Strain, anisotropy of anhysteretic remanence, and anisotropy of magnetic susceptibility in a slaty tuff

Abstract

Finite strain data for the Borrowdale slaty tuffs compare variably with the anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Finite strain, determined from lapilli-rims, shows that slaty cleavage was formed by coaxial flattening with X: Y: Z in the ratio 1.74:1.21 and 0.48. AARM was measured in different coercivity windows to isolate contributions from magnetite of different grain sizes: (a) 0–3 mT for multi-domain (MD), (b) 3–15 mT for pseudo-single-domain (PSD) and (c) 15–60 mT for single-domain (SD). AMS combines petrofabric contributions from silicates as well as magnetite. Magnetite grains may grow, recrystalize or rearrange domains after or during metamorphism and postdate or overlap with the silicate’s fabric evolution. AMS foliation, defined by paramagnetic chlorites, is parallel to slaty cleavage. AARM foliation for SD magnetites is offset clockwise from AMS foliation, which may reflect late crystallization or domain-rearrangement of magnetites in response to a latter noncoaxial increment. AMS fabric-shape consistently corresponds to strain ellipsoids and indicates that the strain-induced AMS fabric is susceptible to the change of oblateness rather than strain intensity. Furthermore, investigation of the different AARM subfabrics and finite strain shows that only SD magnetite’s AARM correlates with finite strain, and weakly at that.