Himalaya-Karakoram-Tibet Workshop 2006
Abstract Submission Template
Geology of the Lesser Himalayan and Higher Himalayan
Crystalline sequences along the Dudh Koshi valley, Eastern
Nepal Himalaya
*Santa Man Rai1, Masaru Yoshida2, Bishal Nath Upreti1,
Prakash Das Ulak1 and Matrika Prasad Koirala1
1Department of Geology, Tri-Chandra Campus, Tribhuvan
University, Ghantaghar, Kathmandu, Nepal
2Gondwana Institute for Geology and Environment, Hashimoto,
648-0091, Japan
*Corresponding e-mail santamanrai@yahoo.com
The 40km sector from Jubing to Gokyo Ri along the
Dudh Koshi valley is occupied by the Lesser Himalayan Sequence
(LHS) in the south and the Higher Himalayan Crystalline
Sequence (HHCS) in the north, with the Main Central Thrust
(MCT) between the above two sequences running at about 2.5km
north from Jubing.
The main rock types in the LHS include phyllite,
metasandstone, Proterozoic granitic augen gneiss, schist, marble
and quartzite. The rocks of the HHCS are amphibolite grade
metamorphic rocks which are intruded by Palaeozoic and Tertiary
granitoids. In the upper section south of Gokyo-Ri, very coarse
grained, highly sheared augen gneisses of about 300m thick are
exposed. This gneiss could be the Cambro-ordovician granite and
are considered to be comparable with the Formation III of the
Tibetan Slab in central Nepal (Le Fort, 1975). The remaining part
of the upper section down to around (1km west of) Panboche are
characterized by the general occurrence of amphibole in biotite
gneisses (Amp-Bt schist) as well as calc gneisses rarely with
marble layers, most possibly referable to the Formation II in
central Nepal (Le Fort, 1975). The middle section surrounding
Namche Bazar is dominated by migmatite with migmatitic biotite
gneiss, augen gneiss, granitic gneiss, all carrying more or less
sillimanite. The lower section, south of Jorsalle is mostly covered
by biotite gneiss with or without garnet and sillimanite.
Tourmaline-bearing biotite-muscovite granite is common
throughout the middle and lower sections, cross cutting the
gneisses and migmatite. Rocks of the middle and lower sections as
a whole are comparable with the Formation I of the west-central
Nepal (Le Fort, 1975).
The gneisses of the HHCS carry dominant foliation and
show a variety of mesoscopic folds. Earliest isoclinal reclined
folds, earlier tight plunging folds, and later open and kink upright
folds are identified. Some of these structures are considered to be
pre-MCT, and some others are found to be syn-MCT deformations.
The SSW and SSE plunging mineral lineation marked by mica and
sillimanite possibly marks the transport direction of the MCT. The
Tertiary tourmaline granite cutting across gneisses suffers upright
folding and carries cleavage structure paralleling the axial surface
of the fold. Analysis of these kinds of relationships is expected to
unravel the tectono-metamorphic evolution of the HHCS.
Reference
1.
Le Fort, P., 1975, Am. J. Sci, 275A, 1-44.
2
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