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

Physics of Geological Processes (PGP) and Department of Geosciences, University of Oslo, PO Box 1047 Blindern, 0316 Oslo, Norway

Correspondence: ^* E-mail: t.s.rohr{at}geo.uio.no

Almandine garnet (Alm_62.6Pyr_11.4Sps_8.4And_6.5Grs_4.0) 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 Hisarøya, 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.

Key Words: Garnet • almandine • HREE • heavy rare earth elements • yttrium • sodium • Lu • Yb • Er • Tm • Dy • YAG • WGC • granulite • eclogite • deformation • fracturing • fracture propagation • diffusion

This article has been cited by other articles:

				A. J. Martin Sub-millimeter Heterogeneity of Yttrium and Chromium during Growth of Semi-pelitic Garnet J. Petrology, September 1, 2009; 50(9): 1713 - 1727. [Abstract] [Full Text] [PDF]