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

Abstract

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.

Mantle versus crustal contributions in crustal-scale magmatic systems (Sesia Magmatic System, northern Italy) from coupling Hf isotopes and numerical modelling

“…The importance of crustal melting in generating silicic magmas in the Sesia Magmatic System was questioned by Barboza et al (1999) and Barboza and Bergantz (2000), mainly based on the observation that the thermal impact on the roof of the MC is limited to less than 2 km of migmatite, and that the metamorphic gradient in the Kinzigite Formation pre-dates the intrusion of the Mafic Complex. This is in agreement with field observations (e.g., Zingg 1990;Sinigoi et al 2011) and recent geochronological data on the Kinzigite Formation showing that the onset of crustal melting pre-dates the emplacement of the SMS by >20 Myr (Ewing et al 2014;Guergouz et al 2018;Kunz et al 2018).…”

Jurassic carbonatite and alkaline magmatism in the Ivrea zone (European Alps) related to the breakup of Pangea

“…The generation of sodic CO 2 -bearing melts from isotopically variable amphibole-rich peridotites during passive rifting explains the relatively low trace-element concentrations and heterogeneous isotopic signature. The Ivrea zone is also famous for isotopically enriched phlogopite peridotites related to crustal metasomatism (Voshage et al, 1987 (Belousova et al, 2002) carbonatitic zircons zircons from Ivrea metapelites (Kunz et al, 2018) zircons from Ivrea metapelites but their local involvement would contribute to the variable Na 2 O/K 2 O ratios (40-0.4) and isotopic signature of the alkaline melts. Finally, the small-scale alkaline magmatism in the Ivrea zone occurs over a distance of ~80 km, hence cannot stem from a single mantle volume but from local mantle regions, in all likelihood the most enriched ones.…”

Lead oxide nanospheres in seismically deformed zircon grains

“…Extended period of high-T metamorphism was suggested by various studies (e.g. Peressini et al, 2007;Guergouz et al, 2018;Kunz et al, 2018).…”