A Kinked Orthopyroxene Megacryst from the Mealy Mountains Anorthosite, Labrador

While attending university I worked for three summers as a field assistant in Labrador for the Geological Survey of Canada under Dr. Ron Emslie mapping parts of the Harp Lake anorthosite body, the Mealy Mountains anorthosite complex and the Ptarmigan Complex in the Red Wine Mountains. While mapping in the Mealy Mountains I picked up a loose, highly kinked, bronzite crystal, which later thin sectioning revealed contained exsolved plagioclase and clinopyxene lamellae along (100) of the orthopyroxene host. Below are three photographs of that specimen. It was about 7 cm long, by 3.5 cm wide by 5 cm thick (before sectioning). 

 

The middle photograph shows a cut and polished surface.   In the  edited version of that photograph provided below I have used pink lines to show the kink zone  boundaries.   I’ve shown some of the (100) exsolution planes with  blue lines.  The (100) exsolution planes run roughly diagonally upwards  from right to left, but changes direction in each kink zone.  

 

 

Below are two photographs of a polished thin section made from this specimen.

The very fine lamellae are exsolved clinopyroxene.  The thicker lamellae are exsolved plagioclase.  Plagioclase grains  (some displaying twinning) can also be seen along the kink boundary.   Some of the fine clinopyroxend lamellae bend as they approach the kink boundary.

The specimen is an aluminous orthopyoxene megacryst –  an example of a Type 1 megacryst  “interpreted to have crystallized within the mantle and at deep crustal levels” (see Emslie, 1975) commonly found in anorthosite intrusions.   Most believe that the aluminum is present in the original orthopyroxene megacryst  as various Tschermak components  (Ca,Mg,Fe)Al2SiO6 and/or as Ca(Ti 3+, Fe3+) AlSiO6 where  trivalent titanium  Ti3+ or trivalent iron Fe3+  is a  cation in the M1 site, before exsolving in a reaction that produces plagioclase and any of  ilmenite,  spinel, rutile and various iron oxides.

Interestingly, while most mineralogy texts and web sites mention that the typical orthopyroxene cleavages is  parallel to (210), with planes intersecting  at ~90̊, with parting on (100) and (010), the specimen shown in the above photographs displays  very good cleavage parallel to the (100) exsolution planes, with  good parting on (010) and (001).

It is worth noting that the specimen  in the above photographs is not representative of orthopyroxene megacrysts from the Mealy Mountains as it is much more kinked than many other crystals that I found.  While mapping the Mealy Mountains anorthosite I found hundreds of large loose bronzite and hypersthene crystals scattered across the surfaces of outcrops, and many examples within the anorthosite.   The largest ones I observed were rounded and larger than a meter in diameter.   Many crystals displayed distorted zones and some displayed kink bands.  None of the other pyroxene crystals that I looked at were as kinked as the specimen shown in this blog posting. 

Others have reported and figured kinked aluminous orthopyroxene megacrysts. Ron Emslie (1975) noted that plagioclase lamelllar structure in alumious orthopyroxenes pyroxenes from anorthosites “is frequently warped and sometimes is crossed by kink bands”, and included a photograph (Fig. 1,E)  from the Morin Anorthosite showing minor kinking.  Emslie (1976) includes as figure 33.7  a photograph of a kink banded specimen from the Mealy Mountains with the caption “Orthopyroxene megacryst showing kink-banded  cleavage. Kink-banded is a common feature in these megacrysts and the enclosing rocks are not deformed.”   Nunn et al. (1986), in a report on the Atikonak River Massif, Western Labrador, for a rock unit comprised of troctolite, leucotroctolite and anorthosite, include a photograph (plate 14, page 136) showing “Strong kink banding in a giant orthopyroxene; the crystal is over 40 cm across.”

Numerous authors have remarked on the kinked orthopyroxene crystals in the Lac-Saint- Jean anorthosite in Quebec.  Benoit and Valiquette (1971) mention that along the road between Saint-Bruno and Larouche, some of the hypersthene crystals are a foot long and that “Les cristaux d'hypersthène déformés au point de montrer des faces cristallines ondulées sont d'observation commune.” [Translation:  Hypersthene crystals distorted to the point of showing wavy crystal faces are a common sight.]  Berrangé (1977), in a discussion of the Lac- Saint- John anorthosite reported that “very large hypersthene crystals are not rare” and included a photograph as Plate IV-B of a large (18 cm x 13 cm)  “Kink banded crystal of  orthopyroxene (En72 —70).”    David  Duguay (2012) in a study of aluminous pyroxene megacrysts from an  outcrop located along the Route du Pont in Arvida, Quebec, part of the Lac-Saint-Jean anorthosite complex, reported plagioclase exsolution lamellae, clinopyroxene exsolution lamellae,  ilmenite exsoloution lamellae, and exsolved  rutile, and kinkbands in the crystals.  Photographs 10C and 10D at page 24 of his thesis show spectacular kinking on par with the specimen shown above from the Mealy Mountains.   Klein and Philpotts (2016, Figure 9.47 (B)) include a photograph of a “Single crystal of orthopyroxene showing kink bands from the Lake St. John anorthosite massif, Quebec.” where the crystal is about 30 cm long by 15 cm wide.

Low (1896) in a report of an anorthosite occurrence along the Romaine River, Labrador, mentions (page 235) that it contains “masses of brown hypersthene, often several inches in diameter.  The hypersthene often exhibits a zig-zag crumpled texture.” and that the anorthosite “holds much hypersthene, often in large masses, some a foot across.”  Low also reported that the zig-zag structure in hypersthene was seen in thin section.  I suspect that Low’s "zig-zag crumpled" hypersthene crystals are kinked aluminous orthopyroxene megacrysts, but can’t find where anyone has studied them since Low visited the site in 1892-95.   Sabina (2003, page 189) mentions the Romaine River ‘zigzag, crumpled’ hypersthene occurrence in her mineral collecting guide for Labrador, but does not appear to have visited the occurrence.
 
Numerous authors have reported on kink bands in orthopyroxene crystals.   All agree that the bending and kinking of bronzite and orthopyroxene is  indicative of the intense deformation and pressure experienced by the host rock .   Berrangé (1977) notes that “ Turner and others (1960) who have studied identical [kink] bands produced in  experimentally deformed orthopyroxenes, and in orthopyroxenes  from the Adirondacks and elsewhere, attribute the kink bands to  post-crystalline plastic deformation by translation gliding on {100} parallel to [001]." Numerous authors have reported on clinopyroxene exsolution along the (100) plane of orthopyroxene.   Most believe that the exsolution of Ca-clinopyroxene from orthopyroxene is aided by deformation (in part because experiments show that highly strained  bronzite inverts to clinoenstatite along the (100) plane).   I’ve provided a number of the more interesting papers below in the list of references.  

Deformation textures such as kink bands in orthopyroxene, bent exsolution lamellae in orthopyroxene, curved cracks in pyroxenes, are a common feature of mantle xenoliths (Mundl et al, 2015; Alifirozva, T.A. and Pokhilenko, L. N. , 2008;  Engvik et al.,  2020).  Kinked pyroxenes are also common in high grade metamorphic rocks.  For example Sturt (1969) mentions that pyroxene crystals in amphibolite facies regional metamorphism “show such features as small faults, kink bands, strain-induced twin lamellae, and in some cases exsolution lamellae” and includes as plate 3B a photograph of a thin section of a pyroxene showing exsolution and kink bands.

 Below is a schematic drawing of an ideal kink band, modified from McLaren and  Etheridge (1976), who conducted a  transmission electron microscope study of naturally deformed orthopyroxene.   The kink band boundaries are A-B and E-D.

The slip planes are normal to the drawing and the slip directions are indicated by the arrows.  Note that the slip planes are bent in the areas ABC and DEF.

As noted by Plummer et al. (2021) “Kinking is a deformation mechanism ubiquitous to layered systems, ranging from the nanometer scale in layered crystalline solids, to the kilometer scale in geological formations.”   Below is a schematic drawing of an ideal kink band, modified from Donath, 1968; Rousell, 1980 and Noël and Archambault, 2006, who all studied kink bands in rock formations.

The fluorescent pink arrows show the relative direction of movement.  Here the rotation and slippage of the lamellae produces voids (blue triangles)  at the end of rotated lamellae along the kink zone boundary.  In the above photographs of thin sections of kinking in the orthopyoxene megacryst, the voids are filled by plagioclase, clinopyroxene and estatite.  The kink zone is shortened by twice the base of the blue triangles.

In a YouTube video on pyroxenes, Professor Kenneth Befus  (2020) includes a bronzite specimen to show the metallic bronze colour of bronzite.  It is almost as kinked as the specimen in my photographs.  

Christopher Brett
Ottawa, Ontario

References and Suggested Reading

Alifirozva, T.A. and Pokhilenko, L. N. , 2008
A variety of textures in mantle xenoliths of peridotites and pyroxenites from Yakutian pipes: petrological interpretation; 9th International Kimberlite Conference Extended Abstract

Anonymous
Orthopyroxene Deformation.  Structure Database.  University of Otago New Zealand. Blog at WordPress.com.
https://structuredatabase.wordpress.com/mineral-deformation/mineral-deformationorthopyroxene-deformation/

Anonymous
Phase transitions and exsolution phenomena in pyroxenes. 
https://www.uni-muenster.de/imperia/md/content/mineralogie/lecture_3_pyroxenes.pdf

Befus, Kenneth, 2020
Mineralogy: Lecture 45, Pyroxenes. Duration: 17:42.  Posted: Nov 16, 2020. https://www.youtube.com/watch?v=TZlMGDYB6xA
[includes photograph of kinked bronzite at 16:00  to 17:30 ]

Benoit , F.-W. et Guy Valiquette, 1971
Region du Lac Saint-Jean (Partie Sud).  Gouvernement du Québec, Ministère des Richesses Naturelles, Direction Generale des Mines , Service de l'exploration Géologique, Rapport Géologique 140  https://gq.mines.gouv.qc.ca/documents/examine/RG140/RG140.pdf

Berrangé, Jevan P.,  1977
Final report, Antoine-La Trappe area, Roberval county [Quebec], report DP 462.  Quebec Ministere Des Richesses Naturelles, Direction Générale  des Mines, 204 pages
https://gq.mines.gouv.qc.ca/documents/examine/DP462/DP462.pdf

Bertrand, Claude, 1963
L'hypersthène alumineux du Lac St. Jean  [Aluminous hypersthene of Saint Jean Lake] Master's thesis.  thèse de maîtrise non publiée. Ecole Polytechnique, Montreal, QC, Canada, 64 pages

Brett, Christopher P., 2014
Layering in the Mealy Mountains Anorthosite Complex, Labrador.  Blog posting dated  November 4, 2014, http://fossilslanark.blogspot.com/2014/11/layering-in-mealy-mountains-anorthosite.html

Brett, C. P. And Emslie, R. F., 1979
Orthopyroxene Megacrysts in anorthosites of the Mealy Mountains. Prog. With Abstr., Ann Meeting Geological Society of Canada

Brey  G. P. , Köhler T. ,  1990
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Bruijn , Rolf H.C. and Philip Skemer, 2014
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Bystricky, M., J. D. Lawlis, S. J. Mackwell, F. Heidelbach, and P. C. Ratteron , 2016
High-Temperature Deformation of Enstatite Aggregates.  Journal of Geophysical Research: Solid Earth, Volume 121, Issue 9 p. 6384-6400 September 2016 https://doi.org/10.1002/2016JB013011

Dewey J.F. 1965.
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Donath F.A. , 1968
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Duguay, David. 2012
Déterminer l’origine des mégacristaux de pyroxène de l’affleurement situé en bordure de la Route du Pont à Arvida .   Mémoire présenté dans le cadre du cours Projet de find’études
Université du Québecà Chicoutimi Avril 2012
https://eweb.uqac.ca/bibliotheque/archives/travaux/030300974/PFE_Memoire_DavidDuguay.pdf

Emslie, R .F. , 1975
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https://rruff-2.geo.arizona.edu/uploads/CM13_138.pdf

 Emslie R. F., 1976
 Mealy Mountains Complex, Grenville Province, southern Labrador. In: Report of Activities, Part A. Geological Survey of Canada, Paper 76-1A, 165–170 https://doi.org/10.4095/119844

Engvik, Ane K., Cornelia Mertens, Claudia A.Trepmann, 2020
Episodic deformation and reactions in mylonitic high-grade metamorphic granulites from Dronning Maud Land, Antarctica. Journal of Structural Geology, Volume 141, December 2020, 104196
 
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Garnet-orthopyroxene  assemblages  as  pressure-temperature  indicators  •.  An  experimental  study  with  applications  to  granulites  from  Enderby  Land,   Antarctica.  Doctoral Thesis.   https://core.ac.uk/download/pdf/33314696.pdf

Harrison, Richard
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