Palaeozoic and Mesozoic tectono-metamorphic development and geochronology of the Orobic chain (Southern Alps, Lombardy, Italy)

The Orobic metamorphic basement and the Early Permian ‘Collio Orobico basin’ are discussed in this Ph.D.thesis. The main goal of the investigations was the establishment of a Variscan-Permian P-T-d-t path and to reconstruct the processes that played a role in the tectonic unroofing and exhumation, that finally lead to the formation of the Permian Collio basin. Important aspects of the pre-Variscan and Mesozoic tectono-thermal development were also revealed. The results of the investigations include: An almost monoclinic east-west striking ‘Orobic fold nappe’ overlying a parautochton has been identified. It is interpreted as a Variscan structure, forms the greater part of the exposed basement and consists of two sequences. The lower sequence, which contains schists and metabasites of volcanic origin, was affected by the Early to Middle Ordovician ‘Sardic’ metamorphicand folding phase and contains slightly peraluminious granodioritic augengneisses which protoliths are Late ‘Sardic’. The upper sequence, a Middleto Late Ordovician volcano-sedimentary complex with rhyoliths, was deposited in an active marginal basin, on ‘Sardic’ metamorphic basement of the lower sequence. The following metamorphic phase, the main phase, jointly affected both sequences. The combination of monazite-xenotime thermochronometry, other geothermobarometer and phase petrographic relationships constrained maximum P at 1.3 Gpa, T around 750 °C and the time of peak metamorphism at 330 to 320 Ma. The formation of the ‘Orobic fold nappe’ has started at Variscan peak metamorphic conditions. Melting of the hydrous phases white mica and biotite took place and caused the buoyant rise of an elongate diapir. Superimposed dextral transpression, made this diapiric ridge extrude from a rootzone and develop into a nappefold, spreading over the parautochton. By far the majority of Variscan tectonic structures that formed during the formation of the nappe fold have an extensional component and/or served as adjustment minimising the potential energy in the gravity field. Although this phase occurred in an orogenic zone deforming under transpression, this phase contributed to crustal thinning, retrogression and exhumation. The Pand T conditions during the culmination of ductile-brittle deformation related to this nappe folding are ~600 °C at ~1.0 Gpa. At 325-305 Ma the ‘Orobic fold-cooled down to the closure temperature of white mica, as the plateaux in the Ar/Ar apparent age-spectra indicate. Ar/Ar-multigrain step heating-results of all biotite and only some white mica samples indicate: (1) A Middle Permian hydrothermal phase, related to late volcanic activity of the Collio basin. (2) A Mesozoic hydrothermal phase, related to a combination of Ladinian magmatic activity and Ladinian to Liassic rifting. The latter phase has re-equilibrated the U-Th-Pb system of monazite at ~322 °C, as revealed by monazite-xenotime thermometry on samples of ‘Gneiss Chiari’. Isotopic Ar/Ar-dating of single detrital white mica grains from the very low-grade metamorphic volcanoclastic sequence of the ‘Collio Orobico’ basin indicated: (1) The source areas of the identified populations of detrital white mica grains are predominantly the Orobic basement units (2) Middleto Late Permian hydrothermal activity (~270-255 Ma) and Mesozoic hydrothermal overprint (~250-180 Ma). (3) A contractional geodynamic setting during deposition, respectively in a molasse basin for the basal conglomerates and an intramontane basin for the ‘basal conglomerates’ and the ‘Collio Orobica’ formation, as indicated and confirmed by the Lag-times of these detrital white mica grains.