TRACE ELEMENT COMPOSITIONAL DIVERSITY OF OLIGOCLASE-LABRADORITE PLAGIOCLASE IN MIGMATITIC GNEISSES OF THE SKAGIT GNEISS, WASHINGTON

Two types of fine-scale leucosomes are identified in migmatitic sillimanite gneiss, biotite ± garnet gneiss, biotite-cummingtonite schist, and amphibolite from the Skagit Gneiss, North Cascades, Washington State, U.S.A. based on trace element, compositional variation in plagioclase and petrographic observations. In-situ analyses using laser ablation-ICP-MS reveal that in type 1 samples, plagioclase Sr content peaks in leucosome interiors and steadily decreases towards leucosome edges, across contacts with wall rock, and ~0.75-1.75 cm into wall rock (525-1150 ppm in leucosome to 415-990 ppm in wall rock). Ba tends to follow a similar pattern (110-350 ppm in leucosome to 80-180 ppm in wall rock), while Eu tends to steadily enrich (0.42-1.55 ppm in leucosome to 0.48-1.95 ppm in wall rock). Plagioclase grains in type 1 leucosomes are texturally identical to those in adjacent wall rock and lack magmatic textures such as euhedrality and oscillatory zoning. Plagioclase in type 2 leucosomes may be similar to, or distinct from, wall rock-hosted grains in Sr, Ba, and Eu content, but no trends between trace element content and proximity to leucosome interior are observed. Texturally, type 2 leucosomes are recognized by either the occurrence of plagioclase with euhedral (and often sieved) cores and oscillatory zoning, or the presence of peritectic garnet + titanite.

Type 1 leucosomes are interpreted as products of metamorphic/metasomatic processes because the distribution of Sr, Ba, and Eu observed among plagioclase grains is likely explained by crack-seal vein development with an evolving fluid composition or anisotropic fluid alteration within flow channels. While the lack of plagioclase euhedrality/oscillatory zoning and peritectic phases does not preclude a magmatic origin, the unique coupling of textural and geochemical results in this type strongly suggest crystallization by subsolidus, rather than magmatic, processes. Type 2 leucosomes are inferred to be igneous in origin because 1) plagioclase textures and peritectic phases observed suggest magmatic crystallization and 2) the lack of progressive, Sr + Ba enrichment and Eu depletion in plagioclase is consistent with elemental fractionation between residual/solid and liquid phases during melt generation and subsequent leucosome crystallization.