In the Anorthosite (very plagioclase-rich gabbro) of Ankafotia in southern Madagascar, orthopyroxene (and black ilmenite : iron and titanium oxide) are surrounded by a corona of garnet, in contact with the Plagioclase. Actually, observation under the microscope shows that it is more complex than that ......(the tip of the hammer, at the bottom right of the photo, gives the scale)
The Ankafotia anorthosite massif forms an elongated white lens (ca. 10km long) in the NS direction in this Google Maps image (see also the Saririaky Massif a little to the south)
Indeed, observation under the microscope (in plane polarised light) reveals that the coronite texture is polymineral. The primary, magmatic minerals (Opx1 and Pl1) are separated by clinopyroxene (Cpx2), a new generation of orthopyroxene and plagioclase (Opx3 + Pl3) and a corona of garnet and quartz (Gt2 + Q). This observation is interpreted as the result of the Opx + Pl = Cpx + Gt + Q reaction being passed in one direction and then in the other. The short side of the picture is about 1mm.
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