THE MCT ZONE FROM THE SIKKIM HIMALAYA : AN EXAMPLE OF A RETROGRESSED MYLONITE ZONE

In the Darjeeling-Sikkim Himalaya, the Main Central thrust (MCT) sheet is carried by two distinct faults, MCT 1 and MCT 2. The lower MCT 2 places Greater Himalayan amphibolite grade Paro-Lingtse gneiss on top of Lesser Himalayan greenschist grade Daling metapelites. The MCT2 is folded by the sub-thrust Lesser Himalayan duplex and is exposed at different structural elevations. At Pelling, in Sikkim, the MCT2 zone is exposed as a ~ 325 m thick NW dipping fault zone that exposes ~ 14 m of hanging wall mylonitized Lingtse gneiss.

The Lingtse protolith shows evidence of amphibolite grade plastic deformation features in quartz and feldspar, including subgrains, deformation lamellae and deformation twins. Spektor chord analysis of protolith and hanging wall mylonite zone (HWMZ) samples reveals grain size reduction of ~ 48% in quartz and ~ 68% in feldspar. There are early overprinting microfractures of at least two different types – thinner, closely spaced healed fractures, and later, thicker, widely spaced fractures filled by recrystallized quartz along which feldspar grains are pulled apart. The mylonitic foliation is also overprinted by late-stage fractures which are dominantly transgranular and are more prominent within the lowest ~ 3 m of the exposed HWMZ. Quartz of the HWMZ, lying farthest from the fault, deforms dominantly by dislocation glide (~ 44%) and dislocation climb (~ 46%); feldspars show evidence of subgrain formation and dislocation glide (~ 38%). At the footwall contact, the dominant deformation mechanisms in quartz are intragranular and transgranular fracturing (~ 35%), dislocation glide (~ 40%) and dislocation climb (~25%). Feldspars in the same zone deform by intragranular (~ 50%) and transgranular (~ 35%) fracturing and dislocation glide (~ 15%). Sericitization of plagioclase feldspars within the HWMZ suggests the presence of fluids during deformation, and is strongest close to the fault. Volume percentage plots of minerals within the HWMZ show inverse relationships between modal proportions of quartz + muscovite and plagioclase feldspars, further supporting sericitization during mylonitization. Based on the microstructures, we estimate temperatures of 250 – 350°C within the fault zone, where the amphibolite grade MCT 2 sheet has undergone fluid-assisted retrogression and deformation.