Microstructural, microchemical and geochronological investigation of two opposite, crustal-scale shear zones in the Garhwal Himalaya (NW India)

Constraining the timing of the activity of the Main Central Thrust zone (MCTz) and the South Tibetan Detachment System (STDS) is one of the major tasks to understand Himalayan tectonics. These shear zones are crustal scale ductileto-brittle structures running all along the belt. The MCTz and STDS are the lower and upper boundary, respectively, of the Greater Himalayan Sequence (GHS), the metamorphic core of the orogen. In some areas the MCTz is a km-wide shear zone, whereas in other areas it is bounded by two distinct thrusts. In Garhwal (NW India), the structurally lower and upper MCTz boundaries are the Munsiari and Vaikrita Thrusts (Valdiya, 1980), respectively. We constrain the time of activity of the Vaikrita Thrust by 39Ar-40Ar dating of biotite and muscovite from two garnet-bearing mylonitic micaschists and one garnet-staurolite-bearing quartzite. Microstructural observations reveal at least three different mica growth stages with muscovite being larger and more abundant than biotite. Mica-1 highlights a relict foliation, only locally preserved, at high angle with respect to the main mylonitic one; mica-2 grew along the main mylonitic foliation; its small flakes are often shredded; mica-3 forms coronitic structures around garnet porphyroclasts. EPMA on muscovite shows restricted chemical variation, with Ti, Mg and Fe contents being systematically lower in muscovite-3. Biotite from micaschists shows no significant chemical variations, whereas in quartzite biotite-2 shows two distinct compositional clusters. Rocks were crushed and sieved. Biotite and muscovite were separated by gravimetry and extensively handpicked. Biotite step ages range between 8.6 and > 12 Ma, and muscovite step ages between 3.6 and > 7 Ma. As all samples are from the same 10 m wide outcrop, "cooling ages" should be equal. Instead, their large variations are petrogenetically controlled. Since chloritization is pervasive in all samples, we use the Ca/K ratio to identify Ar released from mica sensu stricto from the intergrown alteration phases. Furthermore, the Cl/K ratio (Villa et al., 2014) allows us to distinguish mica-3 from mica-2. Handpicking enriched muscovite-3 over muscovite-2. The results are best explained by mica growth along the main foliation around 9 Ma. Coronites formed c. 3 Ma later during retrograde garnet breakdown, as shown by the lack of internal deformation in micas and by microchemical data. The time span for the activity of the STDS is constrained by a little-deformed pegmatitic dyke close to it. Its white mica grew after the ductile deformation and gave a 39Ar-40Ar age > 16 Ma. Thus, the STDS froze c. 7 Ma earlier compared to the main movement phase of the Vaikrita Thrust