Isotopic evidence from the Ivrea Zone for a hybrid lower crust formed by magmatic underplating

Voshage, H.; Hofmann, A. W.; Mazzucchelli, Maurizio; Rivalenti, Giorgio; Sinigoi, S.; Raczek, Ingrid; Demarchi, G.

doi:10.1038/347731a0

Cited by 283 publications

(158 citation statements)

References 26 publications

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“…Contrary to interpretations of early studies (e.g. Fountain 1989;Voshage et al 1990;Sinigoi et al 1991;Quick et al 1994;Schnetger 1994;Henk et al 1997), the now-accepted view is that the intrusion of mafic magmas into the lower crust was not the main cause for HT metamorphism (e.g. Barboza et al 1999;Redler et al 2012).…”

Section: Introductionmentioning

confidence: 47%

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Kunz

Regis

Engi

2018

Contrib Mineral Petrol

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Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.

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Section: Introductionmentioning

confidence: 47%

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Kunz

Regis

Engi

2018

Contrib Mineral Petrol

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“…Schmincke (1981) gives 2.1 km 3 per year for destructive margins; Crisp (1984) estimated 2.9 to 8.6 km 3 per year. An additional, but unqualified, flux may be involved in 'underplating' (Voshage et al 1990) in orogenic areas. These larger estimates require a large flux of recycled destructive margin volcanic or volcaniclastic material into the mantle in order to maintain the observed mass balances.…”

Section: Apparent Crustal Growth Ratesmentioning

confidence: 99%

The persistent myth of crustal growth

Armstrong¹

1991

Australian Journal of Earth Sciences

305

107

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From the extraterrestrial telescopic, space probe, meteorite and returned sample studies of planetary evolution, and terrestrial evidence for early differentiation of core and fluid spheres and continental crust, I feel the conclusion is inescapable that large terrestrial planets of our solar system underwent essentially immediate differentiation into relatively constant-volume core, depleted mantle, enriched crust and fluid reservoirs. Differentiation was an early event, carried rapidly to completion. It is a false premise to have the formation of sialic crust on Earth dragged out over billions of years after hot accretion. The uniqueness of the Earth arises from its size, retention of water and dynamic surface-renewal processes, which have effectively erased all vestiges of the first several hundred million years of its crustal history. A large volume of depleted mantle always existed and an isotopically nearly homogeneous character for mantle and crust in early times was only sustained by rapid convective stirring of the silicate Earth. The sigmoidal continental crust age curve that is recorded in whole rock Nd and zircon U-Pb dates is a predictable consequence of the highly exponential decline in stirring rates. It represents survival, not the original extent of crustal domains. Positive The idea that the Earth's crust has grown is a myth dating from the 19th century and was established as geochemical dogma in the 1950s and 1960s. It has survived by inertia and repetition and endless self-citation. The alternative no-growth view has been sometimes ignored, frequently questioned and downplayed, often cited only as an end member hypothesis (on the presumption that the truth must lie between extremes), and sometimes acclaimed and supported. Evidence in its favour has been accumulating. The growth myth has survived, however, as the consensus.In science, conventional wisdom is difficult to overturn. After more than 20 years some implications of plate tectonics have yet to be fully appreciated by isotope geochemists who advocate crustal growth, and by geologists and geophysicsts who have followed their lead.

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“…However, many of the relatively few exposures interpreted as fossil continental crust-to-mantle transitions have been reinterpreted after rigorous investigations. For example, the Ivrea zone in the western Italian Alps has been cited repeatedly as a model of an exposed pre-Alpine crust-tomantle transition [Mehnert, 1975;Fountain, 1976;Voshage et al, 1990]. Recently, however, it has been proposed that the Ivrea lower crustal section represents a fossil accretionary wedge in which the ultramafic rocks were not located at the crust-to-mantle transition [Quick et al, 1995].…”

Section: Introductionmentioning

confidence: 99%

Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)

Hermann¹,

Müntener²,

Trommsdorff³

et al. 1997

J. Geophys. Res.

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Abstract. An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian gabbro intrusion. The underplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm 3, respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and •-800øC and ended at 0.85 GPa and 600øC and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust-to-mantle section was exhumed at the Adria margin of the Tethys ocean.

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Isotopic evidence from the Ivrea Zone for a hybrid lower crust formed by magmatic underplating

Cited by 283 publications

References 26 publications

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

The persistent myth of crustal growth

Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)

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