HR: 0800h
AN: T11B-0567    [Abstracts]
TI: Spatial Distribution of Magnetic Susceptibility in the Mt. Barcroft Granodiorite, White Mountains, California: Implications for Arc Magmatic Processes
AU: * Michelsen, K J
EM: kmichels@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24061, United States
AU: Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901, United States
AU: Law, R D
EM: rdlaw@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24061, United States
AU: Boyd, J D
EM: jdboyd77@yahoo.com
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901, United States
AU: Ernst, G W
EM: ernst@geo.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA 94305, United States
AU: de Saint-Blanquat, M
EM: michel@lmtg.obs-mip.fr
AF: Universite Paul Sabatier, LMTG, Toulouse, 31400, France
AB: 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 (K_m) 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 K_m data on the Barcroft granodiorite pluton, White Mountains, California are presented. The average K_m 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 K_m is unimodal. The hysteresis parameters of the Barcroft rocks indicate that K_m is controlled mainly by multi-domain magnetite. The contribution of mafic silicates (biotite and hornblende) to K_m ranges from 0.4 to 99%, with an average at about 1.8%. As in many other ferromagnetic (i.e. magnetite- bearing) plutons, K_m 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 K_m. A new K_m map of the Barcroft pluton shows several important features including (a) a low K_m 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 K_m ridge that could possibly result from local mingling between the main granodiorite rock type and syn-plutonic mafic dikes; (d) a broad reverse K_m zonation, i.e.; higher K_m in the centre; and (e) possibly a positive correlation between K_m 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 K_m in a ferromagnetic pluton can be an efficient tool to constrain its internal organization.
DE: 1020 Composition of the continental crust
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting