In the Mirdita Zone, northern Albania, the metamorphic sole occurs as an assemblage of slices interlayered between the ophiolite sequences and the Rubik Complex The slices are represented by garnet-bearing paragneisses and fine- to coarse-grained amphibolites. The metamorphic sole was sampled and studied in the Western ophiolitic belt; for comparison a section in the Eastern ophiolitic belt was also investigated. Four main types of amphibolites were identified, mainly on the basis of bulk rock composition and metamorphic features. Based on thermobarometric estimates, metamorphic peaks at different conditions result within the amphibolite facies field. Temperature and pressure calculated for gneisses are T = 620° ± 20°C; P =0.3-0.4 GPa and in the different amphibolite types are T = 680° ± 20°C; P =0.2-0.4 GPa, T = 740° ± 25°C; P =0.4-0.5 GPa and T = 850° ± 30°C; P = 0.5 GPa, respectively. Based on their geochemical signature, the inferred protoliths for amphibolites are tholeiitic basalts and cumulate gabbros, and Oceanic Island Basalts (OIB), whilst paragneisses probably represent siliciclastic sediments. The deformation history recorded in the amphibolites and paragneisses includes a S1a-S2a composite schistosity developed within the amphibolite facies corresponding to the metamorphic peak conditions. The associated WNW/ESE trending mineral lineations reveal a top-to WNW sense of shear. The subsequent retrograde deformation phase is represented by folds and a S3a crenulation cleavage developed under greenschist facies conditions. No evidences of prograde textural and mineralogical evolution are preserved. The uppermost level of the Rubik Complex is represented by a volcano-sedimentary sequence. It includes acid calcalkaline and basic subaerial pyroclastites, reworked in turbidite flows and slumps. These rocks are in turn affected by a deformation history which occured in two phases; the first one is characterized by the development of isoclinal folds and by a pervasive slaty cleavage under very low-grade metamorphic conditions. The second phase includes open to tight folds and a well developed crenulation cleavage. The petrological and structural evidence from the metamorphic sole of the Albanian ophiolites suggests two main stages during the deformation history. The first stage i.e. the intraoceanic stage, was achieved during the Middle Jurassic intraoceanic thrusting and decoupling of the oceanic lithosphere and is characterized by the development of amphibolite-facies metamorphic rocks; the second stage, i.e. the marginal stage, was related to Late Jurassic-Early Cretaceous obduction of the ophiolites onto the continental margin associated with dismembering and boudinage of the metamorphic sole, and the multiple deformation of the neighbour domains of the continental margin, i.e. the volcano-sedimentary sequence.
Geological and petrological features of the metamorphic sole from the Mirdita ophiolites, Albania
MARRONI, MICHELE
1996-01-01
Abstract
In the Mirdita Zone, northern Albania, the metamorphic sole occurs as an assemblage of slices interlayered between the ophiolite sequences and the Rubik Complex The slices are represented by garnet-bearing paragneisses and fine- to coarse-grained amphibolites. The metamorphic sole was sampled and studied in the Western ophiolitic belt; for comparison a section in the Eastern ophiolitic belt was also investigated. Four main types of amphibolites were identified, mainly on the basis of bulk rock composition and metamorphic features. Based on thermobarometric estimates, metamorphic peaks at different conditions result within the amphibolite facies field. Temperature and pressure calculated for gneisses are T = 620° ± 20°C; P =0.3-0.4 GPa and in the different amphibolite types are T = 680° ± 20°C; P =0.2-0.4 GPa, T = 740° ± 25°C; P =0.4-0.5 GPa and T = 850° ± 30°C; P = 0.5 GPa, respectively. Based on their geochemical signature, the inferred protoliths for amphibolites are tholeiitic basalts and cumulate gabbros, and Oceanic Island Basalts (OIB), whilst paragneisses probably represent siliciclastic sediments. The deformation history recorded in the amphibolites and paragneisses includes a S1a-S2a composite schistosity developed within the amphibolite facies corresponding to the metamorphic peak conditions. The associated WNW/ESE trending mineral lineations reveal a top-to WNW sense of shear. The subsequent retrograde deformation phase is represented by folds and a S3a crenulation cleavage developed under greenschist facies conditions. No evidences of prograde textural and mineralogical evolution are preserved. The uppermost level of the Rubik Complex is represented by a volcano-sedimentary sequence. It includes acid calcalkaline and basic subaerial pyroclastites, reworked in turbidite flows and slumps. These rocks are in turn affected by a deformation history which occured in two phases; the first one is characterized by the development of isoclinal folds and by a pervasive slaty cleavage under very low-grade metamorphic conditions. The second phase includes open to tight folds and a well developed crenulation cleavage. The petrological and structural evidence from the metamorphic sole of the Albanian ophiolites suggests two main stages during the deformation history. The first stage i.e. the intraoceanic stage, was achieved during the Middle Jurassic intraoceanic thrusting and decoupling of the oceanic lithosphere and is characterized by the development of amphibolite-facies metamorphic rocks; the second stage, i.e. the marginal stage, was related to Late Jurassic-Early Cretaceous obduction of the ophiolites onto the continental margin associated with dismembering and boudinage of the metamorphic sole, and the multiple deformation of the neighbour domains of the continental margin, i.e. the volcano-sedimentary sequence.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
