Structural evolution of a quartz–sillimanite vein and nodule complex in a late-to post-tectonic leucogranite, Western Adirondack Highlands, New York

Abstract

Quartz–sillimanite veins and nodules within the carapace of a late- to post- tectonic leucogranite crosscut one another as well as calcsilicate schlieren. These relationships document a fracture-related and hydrothermal origin of the vein and nodule complex. Two dominant orientations (N50E, N20E) are observed with the former being the oldest and most deformed. Both of these sets have undergone deformation, including boudinage of veins to produce nodules. Zircon geochronology fixes the emplacement age of the leucogranite at 1035.1±3.8 Ma and late crosscutting pegmatites at 1034±10 Ma, hence the vein–nodule complex must fall within this interval. Late dikes of leucogranite truncate the complex and document the continued presence of magma during vein–nodule formation. Anisotropy of magnetic susceptibility (AMS) in the leucogranite carapace reveals an approximately horizontal flow direction, within a plane striking N49E and dipping moderately to the northwest. In this regime, quartz–sillimanite veins formed initially as tension fractures in subvertical NNE orientations either as a result of high fluid pressures or rapid sinistral shear along the N50E contact. Progressive sinistral shear rotated the veins counterclockwise causing buckling followed by boudinage and rotation of fragments into near parallelism with the N50E contact. Strain was accommodated by slip between crystals and melt migration with an estimated melt fraction of at least 30%. Multiple episodes of fracturing and vein formation appear to have occurred. Final deformation of the carapace and the vein–nodule complex is envisioned as a flattening against the contact, perhaps as a result of pluton inflation. Melt was still present after this event as evidenced by post-vein granite and pegmatite dikes, commonly with sinistral shear along the dike margin.