High temperature ductile deformation, lithological and geochemical differentiation along the Shagou shear zone, Qinling Orogen, China

Abstract

Lithological and geochemical differentiation in ductile shear zone is essential to understanding element mobility during ductile deformation in the mid-lower crust. The Shagou ductile shear zone characterized with intensive compositional layering is investigated to understand element behaviors during high temperature ductile deformation. Geochemistry and zircon U–Pb chronology suggest the protoliths were magmatic rocks crystallized at ca. 735-723 Ma, overprinted by ductile deformation at ca. 198–184 Ma. Microstructure, quartz fabric, and thermometer coupled with barometer constrain the deformation to occur at 4.50–6.12 kbar and 633–713 °C. Field observations, geochemical and zircon U–Pb geochronological data suggest that the amphibolitic and gneissic mylonites were generated from geochemical differentiation during high-temperature ductile shearing. The geochemical contents of progressively deformed mylonites show systematic increasing or decreasing with enhancing mylonitization. Silica displays remarkably decreasing with enhancing mylonitization, whereas Mg, Fe, Mn, Ti and P tend to migrate to higher strain, more acid environment. REEs, LILEs, HFSEs, Zn and V display negative correlations with increasing mylonitization, suggesting that nearly all the trace elements migrate to lower strain in the amphibolitic mylonites. However, HREEs, HFSEs, Cr, Ni, Zn and V exhibit positive correlations with increasing mylonitization in the gneissic mylonites. It implies that these elements tend to migrate to higher strain, more acid environment during ductile deformation.