Petrologic controls of bulk magnetic susceptibility in the McDoogle pluton, Sierra Nevada, California

Bulk magnetic susceptibility (BMS) is useful for mapping subtle and gradational compositional variations in plutons. The relatively high BMS of Sierran plutons (0.001-0.05 SI Units) has led us to assume that the abundance of magnetite controls their BMS. BMS is an easily measured proxy for color index variations if the amount of magnetite correlates with the overall mafic content of the rock. We tested these assumptions in the McDoogle pluton in the Sierra Nevada by investigating the magnetic mineralogy of seven samples with BMS ranging from ~4 - 45 10-3 SI units. The steep, tabular McDoogle pluton has a complex internal anatomy reflected in compositional variations. Abundant wall-rock inclusions (~35% by vol.) are interpreted to be in situ screens because they define a ghost tectonostratigraphy and contain pre-intrusive solid-state fabrics that are concordant both among screens and with external wall rocks. The pluton thus is interpreted to be a plexus of dikes that grew incrementally by crack-seal. Magnetite and ilmenite were identified by electron microprobe in all seven samples. Mineral modes were obtained by point-counting thin sections. BMS correlates strongly with color index for all but the sample with the lowest BMS. That sample has the highest modal abundance of ilmenite, which more generally correlates negatively with color index. Magnetite (vol. k = 1.0-5.7 SI Units) generally contributes more to BMS than ilmenite (vol. k = 0.0022-3.800 SI Units) (Hunt et al., 1995) and thus a higher proportion of ilmenite yields a lower BMS. Several magnetite textures are present and imply a complex and protracted growth history over the life of the pluton. Ilmenite grains in all samples are resorbed and rimmed by titanite, which is interpreted to reflect late-stage oxidation. Thus, the mapped BMS pattern in the pluton might reflect fO2 variations during crystallization of the pluton rather than bulk composition of the rock. Reaction progress therefore was estimated in each sample by assuming spherical grains and calculating the volume ratio of the titanite rims to ilmenite cores. As BMS increases, the abundance of ilmenite and oxidation reaction progress both decrease. Therefore, oxidation of ilmenite to magnetite appears to exert only a minor effect on BMS. Despite a complex growth history of ferromagnetic phases, BMS of the McDoogle pluton strongly correlates with color index and is useful for mapping subtle and gradational compositional variations.