Role of myrmekite and associated deformation fabrics in controlling development of granitic mylonites in the Pofadder Shear Zone of southern Namibia

Myrmekites developed within metagranites of the Pofadder Shear Zone (PSZ) in southern Namibia were investigated to understand how alkali feldspar deforms under fluid-poor mid-crustal conditions. Myrmekites showed two different morphologies: (1) ‘Wormy’ myrmekite with a fine-grained vermicular structure that occurred closest to the alkali feldspar host; and (2) ‘recrystallised’ myrmekite composed of regularly sized quartz grains that formed 120° boundaries with the plagioclase and occurred along the exterior of alkali feldspar and in contact with the mylonitic matrix. These textures were interpreted to represent two generations of myrmekite growth. Recrystallised myrmekite masses were dismembered and incorporated into the fine-grained matrix. This suggests that myrmekite formation during PSZ deformation contributed to the grain size reduction of alkali feldspar and aided in the generation of the fine-grained polymineralic matrix which accommodated most of the strain. Syn-kinematic reactions, such as myrmekite and muscovite production, were internally balanced and T–MH 2 O pseudosections indicated that progressive deformation could have occurred under closed conditions. The biotite–muscovite–plagioclase–alkali feldspar–quartz equilibrium assemblage was stable with decreasing fluid contents until fluid-saturated or slightly fluid-undersaturated conditions were reached around 2 mol% H 2 O. The variability of the quartz rod spacing and the wide range of results from two-feldspar thermometry and phengite barometry on associated muscovite growth indicates that peak metamorphic conditions were around 500 °C, 0.5 GPa and that myrmekite growth continued as the shear zone was exhumed. The myrmekite-forming reaction is a deformation-induced interface-coupled dissolution–precipitation reaction with relict magmatic plagioclase supplying the needed cationic inputs. These results indicate that deformation within granitic units of the PSZ was aided by the formation and deformation of myrmekite and could have taken place under closed relatively fluid-poor conditions.