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

Re-equilibration of Zircon in Aqueous Fluids and Melts

Natural zircon crystals often show complex secondary textures that cut across primary growth zones. In zircon showing structural damage caused by self-irradiation, such textures are the result of a diffusion- reaction process in which a hydrous species diffuses inwards and “catalyzes” structural recovery. Nanoscale pores develop, solvent elements such as Ca, Al and Fe are gained, and radiogenic Pb is lost. In both aqueous fluids and melts, replacement of zircon with undamaged structure by a coupled dissolution- reprecipitation process can produce similar textures. The reacted domains usually have lower trace element contents and may contain micrometer-sized pores and inclusions of uranium, thorium and/or yttrium phases, originally in solid solution. Both processes have considerable implications for zircon geochronology

Lexical frequency co-determines the speed-curvature relation in articulation

The relation between speed and curvature provides a characterization of the spatio-temporal orchestration of kinematic movements. For hand movements, this relation has been reported to follow a power law with exponent . The same power law has been claimed to govern articulatory movements. We studied the functional form of speed as predicted by curvature using electromagnetic articulography, focusing on three sensors: the tongue tip, the tongue body, and the lower lip. Of specific interest to us was the question of whether the speed-curvature relation is modified by articulatory practice, gauged with words’ frequencies of occurrence. Although analyses imposing linearity a priori indeed supported a power law, relaxation of this linearity assumption revealed that the effect of curvature on speed levels off substantially for lower values of curvature. A modification of the power law is proposed that takes this curvature into account. Furthermore, controlling statistically for number of phones and word duration, we observed that the speed-curvature function was further modulated by an interaction of lexical frequency by curvature, such that for increasing frequency, speed decreased slightly for low curvatures while it increased slightly for high curvatures. The modulation of the balance between speed and curvature by lexical frequency provides further evidence that the skill of articulation improves with practice on a word-to-word basis, and challenges theories of speech production

Zircons from Syros, Cyclades, Greece-recrystallization and mobilization of zircon during high pressure metamorphism.

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution---reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios (01). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y---HREE---Th phases and fluid inclusions, showing variable progress of replacement---recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U---Pb ages. The pristine generation has an age of 802 16Ma from a metagabbro, and 764 21Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 524 08Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U---Pb ages with white mica Ar---Ar ages for the same samples support the conclu-sion that Eocene is the true age of high-pressure metamorphism on Syros. KEY WORDS: zircon dissolution---reprecipitation; zircon U---Pb geochronology; paragonite 39Ar/40Ar dating; Syros meta-ophiolit

Permian detachment faulting and syntectonic magmatism constrained by U-Pb LA-ICP-MS on zircon in the Orobic Anticline, Italy

The Grassi Detachment Fault is an Early Permian, low-angle extensional structure located in the Orobic Anticline. It separates the Variscan Basement in its footwall from the volcanic and sedimentary rocks of the Early Permian Collio Formation in its hanging wall. Its textures indicate a top-to-the-southeast displacement. The footwall basement consist of the Variscan Morbegno Gneiss and two granitoid intrusions, the Val Biandino Quarz Diorite (VBQD) and the Valle Biagio Granite (VBG). The former is syntectonic with respect to the detachment, whereas for the latter, the relation to the detachment is unknown. Volcanic rocks of the Collio Formation in the hanging wall may represent the extrusive part of the magmatic system. In the study area in the western part of the Orobic anticline, several faults and shear zones are exposed: (1) The top-SE Grassi Detachment Fault. It is truncated by the unconformably overlying, post-rift, Late Permian Verrucano Lombardo towards the NW. This reflects the eroded culmination of a Permian metamorphic core complex. (2) The Sasso Rosso Fault, a steeply NW-dipping, brittle normal in the footwall between VBQD and VBG. It is also sealed by the basal unconformity of the Verrucano Lombardo. (3) Several minor south-directed Alpine thrusts, duplicating the lithostratigraphy, including the detachment. (4) The Biandino Fault, a steeply SE-dipping Alpine backthrust, overprinting the detachment as well as the Alpine forethrusts. U-Pb zircon geochronology using LA-ICP-MS yielded concordant ages of 293.2 ± 4.9 Ma for the VBQD and 286.0 ± 4.8 Ma for the VBG. These ages coincide with the beginning of the Collio volcanism and with the emplacement of mafic melts in the lower crust of the Ivrea Zone, indicating that the volcanics, granitoids and mafic intrusions belonged to a crustal-scale magmatic system. Since structural relations indicate contemporaneity of VBQD intrusion and extensional detachment faulting, it results that the Early Permian magmatism occurred in a framework of core-complex style extension