Constraining orogenic volcanism in the Western Alps using volcaniclastic deposits from foreland basins

Lyu, Chenhang 2023. Constraining orogenic volcanism in the Western Alps using volcaniclastic deposits from foreland basins. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This study aims at reconstructing the source-to-sink system of Tertiary volcaniclastic deposits preserved in Alpine foreland basin. The mafic-intermediate syn-orogenic Alpine magmatism is recorded in volcaniclastic deposits in the north Alpine foreland basin (NAFB) in Switzerland and SE France, and in Periadriatic intrusions in N Italy ca. 30 - 40 Ma. The volcaniclastic deposits mainly include volcaniclastic flysch turbidites (regionally named Taveyannaz sandstone and its equivalents varying along the arc), igneous pebbles in conglomerates, block-and-ash flow deposits, primary fallout deposits and lavas. This study focused on (i) the Taveyannaz sandstones in Glarus, Taveyannaz, Aravis and Champsaur, (ii) igneous pebbles in Champsaur and Clumanc, (iii) block-and-ash flow deposits (i.e., andesite breccia) and tuffs in St. Antonin and La Vanade, and (iv) andesitic lavas in Biella. The methods use field observations, quantitative petrographic analysis (grain counting), whole-rock geochemistry (ICP-OES+ICP-MS), clinopyroxene and amphibole geochemistry (SEM-EDS+LA-ICP-MS). This was combined with a new discrimination method for clinopyroxenes and amphiboles based on Principle Component Analysis (PCA) to characterise the compositional variability of volcaniclastic components in foreland basins, to ultimately determine their possible sources in the Alpine orogen. New integrated results reveal two types of volcano-sedimentary environments and igneous affinities, which correspond to the Alpine and Provence volcanic provinces. The volcaniclastic deposits of the Alpine volcanic province (Glarus, W Switzerland, Aravis, and Clumanc) are very likely secondary deposits, whereas the volcaniclastic deposits of Provence volcanic province (Champsaur, Saint Antonin, and La Vanade) are proximal, with common primary deposits. PCA results on clinopyroxenes show high compositional variability of the mafic volcanic components preserved in the samples from Glarus, La Tièche, Taveyannaz and Aravis, with distinct compositions and sources for each single locality. Therefore, the provenance of the volcanic components seems more complex than that proposed in new sediment provenance models based on U-Pb and isotope composition of detrital zircons, which have linked the volcaniclastic deposits only to Adamello and Biella sources. In addition, the quenched amoeboid andesitic clasts were identified in previous and new data from Glarus and Flaine. This is a very significant observation that clearly indicates very limited transport of materials from local submarine effusive volcanism, thus suggesting previously undocumented proximal volcanic activity in the foreland basins. Whole-rock and mineral geochemistry results allow to refine the extension of the Provence magmatism from southern France all the way up to the Champsaur region, where pyroclastic deposits with Provence-like geochemical affinities were identified for the first time. In Clumanc (Barrême foreland basin), volcaniclastic materials with Biella-like affinity are found, which suggests derivation from another proximal volcanic centre of the Alpine volcanic front on the other side of the Alps. This apparently undocumented (fully eroded?) source is the southernmost expression of the erosion of the Alpine volcanic province. Further south, in the Provence volcanic system, the terrestrial proximal pyroclastic deposits in St. Antonin and La Vanade have clear geochemical similarity with Provence magmatism. This work outlines the value of clinopyroxene geochemistry in provenance studies, where volumetrically significant mafic volcaniclastics are preserved in the geological record, most particularly in convergent tectonic settings that are associated with arc and/or orogenic volcanism. This contrasts with the use of detrital zircons, that are predominantly formed in slowly crystallised felsic magmas and have more limited value in sedimentary systems dominated by the erosion of mafic igneous rocks (e.g., andesitic volcanoes). A similar observation was made in the Nankai forearc, Japan, where the PCA of trace elements in detrital pyroxenes reveals complex sedimentary patterns unrecognised with other traditional approaches. Thus, the new provenance method developed in this study can offer an innovative insight into the provenance of mafic-intermediate volcaniclastics globally.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Date of First Compliant Deposit:	20 November 2023
Last Modified:	21 Nov 2023 09:43
URI:	https://orca.cardiff.ac.uk/id/eprint/164081

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