Journal of Petrology · 1992 · 111 citations · 0 references
Major-, minor-, and trace-element zoning have been measured in garnets from four samples of differing bulk composition from the east flank of the Shelburne Falls Dome, western Massachusetts, using ion and electron microprobes. The samples are differentially retrograded, so traditional techniques of rim geothermometry and geobarometry and P-T path analysis yield equivocal results. Trace-element abundances in garnets vary with those of major elements, particularly calcium. Garnets exhibit several types of Ca zoning, each accompanied by a distinct mode of trace-clement zoning. Garnets from low-Ca pelites in the Goshen Formation decrease to low Ca abundances near their rims. This feature is coupled with a decrease in Na/Si and Ti/Si. The outermost few microns of these garnets show a depletion in Sc/Si and an enrichment in Mn/Si, Y/Si, and rare earth element (REE) abundances compared with the garnet core. These variations are ascribed to changes in intensive parameters during garnet growth/re-equilibration, probably a decrease in pressure (< 1 kb) accompanied by a small temperature increase, which led to a decrease in Xgrossular Much of the variation in trace-element content may reflect crystal-chemical effects. In contrast, cores of garnets from intermediate-Ca pelites in the Waits River Formation initially display decreases in grossular content, followed by Ca increases towards their rims. The decrease in grossular content correlates with strong increases in Y/Si, Zr/Si, and REE contents. The Ca ‘inflection’ observed in these garnets coincides with the last appearance of clinozoisite inclusions in garnet. Clinozoisite-compatible elements (Y, Zr, and REE) may be released during breakdown of clinozoisite in an internal metasomatic process, producing some of the trace-element enrichments. Garnets from clinozoisite-bearing pelites in the Waits River Formation exhibit zoning profiles with an increase in Ca towards the rim. An abrupt enrichment in grossular content (δXgrossular = 0.06) occurs near garnet cores in these high-CaO, low-SiO2, high-FeO samples. The Ca increase accompanies small decreases in Li/Si and Na/Si, small increases in Ti/Si and V/Si, and large decreases in Y/Si, Zr/Si, and REE abundance. The large trace-element variations are probably due to an interval of growth of clinozoisite ± accessory minerals separating two distinct garnet-growth events. This garnet also shows Co and Cr increases toward the rim, probably as a result of breakdown of magnetite. Proton-probe microanalysis of minerals in these calc-pelites shows strong affinities of specific trace elements for certain minerals: Y in garnet, Ga and Rb in biotite, Zn and Ga in staurolite, Rb and Sr in muscovite, Sr and Pb in plagioclase, and Nb in ilmenite. Trace-element zoning is shown to be a useful monitor of reaction histories and possibly P-T paths during garnet growth.