The “External Crystalline Massifs” of the Western Alps (Aiguilles Rouges, Mont Blanc, Grandes Rousses, Belledonne, Pelvoux, and Argentera) consist of a polymetamorphic Variscan basement, which was only marginally reworked during the Alpine tectonometamorphic cycle. This contribution focuses on the magmatic and Variscan metamorphic history of the Argentera Massif, the southernmost and largest of the External Crystalline Massifs exposed in Italy. Its evolution is compared to that recorded in the Mont Blanc-Aiguilles Rouges Massif, the other External Crystalline Massif extensively exposed in the Italian Alps, and to that recorded in the Maures-Tanneron Massif of Provence, France, the area of Variscan Europe nearest to Argentera. Some speculations on tectonic models for this evolution are also proposed. The main geological events recorded by these Massifs are similar. They experienced: a) pre-Carboniferous intrusions of both acid and basic magmas; b) a Carboniferous Variscan history consisting of subduction with metamorphic peak at ~700°C and 1.5 GPa, continental collision developing amphibolite-facies metamorphism, anatexis, and emplacement of granitoids; 3) Carboniferous exhumation to shallow crustal levels. A few of the events well documented in the Mont Blanc-Aiguilles Rouges and Maures-Tanneron have not yet been characterized in the Argentera Massif. In particular, the Late Proterozoic emplacement of granitoids in a (meta-) sedimentary sequence documented in the Maures and the Late Ordovician granite plutonism documented in the Mont Blanc Massif, still await to be confirmed in the Argentera by geochronological data, though field observations generally support the occurrence of these magmatic events. On the other hand, the age of the high pressure (HP) metamorphic event documented by eclogites and HP granulites found in all the massifs is well constrained so far only in the Gesso-Stura-Vésubie (GSV) Terrane of the Argentera Massif, where Carboniferous ages at ~340 Ma were obtained for zircons in the Frisson HP granulites. The LP metamorphism and partial melting in the GSV Terrane are not directly dated, but a Late- to Mid-Carboniferous age for migmatization has been proposed on the basis of a zircon lower intercept age obtained from the Meris eclogite. These geochronological data link together in a single orogenic cycle the HP and the amphibolite-facies metamorphisms, and suggest that the evolution of the Variscan belt of the Western Alps and Provence may resemble that of present-day collisional settings, such as the Himalayan belt. Similarities between the Variscan and the Himalayan orogenies include the conditions of HP granulite-facies metamorphism, and the rapid (within 20 Ma) succession of HP peak metamorphism, fast exhumation and widespread late anatexis.
New data on the geological history of the Argentera Massif and comparison with the Mont Blanc – Aiguilles Rouges and Maures – Tanneron Massifs.
COMPAGNONI, Roberto;FERRANDO, Simona;
2011-01-01
Abstract
The “External Crystalline Massifs” of the Western Alps (Aiguilles Rouges, Mont Blanc, Grandes Rousses, Belledonne, Pelvoux, and Argentera) consist of a polymetamorphic Variscan basement, which was only marginally reworked during the Alpine tectonometamorphic cycle. This contribution focuses on the magmatic and Variscan metamorphic history of the Argentera Massif, the southernmost and largest of the External Crystalline Massifs exposed in Italy. Its evolution is compared to that recorded in the Mont Blanc-Aiguilles Rouges Massif, the other External Crystalline Massif extensively exposed in the Italian Alps, and to that recorded in the Maures-Tanneron Massif of Provence, France, the area of Variscan Europe nearest to Argentera. Some speculations on tectonic models for this evolution are also proposed. The main geological events recorded by these Massifs are similar. They experienced: a) pre-Carboniferous intrusions of both acid and basic magmas; b) a Carboniferous Variscan history consisting of subduction with metamorphic peak at ~700°C and 1.5 GPa, continental collision developing amphibolite-facies metamorphism, anatexis, and emplacement of granitoids; 3) Carboniferous exhumation to shallow crustal levels. A few of the events well documented in the Mont Blanc-Aiguilles Rouges and Maures-Tanneron have not yet been characterized in the Argentera Massif. In particular, the Late Proterozoic emplacement of granitoids in a (meta-) sedimentary sequence documented in the Maures and the Late Ordovician granite plutonism documented in the Mont Blanc Massif, still await to be confirmed in the Argentera by geochronological data, though field observations generally support the occurrence of these magmatic events. On the other hand, the age of the high pressure (HP) metamorphic event documented by eclogites and HP granulites found in all the massifs is well constrained so far only in the Gesso-Stura-Vésubie (GSV) Terrane of the Argentera Massif, where Carboniferous ages at ~340 Ma were obtained for zircons in the Frisson HP granulites. The LP metamorphism and partial melting in the GSV Terrane are not directly dated, but a Late- to Mid-Carboniferous age for migmatization has been proposed on the basis of a zircon lower intercept age obtained from the Meris eclogite. These geochronological data link together in a single orogenic cycle the HP and the amphibolite-facies metamorphisms, and suggest that the evolution of the Variscan belt of the Western Alps and Provence may resemble that of present-day collisional settings, such as the Himalayan belt. Similarities between the Variscan and the Himalayan orogenies include the conditions of HP granulite-facies metamorphism, and the rapid (within 20 Ma) succession of HP peak metamorphism, fast exhumation and widespread late anatexis.
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