Crystal Mush Flow in Small Concentrically Expanded Pluton (Castle Crags Pluton; Klamath Mountains, CA, USA)

Understanding of the processes of magmatic fabric formation in crystal‐rich magmas and their reflection in rock magnetic properties are important for understanding pluton formation and intrusion mechanisms. On the example of small concentrically zoned Castle Crags pluton in the Klamath Mountains (CA, USA) we provide reconstruction of the flow/deformation mechanisms of the crystal‐rich magma and pluton growth based on detailed structural mapping and microstructural analysis employing the anisotropy of magnetic susceptibility, microstructural analysis, and crystallographic preferred orientation. Our study reveal microstructural evidence for progressive development of magmatic textures in the pluton core transitioning to submagmatic and eventually subsolidus fabric at the pluton periphery, that is interpreted in terms of the flow/deformation of the crystal mush. The documented magmatic textures are linked to anisotropy of magnetic susceptibility parameters and orientation. The recorded anomalous degree of anisotropy of magnetic susceptibility found in the pluton is attributed to tiling and plastic deformation of magnetite grains by the surrounding phenocrysts. The concentric structure of the pluton resulted from horizontal compaction and margin parallel stretching of the dense crystal mush around the vertically intruding trondhjemite magma in the pluton core. The Castle Crags pluton is interpreted as a concentrically expanded pluton, which grew at least by two increments of granodiorite and trondhjemite magma emplacement.