Accessory mineral behavior during differentiation of a granite suite: monazite, xenotime and zircon in the Sweetwater Wash pluton, southeastern California, U.S.A.

Abstract

Compositional and textural characteristics of the accessory minerals monazite, xenotime and zircon in the Sweetwater Wash granites and related aplites indicate that these phases not only controlled much of the trace-element geochemistry of the suite, but that they also record the melt compositional changes that occurred during magmatic differentiation. Fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare-earth element (REE) depletion. This was accompanied by increasing heavy-REE concentrations until xenotime joined the crystallizing assemblage; subsequently, combined monazite-xenotime fractionation resulted in lowering of the entire REE budget. Zircon, which contains inherited cores that were apparently resorbed and rounded during initial anatexis, was saturated throughout the differentiation history of the Sweetwater Wash suite. Accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales. The differences are best displayed by monazite: on average, crystals in more differentiated rocks (aplites) are relatively depleted in light REE's and contain higher concentrations of the substituting elements U and Th. Zircon displays complimentary increases in Hf and Y, while xenotime shows a slight increase in Th and in Gd/Ho ratios; both phases also exhibit higher U concentrations in aplites than in granites. These differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites, but also within individual thin sections, due to in situ fractionation. On an even smaller scale, strong compositional variations are present within single crystals, possibly due to diffusion-controlled melt-compositional gradients in the regions (a) adjacent to growing major phases, and (b) adjacent to the growing accessory crystal itself. Our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes, but that the scale of observed variations must be carefully considered.