MYRMEKITES: WHAT MICROSTRUCTURES CAN TELL US ABOUT MOUNTAIN BUILDING. A CASE STUDY FROM THE WET MOUNTAINS, COLORADO

The Wet Mountains in southern Colorado developed during the accretion of Laurentia, through the Yavapai (~1.75 Ga), Mazatzal (~1.65 Ga) and Picuris (~1.48 Ga) Orogenies; were affected by deformation events (1.47 and 1.44 Ga); and intrusion of granitic plutons (1.47 to 1.36 Ga). Due to the numerous events affecting the Wet Mountains, there is still an incomplete understanding of the processes that developed them. Myrmekites, a microstructural intergrowth of quartz and sodic plagioclase, have been described throughout the Wet Mountains, but not previously used as an indicator of metamorphic and deformational processes. Myrmekites are hypothesised to form through one of three processes: (1) direct crystallisation from a melt; (2) metasomatic replacement; or (3) exsolution in low- or high-strain environments. This study applied a detailed petrographic analysis of samples from the Wet Mountains to evaluate the varying myrmekite textures to infer how they developed.

Five textural types of myrmekite as well as rounded quartz blebs were found within the studied quartzofeldspathic gneisses and amphibole biotite gneiss. All samples displayed intergranular and within plagioclase textures of myrmekite, with all quartzofeldspathic samples displaying lobate textures, and most also containing bulbous and planar rim myrmekite. The amphibole biotite gneiss contains significantly fewer examples of myrmekite, mostly within plagioclase. The intergranular and lobate textures, due to their relatively smooth shapes, as well as the myrmekites within plagioclase and planar rim myrmekites, most likely reflect formation during exsolution. The bulbous textures would indicate exsolution in areas of localized higher strain. The samples do have significant indicators of deformation (foliation fabrics, deformation twinning and undulose extinction) while lacking evidence for melting and metasomatism. The rounded quartz blebs are suggested to have formed through repeated heating events. This indicates pulses of heating, which can be correlated with pluton intrusion, revealing the complex orogenic and magmatic history of the Wet Mountains.