scholarly journals Characteristics and field relation of Ulleri Augen Gneiss to country rocks in the Lesser Himalaya: A case study from Syaprubesi-Chhyamthali area, central Nepal

2019 ◽  
Vol 58 ◽  
pp. 89-96
Author(s):  
Jharendra K.C. ◽  
Kabi Raj Paudyal
Keyword(s):  
Geological Mapping ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Nepal Himalaya ◽  
Field Relation ◽  
Central Nepal ◽  
Thrust Zone ◽  
Augen Gneiss ◽  
Host Rocks

The distribution of Ulleri Augen Gneiss and its origin in the Lesser Nepal Himalaya adjacent to the Main Central Thrust zone is stilla debate among the geo-scientists. Geological mapping was carried out along the Syaprubesi-Chhyamthali area of central Nepal with the aim to study the field relation, distribution, deformation and metamorphism of the Ulleri Augen Gneiss. During mapping, close traverses were set to observe the field relation and a number of systematic samples were collected for analysis of composition and texture. Some preliminary findings were obtained related to its geological position and distribution. This gneiss is hosted within the Kuncha Formation, the oldest unit of the Nawakot Group in the Lesser Himalaya. It has been evolved within this unit as a tabular form in some places and lenses in other places. It shows both concordant (i.e., sill type) and discordant (i.e., dike type) relationship with the host rock. It is characterized by augen-shaped porphyroblasts of K-feldspar and S-C mylonitic texture showing top to the SW sense of shear. The S-C structures and lineated textures shown by the minerals are associated with the shearing caused by the movement along the MCT during the syn-MCT metamorphic deformation. It is characterized in different types of lithologies such as augen gneiss, banded gneiss and two-mica gneiss. An attempt is made to explain the petrological characteristics and field relation of the Ulleri Augen Gneiss with the host rocks along with structural aspects. Based on the field relation and texture analysis, the evolution of the protolith of this Ulleri Augun Gneiss can be interpreted as a multi-story emplacement within the host rocks during and immediately after the sedimentation.

scholarly journals Geology, geochemistry, and radiochronology of the Kathmandu and Gosainkund Crystalline nappes, central Nepal Himalaya

2001 ◽  
Vol 25 ◽  
Author(s):  
Santa Man Rai
Keyword(s):  
Granulite Facies ◽  
Tectonic Block ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Nepal Himalaya ◽  
Magmatic Origin ◽  
The North ◽  
Multidisciplinary Study ◽  
Central Nepal ◽  
Augen Gneiss

A multidisciplinary study was carried out in the Lesser Himalaya (LH), the Kathmandu Crystalline Nappe (KCN) and the Gosainkund Crystalline Nappe (GCN) in central Nepal Himalaya. Two principal deformations are recorded in both the crystalline nappes and the Lesser Himalaya: ductile, syn-MCTor syn-MT metamorphic deformation marked by microstructures (stretching lineation, S-C structures, and isoclinal folding) and post-MCT/or post-MT metamorphic deformation recorded by a major EW-directed Likhu Khola anticline and by NNE-SSW-directed folds. The Upper Lesser Himalayan rocks close to the Main Central Thrust (MCT) record syn-MCT metamorphic conditions at 750 MPa and 566 °C. The rocks of the KCN record P-T condition from 900 to 720 MPa and 700 to 484 °C, while the GCN rocks were equilibrated at upper amphibolite- to granulite-facies conditions from 890 to 583 MPa and 754 to 588 °C. The P-T conditions and field observations exhibit well-preserved inverted metamorphism between the Upper Lesser Himalaya and the Gosainkund Crystalline Nappe. The augen gneisses from the GCN yielding 486±9Ma U-Pb zircon age and the granites of similar age in the KCN bear similar petrographic and geochemical characteristics and suggest a similar magmatic origin although they belong to different tectonic units. The chemical analyses of the Proterozoic Ulleri augen gneiss of the LH and the granites of the KCN fall within the same compositional field, indicating a magmatic origin of these augen gneisses. 40Ar/39Ar datings on muscovite indicate cooling ages younging systematically from south to north: 22 to 14 Ma in the KCN, 16 to 5 Ma in the GCN, and 12 to 6 Ma in the LH. This systematic younging of muscovite ages does not have any correlation with the present elevation, lithology and tectonic unit and is interpreted as a result of the exhumation of the rock units on the Main Himalayan Thrust (MHT) ramp situated to the north of Kathmandu Valley. Both the KCN and the GCN record a late emplacement history, but the KCN was exhumed earlier than the GCN. The two crystalline nappes presently form a single tectonic block, and the combined uplift of the two nappes occurs on a ramp of a major decollement developed in the upper part of the Indian crust.

scholarly journals Early Proterozoic granite of the Taplejung Window, far eastern Lesser Nepal Himalaya

2003 ◽  
Vol 28 ◽  
Author(s):  
B. N. Upreti ◽  
S. M. Rai ◽  
H. Sakai ◽  
D. R. Koirala ◽  
Y. Takigam
Keyword(s):  
Alkali Feldspar ◽  
Main Central Thrust ◽  
Triangular Diagram ◽  
Nepal Himalaya ◽  
Central Nepal ◽  
Marginal Part ◽  
Normative Quartz ◽  
Augen Gneiss ◽  
Bottom To Top ◽  
Far Eastern

The Lesser Himalayan Sequence of the Taplejung Window in the far eastern Nepal Himalaya can be divided into Taplejung Formation, Mitlung Augen Gneiss and Linkhim Schist (from bottom to top respectively). The window is a large domal shaped anticline plunging to the east. Two-mica granite bodies (Amarpur Granite, Kabeli Khola Granite and Tamor River Granite) have intruded the metasediments of Taplejung Formation. The granite bodies are discordant to subconcordant in relation to the country rocks. Quartz, alkali feldspar, plagioclase, muscovite, biotite and tourmaline are the main constituent minerals of the granite. Generally, the core of granite bodies is undeformed, whereas the marginal part is gneissfied with S-C mylonitic texture showing the top to south sense of shear. This sense of shear is related to the movement along the Main Central Thrust (MCT). All the samples from the granitic bodies fall under the granite field in the normative quartz-alkali feldspar-plagioclase (QAP) triangular diagram. The mineral composition shows that the granite is peraluminous in nature. The Kabeli Khola Granite has yielded a 40Ar/ 39Ar muscovite age older than 1.6 Ga indicating its magmatic age. The granites of the study area can also be correlated with the 1.8 Ga Ulleri type augen gneiss of central Nepal.

scholarly journals Distribution of boron in the rocks of central Nepal Himalaya

2003 ◽  
Vol 28 ◽  
Author(s):  
Santa Man Rai
Keyword(s):  
Partial Melting ◽  
Boron Content ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Nepal Himalaya ◽  
Metasedimentary Rocks ◽  
Central Nepal ◽  
Very High

Boron content in the rocks of central Nepal Himalaya depends upon the lithology and the grade of metamorphism. The concentration of boron is abundant (up to 322 ppm) in the metasedimentary rocks of the Lesser Himalaya. There seems to be a rather good correlation between the boron content in the rocks and the grade of metamorphism. The boron content progressively increases from chlorite to garnet isograds, then it systematically decreases in the staurolite±kyanite, kyanite and sillimanite isograds, respectively. This trend may be related to the inverse metamorphism associated with movement along the Main Central Thrust. The Manaslu leucogranite contains very high amount of boron (950 ppm). The enrichment of boron in this rock may be due to the release of boron from the Lesser Himalayan rocks during the partial melting of the Higher Himalayan Crystallines (Tibetan Slab) as a result of the movement along the MCT. Tourmaline from the Manaslu Granite is also highly rich in boron (8460 ppm).

Assessing Quaternary reactivation of the Main Central thrust zone (central Nepal Himalaya): New thermochronologic data and numerical modeling

Geology ◽  
2009 ◽  
Vol 37 (8) ◽  
pp. 731-734 ◽  
Author(s):  
X. Robert ◽  
P. van der Beek ◽  
J. Braun ◽  
C. Perry ◽  
M. Dubille ◽  
...  
Keyword(s):  
Numerical Modeling ◽  
Main Central Thrust ◽  
Nepal Himalaya ◽  
Central Nepal ◽  
Thrust Zone

scholarly journals Petrographic records of two metamorphic events in the Lesser Himalayan metabasites, Modi Khola section, central Nepal

1970 ◽  
Vol 11 ◽  
pp. 5-12
Author(s):  
Lalu Paudel
Keyword(s):  
Metamorphic Grade ◽  
Classical Concept ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Himalayan Orogeny ◽  
Central Nepal ◽  
Ti Oxides ◽  
Clastic Sediments ◽  
Host Rocks ◽  
Good Agreement

Metabasites of the Lesser Himalaya along the Modi Khola valley in central Nepal are the supracrustal dikes and sills of basic igneous rocks emplaced in the clastic sediments and later metamorphosed together with the host rocks. They contain almost a constant metamorphic mineral assemblage of Ca-amphiboles+plagioclase+biotite+quartz±epidote±chlorite+(Fe-Ti oxides). Amphiboles occur in the form of porphyroblast and recrystallized matrix. The porphyroblasts are zoned with actinolitic cores and hornblende rims. The recrystallized matrix ranges in composition from actinolite in chlorite zone to hornblende in biotite and garnet zones. The cores of porphyroblasts are pre-kinematic and were probably formed prior to the Tertiary Himalayan orogeny. The rims of porphyroblasts and matrix amphiboles are syn-kynematic and were formed during the Upper Main Central Thrust activity in the Tertiary. The systematic changes in amphibole compositions as well as textural characteristics of metabasites are in good agreement with the classical concept of increasing metamorphic grade and intensity of deformation structurally upwards towards the Upper Main Central Thrust in the Lesser Himalaya.           doi: 10.3126/bdg.v11i0.1428 Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 11, 2008, pp. 5-12

scholarly journals Thin-Skinned Tectonics of the Tansen-Pokhara Section, Central Nepal Himalaya

2015 ◽  
Vol 26 ◽  
pp. 15-28 ◽  
Author(s):  
Lalu Prasad Paudel
Keyword(s):  
Middle Miocene ◽  
Field Survey ◽  
Early Pleistocene ◽  
Tectonic Unit ◽  
Lesser Himalaya ◽  
Duplex Structure ◽  
Main Central Thrust ◽  
Nepal Himalaya ◽  
Central Nepal ◽  
Main Boundary Thrust

Geological field survey and structural analysis were carried out in the Tansen-Pokhara section of central Nepal in an attempt to unravel the thin-skinned tectonic geometry of the Lesser Himalaya. The Lesser Himalaya in the area forms a foreland-propagating duplex structure, each tectonic unit being a horse bounded by imbricate faults. The Upper Main Central Thrust and the Main Boundary Thrust are the roof and floor thrusts, respectively. The Bari Gad-Kali Gandaki Fault is an out-of-sequence fault. The Pindi Khola Fault is an antithetic back-thrust developed on the hangingwall of the Bari Gad-Kali Gandaki Fault, and the Kusma Fault is a splay-off of the Phalebas Thrust. Deformation of the Lesser Himalaya occurred in distinct three phases namely pre-Himalayan, Eohimalayan and Neohimalayan. The duplex structure was formed in the Neohimlayan stage in the period between Middle Miocene and Early Pleistocene. J. Nat. Hist. Mus. Vol. 26, 2012: 15-28

scholarly journals Tourmaline chemistry in the Miocene and Paleozoic granites, Central Nepal Himalaya

2004 ◽  
Vol 29 ◽  
Author(s):  
Santa Man Rai
Keyword(s):  
Fluid Phase ◽  
Compositional Variation ◽  
Nepal Himalaya ◽  
Central Nepal ◽  
Tethys Himalaya ◽  
Higher Temperature ◽  
Lower Temperature ◽  
Augen Gneiss ◽  
Almost All ◽  
Host Rocks

Tourmalines are found in almost all the formations, from the Lesser Himalaya to the Tibetan-Tethys Himalaya. They are equally found in the Miocene and Paleozoic granites in Central Nepal. Aplite-pegmatite dykes and Miocene granite from the Higher Himalaya, Tibetan-Tethys Himalaya also contain the tourmaline. The chemical composition of tourmalines from the Miocene granite, Paleozoic granite, and aplite pegmatites is presented in schorlite (Fe)-dravite (Mg)-elbaite (Al) diagram. The tourmalines from the granites are schorlitic (rich in Fe) in composition. No compositional variation between rim and core of the tourmaline in augen gneiss of the Formation III of the Higher Himalaya could correspond to the recrystallization of the tourmaline during the Himalayan metamorphism. The tourmalines from Manaslu and Chhokang granites can be evolved from Mg rich composition in higher temperature towards the Fe rich in lower temperature during the crystallization reflected by the XFe variation. Similarly, the composition of tourmalines is found to evolve from dravite (Mg rich) in the aplite-pegmatite dykes of the Higher Himalaya to schorlite (Fe rich) and elbaite (Al) in the aplite-pegmatite dykes of the Annapurna Formation of the Tibetan-Tethys Himalaya and Manaslu granite. The aplite-pegmatite dykes intruded into the pelitic rocks contain Mg rich tourmalines while Fe rich tourmalines are found in calcareous host rocks. So, the composition of the tourmalines of aplite-pegmatites can not be controlled by the composition of host rocks, but it is indirectly controlled by the composition of fluid phase of magma.

scholarly journals Metabasites petrology and P-T evolution in the Lesser Himalaya, central Nepal

2011 ◽  
Vol 42 ◽  
pp. 21-40
Author(s):  
L. P. Paudel ◽  
T. Imayama ◽  
K. Arita
Keyword(s):  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Himalayan Orogeny ◽  
Central Nepal ◽  
Inverted Metamorphism ◽  
The Matrix ◽  
T Path ◽  
Garnet Zone ◽  
Host Rocks ◽  
First Time

Petrological study was carried out for the first time on the metabasites of the Lesser Himalaya in central Nepal. The metabasites are mostly tholeiitic basalts emplaced in the elastic sediments as supracrustal dikes and sills, and later metamorphosed together with the host rocks. They contain almost a constant mineral assemblage of Ca-amphiboles + plagioclase + biotite + quartz ± epidote± chlorite + (Fe-Ti oxides). Amphiboles in the form of porphyroblasts show chemical zonation with actinolite/magnesio­homblende cores, tschermakite/ferro-tschermakite rims, and magnesio-hornblende margins. The cores of porphyroblasts are pre-kinematic and were probably formed prior to the Tertiary Himalayan orogeny. The porphyroblast rims and the matrix amphiboles are syn-kinematic and were formed during the Upper Main Central Thrust activity in the Tertiary period. The compositions of both the porphyroblast rims and matrix amphiboles change from actinolite in the chlorite zone to magnesium­ hornblende in the biotite zone and totschermakite/ferro-tschermakite in the garnet zone. The systematic changes in amphibole compositions as well as petrographic characteristics of metabasites confirm the classical concept of increasing metamorphic grade structurally upwards to the Upper Main Central Thrust in the Lesser Himalaya. Application of hornblende-plagioclase thermobarometry shows a coherent prograde P-T path in zoned amphiboles. The cores of amphibole porphyroblasts were formed at average peak temperature of ~540"C and at pressure of ~3 kbar. The porphyroblast rims and matrix amphiboles were recrystallized at average peak temperatures of ~570°C in the biotite zone and ~630°C in the garnet zone at pressure of ~6 kbar. The metabasites petrology is in favor of the tectono-metamorphic models that relate the inverted metamorphism with thrusting along the Upper Main Central Thrust and coeval inversion of isoiliem1S. It is suggested that published amphibole cooling ages from the Nepalese Lesser Himalaya based on simples, homogeneous mineralogy should be reinterpreted in view of the presence of polygenetic amphiboles with heterogeneous composition.

scholarly journals Geology and structure of the Pokhara-Piuthan area, central Nepal

1982 ◽  
Vol 2 ◽  
pp. 5-29
Author(s):  
Kazunori Arita ◽  
Daigoro Hayashi ◽  
Mitsuo Yoshida
Keyword(s):  
Metamorphic Grade ◽  
Geological Mapping ◽  
The Other ◽  
Main Central Thrust ◽  
Transitional Area ◽  
Central Nepal ◽  
Other Hand ◽  
Thrust Zone ◽  
Present Area

The geological mapping of the present area revealed that the area is a transitional area between central and western Nepal. The Kali Gandaki - Bari Gad fault which runs obliquely to the general Homalayan trend separates the present area lithostrato- tectonically into the northeast and soutewest units. Both units are in contrast each other in many respects. The northeast unit comprises the Tibetan Tethys, Himalayan gneiss, Main Central Thrust, Baglung and Bihadi zones. The last two are composed of the Midland meta- sediment Group of Riphean to Eocambrian age and autochthonous in character. The Himalayan gneisses are thrust over the Midland meta-sediments by the Main Central Thrust zone forming a klippe in the southwest unit. On the other hand the southwest unit consists mainly of slightly metamorphosed sediments probably of Paleozoic age which are folded and faulted as compared to those of the northeast unit. The metamorphic grade of the latter is obviously higher than the foemer. The present area, particularly the Kali Gandaki. Bari Gad fault is of exteme significance in considering the tectonic flame of the Nepal Himalayas.

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