Magnetic anisotropy as a tracer of crystal accumulation and transport, Middle Banded Series, Stillwater Complex, Montana

摘要

Fabric studies of layeredmafic intrusions have led to improved understanding of the mechanical processes operating in largemagma chambers, including crystal accumulation and crystalmush deformation. Such studies, however, are typically limited by a tradeoff between breadth (number of sites studied, characteristic of field-focused work) and sensitivity (ability to discern subtle fabric elements, characteristic of laboratory fabric analyses).Magnetic anisotropy, if analyzed in a systematic way and supported by single-crystal and petrofabricmeasurements, permits relatively rapid characterization of magmatic fabrics for large numbers of samples. Here we present the results of a study of remanence and susceptibility anisotropy from three transects through the Middle Banded Series of the Stillwater Complex, Montana. All three transects exhibit a magnetic foliation that increases with stratigraphic height up to the top of Olivine Bearing Zone III, consistent with crystal mush compaction. Perhaps more importantly, each transect is characterized by a subtle lineation in the anisotropy of magnetic susceptibility with a consistent direction within that transect. The magnetic lineation directions,which generally coincide with crystallographic preferred orientations of silicate minerals, likely record a pre-compaction fabric. Lineation directions differ from one transect to another, implying that the process generating the lineation - either slumping of a semiconsolidated crystalmush or magma transport - actedon lengthscales of atmost a fewkm.These results demonstrate the sensitivity ofmagnetic anisotropy to petrofabric in mafic rocks.