- An enormous ante-kinematic feldspar!

This quartzo-feldspathic gneiss shows an inconspicuous foliation (S). That is not surprising : there are no sheet minerals (like micas) to visualise it properly.The garnet in the centre of the photo has two dark asymmetric aggregates on its periphery :it is cordierite produced by destabilisation of the garnet during the reaction garnet + sillimanite + quartz + V = cordierite.

The asymmetric arrangement of this cordierite shows that the destabilisation of the garnet occured in a synkinematic context of dextral simple shear.

In the same way, here is a circular white body surrounded by the vertical foliation.his object, roughly spherical, is prior to the foliation that surrounds it: it is said to be . ante-kinematic (like the garnet above).

voir le cristal de F ?

Below is the interpretation of this image and .... the scale: this photo is not that of a feldspar crystal in a thin section of rock observed with a microscope, but that of an anorthosite massif with a diameter of nearly 10 km, observed on a satellite image.(An anorthosite is a gabbro in which the proportion of plagioclase is higher than 90%).

The Saririaky anorthosite massif in southern Magascar was trapped in a simple shear, like the garnet above.

Inside the Massif, the following observation can be made:

voir le Massif ?

The foliation surrounds a centimetric feldspar crystal with an arrangement that is perfectly similar to that observed at the satellite scale. We have a good homothety of the structures, from the outcrop scale (centimeter) to the satellite scale (multi-kilometer).

Locate the massif of Anorthosite of Saririaky in the South of Magascar with Google Maps:


Enlarge the map

(see also a little to the North the Massif of Anorthosite of Ankafotia)

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