Role of myrmekites and associated deformation fabrics in controlling development of granitic mylonites in the Pofadder Shear Zone, Southern Namibia

[摘要] ENGLISH ABSTRACT: The predominantly granitic Pofadder Shear Zone (PSZ) of southern Namibia is a crustal-scale dextral shear zone exhumed from the brittle-ductile transition. A variably deformed megacrystic granite with structurally controlled myrmekite development was studied as a proxy for how alkali feldspar replacement microstructures aid in accommodating strain within granites of the PSZ. Image analysis of the S-C fabric of the protomylonites developed within the granite indicated a sinistral sense of shear and that myrmekites were preferably orientated on the S-plane. The evolving myrmekite morphology from fine-grained vermicular intergrowths to granular aggregates that were easily incorporated into the fine-grained polymineralic matrix, established myrmekite generation as a grain-size reduction and strain-accommodation mechanism within the granitic mylonites. Syn-kinematic reactions, such as myrmekite and muscovite production, were internally balanced and T-MH2O pseudosections indicated that progressive deformation could have occurred under closed conditions. The biotite-muscovite-2-feldspar-quartz equilibrium assemblage was stable with decreasing fluid contents until fluid-saturated or slightly fluid-undersaturated conditions were reached around 2 mol % H2O. Two-feldspar thermometry, quartz deformation textures and phengite barometry on associated muscovite growth revealed peak metamorphic conditions were around 450-500 °C and 0.5 GPa. Thermobarometry results (370 °C to ~500 °C, 0.05 – 0.5 GPa) and the wide variability in quartz rod spacing within the myrmekites support a model of continued myrmekite growth under progressively lower grade conditions as the PSZ was exhumed.Significant myrmekite development was further documented within the polydeformed gneisses surrounding the megacrystic granite and established this strain-induced dissolution-precipitation reaction texture as being of importance within all broadly quartzo-feldspathic gneisses of the PSZ. Increased evidence for retrogression in the field and the great abundance alkali feldspar replacement textures developed within adjacent migmatitic biotite gneisses suggested that some rocks were the site of increased fluid-influx. The preservation of three morphologically and chemically different replacement structures: myrmekites, flame perthites and albitic rims, shows that the composition of the fluid these rocks equilibrated with varied over time. The preservation of lower-temperature/higher-strain rate quartz microfabrics and the clear development of dextral kinematic indicators within the migmatitic biotite gneiss, in contrast to the sinistral S-C fabric of the megacrystic granite gneiss it is in contact with, suggests that there was some degree of lithology-based strain partitioning within these rocks. This study highlights how strain-partitioning and fluid localisation is highly variable but that myrmekites within mid-crustal granitoids can nevertheless develop and contribute to deformation under closed conditions.

[发布机构] Stellenbosch University