Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

The Mary granite, in the East Athabasca mylonite triangle, northern Saskatc hewan, provides an example and a model for the development of non-migmatiti c gneissic texture. Gneissic compositional layering developed through the s imultaneous evolution of three microdomains corresponding to original plagi oclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocry sts were progressively deformed and recrystallized, first into core and man tle structures, and ultimately into plagioclase-rich layers or ribbons. Gar net preferentially developed in the outer portions of recrystallized mantle s, and, with further deformation, produced garnet-rich sub-layers within th e plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyr oxene and garnet (and hornblende if sufficient water was present), which we re, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages inv olving S-C fabrics, S-C-C' fabrics, and ultramylonitic domains. In addition , quartz veins were emplaced and subsequently deformed into quartz-rich gne issic layers. Moderate to highly strained samples display extreme mineralog ical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approx imately 900 degrees C and 1.0 GPa and was metamorphosed at approximately 75 0 degrees C and 1.0 GPa. The igneous rocks crystallized in the medium-press ure granulite field (Opx-Pl) but were metamorphosed on cooling into the hig h-pressure (Grt-Cpx-Pl) granulite field. The compositional segregation resu lted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gnei ssic texture by processes such as these may be the inevitable result of iso baric cooling of igneous rocks within a tectonically active deep crust.