GEOLOGY OF HIGHER HIMALAYA

 

In Nepal, the Higher Himalaya is bounded between north of the Lesser Himalayan zone separated by the Main Central Thrust (MCT and south of the Tibetan-Tethys Himalaya separated by the South Tibetan Detachment System (STDS), consisting of high-grade crystalline rocks including various kinds of gneiss, schist and migmatite extends continuously along the entire length of the country.

•Topographically, in contrast to the geologically defined Higher Himalayan Zone, most of the highest peaks of the High Himalaya in Nepal (33 to 4500 m elevation) including Everest, Manaslu, Annapurna, Dhaulagiri, Kanjiroba, Saipal and Api, are capped with Tibetan Tethyan Zone sediments and the Tertiary leucogranites

•Along the Himalayan range in many areas the Higher Himalayan Zone is separated from the Tethyan sedimentary rocks by intervening bodies of Tertiary leucogranite forming the Higher Himalayan Granites.

 

These granitic bodies obscure the nature of the contact between the rocks of the Higher Himalayan Zone and the Tethys sedimentary rock zone. In the past it was generally considered that the contact between the Higher Himalayan gneisses and the Tethyan rocks was gradational and showed no structural break

•More recently a normal fault system, the Southern Tibetan Detachment Fault System (STDFS) has been recognised. That fault separates the rocks of the two zones (Pêcher, 1991; Burchfiel et al., 1992; Hodges et al, 1992; Brown and Nazarchuk, 1993; Coleman, 1996).

•On the other hand, where the Higher Himalayan Crystalline rocks occupy thrust sheets, as in Eastern Nepal, the width reaches to tens of kilometers.

•This crystalline unit in Nepal extends continuously along the entire length of the country. The north south width of the unit varies from place to place.

 

 

In Nepal Himalaya, this crystalline unit has been variously named by different authors.

•Gansser (1964) used the term “Central Crystallines” for this unit as was first used by Heim and Gansser (1939) in Kumaon.

•In Eastern Nepal, Lombard (1953, 1958) used the term Dalle du Tibet (Tibetan slab) for this unit and later the French workers retained the term and used it along the entire Himalayan range including Nepal Himalaya (Le Fort 1975, 1989; Bordet et al. 1972, Pecher, 1977; Brunel et al. 1979).

•Hagen (1969) has included this unit mainly into his Khumbu nappe and partly upper units of his Kathmandu nappe in Eastern Nepal and root zones of Kathmandu nappes in central and Western Nepal.

•In western Nepal, Frank and Fuchs 1970, Fuchs and Frank, 1970, have described this unit as Upper Crystalline Nappes.

•Japanese workers (Hashimoto 1973; Arita 1983) have included the unit into their Himalayan Gneiss Zone or Himalayan Gneiss Group (Kano 1984) lying above and to the north of their MCT II or Upper MCT, and below the Tethyan sediments.

•Schelling (1989) in eastern Nepal divided this unit into his Junbesi Group, Rolwaling (Khumbu) migmatites and Rolwaling paragneisses. In the present book, we have named this unit as Higher Himalayan Crystalline Zone.

 

 

Far Western Nepal

•The Higher Himalayan Crystalline Zone along the Mahakali River section in Far Western Nepal begins with a sharp contact (thrust contact) with the amphibolites. The section begins with garnet bearing biotite-muscovite gneiss (Arita 1984, Bashyal 1986) believed it as a transitional contact.

•Zone a

•This unit is composed micaceous, garnet-bearing, biotite–muscovite gneiss of migmatitic gneiss of irregular banding. Under the microscope, the gneiss exhibits undulatory quartz, twinned oligoclase, highly pleochroic biotite, bluish gray tourmaline, and less abundant muscovite.

••Zone b

•The migmatitic gneisses are followed upwards by dark garnetiferous biotite gneisses, containing quartz and oligoclase–andesine, garnet, and apatite.

•Zone c

•The Zone c is composed of migmatitic gneisses, consisting of gray aplite bands. They are succeeded upwards by thin bands of lime-silicate rocks. Fine red garnets appear in the quartzose aplite bands.

 

•Zone d

•This zone consists of kyanite–garnet–biotite gneisses at the base and is overlain by a thick quartzite succession. The kyanite forms large stalks, whereas biotite appears as large brown sheets, and subordinate plagioclase.

•Zone e

•This zone is characterized by the increasing abundance in number as well as size of pegmatite and aplite dikes. There also appear some large granite dikes and smaller stocks of a fine-grained muscovite granite. Such granite bodies are well exposed around Api Himal.

•Zone f

•This zone extends almost throughout the western Higher Himalaya. In this zone, highly metamorphosed lime-silicate marble bands alternate with finely stratified psammitic gneisses.

•

•Zone g: Budhi Schists

•The feature of this zone is the occurrence of schist. They are called the Budhi Schists. The Budhi Schists include large biotite porphyroblasts with sieve-like quartz inclusions, and some of the biotite flakes lie obliquely to foliation, indicating their subsequent rotation.

•Zone h: Garbyang Formation

•The low-grade metamorphic rocks of the Garbyang Formation overlie the Budhi Schists. They are very fine-grained, slightly sericitic, arenaceous to calcareous phyllites with dolomitic and green chloritic bands. This formation is not represented in the Mahakali River section.

 

 

 Main Central Thrust Zone

•Heim & Gansser 1938 defined the MCT in Kumaon based on the difference in metamorphic grade between low to medium-grade rocks of the Lesser Himalaya and higher-grade rocks of the Greater Himalaya. However, the fault originally defined by Heim & Gansser 1938 is not the MCT, but a fault within Lesser Himalaya rocks (Valdiya 1980; Ahmad et al. 2000).

•This misidentification symbolizes the challenge that workers have faced in locating the MCT. The metamorphic grade within the Lesser Himalaya increases towards the MCT and at higher structural levels.

•In central Nepal, the metamorphic grade increases from low (chlorite + biotite) to medium (biotite + garnet + kyanite + staurolite) towards the MCT over a north-south distance.

•The highest-grade rocks (kyanite and sillimanite gneisses) are found within the MCT shear zone, i.e. upper Lesser Himalaya. Arita (1983) places two thrusts (MCT I and MCT II) on each side of the MCT shear zone.

 

 

 

Mugu Karnali and Upper Thulo Bheri (Kanjiroba Himal) Section

•between Mugu Karnali River and Kaligandaki River (Fuchs 1967, 1977, 1980, Fuchs and Frank 1970; Frank and Fuchs 1970).

•the Higher Himalayan Crystalline Zone belongs to the Upper Crystalline Nappe of these authors.

•Their Lower Crystalline Nappe corresponds to the MCT zone of Japanese workers Arita 1983).

 

In Mugu Karnali section, Fuchs (1977) places the MCT above his Lower Crystalline Nappe near Khumpa Village.

•The following description of the geology of this area is after Fuchs (1977).

•The exposed base of the gneiss zone consists of well-layered, fine- to medium-grained, banded two-mica gneisses.

•Above these gneisses lie thicker layers of light to grey quartzite and quartzite gneisses followed by garnet-mica-schists to paragneisses.

 

 

Thuli Bheri River section

•In the east along upper Thulo Bheri section or sough of Kanjiroba Himal, Fuchs and Frank (1970) have described the Higher Himalayan Crystalline Zone (Upper Crystalline Nappe) not much unlike the Mugu section.

•Here too the zone ranging in thickness between 5-10 km consists of coarse grained garnet-kyanite gneisses ( staurolite or sillimanite) passing into augen granite-gneiss, ortho- to migmatitic gneisses.

•Calc-gneisses, marbles, and calc mica schists are observed in the upper section.

•Metamorphism reaches up to the amphibolite facies (up to the highest sillimanite substage) and decreases from the top of the section, and finally give way to non- to slightly metamorphosed sediments of Tibetan-Tethys zone.

 

 •Central and Western Nepal

•Kaligandaki Section

•Bordet et al. (1972) for the first time tried to subdivide the Higher Himalayan Crystalline Zone (their Tibetan Slab) and identified 4 units as

•Kyanite-sillimanite gneisses

•Pyroxenitic gneisses

•Banded gneisses

•augen gneisses in ascending order.

 

 

 

Formation I

•The lowest rock succession, appearing north of Dana, is included in Formation I.

•It begins with banded mylonitic quartzites liberally alternating with gneisses, which are intermittently highly micaceous.

•There is also a rare passage of impure (cipolin-like) marbles with the above rocks, while moving north (towards the top). The lenses of segregated quartz are frequent within this succession (Le Fort 1975).

•The previous sequence is followed up-section by the kyanite zone of about 1,400 m in thickness. At its lower levels, the quartzites rapidly disappear together with themicaceous horizons, and there appear large garnets with kyanite.

•Importantly, sillimanite occurs towards the top of this succession.

 

 

 

Formation II begins with a zone of calcareous gneisses, including white, yellow, and blue impure marbles (about 2,000 m thick), which follow the kyanite zone.

•The principal minerals found here are calcite, quartz, hornblende, plagioclase, epidote, almandine, phlogopite, and diopside.

•There is also a small amount of scapolite, anorthite, and vesuvianite.

•Metasomatism is observed near the contact of gneiss and marble.

•A succession of banded gneiss (about 1,600 m) appears above the calcareous gneisses.

•It is characterized by many, very fine and perfectly regular, dark bands of biotite, alternating with less than a centimeter-thick, light bands.

•The calcareous nature of the previous succession does not abruptly disappear, but persists in the banded gneisses as thin carbonate bands. However, the carbonates seldom occur in the overlying succession.

 

 

Formation III commences with a succession of augen gneiss. A minor migmatite zone is observed upstream from Ghasa. Within the pegmatites in the augen gneiss, discovered a number of ruby crystals (1–2 mm in diameter) as well as a few pink andalusite grains in some other pegmatites.

•The final gneisses and calcareous schists constitute a zone of about 300 m in thickness.

•The contact between the augen gneisses and the succession of sandstone, calcareous sandstone, and calcareous schist is perfectly concordant.

• It is well exposed west of Dhampu, above a large alluvial cone, and the contact dips at an angle of about 45° due north.

•Farther north, there are two prominent alternating bands of banded gneiss, within the zone of calcareous gneisses.

•This formation gradually passes upward to the limestone of the Tibetan-Tethys Himalaya.

 

 

 

 

 

 

Dhaulagiri-Annapurna-Manaslu Himalaya, Western Region, Nepal

•The Lower Greater Himalayan Sequence (GHS) has an approximate structural thickness of 4800–6000 m and consists of medium- to high-grade metasedimentary rocks with protoliths equivalent to the Nawakot Complex of the Lesser Himalayan Sequence (Larson and Godin, 2009).

•The lower portion of the Lower GHS comprises quartzites, semipelites and phyllites and an augen-orthogneiss layer (Bouchez and Pêcher 1981). The upper portion of the Lower GHS contains massive quartzites, marbles, dolomitic marbles, metacarbonates and schists (Colchen et al. 1986).

 

 

 

 

The Upper Greater Himalayan Sequence has an approximate structural thickness of 6500–7200 m in the Kaligandaki River and Modi Khola valleys and 8500–9000 m in the Marsyandi River Valley and consists of kyanite- to sillimanite-grade mid-crustal metasedimentary rocks, migmatites and leucogranites (Colchen et al. 1986).

•The Upper GHS of the Dhaulagiri-Annapurna-Manaslu Himalaya is divided into three units described from bottom to top as; Unit I, consisting of kyanite to sillimanite-grade schists, paragneisses and migmatites; Unit II, consisting of diopside-scapolite-grade calc-silicate gneisses and pelitic gneisses and Unit III, consisting of sillimanite-grade orthogneisses, schists and migmatites (Colchen et al. 1986).

 

 

 

 

 

•Alampu Schist

•The Alampu Schists are about 6,000 m thick and consist of well-foliated, interlayered biotite schist, metaquartite , augen gneiss and schist, is a southern extension of the Higher Himalayan Thrust.

•The distinguishing characters of the Alampu Schists are presence of kyanite and garnet in schist.

•The kyanite-hornblend bearing schists are found in some localities deu to the high-pressure metamorphic environment.

•The Alampu Schist grade upwards into the Rolwaling Migmatite.

•The lower boundary of the Alampu Schist is the MCT that separates the Higher Himalayan Crystallines have been thrust over the Lesser Himalayan metasediments in the Rolwaling-Lapchi area.

•The age for the paragneiss is considered as the Pre-cambrian to Cambrian-Ordovician.

 

•Rolwaling Migmatite

•The Rolwaling Migmatites are 5,000 to 6,000 m in thick and predominantly sillimanite-bearing migmatic orthogneiss (primarily granitic composition) with intercalated paragneiss and amphibolite consisting of muscovite, biotite, quartz, feldspar and tourmaline.

•Tourmaline is common constituent throughout the unit. Granitic and pegmatitic veins are common and usually show cross-cut migmatitic structures.

•It is probable that the migmatite of the Higher Himalayan Thrust sheet is the source rock for the Higher Himalayan leucogranite.

•The Rolwaling Migmatites are generally foliated, the foliation being formed by both the alignment of biotite and muscovite flakes.

•The age for the paragneiss is considered as the Pre-Cambrian to Cambrian.

 

•Rolwaling Paragneiss

•The Rolwaling Paragneiss bodies are sharply contact above with the intrusive Rolwaling Granite and below by gradational contact with the underlying Rolwaling Migmatites, with approximately thickness of 6,000 m.

•The Rolwaling Paragneisses consist primarily silliminate bearing, biotite rich paragneiss intercalated with metaquartzite, biotite schist, calc-slicate gneiss, marble, gneiss granitic augen gneiss and granitic gneisses.

•The biotite rich augen gneiss of the Rolwaling Augen gneiss included in the Rolwaling paragneisses is about 600 m in thickness with K-feldspar in augen as much as 10 cm long.

•The pegmatite veins and xenoliths are common in this unit. This unit has extensively intruded by granitic and pegmatitic veins as much as several meters thick, which are general, deformed.

•The contact between the Rolwaling Paragneiss and the underlying Rolwaling Migmatite is gradational.

•The age for the paragneiss is considered as the Pre-Cambrian to Cambrian.

 

Rolwaling Granite

•The Rolwaling Granites are medium-grained leucogranites, consisting of biotite, muscovite, quartz, and feldspar with tourmalines as a common constituent situated at the top of the Higher Himalayan Thrust sheet and exposed in the peak of the Rolwaling Himalaya.

•These granites consist of tourmaling bearing and generally undeformed in nature.

•In the Rolwaling valley granites have a maximum aerial extension of 2,500 to 3,000 m in thickness and shows intrusive relationships with underlying Rolwaling Paragneiss.

•The lower part of the Rolwaling Granite bodies or near about the contact with Rolwaling Paragneisses are characterized with xenoliths.

•The age of the granite intrusion is in Miocene.

 

 Tertiary Leucogranites

•In the Swat and Nayan region (west of Nanga Parbat in the Northwest Himalaya of Pakistan), post-collisional leucogranites range in age from 50 to 35 Ma (Zeitler and Chamberlain 1991).

•These granites could have resulted from melting of the lower crust during tectonic thickening of the Indian plate.

•In the Higher Himalayan sequence of Nepal, Early to Middle Miocene leucogranites (Heim and Gansser 1939; Gansser 1964; Le Fort 1975; Hagen 1969) occur as several plutons, very many dikes, and veins.

•These igneous rocks are mainly concentrated towards the top part of the migmatitic gneisses in the Tibetan slab (Le Fort et al. 1987). They are found in the Annapurna–Manaslu region, where the floor of these intrusives is generally concordant with the underlying rocks.

•This relationship is well seen in the Manaslu granite and its eastern extension called the Chhokang.

 

 

SOURCE

Geology of Nepal Himalaya, Prakash Das Ulak (2016).