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

Geochemical Methods for Sourcing Lava Paving Stones from the Roman roads of Central Italy

This paper documents the results of in situ analysis of 306 lava paving stones and 74 possible source rocks using pXRF. Data were collected from sites both in the city of Rome—on major roads beyond the city (including the Viae Flaminia, Cassia, Clodia, Praenestina and Appia)—and in the city of Ostia. Comparison of the pXRF data with lava compositional data from the geological literature allows broad identification of possible sources. The results point to quite distinctive patterns of exploitation for the city of Rome and Ostia, utilizing the Alban Hills lava flows, and the roads of the middle Tiber Valley, drawing on lava flows associated with the Vico and Sabatini volcanoes. The results show the potential of pXRF to produce data to elucidate the exploitation of lava flows for paving the Roman roads

The growth of large mafic intrusions: Comparing Niquelandia and Ivrea igneous complexes

The Niquelandia Complex, Brazil, is one of the world's largest mafic-ultramafic plutonic complexes. Like the Mafic Complex of the Ivrea-Verbano Zone, it is affected by a pervasive high-T foliation and shows hypersolidus deformation structures, contains significant inclusions of country-rock paragneiss, and is subdivided into a Lower and an Upper Complex. In this paper, we present new SHRIMP U-Pb zircon ages that provide compelling evidence that the Upper and the Lower Niquelandia Complexes formed during the same igneous event at ca. 790 Ma. Coexistence of syn-magmatic and high-T subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma. Syn-magmatic recrystallization during this deformation resulted in textures and structures which, although appearing metamorphic, are not ascribable to post-magmatic metamorphic event(s), but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the Niquelandia and Ivrea Complexes. Melting of incorporated country-rock paragneiss continued producing hybrid rocks during the last, vanishing stages of magmatic crystallization. This resulted in the formation of minor, late-stage hybrid rocks, whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components, which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas. (C) 2012 Elsevier B.V. All rights reserved.Research Support Foundation of the State of Sao Paulo (FAPESP)Brazilian National Research Council (CNPq

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

Provenance determination of trachytic lavas, employed as blocks in the Romanesque cathedral of Modena (Northern Italy), using magnetic susceptibility, and petrographic and chemical parameters

Trachytic stones were used sporadically as building blocks in the Romanesque cathedral of Modena and probably were recycled from Roman artefacts. They come from the Euganean Hills, a Tertiary volcanic Complex close to Padua, and more specifically from Monte Oliveto, Monte Merlo and Monte Lispida. Whereas the role of Monte Oliveto and Monte Merlo as sources of trachytes for Roman Mutina (present-day Modena) has been already documented, the stones from Monte Lispida, used in the cathedral, may represent the first evidence of the contribute to Mutina from this important locality, quarried under the Romans