MAGMA EMPLACEMENT-RELATED STRESS, STRAIN RATE, AND VISCOSITY ESTIMATES FROM A CONTACT AUREOLE

A structural study of the contact aureole of the 200 MPa White Horse pluton (eastern NV) was undertaken to evaluate deformation mechanisms, stresses, strain rates, and viscosities attending magma emplacement. Aureole mesoscale structures include tight to isoclinal overturned and upright folds and a pervasive axial planar foliation. Rocks do not contain a penetrative fabric outside the aureole and are openly folded about sub-vertical axial planes; however, all of the above structures are truncated by the intrusive contact at various scales. A microstructural study of carbonate samples indicates that annealed and emplacement related calcite tectonite fabrics have been preserved. Grain growth is ubiquitous throughout the aureole. However, an irregular distribution of annealed fabrics within the aureole suggests that the thermal evolution did not completely destroy emplacement-related strain textures. Instead, opaque phase(s) commonly associated with finer grained samples may have pinned textures during the time of maximum syn- to post-emplacement aureole heating. Annealed textures consist of millimeter scale calcite and dolomite layers. Calcite is anhedral, grain boundaries are curved to lobate and grain sizes range from 0.1 to 1.5 mm in diameter. Dolomite grains are straight to curved and exhibit a clear shape-preferred orientation (SPO). Tectonite fabrics consist of anhedral calcite (or dolomite), grain boundaries are straight to curved, grains range from 0.02 to 0.7 mm in diameter, inter and intragranular opaque phase(s) are common and a SPO is generally observed in coarser grains. Recrystallized grain size analysis was performed on two calcite tectonite samples (up to ~3000 grains measured per thin section). Results were applied to paleopiezometers and yield stresses ranging from 26 to 29 MPa. Calculated aureole strain rates range from 10^-11 s^-1 to 10^-15 s^-1 and effective viscosities range from 10¹⁵ to 10¹⁹ Pa s. The preferred emplacement model requires in-situ chamber dilation accommodated by ductile deflection of host rocks at these rates + fracturing and removal of the aureole rocks from the map plane.