GARNET GROWTH ZONING IN METAPELITES: NOT SO SIMPLE AFTER ALL, AND REVEALING MORE THAN WE ONCE THOUGHT?

In 1977, the first and third authors published a paper (AJS 277, 1152-1167), on interpretation of garnet zoning and solid inclusions in a classic locality for large garnets from SE Vermont, the Gassetts Schist at Starr Hill, with J.B. Thompson and Peter Lyttle (the second author was not yet born). 2-D zoning was presented for a large (~1 cm) garnet crystal by logical contouring of many (several hundred) quantitative analyses – the process took one year, and produced one of the first detailed 2-D maps of grt compositional zoning. Inclusions of numerous species (qtz, ilm-hem, rt, mag, st, pg-mrg, ms-pg, cld, chl), some in multi-phase clusters, were correlated with grt compositions to create a model for reaction history using concentric grt growth and concurrent inclusion of matrix mineral grains. Subsequent examination with modern WDS chemical mapping techniques at high compositional and spatial resolution reveals that the original mapping for Fe, Mg, Mn and Ca end-members was fine as far as it went, but that tremendously complex zoning in some components (down to micron scales in the cases of Ca and HFSEs such as Y, Sc, Ti, Cr) was not well represented. In rapidly diffusing divalent components, particularly Mg and Mn, some preservation of micro-scale “textural” patterns revealed by Ca and HFSEs exists due to rapid cooling, but the smallest-scale details that remain sharp in Ca maps have been erased by diffusion. Quantitative analytical traverses with spot spacing as little as one micron reveal very sharp concentration gradients over short distance (tens of microns) that have allowed us to apply diffusion modeling to constrain the growth and especially the cooling rates of these garnets, revealing a much shorter duration for the Acadian metamorphism in this area than previously thought. In addition, preservation of small- and medium-scale compositional features in grt interiors reveals that rather than forming in a single continuous growth history, as assumed in the 1977 paper, the large 1-2 cm crystals likely represent tectonized older grt angular fragments welded together in a later metamorphism and ultimately overgrown by a continuous rim. Patches of included matrix represent trapped material between the growing garnet cores that ultimately coalesced. This scenario could only represent very rapid grt growth and cooling.