Strain softening and shear zone localization: an example from the Main Central Thrust in the Mt. Makalu area (Eastern Nepal)

The Main Central Thrust Zone (MCTZ) in Himalaya is one of the largest worldwide shear zones and juxtaposes the medium- to high grade metamorphic rocks of the Greater Himalaya Sequence structurally above the low to mediumgrade metamorphic rocks of the Lesser Himalaya Sequence. We investigate the processes active during the MCTZ evolution in the Makalu area (Eastern Nepal) where Precambrian orthogneiss are ductilely sheared and structural analysis testifies how a progressive transformation from orthogneiss to micaschist, through a transitional zone, is present. Quantitative textural evolution, bulk-rock major and trace element chemical changes and mineral phases chemical changes have been characterized studying representative samples from “pristine” orthogneiss, transitional zone and micaschist. Kinematic indicators point to a top-to-the-south sense of shear. They are represented by S-C fabric, rotated porphyroclasts and mica fishes. Kinematic indicators are progressively much more developed associated with changes of foliation morphology (from a spaced anastomosing schistosity to a continuous) starting from orthogneiss to micaschist. ICP-MS trace elements data confirm the common protolith origin for both the orthogneiss and the micaschist. EPMA studies show a definite increase of phengitic substitution in muscovite, and a change in biotite composition, going from gneiss to micaschist, from Al-high annite to Al-low content phlogopite. Tourmaline in gneiss can be classified as schorlites, with Y crystallographic site fully occupied, and their chemical variability is described by the common (Mg+Fe)2+ + X+←→ Al3++ □ coupled substitution. They are fairly homogeneous, rarely showing thin Fe-rich rims. Tourmalines in micaschist show a sharp zoning, with schorlitic cores, quite similar to tourmaline found in gneisses. Rims have dravitic compositions, with the X site almost fully occupied and a lower total for Y crystallographic site, pointing to the presence of some Li+, accommodated according to the substitution 2Mg2+←→Li++ Al3+. Quantitative modal phase variations have been investigated with image analysis techniques (on BSE-SEM images) pointing to a progressive disappearance of feldspars balanced by increasing of micas. This latter process is assisted by progressive increase of Mg content in white micas, by the transition from annite to phlogopite, and the crystallization of dravitic rims in tourmalines. Microstructural and geochemical analyses support a non-isochemical process of strain softening related to phase transformation from gneiss to micaschist enhanced by fluids infiltration and Mg metasomatism, which progressively favoured the localization of the non-coaxial deformation within the MCTZ.