BULK MAGNETIC SUSCEPTIBILITY AS A PROXY FOR COMPOSITIONAL VARIATION IN PLUTONIC ROCKS: MAGMA MIXING CAN BE A PROBLEM
Samples with 15<CI< 55 and both normal and anomalously low BMS were examined optically and using QEMscan to obtain accurate modal abundances of magnetite, ilmenite, and pyrite. Positive correlation (R2 = 0.95) between BMS and modal magnetite confirms that magnetite is the dominant magnetic mineral in the rocks and that anomalous BMS reflects low magnetite abundance. We hypothesized that low ƒO2 and high ƒS2 caused Fe to enter ilmenite and pyrite instead of magnetite. QEMscan modes confirm that anomalous samples have elevated ilmenite and pyrite abundances. Balanced redox reactions predict that growth of ilmenite at the expense of magnetite also shifts Fe into mafic silicates. Hand-specimen and optical petrography revealed that samples with anomalous BMS have a distinctive heterogeneous texture that suggests incomplete mixing of mafic magma—represented in the pluton by abundant mafic enclaves—with the host granodioritic magma. BMS and modal analysis reveal that the mafic enclaves have low BMS relative to CI (BMS = 2.6E-2 at CI = 68) and high ilmenite and pyrite abundances. We thus infer that (1) the rocks with anomalously low BMS are products of magma mixing, and (2) mixing of mafic and felsic magmas in a growing pluton can cause ƒO2 and ƒS2 variations that degrade the correlation between BMS and composition and thus render BMS unreliable as a mapping proxy.