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

U-Pb mineral ages from northern Labrador: Possible evidence for interlayering of Early and Middle Archean tectonic slices

Previous work has shown that a major part of the Early Archean (>3500 Ma) rocks in Labrador north of Nain were reworked under granulite-facies conditions between 2800 and 2700 Ma. New U-Pb zircon data show that Middle Archean gneisses are more abundant than previously recognized. Their metamorphic histories differ from the history of the Early Archean rocks. Part of the Middle Archean gneiss suite was emplaced directly in granulite facies between 3250 and 3200 Ma and was partly retrogressed at the time of granitoid veining ca. 2990 Ma. Only after ca. 2700 Ma do Early and Middle Archean gneisses have similar histories. We suggest that the terrane model proposed for SW Greenland can be applied to Labrador and that terrane intercalation took place ca. 2700 Ma. The Middle Archean gneisses north of Nain are likely to be tectonic slivers equivalent to the Maggo gneiss of the Hopedale area to the south. Correlation between the Middle Archean terrane of Labrador and the Akia terrane in SW Greenland is suggested