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Tesi etd-02162006-085812


Thesis type
Tesi di dottorato di ricerca
Author
Bracciali, Laura
email address
laurabrac@libero.it
URN
etd-02162006-085812
Title
Mesozoic sedimentary covers of the Ligure Piemontese Ocean: from source areas to the architecture of margins
Settore scientifico disciplinare
GEO/07
Corso di studi
SCIENZE DELLA TERRA
Commissione
Relatore Prof. Rocchi, Sergio
Relatore Dott. Pandolfi, Luca
Parole chiave
  • provenance
  • Northern Apennines
  • Mesozoic
  • detrital white mica geochronology
  • Corsica
  • arenite petrography
  • Alps
  • sediment geochemistry
  • Western Tethys
Data inizio appello
24/03/2006;
Consultabilità
parziale
Data di rilascio
24/03/2046
Riassunto analitico
Provenance studies started in the 19th century with the aim of determining the parent rocks of single minerals or mineral varieties. With the advent of the Plate Tectonic theory, classification schemes were proposed, to quantitatively evaluate sandstone composition, which in turn was used to relate sandstone composition to the tectonic setting of the source areas. The traditional provenance studies based on modal analysis of arenites were enhanced by the application of other methods not limited solely to the sand-sized fraction. After the development/improvement of high resolution analytical techniques, the geochemical features of fine-grained siliciclastic rocks have been fruitfully used in provenance studies, along with the petrographic and chemical analysis of coarse-grained deposits. Moreover, the field of provenance analysis has undergone a revolution with the development of high resolution single-crystal isotope-dating techniques, such as SHRIMP U-Pb dating on zircons and laserprobe 40Ar –39Ar dating. As suggested and recommended in recent works dealing with siliciclastic provenance, a multi-disciplinary approach is highly desirable. In this PhD thesis, a multidisciplinary approach is proposed to face a provenance issue, which combines: i) petrography and geochemistry of conglomerate clasts, ii) modal analysis of arenites, iii) geochemistry of major (XRF) and trace element (ICP-MS) of pelites and arenites, iv) Ar-Ar geochronology of detrital white micas.<br>This approach has been applied to sedimentary rocks in order to unravel a geological problem: the reconstruction of the configuration of continental margins of the Ligure-Piemontese oceanic basin (a branch of Western Tethys opened during Jurassic between the Corsica/Europe and Adria plates). The main goals related to this issue are: i) to unravel the composition of the two margins, which in turn can provide further constraints for their configurations prior to their involvement in the collisional events; ii) to discuss the results in order to evaluate the different models existing for the opening history of the Ligure Piemontese Ocean. <br>Remnants of the Ligure Piemontese oceanic basin, today preserved as tectonic units within the Alpine-Apennine belt (originated after the continental collision between Corsica/Europe and Adria plates during Eocene), display well developed Upper Jurassic to Cretaceous sedimentary successions. The investigated units are: the Err-Platta units from the Central Alps, the Balagne Nappe from Corsica, the Internal and External Ligurian units from Northern Apennines. Paleogeographic reconstructions permit the pristine location of these units in the Ligure Piemontese oceanic basin. The studied sedimentary rocks were all fed by the passive margins of the Ligure Piemontese Ocean, making it possible to investigate the nature of the source areas located in the two conjugate margins.<br>The geochemical investigation of pelites permitted to highlight ‘time’-related first-order chemical large-scale features for the sediments of the Ligure Piemontese oceanic basin. A more mafic character is shown by the Radiolarite formation vs the Calpionella Limestone and Palombini Shale formations sampled in the different investigated units (the three formations representing the pelagic post-rift deposits of the Ligure Piemontese oceanic basin). This difference in composition can be explained considering the erosion and distribution inside the oceanic basin of the rough primary morphologies of the basaltic sea-floor at the time of deposition of the Radiolarites. This possibly resulted in a mafic geochemical signature inherited by the fine-grained siliciclastic sediments from the first post-rift oceanic formation (the Radiolarite Formation). This common oceanic basin-scale chemical feature is preserved and still decipherable thanks to the geochemical features, despite the investigated formations belong to tectonic units from various sectors of the Alpine-Apennine chain that experienced different tectonic/metamorphic histories.<br>The composition of the sedimentary covers from the Ligure Piemontese oceanic basin has been then considered under the ‘space’-related perspective, comparing the units to each other. Along a Corsica-Apennines transect, petrographic data from rudites and arenites have been combined with geochemical data from pelites, and the main features of the source areas (i.e. the continental margins) have been highlighted using the differences in composition inferred from these data. The source area of Corsica and Internal Ligurian units, regarded as the Corsica/Europe continental margin, was made up of the upper part of a continental basement plus its carbonate sedimentary cover. The sedimentary record generated by its erosion is in fact represented by turbidite deposits derived from low-grade metamorphic rocks, granites, acidic volcanic rocks and carbonate platform rocks. The geochemical data relative to the fine-grained siliciclastic fraction of these deposits indicate a composition resulting from the mixing of felsic and mafic sources, with the felsic component being prominent. On the other hand, the source area of the External Ligurian units (the Adria margin) was represented by a complete lithospheric section, from the upper mantle to the siliceous/carbonate sedimentary cover. The erosion of this margin produced turbidite deposits derived from mantle rocks, low- to high- grade metamorphic rocks, granites, acidic volcanic rocks, carbonatic platform rocks and pelagic sedimentary rocks. The geochemical data relative to the fine-grained siliciclastic fraction of these deposits indicate an ultramafic source standing out from the mafic-felsic components also contributing to the sediment. This local second-order chemical feature overprints the first-order time-related one, recognized on a basin scale.<br>Petrographic and geochemical data of rudites and arenites from the Alpine transect, where the Err-Platta nappes represent the Ocean-Continent (Adria) Transition, do not show contrasting evidence with this overall asymmetric compositional picture, and in some cases point to that direction (e.g.: rare occurrence in these deposits of material from possible lower crust/mantle sources). On the whole, the ‘space’-related differences in composition between the Corsica/Europe and the Adria continental margins are thought to reflect different configurations of the two margins that in turn provide further constraints to the understanding of the rifting process. Data collected in this work lend support to an asymmetric opening mechanism for the Ligure-Piemontese oceanic basin, with the Adria margin acting as the lower plate during the rifting processes (west-dipping detachment fault). Concerning this point, the multidisciplinary study of sediments can be considered a valuable tool in paleotectonic reconstructions, especially in those cases in which the regional-scale features of the investigated geological framework are lost due to major successive tectonic events.<br>40Ar –39Ar geochronological data on detrital white micas from the Corsica/Europe and Adria margin-sourced deposits have been presented in this thesis. The age-distribution of the two arenite samples from Corsica and Internal Ligurian units well match the known-from the literature picture defined by the Variscan ages found in the crystalline rocks from the Corsica/Europe basement. The age-distribution of the sample from External Ligurian units partially overlaps the oldest Variscan white mica ages known from the literature for the Adria basement. Hence, this sample gives an original contribution to the geochronological characterization of the Adria continental basement. The geochronological data collected along a Corsica-Northern Apennines transect indicate that: i) the detrital micas occurring in the Mesozoic sedimentary covers of the Ligure Piemontese Ocean were sourced by crystalline igneous and variably metamorphosed basement rocks of Variscan age, and ii) detrital white mica geochronology in provenance studies is a useful tool for the characterization of source areas.<br>After having investigated the compositional features of the sediments from the Ligure Piemontese Ocean, with the aim of characterize their origin, (i.e. the source areas), an attempt is made to explore their fate, i.e. what kind of modifications they undergo when involved in a subduction setting. This is a noteworthy issue, since variably metamorphosed sediments and ocean floor can give a geochemical contribution to the mantle-wedge (and related arc-magmas) in terms of released fluids/elements. The composition of basalts and siliciclastic sediments from the Ligure Piemontese oceanic basin are compared to the corresponding metabasalt/calc-schist from tectonic units metamorphosed under blue schist and eclogite facies conditions. The almost unaffected chemical patterns of the metamorphic equivalent both of the basalt and sediment lend support to those works from the literature suggesting the occurrence of the processes responsible for the mobilization of elements at &#34;deeper&#34; levels than eclogite facies along the subducting slab.<br>In summary, a multidisciplinary approach is definitely to recommend when a provenance study of sediments is pursued. The combined analysis of the different grain size fractions of sedimentary rocks by means of various methods has in fact the potential to provide a comprehensive picture of the case study, being pelites, arenites and rudites able to supply different kinds of geological meaningful informations.<br><br>
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