BULK MAGNETIC SUSCEPTIBILITY AS A PROXY FOR COMPOSITIONAL VARIATION IN PLUTONIC ROCKS: MAGMA MIXING CAN BE A PROBLEM

Plutons commonly contain subtle compositional variations that are difficult to map in the field. Mineral modes and bulk-rock geochemistry are too time consuming and/or expensive to achieve a sufficient data density to map such variations in detail. Bulk-rock magnetic susceptibility (BMS) can be a useful mapping proxy because (1) it can be measured quickly during conventional mapping and therefore a high data density is readily achieved, and (2) BMS commonly correlates with rock characteristics such as color index (CI) and bulk-rock geochemistry (e.g., Coleman et al., 2005, in press; Aydin et al., 2007; Bartley et al., 2008). However, in the McDoogle pluton, Sierra Nevada, CA, Stearns (2009) found high CI rocks with anomalously low BMS; in normal McDoogle samples, BMS = ~4.5E-2 SI units at CI = 35 whereas in anomalous samples, BMS = ~1. 5E-2 SI units at this CI. We examined the anomalous behavior to clarify when BMS can be a satisfactory compositional proxy.

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.