Layering in the Mealy Mountains Anorthosite Complex, Labrador

In my last posting I mentioned that I had seen spectacular layering in anorthosite intrusions in Labrador.  Below are photographs that I took of layering in the anorthosite plutons that form the Mealy Mountains in Labrador.  The person that is the scale in the photographs is Dr. Ron Emslie of the Geological Survey of Canada, who was about average height for a world renowned expert on anorthosites.

Ron described the layering as follows:
“Within leucotroctolite, leucogabbro and anorthosite, igneous mineral layering on a scale of 1 cm to 10 m, or more, is common (Fig. 33. 2). Where rock exposure is good, layers or layer contacts can be found on most clean subvertical rock surfaces more than a few metres high. In leucotroctolite the mineral layering involves differences in proportions of olivine and plagioclase,
whereas in leucogabbro and anorthosite, variations in pyroxene and plagioclase proportions are the chief cause. In addition to mineral layering, manifestation of layers is due to differing plagioclase grain sizes and shapes and in some cases, plagioclase colour particularly in leucogabbro and anorthosite. Dark layers in some anorthositic rocks were found to consist of darker coloured plagioclase than that in the enclosing host rock. Such "dark plagioclase" layers were evident in some places even where the enclosing rock contained a noticeably higher proportion of ferromagnesian minerals. Large areas of the complex have relatively consistent layering attitudes. For example, the northwestern part of the complex has dips to the northwest
and west at angles less than 30 degrees. Other areas have remarkably discordant layer attitudes over short distances and sometimes even within the same outcrop. Such discordance seems to be a primary igneous feature of the crystal accumulation process and is not due to superimposed deformation.”

(Emslie, R. .F (1976), Mealy Mountains Complex, Grenville Province, Southern Labrador,
Report of Activities Part A; Geological Survey of Canada, Paper no. 76-1A; p. 165-170
http://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/fulle.web&search1=R=123988
http://ftp2.cits.rncan.gc.ca/pub/geott/ess_pubs/119/119844/pa_76_1a.pdf )


Christopher Brett
Perth, Ontario

Suggested Readings

Emslie, Ronald F.  (1975),  Nature and Origin of Anorthosite Suites, Geoscience Canada, Volume 2, Number 2, pages 99-104
http://journals.hil.unb.ca/index.php/GC/article/view/2912
   
Emslie, Ronald F. (1980) , Geology and Petrology of the Harp Lake Complex, Central Labrador- an example of Elsonian Magmatism.  Geological Survey of Canada, Bulletin 293, 136 pages
http://data.gc.ca/data/en/dataset/96d2a3bc-c79d-57f1-8169-24e01d431893

Hamilton, Michael A. , Scoates, James S. and Rämö, O. Tapani  (2010)
The Petrology of Anorthosites, Related Granitic Rocks, and UHT Assemblages: a Tribute to Ronald F. Emslie, Can Mineralogist, volume 48, pages 705-710
http://canmin.geoscienceworld.org/content/48/4/705.full