The Coronitic Metagabbros from Queyras
In the Guil valley, in the Queyras (French Alps), the Coronitic Métagabbros display a spectacular metamorphic evolution. The magmatic clinopyroxene(Cpx : brown) is sometimes preserved in the centre of a corona of Glaucophane (Glc : bleu marine), a blue amphibole, separating it from Plagioclase, or rather from what remains of it!
Indeed, under the microscope, it is transformed into fine rods of Lawsonite (Laws) and Jadéite (a sodic Cpx). These three minerals are characteristic of the Blueschists facies (stage 4 on the Path). On the other hand, Actinolite (Act) a green amphibole that crosscuts the magmatic Clinopyroxene and stops at the Glaucophane corona, testifies to the GreenSchists facies conditions. The geometric disposition of this Actinolite suggests that it formed from the magmatic Clinopyroxene (stage 3 on the Path), but before the Glaucophane.
under crossed-polarised light:
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