HR: 0800h
AN: GP21A-0151    [Abstracts]
TI: Magnetic Fabrics, Rock Magnetism, Cathodo-luminescence and Petrography of ``undeformed'' Bambui Limestones from S\~{a}o Francisco basin (Minas Gerais State SE-Brazil): An Integrated Study
AU: * Raposo, M B
EM: irene@usp.br
AF: Instituto de Geoci\^{e}ncias, Universidade de S\~{a}o Paulo, Rua do Lago 562, S\~{a}o Paulo, SP 05508-080 Brazil
AU: McReath, I
AF: Instituto de Geoci\^{e}ncias, Universidade de S\~{a}o Paulo, Rua do Lago 562, S\~{a}o Paulo, SP 05508-080 Brazil
AU: D'Agrella-Filho, M S
AF: Instituto de Astronomia, Geof\'\i ¡sica e Ci\^{e}ncias Atmosf\'{e}ricas, Universidade de S\~{a}o Paulo, Rua do Mat\~{a}o 1226, S\~{a}o Paulo, SP 05508-080 Brazil
AB: Magnetic fabric is a sensitive indicator of rock deformation, frequently developing before visible evidence. Apparently undeformed rocks may in fact be internally deformed. We have performed our studies on apparently undeformed limestone and carbonate shales from nearly-horizontal stratigraphic layers mainly from the basal units of the Neoproterozoic Bambui Group in the southern part of the S\~{a}o Francisco basin. Magnetic fabrics were determined applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AAR). The AAR tensor was less well defined than the AMS fabric, due to the low ferromagnetic mineral content. For almost all sites the AMS and AAR fabric orientations are different, mainly with respect to the lineations, and are also different from that expected for depositional-compaction fabric. Even though the magnetic foliation poles (minimum axes) are vertical and close to bedding poles the magnetic lineations (maximum axes) are very well clustered and compare favorably with the mesoscopic-scale fabrics. In general, the AMS lineation is well developed and follows the trend of the main thrusts and faults while the AAR lineation follows the trend of main regional fold axes. Rock magnetism, cathodo-luminescence, transmitted and reflected light microscopy, and X-ray diffraction analyses reveal that there is a mix of ferromagnetic minerals mainly magnetite and pyrrhotite in the majority of the sites, however, in some sites the ferromagnetic minerals are magnetite and hematite and goethite (?). Fine-grained pyrrhotite and pyrite accompany fine-grained graphite and probably amorphous carbon in some of stylolites, while pyrrhotite is present as larger interstitial masses outside the stylolite-containing areas. The magnetic fabrics of the apparently underformed Bambui limestones are typical of very weakly deformed sediments, in which the depositional-compaction fabric has been partly overprinted by a tectonic one, with minimum susceptibility direction remaining perpendicular to bedding
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting