Yttrium-zoning in garnet and stability of allanite in metapelites from the Main Central Thrust Zone and adjacent higher Himalayan crystallines along the Alaknanda Valley, NW Himalaya

Yttrium (Y)zoning in garnet and the stability of accessory allanite in metapelites from a greenschist to amphibolite facies inverted metamorphic sequence in the Main Central Thrust Zone (MCTZ) and the overlying Higher Himalayan Crystalline Sequence (HHCS) along the Alaknanda Valley in NW Himalaya have been studied. Garnet porphyroblasts from the garnet grade MCTZ metapelites commonly show chemical zoning with a Y-rich core (~3311 ppm) and Y-poor rim (b.d.). Phase diagram modelling for an MCTZ sample shows that the garnet zoning developed during prograde metamorphism in the P–T ranges of ~5.3–7.3 kbar and ~500–585 °C. Accessory allanite (up to 3.45 wt% Y2O3) occurs as inclusion in the rim of garnet porphyroblasts and also in the matrix in most of the MCTZ samples. The Y depletion in the rim of zoned garnets from the MCTZ has possibly resulted from sequestration of Y by allanite, which became a stable phase after the formation of the garnet core. Alternatively, the Y zoning has developed by strong partitioning of the element into garnet with the result that it is mostly incorporated in a narrow core and there is little Y available towards the later part of garnet growth. High-Y annulus occurs in a relatively inclusion-free zone of garnet porphyroblast from an MCTZ rock which has likely developed due to a decrease in the growth rate of the crystal. A garnet from the HHCS metapelites with localised resorption of the rim exhibits weak Y zoning marked by a slight increase in Y at the rim which is attributed to back-diffusion of the element during resorption. With increasing structural level across the MCTZ, the ΣLREE content of allanite first increases then decreases which is complemented by variation in the Al content of the mineral. This compositional variation of allanite appears to be linked to the modal abundance of garnet in rocks.