RECONCILING DIFFERENCES AMONG AMS AND AARM FABRICS IN IGNEOUS ROCKS: DATA AND A NEW APPROACH (THERMAL DEMAGNETIZATION OF THREE ORTHOGONAL COMPONENT ANHYSTERETIC REMANENT MAGNETIZATION (ARM))

Both anisotropy of magnetic susceptibility (AMS) and anisotropy of remanence, particularly anhysteretic remanent magnetization (AARM), data have been used to define fabrics in igneous rocks and thus as a proxy for magma flow patterns to evaluate igneous processes. In terms of relations between AMS and AARM principal axes, normal (AMS and AARM principal axes parallel), inverse (max and min axes flipped), intermediate (max and intermediate flipped), and oblique (intermediate parallel, but other axes non-parallel) fabric relations have been observed in igneous rocks. Inverse AMS/AARM fabrics are typically explained by the prevalence of elongate, single domain particles. Several single plutons and volcanic units reveal a lack of internal consistency between AMS and AARM data at the site level. For example, both the early Eocene Shonkin Sag and Square Butte laccoliths, north-central Montana, yield fabric data that are well defined at the site level; AARM data are usually more dispersed. Most sites have positive T values, implying strong foliations. The degree of anisotropy, P’ (1.01 to about 1.10) is typical of magma flow fabrics. AARM P’ values are always higher than AMS P’ values. There is no consistent relationship between AMS T and AARM T values. Of 15 sites with both AARM and AMS data, three have normal, five have inverse, one has intermediate, and six have oblique fabrics. Ratios of MDFarm to MDFirm imply dominance of MD magnetite in these rocks, although the paleomagnetic signature is not controlled by MD grains and normal fabric relations are not common. Similar behavior is observed in several large volume ash flow tuffs (e.g., San Juan Mountains) and voluminous sills (e.g., Karoo intrusions). AMS studies may lead to false interpretations of magma fabrics if the signal is dominated by contrasting populations of magnetite grain sizes at different sites. The use of three-component thermal demagnetization of ARM, acquired at a range of peak AF fields up to 200 mT, following Lowrie (1990), is being evaluated as a potential means of distinguishing subtle variations in assemblages of low to moderate coercivity phases (e.g., magnetite, maghemite). Understanding the origin of differences between AMS and AARM fabrics should provide a better understanding of magma fabrics and ultimately mechanisms of magma emplacement.