Spatial Distribution of Magnetic Susceptibility in the Mt. Barcroft Granodiorite, White Mountains, California: Implications for Arc Magmatic Processes

The petrographic or chemical zonation of plutons has been widely studied and used to constrain petrogenetic processes and emplacement mechanisms. The time involved in modal data collection, as well as the cost of chemical analyses, makes the search for pluton-scale zoning patterns the exception rather than the norm in magmatic arc studies. Yet, the magnetic susceptibility (Km) of plutonic rocks, both magnetite-bearing and magnetite-free, can be an invaluable tool to quickly assess the internal organization of any pluton. New field observations, new magnetic mineral data and reprocessed Km data on the Barcroft granodiorite pluton, White Mountains, California are presented. The average Km of 660 specimens from 76 stations ranges from 140 x 10-6 [SI] to 75000 x 10-6 [SI] with an average at about 16800 x 10-6 [SI]. The distribution of Km is unimodal. The hysteresis parameters of the Barcroft rocks indicate that Km is controlled mainly by multi-domain magnetite. The contribution of mafic silicates (biotite and hornblende) to Km ranges from 0.4 to 99%, with an average at about 1.8%. As in many other ferromagnetic (i.e. magnetite- bearing) plutons, Km variations reflect different amounts of magnetite which itself results from petrographic variations. This is supported by the positive correlation between major oxide variations (e.g., SiO2, FeO) and Km. A new Km map of the Barcroft pluton shows several important features including (a) a low Km zone in the SW corner of the pluton, near areas that exhibit economic mineralization possibly related to hydrothermal fluids; (b) a few isolated anomalies that may be attributed to transformation of normal magnetite into lodestone; (c) a north south high Km ridge that could possibly result from local mingling between the main granodiorite rock type and syn-plutonic mafic dikes; (d) a broad reverse Km zonation, i.e.; higher Km in the centre; and (e) possibly a positive correlation between Km and topographic elevation (between 5000 and 13000 feet), which could be explained by a higher fO2 at a higher structural level in the chamber. These preliminary results suggest that (1) syn-plutonic diking may play a significant role in the geochemical differentiation of granodiorite plutons; (2) the classic dichotomy between ilmenite-series and magnetite-series of granitoids might, at least to some extent, depend on the exposure level if such intrusions are confirmed to be vertically differentiated and, (3) mapping Km in a ferromagnetic pluton can be an efficient tool to constrain its internal organization.