HR: 1340h
AN: V43A-1105    [Abstracts]
TI: Static vs dynamic processes in mafic magma chambers: AMS and image analysis in the Stillwater Complex, Montana
AU: * Butak, K C
EM: kbutak@siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology Mailcode 4324 Southern Illinois University, Carbondale, Il 62901, United States
AU: Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology Mailcode 4324 Southern Illinois University, Carbondale, Il 62901, United States
AB: Most mafic intrusions are regarded as magma chambers where various processes have led to the formation of a layering. Some of these processes are static (e.g., crystal settling) while others are dynamic (magma flow due to emplacement and/or convection). The 3-D analysis of the petrofabric allows to constrain the nature of the main layering forming processes, yet only few methods allow to gain this information rapidly. We present a new template study, based on the Stillwater mafic rocks, where magnetic fabric and image analysis techniques have been combined to resolve the 3-D petrofabric. The Stillwater Complex, Montana, is an Archean mafic-ultramafic intrusion that consists of 5 main units: the basal series, ultramafic series, and lower, middle and upper banded series. This study focuses on surface oriented samples (pyroxene-rich gabbronorites, plagioclase rich anorthosites, and olivine containing troctolites) collected near the J-M reef, the main PGE deposit of the complex. 3-D digital image analysis is performed on 3 mutually perpendicular, large oriented thin-sections to define the shape-preferred orientation (SPO) of major minerals. Two of the SPO principal axes (X and Z) correspond respectively to lineation and perpendicular to foliation. The layering is defined macroscopically in the field. Preliminary results show that whereas a well-defined mineral foliation is systematically present in all samples, a mineral lineation is present in only a few samples. The magnetic susceptibility of a rock is an anisotropic property defined by the ratio between induced and inducing magnetization of a sample. It is distinct from the paleomagnetic directions of a sample. In mafic rocks, the anisotropy of magnetic susceptibility (AMS) results from the lattice preferred orientation (LPO) of paramagnetic ferromagnesian silicates, and from the shape preferred orientation (SPO) of ferromagnetic oxides and sulfides. The AMS data is used to calculate the magnetic anisotropy ellipsoid with 3 principal axes where K1≥K2≥K3. Preliminary results show that the mean susceptibility (K_m) varies from 13 x 10^-6 [SI] to 40000 x 10^-6 [SI] with an average of 825 x 10^-6 [SI], and suggests that some weak samples are dominated by diamagnetic minerals, and strong samples have a significant ferromagnetic contribution in addition to a paramagnetic contribution. The degree of magnetic anisotropy (P_j) ranges from 1.012 to 1.797 with an average of 1.103. The shape parameter (T) ranges from 0.968 to - 0.714 with an average of 0.450 and suggests that most fabrics are oblate. The combination of the two methods (image analysis and AMS) has allowed to identify 2 fundamental types of magmatic fabrics: (1) oblate fabrics resulting mainly from static magmatic processes in pure-shear (crystal settling, crystal mush compaction); (2) prolate fabrics resulting mainly from dynamic magmatic processes in pure-shear/simple shear (magma flow, crystal mush slumping).
DE: 1518 Magnetic fabrics and anisotropy
DE: 3618 Magma chamber processes (1036)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting