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Show 54 college of mines & earth sciences Bulk magnetic susceptibility (BMS) of rocks is controlled by their mineralogy and bulk chemistry. BMS can be useful for mapping variations in granitic plutons because 1) continuous gradients can be mapped whereas conventional mapping requires discrete categories; 2) BMS is an objective measurement whereas conventional mapping requires subjective judgments; and 3) BMS can be quickly measured with a small hand-held meter during conventional mapping. A BMS survey of the McDoogle pluton (Stearns, 2009, UU M.S. thesis) revealed the expected positive correlation between BMS and color index (CI, the volume percentage of mafic minerals) but with anomalies where BMS is low but CI is quite high. Six samples were collected to study the anomalous behavior, five in which CI (26.5-38.4) and BMS (22.2*10-3 - 50.6*10-3) are well correlated and one sample with the highest CI (50.2) but the lowest BMS (9.75*10-3). The low BMS is hypothesized to reflect abundant ilmenite (FeTiO3) in place of magnetite (Fe3O4) which is ~1000x more magnetic. Magnetite is favored by oxidizing conditions whereas ilmenite requires more reducing condi-tions. The anomalous sample has a distinctive heterogeneous texture that suggests formation by incom-plete mixing of two magmas, one that was Fe-rich and the other Fe-poor. We therefore hypothesize that the anomalous rocks reflect addition to the growing pluton of chemically reduced magma that contained abundant iron. One normal sample contained a mafic enclave that may be a sample of such magma. As predicted by our hypotheses, the enclave has a higher CI but lower BMS than its granitic host. Present data thus are qualitatively consistent with our hypotheses regarding the origin of the anomalous rocks. Quan-titative tests of the hypotheses depend on measuring in both normal and anomalous samples 1) ilmenite and magnetite abundances by QEM and 2) mineral compositions by electron microprobe. PETROLOGIC CONTROL OF BULK MAGNETIC SUSCEPTIBILITY VARIATION IN THE MCDOOGLE PLUTON, CENTRAL SIERRA NEVADA, CALIFORNIA KayeLinda M. Heiner (John M. Bartley) Department of Geology and Geophysics University of Utah UNDERGRADUATE RESEARCH ABSTRACTS |