242-23 An EPMA Study of Garnet-Bearing Pelites and Psammites from the Altnaharra Psammite, Port Vasgo, Northern Sutherland, Scotland

Abstract:

Garnet zoning can act as a chemical archive of changes in temperature, fluid composition, protolith makeup, and pressure conditions during mineral growth. While previous research has captured garnet zoning patterns across Scotland’s Northern Highland Terrane, limited electron probe microanalysis (EPMA) data exists for garnets hosted at Northern Sutherland’s Port Vasgo locality. To address this gap, we sampled loose pelite and psammite from three outcrops of the Altnaharra Psammite. After the rocks were prepared as standard polished thin sections, we used a JEOL JXA-8530FPlus EPMA to generate WDS elemental maps and core-rim transects for thirteen mm- to cm-scale garnets. Targeted elements included Fe, Mg, Mn, Ca, Cr, Ti, Sr, and Y. Finally, MinPlotX was used to convert raw oxide data into end-member mole fractions.

Initial study revealed size-dependent zoning and growth patterns. Large garnets displayed distinct core-rim structures with noticeable diffusion of Mn, Mg, and Ca. Medium garnets preserved abundant oscillatory Ca zoning and localized Cr zoning. Small garnets, primarily hosted in nutrient-limited psammitic lithologies, were often severely retrogressed and displayed significant cation diffusion with an occasional sharp Mn ring near the edge. These small garnets hosted 10–20% higher grossular content than larger pelitic variants, though almandine dominated all samples. Special patterns included a “snowball” garnet and a garnet hosting a jagged core that resembled a lightning bolt in shape. These textures provide evidence for Ca’s low diffusion rate and Port Vasgo’s extreme deformation, while highlighting the potential influence of fluids and protolith heterogeneity on zoning. Future Lu-Hf geochronology is required to constrain the exact timing of these growth events and the likelihood of several distinct garnet populations.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-17130

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