PRESERVATION OF SUB-MILLIMETER HETEROGENEITY OF Y AND CR DURING GROWTH OF PELITIC GARNET

X-ray maps of yttrium and chromium in two garnets from a semi-pelitic schist from central Nepal reveal parallel bands of high and low concentration several microns to several tens of microns wide. This zoning mimics similar concentration bands defined by foliation in the matrix outside the garnets. The garnets apparently incorporated the matrix zoning intact during growth at conditions up to lower amphibolite facies. These observations and interpretations indicate that concentrations remained heterogeneous at the scale of a few microns for Y and a few tens of microns for Cr during breakdown of matrix minerals and growth of garnet. This conclusion extends results of previous work which showed that Y does not homogenize at the centimeter scale and Cr does not homogenize at the millimeter scale during metamorphism to amphibolite facies. Collectively, these results suggest that thermodynamic equilibrium of Y and Cr between minerals in metapelitic rocks at lower amphibolite facies relies on transport of these elements no farther than a few or a few tens of microns, respectively. The Y and Cr concentration bands in the garnets spiral from rim to core, disclosing syn-kinematic garnet growth. Zoning of these elements in garnet is thus a new tool for deciphering the relative timing of deformation and garnet growth.