Stress-induced redistribution of yttrium and heavy rare-earth elements (HREE) in garnet during high-grade polymetamorphism

Abstract

Almandine garnet (Alm62.6Pyr11.4Sps8.4And6.5Grs4.0) exceptionally rich in Y2O3 (1.8–2.3 wt%), Sc2O3 (0.11–0.23 wt%), HREE2O3 (1.9–2.3 wt%), and Na2O (~0.3 wt%) occurs in Precambrian quartzofeld-spatic granulites ( T ~ 860 °C) on Hisaroya, Western Gneiss Region, Norway. The granulites were partially reequilibrated to eclogite-facies ( P = 14.9 ± 1.3 kbar, T = 649 ± 67 °C) and amphibolite-facies assemblages during the Caledonian Orogeny. The granulite-facies garnet is reequilibrated in an outer zone, typically ~4 μm thick, where both Y2O3 and HREE2O3 concentrations decrease to 1.2 wt%, and overgrown by Caledonian garnet with Y2O3 and HREE2O3 both below 0.1 wt%. The granulite-facies cores are also reequilibrated to lower Y2O3 and HREE2O3 (both ~1.8 wt%) along deformation structures including microfaults with horsetails, en-echelon bands, and splaying features around the tip of fractures. Locally, these reequilibrated zones, typically 10–20 μm across, have shoulders with higher Y and HREE (~2.9 wt%) than in the original garnet, suggesting closed-system behavior for these elements. In addition, the garnet locally displays a complex mesh-like pattern with high and low HREE and Y bands, possibly related to strain. Two charge-balancing mechanisms account for Y and HREE incorporation: (1) the yttrium aluminum garnet (Y2Al2O6, YAG) substitution involving incorporation of Al into the tetrahedral site is dominant in the granulite-facies garnet, and (2) coupled substitution with Na into the dodecahedral site is increasingly important during high- P reequilibration. The internal closed-system reequilibration of the granulite-facies garnet may be related to stress concentrations build up in garnet during the eclogite-facies event. Local stress concentrations during propagation of fractures may be the driving force behind the redistribution of elements seen within cores of granulite-facies garnet.