Abstract

Zoned allanite is a minor mineral in highly altered blocks of garnet amphibolite that occur in a metaultramafic melange matrix in the amphibolite unit of the Catalina Schist, southern California. Blocks and matrix alike were affected by a high-T, relatively high-P hydrothermal event, which locally induced partial melting of blocks and apparently was accompanied by considerable trace element metasomatism. In blocks that show evidence for light rare earth element (LREE) enrichment, allanite contains almost all of the LREE in the rock. Most of such blocks are also migmatitic, but some are not.

Allanite from four LREE-enriched samples that have diferent whole-rock REE abundances and fractionations was analyzed with the electron microprobe. Three of the samples are migmatites, and one shows no evidence for partial melting. The analyses were obtained for empirically defined zones of uniform brightness in backscattered electron images from the scanning electron microscope (SEM). The zoned allanite displays large ranges of anticorrelated Ca and LREE contents. Variations of these elements and of Al, Cr, Sr, Fe, Mg, and Mn within zoned grains indicate a fairly continuous nature for many substitutions of M2+ and M3+ cations in allanite. Allanite exhibits two shapes of LREE patterns in the same abundance range. Within single grains, some zones display LREE patterns that are strongly fractionated with respect to adjacent zones. In migmatites, the zones most enriched in REE are not grain cores.

The apparent reverse zoning of allanite in migmatites suggests that LREE were added to the rock volume from an external source during allanite growth. Metamorphic textures and petrologic considerations indicate that at least some of the growth of allanite in migmatites predates the partial melting event. The variations of LREE within grains and between samples may in part reflect local, time-dependent changes of fluid compositions in the subduction-related, hydrothermal and metasomatic system the rocks represent.

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