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

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

Trachytes employed for funerary artefacts in the Roman colonies Regium Lepidi (Reggio Emilia) and Mutina (Modena) (Italy): provenance inferred by petrographic and chemical parameters and by magnetic susceptibility

Roman funerary artefacts belonging to the archaeological collections in Modena and Reggio Emilia, two important Roman colonies (Mutina and Regium Lepidi, respectively) in the Cispadane region (Northern Italy), are made of trachytes from the Euganean Hills, close to Padua. In particular, the petrographic and chemical parameters, besides magnetic susceptibility of archaeological trachytes, suggest Monte Oliveto as their main source; very few of them, however, come from Monte Rosso and Monte Merlo. Surprisingly the trachytes from Monselice, which were used extensively to get flagstones for paving the Roman roads of the Po plain, were not implied in funerary art. Possibly the porosity of stones was determinant in addressing the use of the Euganean trachytes: the stones from Monte Oliveto (and also from Monte Rosso and Monte Merlo), which contain frequent cavities and voids, are porous and might favour body decomposition, like important stones used in antiquity for sarcophagi (e.g. lapis sarcophagus). By contrast, the trachytes from Monselice are dense, not porous, hence harder and more resistant to abrasion, therefore suitable for flagstones. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved

New chemical and petrographic data of some undersature lavas from Nyiragongo and Mikeno (Virunga Western African-Rift-Zaire)

The lavas investigated from Nyiragongo and Mikeno volcanoes (Virunga volcanic field) are leucite - melilite - nephelinites, melilite - nephelinites (Nyiragongo) and leucitites (Mikeno): melilite characterizes the Nyiragongo lavas whereas clino-pyroxene is the prevalent mafic phase at Mikeno. The mineralogy is consistent with the highly alkaline (Ealk = 7.6-15.3 wt.%), potash-rich (K.20/Na20 > 1) and strongly silica undersaturated nature of the lavas. The samples analyzed are more or less fractionated. At Nyiragongo, addition of nepheline and leucite (in a very constant ratio) possibly to a bergalite-type magma may explain the “dilution” of both “incompatible” and “compatible” elements with increasing fractionation. On the contrary, at Mikeno, crystal fractionation processes seem to be dominated by the separation of mafic phases. Geochemical features indicate that the analyzed lavas from Baruta, and those from Nyiragongo and Mikeno were generated from different sources