Abstract

We present laser-ablation microprobe inductively coupled–mass spectrometry analyses of Y, Yb, Er, Dy, and Gd in garnet crystals from high-grade metamorphic and migmatitic rocks from the Sierras Pampeanas of Argentina. These rocks contain large garnet porphyroblasts (≥4 mm) that are compositionally zoned. Rare earth element concentrations in the rims are typically one order of magnitude lower than those in the cores. A notable feature of this zonation is the inversion of Yb/Er and Yb/Dy ratios, from >1 in the cores to <1 in the rims. We show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from Rayleigh fractionation during garnet growth. We also analyzed a small garnet crystal (<2 mm) representative of a garnet population that is associated with migmatitic leucosomes. This crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts. The small crystals, and perhaps part of the porphyroblast rims, are likely to be peritectic products of incongruent melting reactions. We argue that melts formed from garnet-bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earth elements and Y than modeling based on bulk source-rock abundances of these elements would indicate.

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