Abstract
Igneous rocks of the Dora-Maira massif in the Po river headwater region in the Cottian Alps of Piedmont, Italy, are surrounded, and in many places surmounted, by gneiss, marble, and other associated metamorphic rocks of controversial age. The evidence is considered conclusive that the age of the rocks ranges from Permo-Carboniferous to Triassic. Criteria for discrimination of successive periods of metamorphism, including retrograde metamorphism, are reviewed. Carbonatized rocks and phengitic conglomeratic quartzite serve as "metamorphic thermometers" useful locally for discriminating successive periods of metamorphism. Cataclastic effects are also useful for determining time relationships, as are also optical peculiarities of quartz, feldspar, white mica, and biotite in various rocks. It is concluded that the region has been subjected to two major periods of metamorphism, and that the Alpine metamorphism in general was less intensive, and in certain aspects was retrograde compared with the pre-Alpine metamorphism.
Retrograde metamorphism
