The trigger of the Triassic magmatism in the Eastern Alps (about 235 Ma) is still highly debated, and both extensional and compressional theories are invoked. The first is mainly supported by the related extensional geologic structures, while the second family of theories is supported by the chemistry of the better exposed lithotypes and the high sedimentation rate, typical of a back-arc environment, whose arc is supposed to be located east or south of the Adria Plate. We support a not orogenic genesis for the dolomitic Triassic magmatism due to the presence of coeval magmatic products showing not orogenic features, by the position of such magmatic province during Triassic times (edge of the Intra Pangea Dextral Shear; IPDS) and by the sedimentation rate which appears comparable with that of the North America Atlantic coast located along the IPDS. It is interesting to notice that the IPDS, which probably begun to move in early Permian, started its extensional movement 235 My ago. In this work we mainly attempt to evaluate possible interaction of the magmatic products with the carbonate (host rocks) through the study of the sills located in the Costabella Crest (Moena, TN Italy), and with the comparison with other dolomitic Triassic products, then, to give a small contribute about the geodynamic discussion.Generally, the samples show ipoialine to strong porphyric textures and a variable grade of alteration mainly due to disequilibrium between olivine and the melts in the magmatic chamber. In the Total Alkali Silica diagram the sills fall in the alkaline field (from basanite to tefriphonolite), but this partially contrasts with their paragenesis. Indeed, among the phenocrystals the plagioclase appears very abundant and show a bimodal composition (labradorite and anorthite). Carbonates can be present with primary textures to suggest the presence of small amount of carbonatic melts in the magma chamber possibly explaining the isotopic features found in the dolomitic flows where a strong isotopic Sr variability is associated to small Pb and Nd isotopic ones. Apart the strong interaction with the host rocks and the post-emplacement alterations, three groups with different La/Yb ratio can be recognized suggesting a heterogenic source. Finally, to evaluate the supposed subduction-related chemical features, as the negative Nb anomaly, the Triassic samples have been compared with other not arc related magmatic products placed along the IPDS of the Central Atlantic Magmatic Province emplaced at the TJ boundary. Despite the very different grade of alkalinity, locally their incompatible element pattern shows strong similitudes that we relate to past subductions.
The Triassic sills of the Costabella Crest (Moena, TN-Italy) in comparison with coeval Dolomitic magmatites
Velicogna Matteo
;De Min Angelo.
2018-01-01
Abstract
The trigger of the Triassic magmatism in the Eastern Alps (about 235 Ma) is still highly debated, and both extensional and compressional theories are invoked. The first is mainly supported by the related extensional geologic structures, while the second family of theories is supported by the chemistry of the better exposed lithotypes and the high sedimentation rate, typical of a back-arc environment, whose arc is supposed to be located east or south of the Adria Plate. We support a not orogenic genesis for the dolomitic Triassic magmatism due to the presence of coeval magmatic products showing not orogenic features, by the position of such magmatic province during Triassic times (edge of the Intra Pangea Dextral Shear; IPDS) and by the sedimentation rate which appears comparable with that of the North America Atlantic coast located along the IPDS. It is interesting to notice that the IPDS, which probably begun to move in early Permian, started its extensional movement 235 My ago. In this work we mainly attempt to evaluate possible interaction of the magmatic products with the carbonate (host rocks) through the study of the sills located in the Costabella Crest (Moena, TN Italy), and with the comparison with other dolomitic Triassic products, then, to give a small contribute about the geodynamic discussion.Generally, the samples show ipoialine to strong porphyric textures and a variable grade of alteration mainly due to disequilibrium between olivine and the melts in the magmatic chamber. In the Total Alkali Silica diagram the sills fall in the alkaline field (from basanite to tefriphonolite), but this partially contrasts with their paragenesis. Indeed, among the phenocrystals the plagioclase appears very abundant and show a bimodal composition (labradorite and anorthite). Carbonates can be present with primary textures to suggest the presence of small amount of carbonatic melts in the magma chamber possibly explaining the isotopic features found in the dolomitic flows where a strong isotopic Sr variability is associated to small Pb and Nd isotopic ones. Apart the strong interaction with the host rocks and the post-emplacement alterations, three groups with different La/Yb ratio can be recognized suggesting a heterogenic source. Finally, to evaluate the supposed subduction-related chemical features, as the negative Nb anomaly, the Triassic samples have been compared with other not arc related magmatic products placed along the IPDS of the Central Atlantic Magmatic Province emplaced at the TJ boundary. Despite the very different grade of alkalinity, locally their incompatible element pattern shows strong similitudes that we relate to past subductions.Pubblicazioni consigliate
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