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

Abstract

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.

Localization effect on AMS fabric revealed by microstructural evidence across small-scale shear zone in marble

“…The relationship between orientation and magnitude of strain and AMS ellipsoids can then be established only when based on microstructural analysis. In more general terms, the localized deformation plays a key role at all scales, from microstructure to large-scale deformation zones and from brittle to ductile conditions 50 and even in magmatic flow of partly crystallized magma with frequent grain to grain interactions 5,51,52 . That is the effects of localization should be considered whenever interpreting the AMS of rocks where localized deformation can be expected.…”

Protracted Multipulse Emplacement of a Postresurgent Pluton: The Case of Platoro Caldera Complex (Southern Rocky Mountain Volcanic Field, Colorado)

“…Circumferential magmatic to submagmatic fabrics in shallow plutons elsewhere have been interpreted to record concentric expansion of plutons, diapiric magma ascent, laccolithic growth, or a helical magma flow in a posttectonic stress regime (e.g., Machek et al, 2019;McCarthy et al, 2015;Paterson & Vernon, 1995;Trubač et al, 2009;Žák et al, 2011). The domainal nature of the Alamosa River monzonite, however, best corresponds to a convective flow pattern (Gutiérrez et al, 2013;Payacán et al, 2014).…”

Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

“…However, it is always associated with the observation of primary fabric intersection and transposition (e.g., Debacker et al., 2004; Hirt et al., 2004; Housen et al., 1993; Kligfield et al., 1977; Parés, 2015; Satolli et al., 2020), synkinematic C/S like subfabrics evolution (Haerinck et al., 2013; Marcén et al., 2018; Satolli et al., 2020), cleavage or crenulation formation (Haerinck et al., 2013; Housen & van der Pluijm, 1991; Pares & van der Pluijm, 2003) or Riedel (R‐, Rʹ‐, Y‐, P‐) shears occurrence (Narloch et al., 2021). In magmatic rocks rich in phenocryst content in submagmatic state, the flow/deformation of crystal mush accommodated by mechanisms of conjugate crystal alignment domains formation leads to the origin of prolate AMS fabric at high angle to expected flow (Závada, Schulmann, et al., 2009; Závada et al., 2017), the AMS fabric counterintuitive orientation (Silva et al., 2014; Závada, Schulmann, et al., 2009) or to the gradual evolution of strong AMS fabric caused by the PS deformation (Machek et al., 2019). In these cases, the microstructure analysis led to interpretation of the AMS record as the fabrics transpositions and synkinematic subfabrics formation similarly to the liquid plaster in PS, SS, and SZ experiments.…”