Analysis of zircon from deep basement units: Correlation across orogenic cycles in the Southern and Western Alps

Kunz, Barbara E. (2017). Analysis of zircon from deep basement units: Correlation across orogenic cycles in the Southern and Western Alps (Unpublished). (Dissertation, University of Bern, Institute of Geological Sciences)

Full text not available from this repository.

The mineral zircon is an important geochronological and petrogenetic tool in metamorphic geology. Due to its physical and chemical resilience paired with high abundance in crustal rocks and the possibility of precise U/Pb dating it found vast applications. Beside U/Pb dating a broad range of geochemical (REE, Ti-in-zircon, O-, Hf- isotopes) as well as imaging (CL, EBSD, scanning ion imaging) characterisations have been developed over the years, improving the validation and interpretation of geological ages in the context of metamorphic evolution. When combined, zircon characteristics can furthermore be used as a tool for the correlation of metamorphic terranes. The present work shows a case study of metamorphic provenance in the European Alps. The Western and Southern Alps both contain slices of lower continental crust derived from the Adriatic margin. While the Ivrea Zone (Southern Alps) escaped Alpine metamorphic overprint, the Adria-derived continental units in the Western Alps experienced Alpine metamorphic imprint of varying intensity. This raises an important question: is it possible to correlate the slices of Adria-derived continental crust back to their original position within the Palaeo-Adriatic margin prior to rifting, subduction, and collision? In order to investigate this issue, this study uses textural information of zircon combined with U/Pb data and trace element characteristics. To allow correlation within the dataset and ensure sufficed zircon yield during mineral separation, metasediments (metapelites) and associated leucosome have been collected. Zircon were texturally characterised via CL-images and analysed by LA-ICP-MS to obtain U/Pb dates, Th/U ratios, Ti-in-zircon concentrations and trace elemental signatures.
In the first part of the thesis zircon characteristics from the source region i.e. the Kinzigite Formation (Ivrea Zone) have been established. In this unit a continuously metamorphic field gradient from amphibolite to granulite facies conditions is exposed. This allows studying the evolution of zircon with increasing metamorphic grade and corresponding crustal depth. The results show the evolution from primarily detrital grains, in the mid amphibolite facies, towards completely newly grown metamorphic zircon in the granulite facies. Furthermore, the study strengthens the evidence for a protracted high-temperature history of the Ivrea Zone in the late Palaeozoic (320–260 Ma). The highest-grade samples in Val Strona di Omegna show large scatter in their zircon data with little to no correlation in U/Pb dates, textures and/or trace element geochemistry. This indicates a decoupling these zircon characteristics potentially caused by the long-lasting (>50 Myr) high‑temperature (>850°C) conditions.
In the second part of the thesis zircon from the Adria-derived units of the Western Alps have been studied and characterised. In these units, Permian metamorphism has, up to now, only been confirmed by very few studies. This study presents a comprehensive overview of Permian U/Pb zircon ages defining an age range between 286 and 266 Ma for high-temperature metamorphism. Trace element thermometry yields temperatures of 580–890°C for Ti-in-zircon and 630–850°C for Zr-in-rutile, which are in good agreement with Temperature estimates from previous studies in the Adria-derived units.
In order to correlate and compare their Permian metamorphic evolution and infer Permian crustal positions for the Adria-derived units, the third part of this thesis combines the results from Ivrea Zone and Adria-derived units. Crustal slices that predominantly preserve the Permian high‑temperature metamorphic imprint – e.g., the Seconda Zona Dioritico Kinzigitica and Valpelline Series – permit for the identification of numerous similarities (zircon, mineral assemblage, P–T conditions) with the Kinzigite Formation in the Ivrea Zone. However, in areas with a pervasive Alpine overprint this comparison is less straightforward. By comparing zircon textures, U/Pb dates and trace elemental data it is possible to match even strongly overprinted samples. When metamorphic grade and crustal position between the Kinzigite Formation (Ivrea Zone) and observations in the Adria‑derived units are compared it can be concluded that all studied samples have a similar origin in the lower continental crust. Furthermore, it is possible to distinguish between different metamorphic grades among the Adria-derived units. Samples from the central Seconda Zona Dioritico Kinzigitica, Eclogitic Micaschist Complex and Mt. Emilius Klippe show zircon characteristics, which mainly correspond to zircon found in the amphibolite facies. The NE Seconda Zona Dioritico Kinzigitica, SW Seconda Zona Dioritico Kinzigitica and Valpelline Series are, however, more comparable to conditions in the amphibolite to granulite facies transition in the Ivrea Zone. Therefore, it is feasible to conclude that (at least until the Permian rifting) the metamorphic evolution and P–T–t conditions of the lower continental slices in the Western Alps have been very similar to ones in the Ivrea Zone.
The approach of metamorphic provenance used in this thesis shows that combining textures of zircon growth/resorption, combined with U/Pb dates and trace elemental data can provide a reliable and robust tool for the correlation of tectonic fragments across orogenic cycles.

Item Type:	Thesis (Dissertation)
Division/Institute:	08 Faculty of Science > Institute of Geological Sciences > Petrology 08 Faculty of Science > Institute of Geological Sciences
UniBE Contributor:	Kunz, Barbara Eva
Subjects:	500 Science > 550 Earth sciences & geology 500 Science
Language:	English
Submitter:	Barbara Eva Kunz
Date Deposited:	10 Jan 2018 09:04
Last Modified:	05 Dec 2022 15:04
Related URLs:	Dataset
Uncontrolled Keywords:	Zircon, Geochronology, Ivrea Zone, Southern Alps, Western Alps, Metamorphic provenance
URI:	https://boris.unibe.ch/id/eprint/99128