THE ORIGIN OF MYRMEKITE IN SHEARED GRANITOIDS

Myrmekite is an albite and quartz symplectite that commonly rims K-feldspar grains in sheared granitoids. A density contrast and fluid assisted diffusion allows for replacement of K-feldspar by plagioclase with excess quartz produced by this replacement forming the flower texture characteristic of myrmekite. This texture has been interpreted as (1) a reaction under partial melt or partial crystallization conditions, (2) a deformation-induced, solid-state reaction, or (3) a product of both (1) and (2). In order to test these different interpretations, we conducted a microstructural analysis of myrmekite in a shear zone that shows evidence of deformation under magmatic to solid-state conditions. The Bitterroot shear zone (BSZ) is a ~100 km long and ~500 m thick granitic shear zone that developed during the Paleocene-Eocene exhumation of the Bitterroot metamorphic core complex and the eastern edge of the Idaho batholith in southwestern Montana. The BSZ is N-S oriented, dips 20-40°E, and carries a strong lineation oriented ~N110°, with a consistent top-to-east sense of shear. The BSZ displays a deformation gradient from a lower region of magmatic and high-temperature fabric where quartz grains display a low aspect ratio, grading into well developed, low-temperature mylonite, where quartz grains show a high aspect ratio or form ribbons. This well-defined structural gradient make this system ideal for studies of myrmekite generated in shear under a wide range of conditions. The study of ~500 myrmekite grains from ~15 thin sections distributed across the BSZ shows that myrmekite is ubiquitous at all levels of the shear zone. When normalized to the surface area of myrmekite-bearing K-feldspar grains, myrmekite is less abundant in the high-temperature region of the BSZ, and increases by approximately 20% in the overlying, low-temperature mylonite. Furthermore, the population density of myrmekite is higher along porphyroclast edges that are perpendicular to the incremental shortening direction as determined from the well established shear sense. Our results suggest that deformation plays a role in the production of myrmekite under a wide range of conditions. The positive correlation between myrmekite occurrence and increasing finite strain suggests that myrmekite generation is enhanced by the accumulation of strain under solid-state conditions.