Origin and age of zircon-bearing chromitite layers from the Finero phlogopite peridotite (Ivrea–Verbano Zone, Western Alps) and geodynamic consequences

Zanetti, A.; Giovanardi, Tommaso; Langone, Antonio; Tiepolo, Massimo; Wu, Fu‐Yuan; Dallai, Luigi; Mazzucchelli, Maurizio

doi:10.1016/j.lithos.2016.06.015

Cited by 45 publications

(56 citation statements)

References 85 publications

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“…In addition, the good fitting existence between geological data and model predictions of post‐collisional extension and the ages of HT‐LP metamorphic rocks and continental gabbros emplacement clearly indicate the persistence of a high thermal state of the lithosphere from the Permian until 230–220 Ma (Figures , , and ). The duration of igneous activity in the Ivrea–Verbano Zone further supports this interpretation (Klötzli et al, ; Langone et al, ; Peressini, Quick, Sinigoi, Hofmann, & Fanning, ; Schaltegger et al, ; Zanetti et al, ). Furthermore, a time span of 20–30 Myr (from 230 to 200 Ma) is a too short period to complete the thermo‐mechanical requilibration of the lithosphere.…”

Section: Discussionsupporting

confidence: 62%

What drives Alpine Tethys opening? Clues from the review of geological data and model predictions

et al. 2018

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Permo‐Triassic remnants (300–220 Ma) of high‐temperature metamorphism associated with large gabbro bodies occur in the Alps and indicate a high thermal regime compatible with lithospheric thinning. During the Late Triassic–Early Jurassic, an extensional tectonics leads to the break‐up of Pangea continental lithosphere and the opening of Alpine Tethys Ocean (170–160 Ma), as testified by the ophiolites outcropping in the Central–Western Alps and Apennines. We revise geological data from the Permian to Jurassic of the Alps and Northern Apennines, focusing on continental and oceanic basement rocks, and predictions of existing numerical models of post‐collisional extension of continental lithosphere and successive rifting and oceanization. The aim is to test whether the transition from the Permo‐Triassic extensional tectonics to the Jurassic opening of Alpine Tethys occurred. We enforce the interpretation that a forced extension of 2 cm/year of the post‐collisional lithosphere results in a thermal state compatible with the Permo‐Triassic high‐temperature event suggested by pressure and temperature conditions of metamorphic rocks and widespread igneous activity. Extensional or transtensional tectonics is also in agreement with the generalized subsidence indicated by the deposition of sedimentary successions with deepening upward facies occurred in the Alps from the Permian to Jurassic. Furthermore, a rifting developed on a thermally perturbed lithosphere agrees with a hyperextended configuration of the Alpine Tethys rifting and with the duration of the extension necessary to the oceanization. The review supports the interpretation of Alpine Tethys opening developed on a lithosphere characterized by a thermo‐mechanical configuration inherited by the post‐Variscan extension which affected Pangea during the Permian and Triassic. Therefore, a long‐lasting period of active extension can be envisaged for the breaking of Pangea supercontinent, starting from the unrooting of the Variscan belts, followed by the Permo‐Triassic thermal high, and ending with the crustal break‐up and the formation of the Alpine Tethys Ocean.

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Section: Discussionsupporting

confidence: 62%

What drives Alpine Tethys opening? Clues from the review of geological data and model predictions

et al. 2018

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“…A quite similar scenario has been proposed to explain the origin of zircons in the chromitites of the Tumut ophiolite complex in southeast Australia [27] and the Cedrolina chromitites in Brazil [28]. The zircons we studied bear some resemblance to those found in the Early Jurassic Finero alpine chromitites (western Alps) wherein zircon genesis was ascribed to direct crystallization from the melt that formed the Finero chromitites during the Early Jurassic [81,82]; those anhedral to subhedral zircons occur in interstitial positions between Cr-spinel and olivine and/or orthopyroxene [65], forming aggregates of up to four crystals [64]. The Finero zircons are inclusion-free with low CL signals, no internal texture and high Th/U ratios, thus resembling the zircons in the Ayios Stefanos chromitites [64,82].…”

Section: An Inductive Reasoning Approach To the Potential Origin Of Zsupporting

confidence: 74%

“…The zircons we studied bear some resemblance to those found in the Early Jurassic Finero alpine chromitites (western Alps) wherein zircon genesis was ascribed to direct crystallization from the melt that formed the Finero chromitites during the Early Jurassic [81,82]; those anhedral to subhedral zircons occur in interstitial positions between Cr-spinel and olivine and/or orthopyroxene [65], forming aggregates of up to four crystals [64]. The Finero zircons are inclusion-free with low CL signals, no internal texture and high Th/U ratios, thus resembling the zircons in the Ayios Stefanos chromitites [64,82]. However, the Finero zircons may be up to 600 μm across and sometimes are found as euhedral inclusions in Cr-spinel and olivine, with low Y contents [64].…”

Section: An Inductive Reasoning Approach To the Potential Origin Of Zmentioning

confidence: 99%

“…If the same scenario is applicable for Othris, then the time interval between gabbro intrusion in the chromitites and metamorphism/serpentinization should also be limited. So, it is likely that the examined zircons were modified first and then underwent serpentinization-related metamictization, possibly in fluid-assisted conditions (e.g., [82]). …”

Section: Zircon Re-equilibration With Deuteric Fluidsmentioning

confidence: 99%

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Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece: Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites

et al. 2016

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Several small chromium (Cr) ore bodies are hosted within a unit of tectonically thinned dunite in the retired Ayios Stefanos mine of the western Othris ophiolite complex in Greece. Chromium ores consist of tectonically imprinted bodies of semi-massive to massive, podiform and lenticular chromitites composed of chromian spinel [Cr-spinel] and mid-ocean ridge basalts (MORBs). Several Cr-spinel crystals in these ores exhibit imperfect zones made up of spinel hosting oriented lamellae of Mg-silicates (mostly chlorite) locally overgrown by porous domains along grain boundaries and fractures. From the Cr-spinel core to the lamellae-rich rim Cr#, Mg# and Fe 3+ # generally increase (0.68-0.87, 0.78-0.88 and 0.55-0.80, respectively), whereas from the core or the spinel zones with oriented lamellae to the porous domains Mg# and Fe 3+ # generally decrease (0.45-0.74 and ≤0.51, correspondingly). The lamellae-rich rims formed at oxidizing conditions, whereas the porous rims resulted from a later reducing event. Several tiny (≤30 µm), subhedral to anhedral and elongated Zr-bearing silicate mineral grains were discovered mainly along open and healed fractures cutting Cr-spinel. Most of the Zr-bearing silicate minerals (30 out of 35 grains) were found in a chromitite boulder vastly intruded by a complex network of gabbroic dykes. The dominant Zr-bearing silicate phase is by far zircon displaying a homogeneous internal texture in cathodoluminescence (CL) images. Raman spectroscopy data indicate that zircons have experienced structural damage due to self-irradiation. Their trace-element contents suggest derivation from a plagioclase-bearing, low-SiO 2 intermediate to mafic source. Combined micro-textural and minerochemical data repeat the possibility of zircon derivation from limited volumes of high-T fluids emanating from the gabbroic intrusions. Once zircon is precipitated in cracks, it may be altered to Ca-rich Zr-bearing silicate phases (i.e., armstrongite, calciocatapleiite). Almost all zircons in these samples show evidence of gains in solvent compounds (CaO, Al 2 O 3 and FeO) possibly due to re-equilibration with late deuteric fluids.

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“…According to several authors, after the early Permian thermal peak, the whole crustal section underwent cooling and exhumation (e.g., Brodie et al, ; Siegesmund et al, ). Recent studies suggest that the cooling and exhumation history of the IVZ was interrupted by discrete Triassic‐Jurassic heating events, as documented by zircon, monazite, and rutile U‐Pb dating (Ewing et al, ; Langone et al, ; Schaltegger et al, ; Smye & Stockli, ; Vavra et al, ; Vavra & Schaltegger, ; Zanetti et al, , ). Based on the U‐Pb dating of rutile from the high‐grade metamorphic rocks (granulites) of the central and southern part of the IVZ, some authors suggest that discrete regional‐scale thermal pulses affected the lower structural levels of the IVZ in response to the Mesozoic hyperextension associated with rift/post‐rift processes at the Adriatic margin (Ewing et al, ; Smye & Stockli, ).…”

Section: Introductionmentioning

confidence: 99%

Zircon U‐Pb Dating of a Lower Crustal Shear Zone: A Case Study From the Northern Sector of the Ivrea‐Verbano Zone (Val Cannobina, Italy)

et al. 2018

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A geochronological study was performed on zircon grains from a middle‐lower crustal shear zone exposed in the northern sector of the Ivrea‐Verbano Zone (Southern Alps, Italy) for the first time. The shear zone developed at the boundary between mafic rocks of the External Gabbro unit and ultramafic rocks of the Amphibole Peridotite unit. It is ~10–20 m wide, can be followed along a NE strike for several kilometers, and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin sections and as separates from three representative outcrops along the shear zone. Zircon grains are more abundant in the shear zone compared to wall rocks and are generally equant, rounded to subrounded with dimensions up to 500 μm. U‐Pb data are mainly discordant, and the apparent 206Pb/238U dates show a large variation from Permian to Jurassic. Isotopic data, combined with microstructural, morphological, and internal features of zircon, reveal an inherited age component and suggest partial zircon recrystallization under high‐temperature conditions during Late Triassic to Early Jurassic. High‐temperature deformation in the shear zone, at lower crustal levels, was coeval with amphibolite to greenschist facies mylonitic deformation at upper crustal levels and is inferred to be related to Mesozoic rifting processes at the Adriatic margin.

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Origin and age of zircon-bearing chromitite layers from the Finero phlogopite peridotite (Ivrea–Verbano Zone, Western Alps) and geodynamic consequences

Cited by 45 publications

References 85 publications

What drives Alpine Tethys opening? Clues from the review of geological data and model predictions

What drives Alpine Tethys opening? Clues from the review of geological data and model predictions

Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece: Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites

Zircon U‐Pb Dating of a Lower Crustal Shear Zone: A Case Study From the Northern Sector of the Ivrea‐Verbano Zone (Val Cannobina, Italy)

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