Distinction between tectonic mixing and mass transfer processes in a ductile shear zone

Abstract

A petrological, geochemical and microtextural study of an upper amphibolite facies shear zone, developed in interlayered mafic and felsic layers, permits the relative roles of tectonic mixing and fluid-assisted mass transfer processes to be determined. Geochemical evidence indicates that the chemical changes in the deformed rocks result from mixing of mafic and felsic layers together with fluid-assisted mass transfer within the shear zone. During mylonitization, most major elements and some trace elements (LREE, Rb, Sr, Ba, Cu, Ni) exhibited mobile behavior. The HREEs, Ti, V, Sc, Co and Fe, on the other hand, were immobile. Based on mass conservation of these elements, a two-component mixing model using mafic and felsic rocks from outside the shear zone as end-members explains the major and trace element data. The chemical composition of the felsic mylonite is modeled by mixing 12±5% mafic rock and 88±5% felsic rock, whereas, the mafic mylonite is modeled by mixing 55±4% mafic rock and 45±4% felsic rock. A closed system mixing model yields a good fit for immobile elements, such as HREEs, Ti, V, and Sc, but significant chemical differences between the calculated data and observed data are explained by fluid-assisted mass transfer of mobile elements under open system conditions. A 24% volume loss in the felsic mylonite and 13% volume loss in mafic mylonite are derived from the mass balance, assuming the HREE, Ti, V and Sc were immobile.