Mechanical response, chemical variation, and volume change in the Brookneal and Hylas shear zones, Virginia

Abstract

Changes in volume and chemical composition were compared with strain indicators measured in two Alleghanian shear zones in granitoid plutons within the Virginia Piedmont. Both shear zones have measurable kinematic indicators that consistently record dextral strike-slip movement. The Brookneal zone, in the Melrose Granite, exhibits one ductile deformation event whereas the Hylas zone, in the Petersburg Granite, records both brittle and ductile events. Comparison of the chemical variation and volume change for each of the shear zones yields several surprising similarities given the differences in deformational history and structural style of the two zones. Overall, both zones experienced volume decreases during deformation from undeformed to ultramylonitic. Silica, K, and U increased in both zones with decreases in Fe, Mg, Sr, Ti, Mn, Y, Nb, Zr. Changes in element behavior with incremental changes in strain are different in each zone and appear to be directly related to the style of deformation and strain processes operating at each deformational stage. Chemical data from the Brookneal shear zone clusters into three groups corresponding to weak deformation, moderate strain, and ultramylonite. Deformation in the Hylas shear zone is more complex and abrupt changes in volume and element behavior may be related to brittle events late in the deformation history. The choice of immobile elements for mass balance and volume calculations was based on the petrography of the samples. The concentration of titanite in the Melrose Granite changes from several percent in the undeformed rock to absent with very minor Ti oxides in the ultramylonite. A similar decrease in most accessory minerals takes place in the Hylas zone; however monazite increases significantly. Titanium mobility is implied in the results of the chemical analyses and volume calculations. Further, it is postulated that the fluid source for the shear zones includes dehydration reactions in the granites themselves and may include other sources, especially in the case of the Hylas zone. Pressure driven fluid movement and mechanical response of the deforming rocks may be responsible for some of the observed variations in the chemistry and volume of the zones.