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

Steady state creep of fine grain granite at partial melting

Steady state creep under constant stress has been measured in a fine grain granite (aplite) from Schauinsland, Germany for the temperature range of partial melting 860°–1060°C, under a hydrostatic pressure of 4.2 kbar, and at low shear stresses of 5–50 bar. The apparatus used is described briefly. Rheological measurements are complemented by microscopic investigations. With a melt fraction of up to about 20%, creep can be described by a power law with a stress exponent of 3-4 and an activation energy of 80 kcal/mole, typical for creep in solids. Above 1010°C or 20% melt, the activation energy increases rapidly to a value of 200 kcal/mole simultaneously with a rapid increase of the melt fraction and a decrease of feldspar content. From the grain structure and from etching tests it is concluded that quartz contributes little to the plastic deformation which is governed mainly by the stress and temperature induced recrystallization of feldspar. The large temperature dependence of the creep rate above 1010° C may be caused by the decreasing area of grain contacts and consequent rise in local stress. These results support those of Arzi (1974) and Roscoe (1952).           ARK: https://n2t.net/ark:/88439/y060550 Permalink: https://geophysicsjournal.com/article/258 &nbsp