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Subject:

Re: sector-zoned zoisite in eclogite

From:

John Rosenfeld <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Thu, 13 Dec 2001 15:31:58 -0500

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text/plain

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text/plain (81 lines)

Dear Javier,

I [Rosenfeld, J. L. (1985). Chapter 21: Schistosity in Preferred
Orientation in Deformed Metals and Rocks: An Introduction to Modern
Texture Analysis. H.-R. Wenk, ed., Orlando, Academic Press, Inc.:
441-461] mentioned sector-zoned zoisite (p. 446-447) in the context
of a study devoted to the origin of schistosity - no chemical data.
For context, see also: Rosenfeld, J. L. (1987), "Rotated Garnets" in
Encyclopedia of Structural Geology and Plate Tectonics. C.K.Seyfert,
Editor, Volume X of Encyclopedia of Earth Sciences Series, R.W.
Fairbridge, Series Editor, New York, Van Nostrand - Reinhold:
702-709. This specimen, collected in 1963 from near the east end of
Lake Ritom in the Val Piora comes from a classical area on the south
side of the Gotthard Massif, Switzerland, studied by Grubenmann, Paul
Niggli's teacher (Grubenmann, 1888, Mitt. thurg. natf. Ges. 8, p.
15-39). The area is just west of the area studied for his
dissertation by my late friend, Martin Frey of Basel (Frey, 1969,
Beitr. Geol. Karte dur Schweiz). It may be that Grubenmann noted the
phenomenon in his study of the Val Piora area, as my own experience
demonstrated to me that his petrographic descriptions of the rocks
were superb and of timeless value! It'd be worth looking it up to see
what he had to say. I have a large specimen of the specimen (alpha
58a) in which I observed the sector-zoned zoisite. Should you need a
sample applicable to your research, I'd be happy to provide it. In
this case it is clear that the sector-zoning is a primary
synmetamorphic, syntectonic feature and representative of
disequilibrium metamorphism synchronous with the syntectonic snowball
garnets studied by Krige and featured in Eskola's section of Die
Entstehung der Gesteine. This evidence of sector-zoning should raise
a red flag for those who rely too much on equilibrium interpretations
of silicate mineral assemblages in regionally metamorphosed rocks at
grades as high as the kyanite zone.

*****

>Dear all,
>
>I've read about chemical aspects of sector zoning in zoisite (variations in
>Fe3/Fe3+Al ratios: Deer, Howie & Zussman, 1986)... Going further, does
>anybody have any opinion about the origin(s) of this rare feature?
>
>I've found sector-zoned zoisite porphyroclasts in poorly deformed eclogite.
>Zoisite coexists with omphacite porphyroclasts and garnet, which is
>arranged in honeycomb patterns (surrounding omphacite and zoisite
>clasts/domains).
>
>It seems therefore that eclogitization took place in this rock at nearly
>static conditions, with low-range chemical diffusion... Is then zoisite
>reflecting some chemical inhomogeneity in a pristine (now diappeared)
>phase? ... I'm thinking about possible An-rich, (zoned?) igneous
>plagioclase.
>
>PS:... Trace amounts of trivalent cations in the former phase can also
>account for tiny rutile inclusions, arranged in preferred orientations
>within zoisite clasts.
>
>
>Thanks,
>
>-------------------ooo---------------------
>Javier Rodríguez
>Dpto. Mineralogía y Petrología
>Facultad de Ciencias, Univ. del País Vasco
>Apdo. Correos 644
>E-48080 Bilbao, Spain
>
>Tfn: + 34 94 601 5456
>Fax: + 34 94 601 3078
>e-mail: [log in to unmask]
>===========================================

--
John L. Rosenfeld
Department of Earth & Space Sciences
University of California, Los Angeles
Los Angeles, California 90095-1567

Phone: 310-825-1505
e-mail: [log in to unmask]
website: <http://www.ess.ucla.edu/facpages/rosenfel.html>

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