TRACE ELEMENT GEOCHEMISTRY OF QUARTZ IN THE SKAGIT MIGMATITES

The Skagit migmatites are part of the North Cascades Crystalline Core, which underwent Barrovian metamorphism in the Late Cretaceous to Early Tertiary. Mechanisms of migmatite formation are debated and range from anatexis to subsolidus recrystallization to metasomatism to igneous injections.

We used LA-ICP-MS to quantify trace elements (Li, Al, Ti, Mn, Fe, Ga, and Ge) in quartz in leucosomes and adjacent mesosomes to better understand the processes that formed the migmatites. Trace element concentrations varied across the samples, but were generally around 15-30ppm for Ti, 20-40ppm for Al, 1-5ppm for Li, 0.5-1.5ppm for Ge, 0.4-1.0ppm for Fe, 0.1-0.5ppm for Mn, and 0.03-0.5ppm for Ga.

Most samples had slightly higher Ti, Al, and Li concentrations in the leucosomes compared to the mesosomes, but slightly lower Ge concentrations, although two samples had no difference in trace element concentrations between the leucosomes and mesosomes. In one sample, two leucosomes separated by only 0.5 cm had markedly different Ti concentrations of 17 and 34 ppm, a larger difference than observed in any other sample, while the adjacent mesosome had 17ppm Ti.

Where trace element concentrations in leucosomes and adjacent mesosomes are similar, an isochemical sub-solidus model for migmatite formation is favored, although post-leucosome metamorphism is not ruled out. Where trace element concentrations decrease outward from leucosome center into mesosome, an open system injection of magma or fluids is suggested. Leucosome types varied across outcrops, as well as within single samples, suggesting multiple mechanisms of formation of leucosomes in the Skagit migmatites. Ti in quartz temperatures in the range of 630-730°C are suggested based on pressures between 6 and 8 kbar.