The origin of partial-disequilibrium major- and trace-element zoning in garnets from the Picuris Mountains, NM

Abstract

New evidence from X-ray mapping, quantitative major- and trace-element analyses, and garnet-inclusion suites suggests that the disequilibrium behavior in garnets from the Picuris Mountains, NM, is the result of changes in matrix assemblages at a very local scale (i.e., within the immediate vicinity of each growing porphyroblast). In addition, detailed petrologic study of metapelites with a range of bulk compositions reveals a wide variety of partial-disequilibrium garnet zoning patterns not previously known to exist. The complicated zoning patterns of garnets from samples containing the assemblage garnet + biotite + muscovite + quartz + plagioclase can be explained by a local change from a garnet-forming to a garnet- and biotite-forming reaction, coupled with the breakdown of allanite, the nucleation of apatite and monazite, and the temporary presence of xenotime. This interpretation is supported by mass-balance calculations that show that the breakdown of less than one modal percent allanite could supply the spikes in trace-element concentration observed in garnet. This study also provides plausible explanations for the unexpected zoning patterns found in garnet-staurolite schists and one calc-schist. The implications for petrologists of cryptic disequilibrium like that recorded by garnets of the Picuris Range are many fold. In particular, the incautious application of geothermometers and geobarometers, especially those which utilize accessory minerals, could lead to seriously flawed results