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) vi Si iv = Al vi Al iv ) between the ideal muscovite and celadonite end-members, which controls the chemistry of phengites, is thought to be particularly sensitive to pressure at low-temperature conditions and serves as a qualitative geobarometer (Guidotti and Sassi 1986). Although, the b-spacing values of white micas have been widely used to estimate the palaeopressure in low-grade terrains all over the world, its application in the Himalaya is relatively rare. In the present study, we present the b-spacing values of white mica across Pokhara-Butwal road and Kali Gandaki valley sections of the Lesser Himalaya in central Nepal, and discuss their tectono-metamorphic implications. Geological setting The Lesser Himalaya in the Pokhara area is divided into four tectonic units from south to north: Parautochthon with Palpa Klippe lying between the Main Boundary Thrust (MBT) and the Bari Gad-Kali Gandaki Fault (BKF), Thrust Sheet I (TS I) situated between the BKF and the Phalebas Thrust, Thrust Sheet II (TS II) bounded by the Phalebas Thrust and the Lower MCT, and the MCT zone bounded by the Lower MCT and the Upper MCT, respectively. The Lesser Himalaya mostly comprises low-grade metamorphic rocks such as slates, phyllites, quartzites, and dolomites (Nawakot Complex) of the Precambrian age. The Nawakot Complex rocks are unconformably overlain by the Gondwana and post- Gondwana sedimentary rocks (Tansen Group) in the Parautochthon (Sakai 1983). Metamorphic grade gradually changes from diagenesis to lower anchizone in the Parautochthon to upper anchizone in the TS I, epizone in the TS II and garnet zone in the MCT zone (Paudel 2000). Sampling and analytical techniques A total of 320 pelitic rocks with unique assemblage "Mica+Chl+Ab+Qtz" (Chl-zone) and lacking detrital white mica were used to measure b-spacing values. A few samples from the northern part of the TS II are from Bt-zone and samples from the MCT zone belong to Grt-zone. Measurement was done on <2µm powder fraction of each sample. Diffractometer setting was constant for all the samples (Rigaku Geigerflex diffractometer, Cu cathode, Ni filter, 35 kv tube voltage, 20 mA current, time constant=2 sec, scatter slit=1° , receiving slit=0.3 mm, divergence slit=1° ). The 63-59.5° 2θ range was scanned at 0.25 2e/min, and b was determined from (060) peak (approx. 61.5° 2θ) using the (211) quartz reflection (approx. 60° 2θ) as an internal standard. Mean b- spacing values were calculated from five repeated measurements for each sample. Results and implications The relationship between the b-spacing values with the composition of white mica was accessed by compositional analysis of white mica in the same samples. A fairly good positive linear correlation was found between the two (Figure 1). This indicates that the b-spacing values serve as indirect measures of phengite content of white mica in the Lesser Himalaya and hence the pressure condition. The plots of b-spacing values versus the whole rock

The b-spacing values of white mica from low-grade metapelites of central Nepal Lesser Himalaya and their tectono-metamorphic implications