Strain Distribution in a Partially Molten Crust: Insights from the AMS Study of Carlos Chagas Anatexite, ARAÇUAI Belt (se Brazil)
Abstract
Anatectic domains, characterized by abundant migmatites, represent portions of the middle crust that have been partially molten. The Carlos Chagas anatexite is a unit that includes diatexites, metaxites and anatetic, peraluminous granites. It is localized in eastern domain of the Araçuaí Belt, which was formed during the amalgamation of West Gondwana by the collision of the Sao Francisco and Congo cratons. This orogenic segment underwent a synkinematic high temperature (>750oC)-low pressure (~ 6MPa) metamorphism that causes widespread partial melting of the middle crust. The Carlos Chagas unit is composed by quartz, feldspar, garnet, biotite, sillimanite, ilmenite, rutile and cordierite. U-Pb data indicates that its crystallization occurred c. a. 574 ~ 3 Ma. The main feature of this migmatite is the presence of a pervasive magmatic foliation, which is marked by the preferential alignment of biotite, alkali feldspars and plagioclase. At the grain scale, quartz displays evidence of interstitial crystallization and few solid-state deformation fabrics are observed. We used anisotropy of magnetic susceptibility (AMS) as a tool for recovering mineral fabric and thus the flow field of the Carlos Chagas anatexite. Magnetic properties of 153 samples were measured. They yield dominantly low values of the bulk magnetic susceptibility (km <360 x 10-6 SI), low anisotropy degrees (1.03 - 1.19), and the mean shape parameter ranges from - 0.3 to 0.8, with oblate ellipsoids (T > 0) being dominantly more frequent than prolates ones. These magnetics characteristics are consistent with biotite being the dominant carrier of the AMS. Magnetic foliations and lineations suggest two main structural patterns. The northern portion of the studied area shows shallowly plunging lineations (02° - 20°) to SE (140°-120°) or NW (340°-300°), while the foliation strikes NW-SE with shallow dips (03°-10°). Local subvertical foliation dips (70°) are due to NE-SW trending transcurrent shear zones. The southern region shows complex magnetic fabric patterns. Magnetic lineation plunges range from 05° to 58°, in varied directions, while the foliation strikes from NW-SE to NE-SW with low to high dips, lacking a statistical preferred orientation. Using electron backscatter diffraction (EBSD) analysis we investigated the correlation of the biotite crystallographic fabrics with the AMS fabric and structural field measurements of the Carlos Chagas anatexite. EBSD data for biotite document a correspondence between the <001> direction of biotite and k3 (as well as macroscopic foliation). In hot orogens such as the orogenic sector studied here, strain localization is not efficient and rocks are deformed homogeneously. Field data, AMS and EBSD studies suggest that, in the anatectic unit as well as in the adjacent units of the Araçuaí belt, the Africa-South America convergence resulted in the development of a complex, 3D strain field in huge volumes of magmatic and anatetic rocks, which hinder a simpler, high-temperature plastic flow regime.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMGP21A1123C
- Keywords:
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- 1518 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic fabrics and anisotropy;
- 1519 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic mineralogy and petrology;
- 5112 PHYSICAL PROPERTIES OF ROCKS / Microstructure;
- 8108 TECTONOPHYSICS / Continental tectonics: compressional