Evaluación de la movilidad de elementos mayoritarios y trazas durante la milonitización de rocas del basamento de las Sierras Australes de Buenos Aires.

LAURA GRECCO,; SERGIO DELPINO,; DANIEL GREGORI,; LUIS DIMIERI

Revista de La Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 2000 vol. 55 p. 352 - 362

0004-4822

Several authors have demonstrated that the distribution of major and trace elements can bemodified by procceses such as meteorization, metamorphism or hydrothermal alteration. The mobility of chemical elements in ductile shear zones have been little studied, and the papers that deal with these problem are relatively recent. The mobility of the chemical elements in shear zones will depend on a number of variables, mainly the mineralogical constitution, texture and chemical composition of the protolith, and the deformation regime. We present the preliminary results of a study on the mobility of the chemical elements associated with the development of ductile shear zones, on granitic rocks from the Sierras Australes of the Buenos Aires basement. Mylonitization caused mobilization of almost all major and trace elements, including rare earths. The mobility of elements is mainly a function of relicts, recrystallizecl and neocrystallized minerals. At the lowest degrees of deformation, quartz and constituents of mica (Si, Al, Cr, Rb, Ba y Cs) proved to be the less mobile elements. Intense deformation (ultramylonites) promoted extreme mobility of almost all elements, except Si. In all cases LREE decreased with the increasing deformation, but HREE showed a different behaviour, increasing with the intensification of deformation in one of the examples presented here, and decreasing in the other. Precipitation of HREE in enrichecl mylonites, might be tentatively adscribed to the crystallization of accesory minerals during deformation.