Metamorphism in Himalaya

A Review of Case Studies
Mid Term Assignment of Geology of Nepal (GEO 513)
Metamorphism in
Himalaya
Submitted By:
Sujan Raj Pandey
Roll No.: 05
M.Sc.1st Semester, 2078
Submitted To:
Dr. Ranjan Kumar Dahal
Associate Professor
Central Department of Geology
Kritipur, Kathmandu

General Outline
Himalaya (Review)
Metamorphism in Himalaya & its types
 Metamorphic Events in the Himalaya
1.Subduction Metamorphism or Pre-Himalaya Metamorphism (M0)
with case studies.
2.Collision Metamorphism of Himalaya with case studies
• Metamorphism above the MCT, in the Higher Himalaya Crystallines (Eo-
Himalayan, Neo-Himalayan & Late-Himalayan Metamorphism and
Polyphase metamorphism)
•Metamorphism below the MCT, in the northern part of Lesser Himalaya
(Inverted Metamorphism & Polyphase Metamorphism)
 Inverted Metamorphism Models
 Conclusion

Himalaya: A Review
Himalaya is the highest and youngest orogenic belt, resulting
from the continuous and still active collision between Indian plate
and Eurasian Plate.
The collision is a product of northward convergence of Indian
Plate, as characterized by
– subduction of Neo-Tethys Ocean.
– subduction of Indian continental margin,
– thickening of Indian margin &
– thrusting and redoubling of Indian crust prior to collision.

Northward drift of India
(Patriat and Achache,
1984)

 The Himalayan orogen is characterized by a north-dipping,
southward-propagating, crustal scale thrust sequence.
 Major thrusts delimit three distinct units that are, from south to
north ;
– the Siwaliks foreland fold-and-thrust belt between the Main
Frontal (MFT) and the Main Boundary (MBT) thrusts;
– the metasediments of the Lesser Himalaya between the MBT
and the Main Central Thrust (MCT);
– the Greater Himalayan crystalline thrust sheet with overlying
Tethyan sediments separated from Tibetan Tethys Himalaya
zone by the extensional South Tibetan Detachment System
(STDS).
– (Gansser, 1964; Le Fort, 1975)

Fig 1 : Tectonic and geological map of the Himalaya
Source: Rasoul Sorkhabi (2010), Himalayan Journal 66

Fig 2 : Geological cross section of the Himalaya.
Source: Rasoul Sorkhabi (2010), Himalayan Journal 66

Metamorphism in the Himalaya & its types
 Metamorphism is the process of mineralogical and structural
changes of igneous , sedimentary , or already metamorposed
rocks in their solid state in response to physical and chemical
conditions.
 In Himalaya, several metamorphic events are overprinted on
the rocks of Lesser Himalayan Sequence and Higher
Himalayan Crystalline.
 Metamorphism types that are common on Himalaya orogeny
are;
 Barrovian metamorphism.
 Regional metamorphism
 Contact metamorphism
 Burial metamorphism
 Prograde to Retrograde metamorphism &
 Famous Himalayan inverted metamorphism.

Barrovian Metamorphism
 Barrovian metamorphism is the most commonly encountered.
It occurs in intense tectonic conditions associated
with Volcanic Arcs, and Major Mountain Building.
 Barrovian metamorphism produces the most common
metamorphic rocks.
 Barrovian metamorphic sequences produces the following
rocks, in order: slate, phyllite, schist, and gneiss.

 If we observe a terrane of increasing metamorphic intensity,
beginning with a mafic parent, like Basalt or Gabbro, the parent
undergoes a systematic sequence of mineralogic and textural
changes as Basalt >> Greenschist >> Amphibolite >> Granulite
 In Barrovian metamorphism there are three facies. Commonly
we call then low, middle, and high grade metamorphism, but
geologically they are the greenschist facies, amphibolite
facies, and granulite facies.
a. Barrovian Metamorphism

 Since the collision at 55 Ma, the shortening due to convergence
produced intra-upper crustal shear zones that thickened the
Indian crust to its present thickness of 70 km (Le Fort, 1975a;
Molnar, 1988; Zhao et al., 1993).
 A main consequence of this thickening is the concentration of
radioactive heat-producing elements (K, U, Th) possibly
responsible for the development of a Barrovian metamorphism
and associated partial melt within the HHC.
 In Central Nepal, Kathmandu region rocks display normal-way-
up Barrovian metamorphism i.e. stratigraphically downward
from unmetamorphosed Devonian limestone, one finds sericite
and chlorite in Tistung Formation, prominent of biotite in
Bhimphedi group and garnet in Raduwa formation suggesting
that metamorphism increases with depth.
Barrovian Metamorphism in the Himalaya

b. Regional Metamorphism
 Large scale metamorphism that occurs during mountain
building.
 This type of metamorphism occurs over large areas that were
subjected to high degrees of deformation under differential
stress.
 The differential stress usually results from tectonic forces that
produce a compression of the rocks.
c. Contact Metamorphism
 Occurs adjacent to igneous intrusions and results from high
temperatures, low pressure metamorphism.
 Associated with the granite intrusion in Himalaya such that
they have metamorphosed the surrounding country rocks ( not
in case of leucogranite).

d. Burial Metamorphism
 The sediments are buried in deep sedimentary basin at
converging plate margins, may attain temperature of several
hundred degrees with high water pressure.
 Himalaya exhibit burial metamorphism in relation to
convergence and subduction of Indian Plate
 The rocks are subject to high pressure and exhibit
glaucophane and blueschist metamorphism of Tethyan
sediments.
 Also, earlier burial metamorphism event are recorded in
Nawakot Group, Lesser Himalaya where in chlorite grade
metamorphism exhibit (mark chlorite grade is also a result of
retrograde metamorphism )

e. Prograde Metamorphism
 It occurs as the temperature and pressure are increased on
the rock.
 As the pressure and temperature increase, a rock of a given
chemical composition is expected to undergo a continuous
series of chemical reactions between its constituent minerals
and any fluid phase present to produce a series of new
mineral assemblages that are stable at the higher pressures
and temperatures.
 Higher Himalayan Crystalline (HHC) and (LH) both show one
prograde metamorphism.
 The grade reaches up to kyanite- sillimanite in HHC while
garnet grade prograde inverted metamorphism in LH.

f. Retrograde Metamorphism
 The process of conversion of high grade metamorphic rocks
(high pressure - high temperature) to the low grade
metamorphic rocks (low pressure-low temperature) is known
as the retrograde metamorphism.
 In Himalaya, presence of biotite in the rims or fracture of
garnet is indicator of retrograde metamorphism. It can be found
along MCT ductile shear zone associated with inverted
metamorphism.
 Also, formation of chlorite in garnet bearing schist(bulk rock)
due to drop of pressure and temperature are also found in
Himalaya.
 It result from late stage retrogression marking the end of ductile
shearing in the MCT zone.

g. Inverted Metamorphism
 High grade metamorphic rocks(high pressure-high
temperature) overlie the low grade metamorphic rocks(low
pressure-low temperature).
 Kyanite-garnet bearing gneiss overlies the garnet biotite
bearing schist.
 In Himalaya , this metamorphism is well known as low grade
rocks(slate, phyllite ,schist ) overlian by high grade
metamorphic rocks(gneiss , migmatite).
Fig 3 : A block of Inverted metamorphism in Himalaya

Geological Division Associated rock and metamorphism Event
TRANS HIMALAYA
[AGE 55 to30 mya ].
upper cretaceous volcanic and granitic
rock , calc alkaline granite, alkaline granite
Magmatism
ITSZ [AGE 55 mya] Tholeiitic volcanic rocks, high pressure
metamorphic rock {Green schist}
Subduction or M0
metamorphism
Tethyan Himalaya [AGE
45 to 35 mya]
granite and gneiss, white leucogranite
Silurian age {muthquartzite}, ophiolite
melanges, Nilgiri limestone, Kamdehan
shale, cretaceous age flysh sandstone
Collision (Eo-
Himalayan or M1 )
metamorphism
The higher Himalaya
[AGE 24 to 17 mya]
central crystalline zone
Proterozoic to Cambrian age igneous and
metamorphic rock, white leucogranite of
Miocene age.
Collision (M1, M2 &
M3 phase)
metamorphism
Lesser or lower
Himalaya [AGE 11 to7
mya]
metamorphosed sedimentary rocks
quartzite, marble, slate, phyllite, schist,
gneiss, volcanic and granitic rocks of
Proterozoic to Cambrian {2000-500 mya}
and low temperature metamorphic rocks
Pre- Himalaya (M0),
Neo Himalaya (M2)
and late Himalaya
(M3) metamorphism.
Sub Himalaya or Siwalik
[AGE 2.6 to recent]
Conglomerate, sand stone and mud stone
of Miocene
Diagenesis
Metamorphic Events in the Himalaya

Metamorphic Events in the Himalaya
 The Himalaya is characterized by a typical backbone of
metamorphic rocks associated with magmatism and burial or
regional, which are mostly dominant between the STDS and
the MCT zone.
 Major metamorphic events affecting the Indian plate are
associated with the subduction and collision of the Indian
plate beneath/or with Eurasian Plate, i.e.
1. Subduction metamorphism of high pressure & low
temperature (HP-LT).
2. Collision metamorphism of medium pressure- medium
temperature (MP-MT)

1. Subduction Metamorphism or Pre-Himalaya Metamorphism
(M0)
 Corresponds to HP-LT metamorphism event, affecting rocks
located near to Indus-Tsangpo suture, in the NW Himalaya.
 Here, two events are concerned.
a. metasedimentary units and associated mafic lenses that
record a blueschist metamorphism in front of the Tethyan
intra-oceanic subduction, of Cretaceous
b. Internal crystalline massif; Tso Morari, Kaghan and Stak
massifs, overprint by subduction metamorphism following
burial and exhumation marked by facies transition from
eclogite to amphibolite to greenschist facies.
 In Central Nepal, normal burial (chlorite-grade) metamorphism
associated with pre-Himalaya is recorded in Nawakot Group,
Lesser Himalaya.

1A. Emplacement and exhumation of Ultra High Pressure
(UHP) eclogites. ( A review from upper Kaghan Valley)
 Internal crystalline massif; Tso Morari eclogites , Kaghan
eclogites and Stak eclogitic massifs shows peak metamorphic
condition range;
– Pressure : 3 Gpa to 4 Gpa
– Temperature: 600°C to 730°C &
– Burial to a depth of at least 90-100km.
 During their exhumation, these rocks were partially
retromorphosed.
• First at the blueschist/ eclogite transition (glaucophane
appearance at approx. 2GPa, 580°C).
• Then in amphibolite facies (approx. 1GPa, 650°C).
• Finally, in the green schist facies (approx. 0.4GPa, 400°C)

Source: Y. KANEKO et.al (J. metamorphic Geol., 2003, 21, 589–599)
Fig : Locations of the coesite-bearing gneiss outcrops indicated by stars in Kaghan Valley with geological
map of the area.

Field Occurrence & Petrography of UHP rocks in Kaghan
Valley:
 Coesite-bearing eclogite, pelitic gneiss and felsic gneiss from
Higher Himalayan Crystalline rocks in the upper Kaghan
 Eclogites exhibit : Grt + Omp + Coe ⁄ Qtz + Phn + Rt ± Amp ±
Ep with minor zircon and apatite assemblage.
 Strong amphibolitization has destroyed the primary texture
and mineralogy along margins and cracks.
 Coesite-bearing felsic gneiss wraps around the
amphibolitized eclogite and hosts amphibolite layer.
 The country gneisses are highly retrograded compared to the
eclogites; garnet is replaced by chlorite, and rutile by
ilmenite.

 except for country rocks show no apparent evidence of UHP
conditions, relics of UHP minerals such as coesite inclusions
in zircon.
U–Pb isotopic analysis for zircon core, mantle and rim by
SHRIMP dating:
 The data points from the zircon cores yield 206Pb ⁄ 238U
ages of 253–170 Ma, suggesting protolith magmatic activities
at Late Permian to Middle Jurassic.
 The zircon mantle contains quartz inclusions, clear indicative
of their growth under non-UHP conditions and gives an
average age of c. 50 Ma.
 The coesite bearing zircon rim, suggesting formation under
UHP conditions, yields an age of 46.2 ± 0.7 Ma

UHP- peak P-T condition:
 Peak metamorphic
conditions of the Kaghan
UHP rocks are estimated
as 700–770°C and 27–32
kbar which is equivalent to
90-110km in depth.
 Retrograde textures and
minerals are common in
UHP eclogites.
 Retrograde P–T conditions
of 655 ± 55 ° C and
pressures of 11 ± 1.4 kbar
Fig : Pressure–temperature conditions of the Kaghan
UHP rocks. P–T condition of the upper Kaghan gneiss
(Trelor, 1995) shows retrograde condition.

Interpretations from Studies of UHP rocks of upper Kaghan Valley:
 It has been suggested that the UHP metamorphism occurred
before the Himalayan orogeny and the origin of the UHP
eclogites were related to obduction of the Kohistan arc (Searle
et al., 1999, 2001).
 UHP rocks in collisional orogens are extensively overprinted by
Barrovian-type metamorphism at mid-crustal depths during
their exhumation, this extensive hydration recrystallization
event obliterates most UHP records (Liou et al., 1998).
 Transition from eclogite facies to amphibolitic metamorphic
facies.
 UHP eclogites shows burial subduction metamorphism followed
by the retrograde metamorphism as evident for polyphase
metamorphism in Himalaya.

1B. Pre-Himalaya deformation and metamorphism in Lesser
Himalaya ( Findings from Central Nepal).
 Studies along the Lesser Himalaya of Central Nepal, emerge the
pre-Himalayan D1 & D2 deformation phase and M0
metamorphism event.
 D1. Pre-deformational compositional layering (S0) has been
preserved throughout the Lesser Himalaya. The first
deformational event (D1) is marked by the dominant bedding-
parallel foliation (S1) in the Nawakot Complex.
 The S1 is probably the result of bedding-parallel flatening due to
syn-sedimentary loading.
 D2. The D2 event corresponds to the deformation of the S0 and
S1 producing drag and isoclinal folds (F2) with NNE±SSW
trending axes.

Fig : Photograph showing west vergent F2 drag fold formed by the deformation of bedding (S0)
and S1 foliation observed in the Thrust Sheet I along the Kali Gandaki river valley south of
Phalebas.
(Source: Paudel L.P*, Arita K, 1999)

Deformation in Lesser Himalayan Zone.

 There is a sharp discontinuity in the illite crystallinity (IC)
values across the unconformity between Nawakot Complex
and Tansen Group.
 Indicating Nawakot Complex had already been heated up to
the anchizone (prehnite ± pumpellyite facies).
 Heating event resulted in M0 metamorphism in Lesser
Himalaya as a result of burial metamorphism showing chlorite
grade mineral assemblage.
 And, may be related to D1 which produced bedding parallel
foliation in the Nawakot Complex.
 Thus, Lesser Himalaya shows anchizone grade prograde
metamorphism (D1) as a result of burial metamorphism.

2. Collision Metamorphism of Himalaya
 Corresponds to MP-MT metamorphism event including the
metamorphic base of the Tethyan series, above and north of
the STDS.
 The collision metamorphism is associated with the thrusting
and shearing of Indian Plate. Main Central Thrust (MCT) was
the first thrust to break the Indian Plate.
 Hence, collision metamorphism is mostly related to the MCT
zone differing the metamorphism above and below the MCT.
 The MCT zone is a heterogeneous shear zone, up to several
km thick, affecting mainly the cover series of Lesser Himalaya.
 Below the MCT, the metamorphism is characterized by famous
inverted Himalayan metamorphism.

 Three main tectono-metamorphic stages of different imprints
from one part of the belt to another are delineated as;
• Eo- Himalayan metamorphism or M1 metamorphism,
• Neo- Himalayan metamorphism or M2 metamorphism &
• Late- Himalayan metamorphism or M3 metamorphism.
2.A Metamorphism above the MCT, in the Higher Himalaya
Crystallines

Eo- Himalayan metamorphism or M1 metamorphism
 M1 metamorphism is a prograde metamorphism associated
with shearing at the base of the upper crust of Indian plate
resulting in underthrusts or early burial of HHC under units of
the Tethyan Himalaya and internal nappes before the end of
thermal relaxation of the thickening Himalayan pile.
 M1 is dated between 44Ma and 33Ma.
 High P/T condition metamorphism (1.5 to 2.0 GPa and 700°C).
 Amphibolite to granulite facies transition and Barrovian-type
metamorphism.
 Kyanite grade prograde metamorphism while garnet are rich in
pyrope.

Neo- Himalayan metamorphism or M2 metamorphism
 M2 metamorphism is a retrograde metamorphism above
MCT zone associated with HHC where, HHC is a partial melt
or has become hot being underthrust below Tethyan
sediments such that exhumation of HHC occurred over
thrusting LH sequence.
 M2 is linked to south vergent movements on the MCT and to
the formation of the Himalayan leucogranites.
 M2 is dated between 23Ma and 16Ma.
 P/T condition show 0.8 to 1.0 GPa and 750°C to 800 °C).
 Corresponds to thermal peak and beginning of retrograde
metamorphism path as marked by migmatisation.

Late- Himalayan metamorphism or M3 metamorphism
 M3 metamorphism is dated in the HHC from 14 Ma to 10Ma.
 Corresponds to metamorphic equilibrium at the transition from
low-pressure amphibolite to high temperature greenschist
facies
 Marking cooling from 750-800 °C (M2) to 450-500 °C (M3)
 Pressure slightly decreases from 0.5GPa to 0.3GPa
 Replacement of garnet and biotite by chlorite.
 M3 starts at the end of HHC high temperature exhumation and
marks the end of the ductile shearing in the MCT zone.

Polyphase metamorphism in the Higher Himalaya (Findings
from Tansen-Pokhara Section)
 In Higher Himalayan Crystalline, metamorphic event can be
observed; a high P/high T kyanite grade-Barrovian type
metamorphism (Eo Himalayan, Caby et al., 1983) followed bya
later lower P/ high T sillimanite grade metamorphism (Neo
Himalayan)
 Three metamorphic events are recognized in the Higher
Himalaya of the Tansen ± Pokhara section;
 The first metamorphic event (M1) was a high P/high T
amphibolite facies prograde metamorphism as in the other
sections of central Nepal. The kyanite and pyrope-rich garnet
cores belong to the M1
 M1 followed by sillimanite grade metamorphism (lower P/high T)
is not observed in Tansen – Pokhara section.

Polyphase metamorphism in the Higher Himalaya (findings
 However M2 is shown by widespread retrogressive
metamorphism.
 The Fe-rich and Al-poor white micas from higher Himalayan
gneisses probably do not coexist with the kyanite and pyrope-
rich cores of garnet so they must have crystallized or re-
equilibrated at lower metamorphic conditions.
 The retrograde zoning profiles at the margins of garnets from the
Higher Himalaya were formed by resorption due to retrogression.
 Wide-spread development of dynamic textures such as
fractured and bent kyanite blades and elongated garnet
porphyroblasts with pressure shadows shows that the M2 was
related to the thrusting along the upper MCT.

Polyphase metamorphism in the Higher Himalaya (findings
 Therefore, the inversion of isograds from the garnet zone in the
Lesser Himalaya to the kyanite zone in the Higher Himalaya is
only apparent due to thrusting along the UMCT.
 The replacement of garnet and biotite by chlorite is the third
retrogressive metamorphic event (M3), occurred during
exhumation.
 Therefore, the Higher Himalaya of Tansen – Pokhara section,
Central Nepal exhibit three phase of metamorphism viz.. M1
followed by M2 and later M3.
 Thus, there is polyphase metamorphism in Central Himalaya.

Metamorphic events in the central Nepal

2.B Metamorphism below the MCT, in the northern part of
Lesser Himalaya.
 Below the MCT zone, the metamorphism is characterized by the
famous inverted Himalayan metamorphism, which is observed
through out the Himalaya.
 Here, the Lesser Himalaya formation of higher metamorphic
grade, found in the MCT zone, lie above fewer & fewer
metamorphic formations southwards, down to the hanging wall
of MBT.
 There is a steady increase in metamorphic grade from south to
north marked by the successive appearance of chlorite, biotite,
garnet , kyanite and staurolite; imprinting prograde
metamorphism.

Fig: Inverted metamorphism in the Lesser and higher Himalaya of central
Nepal, across the Manaslu massif. Source Modified from Pêcher, 1989

 The minerals such as chloritoid, staurolite or garnet, and also
quartz display consistently top to south syn-shearing
recrystallisation microstructures.
 The inverted metamorphism concerns the M2 metamorphic
event which began with the initiation of MCT and over-thrusting
of HHC on the Lesser Himalaya along the MCT.
 This metamorphism is accumulated to the footwall of MCT such
that maximum pressure and temperature reaches up to
1.1Gpa and 600°C
 At the footwall of MCT, relatively high grade (garnet- staurolite-
kyanite) overlies or superimposed to less metamorphic ones
(biotite-chlorite).
 The garnet exhibit snowball texture.

A case study of “Inverted metamorphic zonation in the
hanging and foot walls of the Mahabharat Thrust, Kathmandu
Trishuli area, central Nepal.”
 Background:The Kakani- Trishuli area is covered by high-
grade metamorphic crystalline rocks of the Kathmandu Nappe
lying over the low-grade metasedimentary rocks of the Lesser
Himalaya along the Mahabharat Thrust (MT) (Hagen 1969,
Arita et al. 1973; Stöcklin 1980; Stöcklin and Bhattarai 1981).
 The MT is interpreted as a direct continuation of the MCT.
 The Lesser Himalayan rocks in the study area belong to the
Kunchha Formation, Benighat Slate and the Robang
Formation of the Nawakot Complex.
 The Kathmandu Nappe (Kathmandu Complex) comprises the
Kalitar Formation, Gneiss Zone, Tistung Formation, Sopyang
Formation and the Chandragiri Limestone.

 The study shows inversion of metamorphic zones at the foot
and hanging walls of the MT.
 Four metamorphic zones are identified based on index
minerals in the study area viz.. Chlorite zone, Biotite zone,
Garnet zone and Sillimanite zone.
Chlorite zone:
• The chlorite zone is observed in the southern part around
Mudku Dhoka region.
• This zone comprises upper part of the Tistung Formation,
Sopyang Formation, and the Chandarigiri Limestone.
Biotite zone:
• The biotite zone is observed both in southern and northern
parts of the study area.
• In the northern part, it covers both the Kunchha Formation
and the Benighat Slate.
Continue..

Continue..
• In the southernpart, the biotite isograd passes from south of
Tinpiple and covers both the lower part of the Tistung
Formation and the gneiss zone.
Garnet zone:
• The garnet zone is found only in the northern part near the
MT. The garnet isograd passes almost parallel to the Tadi
Khola and do not follow the lithological boundaries.
• It covers both the upper part of the Benighat Slate, Robang
Formation below the MT as well as the Kalitar Formaiton and
gneiss zone above the MT.
Sillimanite zone:
• The middle part of the study area is entirely covered by the
sillimanite zone.
• The sillimanite isograd is observed in between Kakani and
Ranipauwa in the southern part and near Belkot in the north.
• The sillimanite zone covers the gneiss zone.

Fig: Metamorphic zonation map of the
study area
Source: Bulletin of the Department of Geology, Tribhuvan University, Kathmandu,
Nepal, Vol. 14, 2011, pp. 51–58

Findings:
 The disposition of metamorphic zones in the Kathmandu-Trishuli
area do not a follow a pattern expected in a normal
metamorphism.
 The oldest unit of the Lesser Himalaya, i.e., the Kunchha
Formation belongs to the biotite zone whereas the younger units
(Benighat Slate and Robang Formation) belong to the garnet
zone showing a clear evidence of inverted metamorphic
zonation at the footwall of the MT.
 Above the MT, the garnet zone at the base is followed
stratigraphically and structurally upwards by the sillimanite zone.
 The kyanite zone is missing here indicating high T/low P
metamorphism.

Findings:
 Therefore, the inverted metamorphic sequence seems
continuous from the footwall to the hanging wall of the MT.
 However, in the southern part, the metamorphism is normal,
i.e., the grade of metamorphism decreases from biotite zone in
the Tistung Formation to chlorite zone in the Chandragiri
Limestone.
 Hence, the inverted metamorphic zonation at the hanging wall
of the MT may be related to the high temperature contact
metamorphism by pegmatite injection.
 But, the inverted metamorphism at the footwall needs an
explanation; as which can be explained by hot iron” model of
Le Fort (1975).

1.The Hot iron model (Le Fort ,1975) for explaining inverted
metamorphism at the footwall of MCT/ MT
Fig: The flat-iron model after Le Fort, 1975
 A thermal model that links the exhumation of HHC, the
movements on the MCT and the formation of Himalaya
leucogranites and the inverted metamorphism.
Inverted Metamorphism Models

 A thermal model for oceanic subduction zones, where the
HHC are considered as deep and hot nappe (>700°C) that
over thrust nearly instantaneously on the cold Lesser
Himalaya and heated by conduction from the HHC, leading to
following results:
a) The partial cooling of the base of HHC that explains the
retrogressive metamorphic PT pattern in its lower part
(footwall of the MCT), although its upper part keeps
warmer and at lower pressure ( presence of sillimanite).
b) The warming of the Lesser Himalaya below the thrust,
which leads to its partial dehydration; the fluid transferred
to the HHC induces the partial migmatisation of its middle
part, and, formation of Himalaya leucogranites (Le Fort,
1975).
Continue..

 This is the first thermal model is based upon the Central Nepal
observation as in our case study.
 It emphasizes on the relationship between the MCT motion
and the reversal of the isotherms, which results in the reversal
of the isograds. Here, metamorphism postdate the shearing.
 In this model, erosion is a passive phenomenon and suggests
that the inverted metamorphism develops on both side of
MCT.
 Therefore, it can be concluded that the inverted
metamorphism at the footwall of MCT is a result of thrusting
where rate of thrusting is greater than the thermal relaxation
of the HHC i.e inverted thermal gradient.
Continue..

2. Ductile Shear as a Cause of Inverted Metamorphism:
Example from the Nepal Himalaya
 Another approach to explain inverted metamorphism in
Himalaya is a “Ductile-flow model”.
 In this approach, a physical process inverts a preexisting right-
way-up metamorphic sequence.
 This include a model of folding preexisting isograds (Tilley
1925; Heim and Gansser 1939; Bhattacharya and Das 1983;
Searle and Rex 1989), or a model of imbricate thrusting of
individual slices from different structural levels and thus
different metamorphic grades (Bordet 1961; Brunel and
Kienast 1986; Treloar et al. 1989).
 Here, HHC are expelled between the two major faults, but with
more ductile rheology and that the formation of a large –scale
fold would also induce the folding of isograds.

Continue..
Fig: Generalized ductile flow model.
The higher Himalaya crystalline (GHS) act as large shearing zone, bracketed by
two major shear surfaces (MCT and NHF). Some variation of the shear velocity on
the foliation surfaces lead to the folding of isograds. These models necessitate
that the deformation should postdate the metamorphic peak.
Source: Himalaya Tibetan Collision.

Fig :Schematic diagram of ductile shear model. Simplified illustration of how ductile
shear can invert isograds within shear zone. Shear fabric is penetrative within the shear
zone and sub-parallel to the boundarys of the shear zone. Shaded areas represent
zones of different metamorphic grade. CH = chlorite; ST = staurolite; KY = kyanite; SIL =
sillimanite.

 The metamorphic grade of rocks within the MCT zone of eastern
Nepal increases toward higher structural levels.
 Rocks of the lowermost MCT are garnet-grade, but the presence
of biotite and/or chlorite suggests the low PT end of garnet-
grade metamorphism. Garnet can be found in pelitic rocks at all
levels within the MCT zone.
 These low-end garnet-grade rocks are overlain by rocks
containing staurolite. Kyanite occurs in mid-level rocks of the
MCT zone.
 The upper levels of kyanite-bearing rocks contain fibrolitic
sillimanite and are overlain locally by rocks with prismatic
sillimanite.
 Metamorphic isograds are subparallel to the lithologic contacts
and shear foliation within the rocks.

Findings:
 In eastern Nepal structural and petrologic evidence most
strongly suggests that a model of ductile shear across a -5-10
km wide zone played the major role in inverting a pre-existing
metamorphic sequence.
 This model explanation creates an inverted metamorphic
sequence consistent with observations of: (1) a zone of
distributed deformation; (2) metamorphic textures overprinted
by deformation; and (3) metamorphic isograds sub-parallel to
shear fabric.
 Though inverted metamorphism may occur in different geologic
settings due to different causes, one should strongly consider
the role of ductile shear if the inverted metamorphism is
coincident with a thrust-sense shear zone.

Fig :Map and cross-section of
MCT zone in eastern Nepal.
Cross-section shows location of
metamor-phic isograds. SI =
sillimanite; KY = kyanite; ST =
staurolite; GA = garnet. Strike
and dip symbols repre-sent
orientations of the penetrative
share fabric. The shear fabric is
sub-parallel to the lithologic
contacts (see cross-section).

3. Channel Flow Model ( A example from Kangchenjunga
region)
Fig : Application of channel flow model to the Kangchenjunga region. After Searle &
Szulc, 2005
1. Deepening and prograde metamorphism; 2. sillimanite peak ; 3.partial melting,
migmatisation; 4.crystallization and emplacement of leucogranites; 5. extrusion of HHC
within the channel flow; 6. recumbent folding of isograds; 7. propagation of MCT
toward the structural base. The isograd reversal results, of their folding subsequently to
their emplacement.
Source: Himalaya Tibetan Collision.

Channel Flow Model ( A example from Kangchenjunga region)
 This model implies that 20Ma after the beginning of the collision,
i.e. 30-35Ma, the HHC underthrust below the Tibet became hot
enough (>700-750°C) that began to melt.
 Based on expt, a fraction of few percentage points of liquid
magma is enough to reduce viscosity one or two order of
magnitude and to reach the critical value of 1019 Pa/s necessary
for the migration of matter in the channel.
 The driving force to the flux within the channel is gravitational
potential difference between the thickened Tibet and the
Himalayan front.
 Between 23-18Ma, the highly effective erosion initiated by
summer monsoon, created a localized appeal of material.

Channel Flow Model ( A example from Kangchenjunga region)
 Thus, allowing the extrusion of the channel between two shear
planes, the MCT at the base and STDS at the top.
 The inverted geometry of the metamorphic isograds is explained
by variation in the intensity of searing and the speed differential in
the channel, which gradually became thicker and thicker and
incorporated units of its footwall ( Lesser Himalayan units).
 Hence, cannel flow postulates that, “exhumation is
aconsequences of partial melting and that erosion allows the
continuous exhumation of the heat softened unit.
 These models favor volume forces rather than boundary forces
and suggests exhumation of HHC is passive, not active.

Polyphase metamorphism in the Lesser Himalaya (Findings
from Ulleri Augen Gneiss, based on study from Central
Nepal)
Geological Settings
The study area extending from Syaprubesi in the east to the
Chhyamthali area in the west belongs to the Kuncha Formation,
the oldest unit of the Nawakot Group (Stöcklin, 1980). The Ulleri
Augen Gneiss is another major lithological unit exposed within
the succession of the Kuncha Formation.
Kuncha Formation
 Kuncha formation consists of mainly chlorite biotite grade grey
phyllite/schist, interlayered with fine-grained
metasandstone/quartzite and green metabasics (amphibolites).
The metasandstone is monotonous, thickly bedded, medium-
grained, grey interparting with highly deformed phyllite.
.

 The Ulleri Augen Gneiss is felsic augen gneiss that was
intruded in the Kuncha Formation.
 The Ulleri Augen Gneiss is exposed within the schist, phyllite
and metasandstone of the Kuncha Formation of the Lesser
Himalaya.
 It can be correlated with the Melung Salleri Augen Gneiss or
Phaplu Augen Gneiss of eastern Nepal whereas similar type of
augen gneiss has been mapped in the Chainpur area of Far-
Western Nepal.
 At the Jharlang area the lithological contact between the gneiss
and host rock is represented by a shear zone which is mapped
as the Jharlang shear zone
Ulleri Augen Gneiss
Continue..

Continue..
Deformation and Metamorphism
 The rocks of the region are highly deformed, sheared and
metamorphosed.
 Shear sense indicators like asymmetric augen of quartz and
feldspar and S-C fabric developed in the rocks show top-to-the
south sense of shearing in the rocks
 Metamorphic events in study area are supported by the
presence of metamorphic minerals such as chlorite, biotite and
garnet, respectively.
 In the study area, the grade of metamorphism increases from
south to north, approaching the MCT is evident for inverted
gradient i.e inverted metamorphism.

Continue..
 Also, development of biotite
at the rims of garnet and the
formation of chlorite within
the schist indicates
retrograde
metamorphism.
 Thus, the area is affected by
poly-phase metamorphism
such that it is evident from
garnet porphyroblast which
shows the inclusions of
quartz aligned along the
internal foliation which is
across the direction of the
major external foliation of
the schist.
Fig. : Photomicrograph (UCN 4X) showing the
garnet schist (sample taken from Syabrubesi
area). Garnet has the inclusions of quartz with
internal foliation.

Findings:
 Ulleri Augen Gneiss is granitic protolith consisting of mineral
assemblages of quartz, orthoclase, plagioclase, muscovite
and biotite, emplaced within Kuncha Formation of Lesser
Himalaya.
 Based on the field relation and texture analysis, the evolution
of the protolith of this Ulleri Augen Gneiss can be interpreted
as a multi-story emplacement within the host rocks during
and immediately after the sedimentation (Pre-Himalaya
Metamorphism?)
 It can ultimately be linked with the movement of the Higher
Himalayan crystalline rocks through the MCT over these
Lesser Himalayan rocks.

Findings:
 The adjacent rocks gets assimilated and has suffered from
contact and regional metamorphism.
 The movement associated with MCT has resulted the
inverted metamorphism and retrograde metamorphism along
the Ulleri gneiss and adjacent rocks.
 The Lesser Himalayan rocks along with the gneiss unit in the
Syabrubesi area close to MCT shows the effect of poly-phase
metamorphism (Rai et al., 2017).

Two metamorphic events in the Lesser Himalayan metabasites:
 The Lesser Himalayan meta-basites contain almost constant
mineral assemblage of Ca-amphiboles, biotite, plagioclase,
quartz, epidote, chlorite and Fe-Ti oxides.
 Modal percentage of amphiboles and biotite increases, and
that of epidote and chlorite decreases from south (structurally
lower part) to north (structurally higher part) towards the
Upper MCT in the LH.
 Amphiboles in the form of porphyroblast are chemically zoned
with actinolite cores and hornblende/tschermakite rims
showing their two stage metamorphic growth. The actinolite
cores are pre-kinematic and were formed most probably prior
to the Tertiary Himalayan orogeny.

 Hornblende forming matrix and rims of porphyroblasts are syn-
kinematic and were formed during shearing along the Upper
MCT in the Tertiary.
 The systematic compositional changes of syn-kinematic
amphiboles and textural characteristics confirm the classical
concept of increasing metamorphic grade northwards
(structurally upwards) in the LH.
 Similarly, metamorphism study of Jhyallaphyay- Barpak area of
Gorkha district shows rocks have been metamorphosed to
greenschist facies and epidote-amphibolite facies in the Lesser
Himalaya and epidote-amphibolite facies with the inverted
metamorphic gradient.
 Thus, two metamorphic events namely as syn-tectonic
prograde metamorphism (M1) and post-tectonic retrograde
(M2) have been identified from the study area.

Conclusion
 Himalaya shows various metamorphic imprints associated with the
subduction and collision related metamorphic event.
 Being on shelf of these event, wide variety of metamorphism types
are encountered. Among them the inverted metamorphism
associated with the MCT zone is observed all over the Himalaya.
 Inverted metamorphism is resemble by chlorite, biotite, garnet
assemblage of Lesser Himalaya which are overlain by sillimanite
grade.
 To describe the inverted metamorphism; models like hot iron,
ductile flow, channel flow models are brought in use
 On relation to inverted metamorphism, retrograde metamorphism
are also associated, with most common example of biotite
formation on the rims of garnet or formation of chlorite in schists.

Conclusion
 Poly-phase metamorphism is evident from garnet
porphyroblast which shows the inclusions of quartz
aligned along the internal foliation which is across the
direction of the major external foliation of the schist.
 Polyphase metamorphism can be observed in both
Higher Himalayan rocks and Lesser Himalayan rocks.
 In relation to subduction metamorphism, eclogite
metamorphic facies are result of burial metamorphism
during subduction which later exhibit the amphibolite-
greenschist facies transition when exhumed.

References
Dhital, M. R. (2015). Geology of the Nepal Himalaya: regional perspective of the
classic collided orogen: Springer.
Harrison, T. M., Grove, M., Lovera, O. M., Catlos, E., & D’Andrea, J. (1999). The
origin of Himalayan anatexis and inverted metamorphism: Models and constraints.
Journal of Asian Earth Sciences, 17(5-6), 755-772.
Hubbard, M. S. (1996). Ductile Shear as a Cause of Inverted Metamorphism:
Example from the Nepal Himalaya. The Journal of Geology, 104(4), 493-499.
K.C, J., & Paudyal, K. R. (2019). Characteristics and field relation of Ulleri Augen
Gneiss to country rocks in the Lesser Himalaya: A case study from Syaprubesi-
Chhyamthali area, central Nepal. Journal of Nepal Geological Society, 58(0), 89-
96. doi:10.3126/jngs.v58i0.24577
Kaneko, Y., Katayama, I., Yamamoto, H., Misawa, K., Ishikawa, M., Rehman, H., . .
. Shiraishi, K. (2003). Timing of Himalayan ultrahigh‐pressure metamorphism:
Sinking rate and subduction angle of the Indian continental crust beneath Asia.
Journal of Metamorphic Geology, 21(6), 589-599.

References
Mascles, G., Pecher, A., Guillot, S., Rai, S. M., & Gajurel, A. P. (2013). The
Himalaya-Tibet Collision. Nepal Geological Society, 30, 93.
Oli, L. M., Paudel, S., & Paudel, L. P. (2019). Metamorphism of Jhyallaphay-
Barpak-Bhachchek area of Gorkha District, Lesser Himalaya and Higher Himalaya,
Central Nepal. Journal of Nepal Geological Society, 58, 119-130.
Paudel, L. P., & Arita, K. (2000). Tectonic and polymetamorphic history of the
Lesser Himalaya in central Nepal. Journal of Asian Earth Sciences, 18(5), 561-
584.
Thapaliya, S. P., & Paudel, L. P. (2011). Inverted metamorphic zonation in the
hanging and foot walls of the Mahabharat Thrust, Kathmandu-Trishuli area, central
Nepal. Bulletin of the Department of Geology, 14, 51-58.

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