The geology between Khimti Khola and Likhu Khola valleys: a field trip along the Numbur Cheese Circuit (central-eastern Nepal Himalaya)

This paper describes a 11-days field trip along the Numbur Cheese Circuit (NCC), running along the Khimti Khola and Likhu Khola valleys in central-eastern Nepal Himalaya. The purpose of this guide is to introduce the most interesting geological aspects of this sector of the Himalaya through observations of selected outcrops, samples and view-points along the NCC. The NCC trek crosses a thick portion of the Greater Himalayan Sequence (GHS), divided into a lower (GHS-L) and an upper (GHS-U) portion. Along this path, the GHS-L consists of Grt ± St ± Ky -bearing two-micas micaschist, associated to metric to decametric -thick levels of two-micas quartzitic gneiss, Grt + Zo granofels and small lenses of impure marble. In its lower structural levels, the GHS-L shows intercalations of phylladic Ank-bearing micaschist (+ greenish Bt), whereas in the uppermost structural levels evidence of partial melting are locally observed. Upwards, the GHS-U (corresponding to the Higher Himalayan Crystallines) consists of Grt + Bt + Kfs + Ky/Sil anatectic paragneiss (i.e. Barun Gneiss) and Kfs + Bt + Sil ± Grt paragneiss (i.e. Black Gneiss) characterized by Qtz + Sil nodules. Metric to plurimetric -thick layers of calc-silicate granofels and impure marble are locally intercalated. Networks of leucogranitic and pegmatitic dikes occur in the upper structural levels of the GHS-U, and are spectacularly exposed in the highest peaks. From a structural point of view, in the NCC area, the GHS is dominated by lithological boundaries and foliations dipping towards the north. The Main Central Thrust Zone, namely the shear zone related to the exhumation of the high-grade GHS on the lower Lesser Himalayan Sequence (LHS), is roughly centered on the intensively top-to-the-S sheared GHS-L. Locally, evidences of late top-to-the-NE extension has been observed in the GHS-L.