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

摘要

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.