Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Ch (a) over cap tillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Gamet-glaucophane-phengite-paragonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Ch (a) over cap tillon slice. Paragneiss and micaschist probably derive from pre-Alpine "kinzigites"; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T less than or equal to 560 degreesC and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Ch (a) over cap tillon slice is very similar to the Eclogitic Micas-chists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Ch (a) over cap tillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.
Geology and petrology of the Austroalpine Châtillon Slice, Aosta Valley, Western Alps.
Rolfo F;
2004-01-01
Abstract
Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Ch (a) over cap tillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Gamet-glaucophane-phengite-paragonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Ch (a) over cap tillon slice. Paragneiss and micaschist probably derive from pre-Alpine "kinzigites"; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T less than or equal to 560 degreesC and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Ch (a) over cap tillon slice is very similar to the Eclogitic Micas-chists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Ch (a) over cap tillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
