Dating protracted fault activities: microstructures, microchemistry and geochronology of the Vaikrita Thrust, Main Central Thrust zone, Garhwal Himalaya, NW India

The timing of shearing along the Vaikrita Thrust, the structurally upper boundary of the Main Central Thrust zone (MCTz), was constrained by combined microstructural, microchemical and geochronological investigations. Three different biotite-muscovite growth and recrystallisation episodes were observed: a relict mica-1; mica-2 along the main mylonitic foliation; mica-3 in coronitic structures around garnet during its breakdown.
Analyses of biotite by electron microprobe show chloritization, and bimodal composition of biotite- 2 in one sample. Muscovite-2 and muscovite-3 differ in composition from each other.
Biotite and muscovite 39Ar-40Ar age spectra from all samples give both inter-sample and intra- sample discrepancies. Biotite step ages range between 8.6 and 16 Ma, muscovite step ages between 3.6 and 7.8 Ma. These ages cannot be interpreted as "cooling ages", as samples from the same outcrop cooled simultaneously. Instead, Ar systematics reflect sample-specific recrystallisation markers. Intergrown impurities were diagnosed by Ca/K ratios. Age data of biotite were interpreted as a mixture of true biotite-2 (9.00±0.10 Ma) and two alteration products. The negative Cl/K-age correlation identifies a Cl-poor muscovite-2 (>7 Ma) and a Cl-rich, post-deformational, coronitic muscovite-3 grown at ≤5.88±0.03 Ma. The Vaikrita Thrust was active at least from 9 to 6 Ma around 600 °C; its movement ceased by 6 Ma.