************************************
> *Bernardo Cesare*
> Dipartimento di Geoscienze, Universita' di Padova
> Via G. Gradenigo, 6, I-35131 PADOVA  ITALY
> Tel: ++39-049-8279148 <+39%20049%20827%209148> Fax: ++39-049-8279134
> <+39%20049%20827%209134>
> email*:* [log in to unmask]
> MicROCKScopica: *http://www.microckscopica.org
> <http://www.microckscopica.org>*
> InsiemePerWamba: *http://www.insiemeperwamba.org
> <http://www.insiemeperwamba.org>*
> *******************************************************************
>
> *"The only real failure is the failure to try, and the measure of success
> is how we cope with disappointment, as we always must."*
>  From the movie: The Best Exotic Marigold Hotel
>

-- 


Disclaimer - This e-mail is subject to UWC policies and e-mail disclaimer 
published on our website at: 
https://www.uwc.ac.za/Pages/emaildisclaimer.aspx