Noncoaxial K‐feldspar and AMS subfabrics in the Land's End granite, Cornwall: Evidence of magmatic fabric decoupling during late deformation and matrix crystallization

Abstract

A comparative study of the anisotropy of magnetic susceptibility (AMS) and K‐feldspar phenocryst fabrics in the Land's End granite demonstrates that the AMS fabric predominantly reflects late magmatic deformation. Tensor analysis of the K‐feldspar fabric shows a complex pattern, characterized by meter‐scale variations in orientation, symmetry, and intensity, mainly related to heterogeneous flow of the phenocryst‐rich magma during emplacement. In contrast, the AMS fabric is predominantly homogenous, subhorizontal, and oblate, and is stable at the pluton scale. Quantitative microstructural analysis suggests that the AMS fabric is controlled by deformation of the partially crystallized matrix, resulting from the combined effects of late internal adjustment within the pluton and regional deformation. A general model of fabric development associated with a vertical pure‐shear overprint on a variable vertical fabric is evaluated by numerical modeling. The study demonstrates how the memory of different fabric elements may be dependent upon their grain size, crystallization sequence, and recorded previous strain.

Some remarks on fabric overprints and constrictional AMS fabrics in igneous rocks

“…We show that the variations of fabric symmetries and degrees of AMS anisotropy may result from complex deformation overprints. Such a possibility was already supported by independent structural studies (Kratinová et al 2007(Kratinová et al , 2010Parry et al 1997;Schulmann et al 2009). …”

“…Note that for some studies of natural samples/rocks, the amount of magnetic grains that are not randomly oriented may vary (see Kratinová et al 2010). Thus, the value of P, the anisotropy degree, may be somewhat uncertain.…”

“…In magmatic rocks, deformation overprints of various origins are recognized by several authors dealing with structural studies in the field (e.g., Ž ák et al 2005Sen and Mamtani 2006). However, the magnetic fabric method provides only a record of the bulk strain, and therefore, the maps of the anisotropy of magnetic susceptibility ellipsoid shapes and anisotropy degree may provide confusing information as shown for instance by Kratinová et al (2010).…”

Anisotropy of magnetic susceptibility of the Pyrenean granites

“…However, recent studies suggest that during the emplacement (magmatic conditions) some deformation phases may overprint each other contributing to the finite strain ellipsoid and, thus, the study of AMS alone does not necessarily unravel the complete deformational history of the granite (Schulmann & Ježek, 2012). In addition, the correspondence between deformation axes from felspar and biotite is not always univocal (Kratinová et al, 2010;Román-Berdiel, Pueyo-Morer, & Casas-Sainz, 1995). In this sense, some work in weakly deformed sedimentary rocks suggests the importance of magnetic subfabric separation (Oliva-Urcia et al, 2009) to disentangle the deformation history.…”

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

“…Bouchez et al, 2006), whereas younger plutons (Dartmoor, St Austell, Land's End) exhibit only magmatic state fabrics (e.g. Kratinová et al, 2010); both fabrics indicate some degree of coupling with extensional host rock deformation.…”