1

Tectonochemistry and P-T Conditions of Ramgarh and Almora Gneisses from

Askot Klippe, Kumaun Lesser Himalaya

Biraja P. Das

1, Mallickarjun Joshi

1,* and Ashutosh Kumar

1

1Centre of Advanced Study in Geology, Institute of Science, Banaras Hindu University,

Varanasi-221005

Abstract: The inter se chemical and petrological correlation of metamorphic nappes and klippes overlying the

Proterozoic sedimentary units in the Kumaun Himalaya is still debated. The Ramgarh and Almora gneisses,

not previously distinguished in the Askot Klippe, show distinct field, petrological and chemical signatures

markedly similar to the tectonostratigraphic disposition of the Almora Nappe. A negative Eu anomaly in the

Ramgarh gneisses indicates lesser plagioclase fractionation while the Eu anomaly in the Almora pelitic

gneisses was is likely to have been controlled by feldspar crystallization in restites. During the anatexis at >

776o

C temperature and >6.6 kbar pressure, the melt moved slightly away to its crystallization sites. The

Rb/Sr ratio ~0.54 and Nb ~10 ppm is consistent with the granodioritic composition. The negative Sr anomaly

in the underlying Ramgarh granitic gneisses indicates a distinct mantle source/plagioclase fractionation with

a notable correspondence to other late orogenic granites, particularly the basement Ulleri gneisses from the

Nepal Himalaya. Ramgarh gneisses plot in the late- and post-COLG field. The Askot ensemble is likely to be

the tectonometamorphically-reworked basement, viz. the Ramgarh Group along with its metapelitic cover of

the Almora Group, together comprising southward thrust remnants of the leading edge of the Indian Plate

that collided with Tibet during the Tertiary Himalayan orogeny.

Keywords: Almora gneiss, Ramgarh gneiss, Almora Nappe, Askot Klippe, Geochemistry, Thermobarometry.

Email:mallickbhu@gmail.com

1 Introduction

Himalayan continental collision was responsible for the geological reworking of the rocks comprising the

leading edge of the Indian Plate. Sequentially deciphering India‟s palaeoposition during the geological past and its

location vis a vis the ever-changing configurations of supercontinents demands that the Pre-Tertiary lithological and

structural elements are studied in a time-space perspective. Association and dissociation of supercontinents impelled

by tectonic processes have a known periodicity of 400-500Ma (Wilson, 1963; Nance et al., 1988). Columbia, one of

the oldest supercontinents in the geological history of the earth, coalesced between 2.1 to 1.7Ga ago (Rogers, 2000;

Condie, 2002; Rogers and Santosh, 2002; Meert and Santosh, 2017).

Study of the Lesser Himalayan igneous, sedimentary and metamorphic sequences has a key role in

supercontinent reconstructions in southeast Asia, which has been attempted by Gansser (1964), Le Fort (1975),

Valdiya (1980, 2009), Yin (2006) and others. The Lesser Himalayan rocks have undergone a tortuous deformation

history, culminating in three major orogenic parallel thrusts, viz. the Main Central Thrust, the Munsiari Thrust and

the Ramgarh Thrust, along with the brittle crustal scale North Almora Fault (Joshi, 1999, Joshi et al., 2018),

intersecting the area.

This article is protected by copyright. All rights reserved.

This article has been accepted for publication and undergone full peer review but has not been

through the copyediting, typesetting, pagination and proofreading process, which may lead to

differences between this version and the Version of Record. Please cite this article as doi:

10.1111/1755-6724.13814.

2

The Lesser Himalaya essentially consists of Pre-Himalayan or Pre-Tertiary sedimentary (Valdiya, 1980, 2009;

DeCelles et al., 2001) and igneous rocks that are capped by detached tectonic outliers essentially comprising a

metapelitic cover that invariably overlies mylonitized igneous rocks, with an intervening thrust.

Despite several workers arguing for Pre-Tertiary orogenic events (e.g. Fuchs, 1968; Srikantia, 1977;

Bhargava, 1980, 2011; Bhargava and Bassi, 1994; Valdiya, 1995; Joshi et al., 1994 ; Joshi, 1999; Gehrels et al.,

2003; Joshi and Tiwari, 2004, 2009) in the Lesser Himalaya, the idea somehow still does not have a wide

acceptance. In fact, Baig et al. (1988) proposed a late Cambrian orogeny from the Pakistan Himalaya, while Gehrels

et al. (2003) suggested a Late Cambrian-Middle Ordovician thrusting from the Nepal Himalaya. Gehrels et al.

(2006) discussed regional metamorphism that at least reached garnet-grade, and associated regional folding related

to an Early Palaeozoic tectonic event, ratifying Joshi et al. (1994) and Joshi and Tiwari (2004), who documented a

Pre-Cambrian amphibolite facies metamorphic event from the adjacent Kumaun Himalaya.

There is evidence suggesting that the Palaeoproterozoic (1800-1900Ma) has been a particularly active period

in terms of igneous activity in the Lesser Himalayan sequences. Though the area has not been extensively dated

employing well constrained U-Pb and Pb-Pb dating, Kohn et al. (2010) suggested a date of 1830±50Ma based on U-

Pb zircon dating for the edge of a palaeocontinental arc in the Lesser Himalaya. A crystallization age for a granite–

granodiorite gneiss has been determined at 1856.6±1.0Ma (±19Ma), and a crystallization age of 1878±2.4Ma

(±19Ma) has been suggested for a biotite augen gneiss, using U-Pb zircon dating for the Askot area in the Kumaun

Himalaya by Mandal et al. (2016).

The present study is essentially focused on two aspects, viz. the tectonochemistry of the gneissic rocks

comprising the igneous basal part of the Askot Klippe, and the chemistry and the petrotectonic evolution of a

hitherto unexplored metamorphic cover for its relationships with rocks of similar stratigraphic disposition within the

Himalaya, for better regional comprehension, in the context of its possible relationships with the Columbian

supercontinent.

During the Tertiary continental collision, the Main Central Thrust transported the quartzite-schist-granite

gneiss ensemble southwards from its root zone in the Higher Himalaya (Heim and Gansser, 1939; Valdiya, 1980,

2010 and others). Askot Klippe is one of the smaller remnants of this large thrust sheet, similar in its architecture to

the Almora Nappe, which is the southernmost extension of a once continuous larger thrust sheet (Fig.1a).

3

Fig. 1. Simplified geological map of the Kumaun Lesser Himalaya (modified after Valdiya, 1980; Joshi, 1999).

4

(a), Various thrust contacts. Black open rectangle indicates the study location; (b), Lithological map of the studied location, characterizing various thrust

contacts and lithologies from bottom to top.

Striking resemblance in lithology, structural history, tectonic setting and metamorphic evolution renders the

crystallines of Almora Nappe and Baijnath, Askot, Chhiplakot and Lansdowne klippes almost indistinguishable

(Valdiya, 1980; Celerier et al.2009; Patel et al. 2011c, 2015).

The Ramgarh gneisses can be correlated largely after Valdiya (1980, 1983 and 2010) and Singh (2010) with

other gneisses along the Himalayan strike. Thus, the Ramgarh gneisses can be correlated with the Bhatwari

Formation farther north in the Kumaun Himalaya, the Himachal Himalaya, the Dhauladhar gneisses in Kashmir, the

augen gneisses of the Bhimphedi Group in western Nepal, the Ulleri gneisses in Central Nepal (Le Fort and Rai,

1999), migmatitic gneisses of the Tumlingtar Group in Eastern Nepal, the Bomdila Mylonitic Gneisses in the

western Arunachal Himalaya (Singh, 2010), the Lingste augen gneiss of the Darjeeling-Sikkim Himalaya (Sinha-

Roy, 1980) and with gneisses of the Shumar Formation of the Bhutan Himalaya (Dasgupta, 1995).

This present attempt addresses the petrochemistry of Askot Klippe, in order to understand the evolution of the

area, and to discriminate between the petrotectonic settings of the Ramgarh and the Almora gneisses. We focus on

deciphering the compositional similarities of the Ramgarh granite gneisses with similar granites from other parts of

the Himalaya and the world, in order to understand their tectonic setting and regional correlations.

2 Geology of the region

Askot Klippe resulted from the southward thrusting of the metamorphics from the Higher Himalayan

Crystallines over the Main Central Thrust (MCT). Valdiya (1983), however, suggested that these basement gneisses

have been thrust southwards. It is strikingly similar to the Almora Nappe, both in terms of structure and

metamorphism. The Klippe has a slightly asymmetric structural disposition, with the northern limb dipping 30-35o

due south while the southern limb dips 20-25o due north. The longer axis of the rhomboid Askot Klippe on the map

is oriented roughly E-W.

For the first time, we present evidence that Askot Klippe consists of two tectonostratigraphic units, viz. the

basal Ramgarh Group and the overlying Almora Group. We mapped an asymmetric overturned antiform in the

central part of the klippe that exposes pelitic gneisses, with its southern limb steeply dipping (50o-71

o) due north and

the northern limb dipping relatively gently (22o-35

o) due south (Fig. 1b). The absence of an adjacent synform could

be explained through truncation by the Almora Thrust. The fold axis of this doubly-plunging fold is oriented broadly

E-W and it exposes K-feldspar-sillimanite-cordierite gneisses, reported here for the first time. The Shear sense

indicators suggest that the general sense of movement for mylonites in the klippe is from top-to-south. We consider

the top-to-north sense of shears recorded by Mandal et al. (2016) in the southern contact of the Klippe to be minor

exceptions due to lagging behind by some tectonic slices within the dominant top-to-south tectonic transport in the

shear zone, likely due to a back thrust. In Askot Klippe, like the Almora Nappe, the outer periphery of the klippe

comprises Ramgarh Group rocks, which are separated from the underlying sedimentary rocks of the Damtha and

Tejam groups by the Ramgarh Thrust (RT) of Joshi (1999) in its northern and southern flank, as identified in the

Almora Nappe. Like the Almora Nappe, Almora Group rocks exposed in the central parts of the nappe are separated

from the underlying Ramgarh Group rocks by the Almora Thrust (AT) in its northern and southern flank.

Based on the rock types, the Almora and Ramgarh groups are separated by the Ramgarh Thrust with northern

and southern exposures. The Basal Shear Zone of the Almora Nappe is characterized by the presence of granitic

mylonites and ultramylonites of greenschist facies and rare proto-mylonites. The Ramgarh Group is characterized by

the presence of dark grey granitic gneisses, whereas the Almora Group is distinguished by the presence of light

coloured gneisses. Gneisses of the Almora Group are pelitic in nature and comprise garnet, cordierite, kyanite,

sillimanite and K-feldspar. The Askot Klippe is bound in the north by schistose phyllites and limestones of the

Mandhali Formation, locally known as the Tejam Group (Valdiya, 1980; Celerier et al. 2009), and to the south by

quartzites of the Jaunsar and Berinag formations (Valdiya, 1980).

5

Large scale displacement over a single thrust resulted in the larger Almora Nappe and a number of smaller

klippes in the Kumaun Lesser Himalaya, such as the Askot Klippe and the Chhiplakot Klippe. The Askot Klippe

tectonically rests on the dolomites and the dolomitic limestones of the Damtha and Tejam groups. The Ramgarh

Group consists of ultramylonites, mylonites and protomylonites, after the granite gneisses that are exposed at the

base of the klippe. These are followed upwards by mylonitized schistose phyllites, quartzites and/or schistose

quartzites. Further up the section, the equivalent of the Almora Thrust is marked by the first appearance of garnets

and chlorite-biotite-muscovite-garnet schists, garnet-biotite-kyanite schists, K-feldspar-sillimanite gneisses

interbanded with psammitic bands and amphibolite sills. A post-orogenic intrusive granitic-granodioritic body which

appears gneissose due to mylonitization has also been observed.

3 Petrography

3.1 Almora Group

The Almora Group rocks are essentially pelitic, comprising white, greenish grey, bluish grey and brown, fine

to medium-grained schists, micaceous quartzites, amphibolites and gneisses (Fig.2). Different varieties of schist are

recognized, such as chlorite schists, biotite-sericite schists, graphitic schists and garnet-mica schists. The schistosity

is defined by the preferred alignment of the micas. Locally-developed porphyroblastic garnets are quite common,

particularly in the mica schists. The pelitic augen gneisses occur in the central parts of the klippe, in an anticlinal

exposure (Fig. 2a).

Fig. 2. Megascopic photographs of the Almora Group.

(a), Feldspar porphyroblasts in augen gneiss; (b), South Almora Thrust near Ogla, showing the underlying Ramgarh mylonites and the mylonitized garnet

mica schists of the Almora Group; (c), Asymmetric quartzo-feldspathic veins showing top-to-south movement in the Almora gneisses. Arrow shows the

direction of shearing; (d), Contact of gneiss and amphibolite.

6

The pelitic gneisses are rich in biotite and contain porphyroblasts of plagioclase and K-feldspar. The schists

are interbanded with quartzites and gneisses. Near the periphery of the klippe, the low grade schists are underlain by

protomylonites, mylonites and ultramylonites, after the dark-coloured Ramgarh gneisses. The porphyroblastic

pelitic gneisses exposed in the core of the klippe are mylonitized towards the periphery with the degree of

mylonitization increasing as the equivalent of the the Almora Thrust is approached. The augen gneisses of the

Almora Group in the Askot Crystallines have a distinctive character, and consist of large porphyroblastic feldspar

augens. The high grade K-feldspar-sillimanite gneisses are restricted to the central parts of the klippe. The thick pile

of gneisses is exposed between the schists due to a later overturned folding event. The locally-interbanded nature of

schist-gneiss-schist is observed, suggesting melting in the more albitic layers that produced the gneisses.

Microscopically, the mica schists and garnetiferous mica schists are characterized by the dominant S2

schistosity that defines the regional trend. Relicts of an older schistosity (S1) are also present. The schistosity is

defined by the preferred orientation of chlorite, biotite and muscovite. In the pelitic gneisses of the Almora Group,

plagioclase, garnet, quartz and rare kyanite blades and sillimanite needles are present within the K-feldspar. The

plagioclase occurs as laths and shows lamellar twinning. Polysynthetic twinning is observed. In places, the

plagioclase grains are highly saussuritized and converted to dense aggregates of sericite, epidote and calcite. The

feldspar length is truncated by the S-plane and the mica flakes are truncated by the foliation plane. The S-C fabrics

are well-developed in the garnet-mica schists of the Almora Group (Fig. 3b). The plagioclase generally shows

polysynthetic twinning and, in places, oscillatory zoning. Biotites are irregularly-shaped and heterogeneously

distributed in the rock, and occur as uneven masses. Myrmekitic intergrowths of plagioclase with quartz are

common. The other accessory minerals include epidote, zircon, apatite, sphene, tourmaline, clinozoisite and

opaques.

The major mineral assemblages are;

1. Chlorite- biotite- muscovite- garnet- quartz

2. Biotite- muscovite- garnet- quartz

3. Biotite- muscovite- garnet- plagioclase- quartz

4. K-feldspar-sillimanite-garnet-biotite- plagioclase ± cordierite – quartz

7

8

Fig. 3. Photomicrographs of the Almora Group a, c are in XPL and b, d are in PPL.

(a), Random orientation of biotite flakes (removed from Ramgarh Group); (b), shows the S-C fabric; (c),show the Garnet porphyroclast surrounded by

retrograded chlorite with symmetric neutral pressure shadow; (d),shows theTourmaline crystal embedded in the ground mass of feldspar; (e),shows the

Garnet-biotite reaction in PPL; (f),shows theSame in XPL; (g) and (h) show synkinematic and postkinematic garnet in the kyanite-garnet-mica schists of

the Almora Group.

Chlorite in the schists of the Askot Crystallines is generally a retrogression product, and chloritized garnet is

common at the base of the Almora Group rocks, comprising the shear zone of the Almora Thrust . Muscovites and

biotites of two generations, viz. one of coarser size, define the S2-foliation and the other cross-cutting finer one

defines a later foliation. Biotite grains are strongly pleochroic from yellow-brown to deep reddish-brown. Randomly

oriented biotite flakes in the pelitic gneisses suggest that, due to extreme temperature, partial melting was initiated,

and the physical conditions of the rock changed to a semi-plastic state with consequent hydrostatic pressure.

Two types of garnet have been observed in the pelitic gneisses of the Almora Group. Garnet–I are syn-

kinematic crystals that developed during the deformation. The sigmoidal disposition of the quartz grains in the

central part of the garnet indicates rotation during growth. Internal schistosity (Si) in the core is oblique to the

external schistosity (Se), while Si close to the garnet rim is parallel to Se (Fig. 3c). Garnet-II crystals, with an

idioblastic outline are are post-kinematic and developed after the cessation of deformation (Fig. 3g and Fig. 3h).

This suggests that the garnet core developed during a synkinematic recrystallization, while the inclusion-free rim

developed when the directional stresses ceased. Evidently the metamorphism in the area outlasted the deformation.

The micas and other minerals appear to flow around the garnet. Hematite is observed along the cracks and margins

of the garnet. Inclusions of quartz and iron oxides are commonly observed in the garnets. Highly chloritized garnets

are observed, due to retrogression near the thrust (Almora Thrust) at the base of the Almora Group.

The pelitic gneisses of the Askot Klippe are medium-to-coarse grained, crudely foliated, biotite gneisses.

Foliation is defined by planar blebs and streaks of biotite. The main constituents are garnet, cordierite, quartz, K-

feldspar, plagioclase, biotite and sillimanite. Random orientation of biotite is observed due to the development of

hydrostatic stresses during anatexis in the gneisses at peak metamorphic conditions. Microperthites in the pelitic

gneisses indicate locally strong deformations and interlocking of the grains. Cordierite is here reported for the first

time from any of the klippes/nappes in the Kumaun Lesser Himalaya. The reaction between garnet and biotite

indicates the onset of granulite facies metamorphism in the pelitic gneisses (Fig. 3e and Fig. 3f). Plagioclase is

mostly subhedral and euhedral and commonly shows albite, albite-carlsbad and albite-pericline complex twinning.

In some of the samples, the laths of plagioclase are truncated by the S-planes. Inclusions of quartz, biotite,

muscovite, sericite and zircon are observed within the plagioclase. Plagioclase also occurs as small grains and as

inclusions within the biotite.

3.2 Ramgarh Group

Ramgarh Group rocks are essentially granitic and have been subjected to intense mylonitization, leading to the

development of mylonites and ultramylonites under greenschist facies conditions with the characteristic

development of chlorite (Fig.4).

9

Fig. 4. Megascopic photographs of the Ramgarh Group.

(a), Greenschist facies ultra-mylonite; (b), South Ramgarh Thrust (SRT) contact between carbonaceous shale of the Damtha and Tejam groups and proto-

mylonites of the Ramgarh Group; (c), Microfolding in mylonites; (d), Granitic gneiss of Ramgarh.

The rocks of the Ramgarh Group, which constitute the basal part of the klippe, show more intense shearing

compared to those of the overlying Almora Group. Broadly speaking, the degree of mylonitization increases as the

basal Ramgarh Thrust is approached. The mylonites are generally dark in colour, due to the predominance of

ferromagnesian minerals. Chlorite-rich mylonites and ultramylonites occur after the mica schists are exposed at the

base of the Almora Group rocks overlying the Ramgarh mylonites in the Ogla-Rangaon section.

Under the microscope, the granite gneisses of the Askot Klippe can be seen to be medium-to-coarse grained

mylonitized biotite gneisses, characteristically lacking garnets. Orthoclase is usually fractured with veins of quartz

and plagioclase showing combination twinning and strong saussuritization. Mortar texture is commonly developed,

due to intense mylonitization (Fig. 5b). In some places, myrmekitic intergrowths are characteristically developed

(Fig. 5c). Microfolding is common. Porphyroblastic crystals of orthoclase and subordinate plagioclase which are

twinned and highly saussuritized can be observed. The main mineral assemblages, viz. quartz, feldspar, muscovite

and biotite, are unevenly distributed within the rocks. Near the Almora Thrust, mortar texture is displayed by K-

feldspar. Chessboard twinning in plagioclase is also common. The mineral assemblages are:

1. Muscovite-biotite-plagioclase-quartz.

2. Muscovite-biotite-plagioclase-K-feldspar-quartz

3. Muscovite-biotite-plagioclase-K-feldspar-quartz.

10

Fig. 5. Photomicrographs of the Ramgarh Group.

(a), Chessboard twinning in plagioclase; (b), Mortar texture in K-feldspar; (c), Myrmekite intergrowth; (d), Green biotite.

Quartz occurs as fresh grains with sharp outlines, having inclusions of biotite, muscovite, chlorite, sericite,

euhedral zircons and opaques. Quartz also occurs as inclusions within the K-feldspar, plagioclase and mica.Brown

biotite is the dominant mica in these rocks. Biotite, muscovite and quartz fiber are the common minerals developed

in the asymmetric pressure shadow zones around the feldspar porphyroclasts. The K-feldspar has undergone

sericitization, likely as a result of ingress of water during shearing. Inclusions of quartz, biotite, muscovite, sericite

and zircon are observed within the plagioclase.

The grains of plagioclase are highly saussuritized near the marginal area. The plagioclase shows chessboard

twinning (Fig.5a) in some of the samples, due to local shearing. Metasomatism in local shear zones is brought about

by hydrothermal fluids, that reduce the temperature and result in such tiled textures (Exner, 1949). Microlithons

retaining the original fabric of the precursor rock are also present. The rocks of the Ramgarh Group are mainly

granitic and characteristically devoid of any garnet. The presence of green biotite in the mylonites of the Ramgarh

Group (Fig.5d) indicates the basic character of the rocks, with a relatively higher Fe2O3 content and (FeO+MgO)

and a lower Al2O3 content (Sakai, 1981). The green biotites are formed at a slightly lower temperature and pressure

than the brown biotites (Sakai, 1981).

4 Petrochemistry

The pelitic gneisses and the granitic mylonites of both the Almora and Ramgarh groups were analyzed for

their geochemical characteristics. A total of thirty samples were subjected to whole rock analysis using XRF

(SIEMENS SRS 3000 at the Wadia Institute of Himalayan Geology, Dehradun, India), to determine the major oxide

chemistry of the rocks. Eleven of the samples were analyzed at the Activation Laboratory, Canada, for both the trace

11

element characteristics and the REE behaviour of the rocks. The values for these elements were determined by

fusion ICP-MS. Cu, Pb, Zn, Ni, Ag, As, Sb, W, Cr and Sn content were also analyzed (Table 1). The Almora and

Ramgarh gneisses showed distinctive geochemical characteristics. In the Ramgarh granitic gneisses, the ∑REE

ranges from 152.24 to 265.53, while in the Almora pelitic gneisses the ∑REE ranges from 144.91 to 155.12. Thus

the ∑REE is significantly lower for the Almora pelitic gneisses, which clearly distinguish the two geochemically.

Table 1 XRF analysis of the selected samples from the Ramgarh and Almora groups

Ramgarh Group Almora Group

sample

s

CB-5 DS-4 KD-2 KP-2 LG-

26

CB-8 LG-5 OR-

11A

KD-4 RN-

11

RN-

12

OA-

10

ON-3 ASK-

79

SiO2 62.01 69.5

5

73.11 72.4

5

65.94 70.43 61.03 70.35 70.06 73.46 74.48 71.13 72.31 62.8

Al2O3 17.89 15.4

8

15.31 14.1

0

16.70 16.54 18.55 15.83 15.67 14.09 13.72 14.83 16.46 17.03

FeOT 4.67 2.88 2.19 2.92 3.46 2.00 5.12 2.46 2.75 1.95 2.02 2.63 1.45 5.45

MnO 0.05 0.03 0.01 0.03 0.04 0.02 0.07 0.02 0.03 0.02 0.02 0.02 0.01 0.08

MgO 2.34 0.45 0.72 0.34 1.39 0.37 2.46 0.35 0.46 0.23 0.20 2.15 1.73 1.25

CaO 2.10 1.19 0.41 0.93 2.71 0.35 2.51 1.26 1.16 0.89 0.71 0.18 0.17 3.6

Na2O 3.88 2.63 3.93 2.67 3.10 2.80 2.71 2.40 2.82 2.77 2.51 0.59 0.55 1.55

K2O 2.88 5.98 2.77 5.20 3.74 6.13 3.55 6.03 5.37 5.06 4.85 4.49 5.15 5.4

TiO2 0.61 0.34 0.28 0.32 0.50 0.22 0.71 0.31 0.36 0.21 0.20 0.30 0.1 0.65

P2O5 0.20 0.16 0.11 0.13 0.15 0.12 0.20 0.15 0.16 0.13 0.13 0.10 0.13 0.21

Total 98.69 99.5

2

100.6

6

99.8

8

99.51 100.4

9

100.1

2

100.8 100.9

1

99.92 99.93 98.69 100.4

1

101.62

Rb 126.0

0

na na na 148.00 na 207.0

0

415.00 304.0

0

460.00 487.00 331.00 na na

Sr 293.0

0

na na na 345.00 na 269.0

0

45.00 142.0

0

33.00 28.00 4.00 na na

Y 25.00 na na na 17.00 na 26.00 26.00 25.00 26.00 29.00 20.00 na na

Zr 144.0

0

na na na 101.00 na 175.0

0

165.00 131.0

0

99.00 106.00 99.00 na na

Nb 10.00 na na na 6.00 na 11.00 13.00 10.00 10.00 11.00 8.00 na na

La 56.10 na na na 34.50 na 59.80 50.30 47.70 30.00 29.10 33.60 na na

Ce 109.0

0

na na na 65.90 na 117.0

0

103.00 94.70 63.10 61.40 67.30 na na

Pr 12.20 na na na 7.47 na 13.20 11.50 10.80 7.26 7.03 7.62 na na

Nd 43.30 na na na 26.20 na 47.10 40.00 38.30 25.20 24.10 26.70 na na

Sm 7.70 na na na 5.00 na 8.50 8.40 7.10 5.80 5.70 5.70 na na

Eu 1.51 na na na 1.15 na 1.64 0.72 0.89 0.37 0.34 0.27 na na

Gd 5.60 na na na 4.10 na 6.30 7.10 5.50 5.30 5.50 5.00 na na

Tb 0.80 na na na 0.60 na 0.90 1.00 0.90 0.90 1.00 0.80 na na

Dy 4.50 na na na 3.20 na 4.70 5.20 4.60 5.20 5.40 4.00 na na

Ho 0.80 na na na 0.60 na 0.90 0.90 0.80 0.90 0.90 0.60 na na

Er 2.40 na na na 1.60 na 2.50 2.10 2.00 2.00 2.30 1.70 na na

Tm 0.34 na na na 0.22 na 0.35 0.28 0.29 0.26 0.30 0.23 na na

Yb 2.10 na na na 1.50 na 2.30 1.60 1.70 1.40 1.60 1.40 na na

Lu 0.33 na na na 0.20 na 0.34 0.24 0.22 0.21 0.24 0.20 na na

12

Hf 4.10 na na na 3.10 na 5.30 5.60 4.10 3.60 3.90 3.30 na na

Th 20.20 na na na 13.50 na 24.60 33.70 27.40 28.10 28.30 23.50 na na

For the Almora pelitic gneisses, the (La/Lu)N was restricted to between 12.60 and 17.45, while for the

Ramgarh granitic gneisses it varied from 17.66 to 22.53, again characterizing the two clearly. However, both of

them indicate a typical continental margin setting (Cullers and Graf, 1983). To determine the tectonic setting, both

the rock groups were plotted on the diagram of Pearce et al. (1984).

The Eu/Eu* ratio in the Ramgarh granitic gneisses varied from 0.29 to 0.78, showing a moderate negative Eu

anomaly. In contrast, the Eu/Eu* ratio for the Almora pelitic gneisses varied from 0.15 to 0.54, showing a more

pronounced negative Eu anomaly for the rocks. The negative Eu anomaly for the Almora pelitic gneisses (Fig.8a)

signifies a somewhat higher degree of fractionation in the plagioclase. The Eu anomaly is controlled by feldspar,

particularly in felsic magmas, as Eu (present as Eu2+

) is compatible in plagioclase and potassium feldspar in contrast

to Eu3+

, which is incompatible. Thus, the removal of feldspar from a felsic melt by crustal fractionation or the partial

melting of rocks, in which feldspar was retained in the source to give rise to a (-ve) Eu anomaly in the melt

(Rollinson, 1993), is likely.

13

Fig. 6. Harker diagrams showing SiO2 variation with major oxides of Almora and Ramgarh Gneisses from Askot Klippe. Red circles- Ramgarh gneisses; Blue diamonds-Almora gneisses; Green triangles- Debguru Porphyroids; Black square-Ulleri gneiss.

Fig. 7. Harker‟s diagram showing SiO2 variation with trace elements of Almora and Ramgarh gneisses of Askot crystallines.

Symbols are same as in Fig. 6.

14

Fig. 8. Chondrite-normalized (value after Boynton, 1984) REE pattern of Almora and Ramgarh gneisses.

15

(a), Average REE abundances of granite gneiss compared with patterns of late-Orogenic, post-Orogenic, Anorthosite Rapakivi Granite and Alkaline Ring

Complex Granite of Rogers and Greenberg (1990); Granodioritic gneiss, Monzo-granitic gneiss and Syeno-granitic gneiss of S. Liu et al (2005); Even-

grained biotite granite and Coarse-grained amphibole granite of L.A Neymark et al (1994);(b), Spidergram normalized to primordial mantle (value after

Wood et al. 1979). The Ramgarh data compare well with the average late-Orogenic granites of Rogers and Greenberg (1990).

However, in the Askot Klippe the pelitic schists imperceptibly grade into the Almora pelitic gneisses and the

reactions muscovite + albite + quartz = K-feldspar + sillimanite + H2O (Thompson 1974) and muscovite +quartz =

sillimanite+K-feldspar+H2O (Turner and Verhoogen, 1960 and Winkler, 1978) have been petrographically identified

in the thin sections. Thus, the large negative spike of the Eu anomaly is likely to be a reflection of the removal of the

melt from the adjacent schists, that eventually solidified as the gneisses. This suggests that the source rocks, viz.

schists, are likely to have melted, leaving a feldspar rich restite and producing a melt with a negative Eu anomaly.

The Rb/Sr ratio varies from 0.43 to 9.22 for the Ramgarh gneisses and from 13.94 to 82.75 for the Almora

gneisses, which once again distinguishes the two. The unexpected increase in the Rb/Sr ratio and decreasing Sr

content in the Almora gneisses can be related to fractional crystallization.

In contrast, samples of the Ramgarh Group of rocks viz., CB-5, LG-26, LG-5 show a decreasing Rb/Sr ratio

and increasing Sr content. The average Rb/Sr ratio is ~0.54 and Nb ~10 ppm in these three samples, characterizing a

granitic magma that produced the granodiorite (Henderson, 1980).

In the Harker variation diagrams, Al2O3, CaO, TiO2, P2O5 and FeOt showed a negative relationship with the

SiO2, while MgO, Na2O and K2O did not show any relationship with the SiO2 (Fig. 6). The A/CNK and K2O/Na2O

and the trace elements Ni and Cr did not show any relationship with the SiO2, while the trace element Rb showed a

positive relationship with the SiO2 and Sr showed a negative correlation with the SiO2. The mg# showed a negative

correlation with SiO2 for the Ramgarh gneisses while it showed no relationship with the SiO2 for the Almora

gneisses (Fig.7). Some of the pelitic gneisses of the Almora Group, viz. OA-10 and ON-3, have high Al2O3 and

K2O, but low CaO and Na2O. Such variations in the pelitic gneisses are also known from other parts of the world

(Chinner, 1960, Malisa, 2005). Low Ca content in the granitic gneisses of the Ramgarh Group suggests that these

granites may have anorogenic tectonic affinities (Chappell and White, 2001).

Both the Ramgarh and Almora gneisses show large REE variations from the primordial mantle composition

(Fig.8), but have been treated separately here, consistent with the field and petrographic evidence elaborated

previously. The Ramgarh gneisses are likely to have evolved from assimilation and fractional crystallization, while

the K-feldspar-sillimanite-cordierite bearing Almora gneisses formed due to melting under the highest metamorphic

grade conditions. Both groups have a negative Sr anomaly and a negative Nb anomaly. The negative Sr anomaly

indicates the distinctive mantle source/plagioclase fractionation. In some of the gneisses of the Ramgarh Group, the

Ni concentration is between 20-40 ppm, indicating a modern ocean arc setting (Smith et al., 1997). The relative

enrichment of LILEs to REEs and the depletion of Nb and P is suggestive of the arc environment for the genesis of

the Ramgarh gneisses, and is expected to be the configuration of the bulk continental crust. On the Rb/30 vs. Hf vs.

Ta*3 plot of Harris et al. (1986), the granitic rocks of the Ramgarh Group lie within the field of the volcanic arc and

the late- and post-COLG field (Fig. 9). The gneisses of the Almora Group lie within the fields of syn-collisional and

within-plate tectonic settings, which appear consistent with their metamorphic origin.

16

Fig. 9. Geotectonic classification of Almora and Ramgarh gneisses of Askot crystallines (after Harris et al., 1986).

Of the four types of peraluminous granites identifiable in the A-B multicationic diagram of Villaseca et al.

(1998), modified after Debon and Le Fort (1983), granite gneisses of the Askot Klippe show two distinct clusters,

viz. the highly felsic peraluminous granites (f-P) equivalent to the Almora Group and the highly peraluminous

granitoids (h-P) of the Ramgarh Group. The Almora gneisses show most of the characteristics of the highly felsic

peraluminous granitoids and show a marked absence of mafic minerals. These ly close to their source region and

contain cordierite, almandine-pyrope garnet and sillimanite, as described by Villaseca et al. (1998). The Ramgarh

gneisses, however, are characterized by an increase in peraluminosity with increasing mafic content, consistent with

field observations (Fig. 10). Surprisingly, none of the Ramgarh gneisses plot in the metaluminous field, as reported

by previous workers (Rao and Sharma, 2009; Mandal et al., 2016).

17

Fig. 10. Distinct nature of the Almora gneisses and the Ramgarh gneisses in the tectonic discrimination diagram of Villaseca et

al. (1998, modified after Debon and Le Fort, 1983).

In the tectonic discrimination diagrams of Pearce et al. (1984), the Almora gneisses invariably plot in the Syn-

COLG field with only one exception plotting in the WPG field, which is consistent with their behaviour in the other

diagrams discussed above. The Ramgarh gneisses, however, show a more mixed nature, falling in the VAG, Syn-

COLG, and at the boundary with the WPG field (Fig. 11).

18

Fig. 11. Tectonic discrimination diagrams after Pearce et al. (1984) for the Almora and Ramgarh gneisses. Symbols are same as

Fig. 8

The Na2O+K2O – Cao vs. SiO2 plot of Frost et al. (2001) also clearly distinguishes the calc-alkalic S-type

Almora gneisses from the alkali-calcic I-type Ramgarh gneisses (Fig. 12). The S-type granites are characterized by

their peraluminous nature, while the I-type granites are almost invariably metaluminous. That the Almora gneisses

are of pelitic origin and distinctly different from the Ramgarh gneisses is once again brought out by their plotting in

the S-type field in the Frost et al. (2001) diagrams. Present analyses for the Ramgarh gneisses, however, plot in the

I-type field, which is generally the metaluminous field.

19

Fig. 12. Na2O+K2O – Cao vs. SiO2 plot of Frost et al. (2001), clearly distinguishing the calcic S-type Almora gneisses from the

alkali-calcic I-type Ramgarh gneisses.

5 Mineral chemistry and phase equilibria study of the Almora Group gneisses

The mineral chemistry for the gneisses of the Almora Group was carried out employing the Electron Probe

Micro Analyzer (EPMA) in the EPMA Laboratory, Department of Geology, Institute of Science, Banaras Hindu

University. Polished thin sections were coated with a 20 nm thin layer of carbon for electron probe micro analyses

using the LEICA-EM ACE200 instrument. The CAMECA SXFive instrument was operated by SXFive Software at

a voltage of 15 kV and a current of 10 nA, with a LaB6 source in the electron gun for generation of the electron

beam (Table 2).

20

Table 2 Probe analysis of selected mineral grains from the Almora Group

ASK-79 DO3

Oxides (wt. %) Grt (c) Grt (r) Bio (m) Bio (m) Plag (m) Plag (m) Crd (m) Crd (m) Grt (c) Grt (r) Bio (m) Bio (m) Plag (m) Plag (m)

SiO2 36.17 36.39 34.69 34.79 62.21 61.24 46.47 43.98 36.36 36.25 33.19 34.45 61.54 61.62

TiO2 0.23 0.18 2.03 1.98 0.02 0.00 0.00 0.00 0.00 0.00 0.95 1.09 0.00 0.00

Al2O3 20.21 19.87 16.30 16.60 22.08 21.80 31.88 33.54 20.80 20.74 20.67 18.94 23.08 22.75

Cr2O3 0.11 0.11 0.19 0.07 0.08 0.05 0.00 0.00 0.00 0.02 0.01 0.10 0.00 0.00

FeO 18.94 21.18 26.10 25.77 0.01 0.00 8.74 9.23 37.27 31.75 22.32 24.77 0.00 0.00

MnO 9.06 6.36 0.39 0.52 0.00 0.00 0.00 0.00 0.22 3.59 0.00 0.25 0.01 0.00

MgO 0.19 0.28 5.22 5.46 0.00 0.00 9.36 8.78 2.16 1.16 6.86 6.01 0.00 0.00

CaO 14.85 14.35 0.18 0.07 3.91 3.67 0.00 0.00 4.12 6.13 0.21 0.13 4.90 4.33

Na2O 0.01 0.01 0.13 0.06 10.01 9.90 0.02 0.00 0.00 0.00 0.16 0.09 9.34 9.78

K2O 0.01 0.01 8.99 8.84 0.24 0.19 0.00 0.00 0.00 0.00 9.81 8.66 0.03 0.04

P2O5 0.08 0.04 0.19 0.11 0.03 0.05 0.00 0.00 0.00 0.00 0.09 0.11 0.00 0.00

Total 99.86 98.78 94.39 94.27 98.59 96.90 96.47 95.53 100.92 99.64 94.27 94.60 98.90 98.52

Oxygen atoms 12.00 22.00 8.00 18.00 12.00 22.00 8.00

Si 2.91 2.95 5.55 5.55 2.80 2.80 4.90 4.71 2.93 2.95 5.22 5.42 2.76 2.78

Ti 0.01 0.01 0.24 0.24 0.00 0.00 0.00 0.00 0.00 0.00 0.11 0.13 0.00 0.00

AlIV

0.09 0.05 2.45 2.45 1.17 1.18 3.10 3.29 0.07 0.05 2.78 2.58 1.22 1.21

AlVI

1.84 1.86 0.63 0.67 0.00 0.00 0.87 0.94 1.91 1.95 1.05 0.94 0.00 0.00

Cr 0.01 0.01 0.02 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00

Fe3+

0.12 0.11 0.00 0.00 0.00 0.00 0.00 0.00 0.08 0.05 0.00 0.00 0.00 0.00

Fe2+

1.15 1.33 3.49 3.44 0.00 0.00 0.77 0.83 2.43 2.12 2.94 3.26 0.00 0.00

Mn 0.62 0.44 0.05 0.07 0.00 0.00 0.00 0.00 0.02 0.25 0.00 0.03 0.00 0.00

Mg 0.02 0.03 1.24 1.30 0.00 0.00 1.47 1.40 0.26 0.14 1.61 1.41 0.00 0.00

Ca 1.28 1.25 0.03 0.01 0.19 0.18 0.00 0.00 0.36 0.53 0.04 0.02 0.24 0.21

Na 0.00 0.00 0.04 0.02 0.87 0.88 0.00 0.00 0.00 0.00 0.05 0.03 0.81 0.85

K 0.00 0.00 1.84 1.80 0.01 0.01 0.00 0.00 0.00 0.00 1.97 1.74 0.00 0.00

XMg 0.02 0.02 0.26 0.27 0.00 0.00 0.66 0.63 0.10 0.06 0.35 0.30 0.00 0.00

XFe 0.98 0.98 0.74 0.73 0.00 0.00 0.34 0.37 0.90 0.94 0.65 0.70 0.00 0.00

XCa 1.00 1.00 0.02 0.01 0.18 0.17 0.00 0.00 1.00 1.00 0.02 0.01 0.23 0.20

21

The biotites of the gneisses of the Almora Group are siderophyllites enriched in Al2O3 and FeO, but depleted

in SiO2 and MgO. The Fet/Fe

t+Mg in biotites of gneisses vary from 0.65 to 0.74. The MgO content of biotite in the

rock varies from 5.22 to 6.86 wt %, whereas the Al2O3 content varies from 16.30 to 20.67 and the FeO content

varies from 22.32 to 26.10 (Fig. 13a and Fig. 13b). The siderophyllite variety of biotite is generally not known from

Lesser Himalayan metapelitic sequences. Although siderophyllite is generally not a common mineral in gneisses of

metapelitic origin, it has nonetheless been recorded in the garnet-orthopyroxene-cordierite-biotite-quartz-

plagioclase-perthite ± sillimanite assemblage from the Karimangar area of the Eastern Dharwar Craton, India

(Sharma and Prakash, 2007). Siderophyllite is also recorded from pelitic gneisses from the Gois Magmatic Arc,

Brazil (Navarro et al. 2013). Interestingly, the Karimangar assemblage is identical to the pelitic gneisses from the

Askot area, albeit with the difference that orthopyroxene is inferred at the reaction boundary between the garnet and

biotite in the current area under discussion.

Fig. 13. Chemical discrimination diagram of biotites from the Almora Group.

(a), Fet/Fe

t+Mg vs. Al

ivdiagram (after Deer et al., 1963) for biotites of Almora gneisses; (b), FeO-MgO-Al2O3 diagram of biotites of Almora gneisses.

Fields are after Abdel-Rehman (1994).

The BSE image and its subsequent compositional plot of garnet shows the zoning profile of Ca, Mg, Mn and

Fe from core to rim. XCa represents Ca/(Ca+Mg+Fe+Mn), XMg represents Mg/(Fe+Mg), XFe represents Fe/(Fe+Mg)

and XMn represents Mn/(Ca+Mg+Fe+Mn). Garnet zoning in the metapelitic gneisses of the Almora Group shows the

variation in Ca, Mg, Fe and Mn from core to rim (Fig. 14). XAlm shows a slightly bowl-shaped profile and increases

from core to rim. However, XPy and XGros are more or less flat. On the other hand, XMn shows a slightly plateau-

shaped profile and decreases from core to rim. Partial re-equilibration in the P-T conditions of the metapelitic

Almora gneisses resulted in some flattening in the elemental composition profile of the garnets.

22

Fig. 14. BSE image and zoning profile of garnet-sillimanite-K-feldspar-cordierite bearing gneisses of the Almora Group.

Phase section modelling is one of the better tools to compute the P-T conditions. The reliability of the results

is based on the internally consistent thermodynamic dataset. The phase section model was drawn by the Perple_X

6.7.5 (database: hp02ver.dat) (Connolly, 2005) in the NCMnKFMASHT system for the garnet-cordierite-K-

feldspar-sillimanite bearing gneisses of the Almora Group. The amount of H2O taken for the calculations of phase

section modelling was estimated by the „Loss On Ignition‟ (LOI) value in the whole rock analysis. The phase section

modelling was drawn with the help of the internally consistent thermodynamic dataset and equation of state for H2O

(Holland and Powell, 1998). The solution models of Bio (HP), Gt (HP), hCrd, Chl (HP), Mica (CHA), Ilm (WPH),

melt (HP) and feldspar are used to draw the phase section diagram (Table 3). Compositional isopleths have been

drawn in the calculation to decipher the P-T path. The isopleth contours for the garnet were calculated in the

23

cordierite-K-feldspar-garnet bearing gneisses as shown in Fig. 15. Through the intersection of the XFe, XCa and XMg

isopleths, the temperatures and pressures are estimated.

Fig. 15. P-T pseudosection of the sillimanite-K-feldspar bearing gneiss of the Almora Group in the NCMnKFMASHT system.

The bulk composition in weight percentages is calculated based on the XRF data. Compositional isopleth contours constructed relate to garnet for

grossularite content, pyrope content, almandine content and spessartine content. Thermal peak P-T conditions of schists and gneisses of the Almora Group

with the help of core and rim compositions for the garnet from the rocks of the Almora Group by the intersection of XMg, XFe and XCa of the gneisses of the

Almora Group. Solid Square represents the peak P-T point.

The software PTCALC (Ferry and Spear, 1978 and Lal, 1991) and QBASIC (after Wu et al., 2004) were used

to estimate the average temperature and pressure from the mineral chemistry of the rocks from the study area. In the

observed petrogenetic grid system, the garnet-cordierite-biotite gneisses yielded average temperatures around 776oC

and pressures around 6.6 kbar, as calculated by preferred thermobarometric models (Thompson, 1976; Holdaway &

Lee, 1977; Ferry and Spear, 1978; Perchuk and Lavrenteva, 1983) for temperature and Lal, 1991 and Wu et al., 2004

for pressure. These P-T conditions are the minimum estimates of the P-T reached by the gneisses. This is

corroborated by the temperatures estimated for the core and rim of the garnets, viz. 665oC and 776

oC respectively

(Table 4). The corroded garnet-biotite contact suggests the onset of granulite facies metamorphism, which is

corroborated by temperature estimates of around 776oC for the garnet rims.

24

The data were intrapolated within the Petrogenetic Grid System (PGS) (modified after Spear et al., 1999),

which is a closed system with the calculated pressure-temperature path (Fig. 16).

Fig. 16. Simplified petrogenetic grid showing the P-T conditions of the core and rim of the selected sample in the system Na2O-

CaO-MnO-K2O-FeO-MgO-Al2O3-SiO2-H2O(KFMASH). (Modified after Spear et al. (1999)).

Table 4 P-T conditions of Almora pelitic gneisses

Geothermometry based on Garnet – Biotite geothermometry estimated at 6 kbar pressure

Models Calculated temp in oC

Core Rim

Thompson (1976) 651 710

Holdaway& Lee (1977) 664 683

Ferry & Spear (1978) 615 725

Pigage and Greenwood (1982) 662 817

Hodges & Spear (1982) 722 934

25

Perchuk et al. (1985) 749 931

Perchuk and Lavrenteva (1983) 666 811

Aranovich et al. (1988) 698 880

Hoinkes (1986) 589 609

Bhattacharya et al. (1992); (H&W) & (G&S) 634 660

Average temperature 665 776

Geobarometry based on Garnet - Cordierite - Sillimanite - Quartz geobarometry estimated at 600oC

temperature.

Models Calculated pressure in kbar

Core Rim

Thompson (1976) 5.9 7.0

Wells (1979) 6.1 7.2

Nichols et al. (1992) 6.2 7.1

Perchuk et al. (1985) 6.3 6.9

Lal (1991) 6.0 6.5

Newton & Wood (1979) 6.4 5.2

Harris & Holland (1984) 6.0 6.0

Newton (1978) 6.4 6.4

Aranovich & Podlesskii,(1983,1989) 6.2 5.8

Aranovich & Podlesskii,(1983,1989) 6.5 7.9

Average pressure 6.2 6.6

Based on GBPQPI estimated temperature in 600 degree C

Models Calculated pressure in kbar

Wu et al. (2004) 6.2 6.5

The temperature of Ferry and Spear (1978) and pressure of Lal (1991) were chosen, based on the petrographic

observations and the petrogenetic grid. The petrogenetic grid is one of the easiest ways to document changes

through mineral reactions during the progress of metamorphism. The estimated temperatures and pressures were

plotted in the KFMASH system (Fig. 16). The persistently corroded boundary between garnet and biotite suggests

dehydration melting of biotite under the ambient P-T conditions and the estimated P-T estimates are clear evidence

for the onset of granulite facies metamorphism. These gneisses are characterized by the common presence of fresh

muscovite and biotite overprinting the regional foliation. The recrystallization of the fresh later-generation micas is

interpreted as being due to their development past the peak metamorphic conditions during the regressive arm of the

prograde metamorphism. The temperature of 776o

C was recorded when the rock was cooling. The system was a

closed system and flushed with water. Upon the solidification of the anatectic melt, slightly away from the restite,

water was released and used up in giving rise to the fresh alteration of sillimanite-K-feldspar to coarse high-T

muscovite. There was little melt in the system due to the intermittent presence of hydrous minerals in the rock, and

consequently the melt did not escape the system in a sizable quantity. This segregated H2O was one of the reactants

for the later development of fresh assemblages containing coarse muscovite.

6 Discussion

26

In Askot Klippe, the SiO2 shows a discrete relationship with most of the trace elements, such as Rb, Sr and Ni

in both the Almora and Ramgarh group gneisses. The SiO2 vs. Al2O3 and K2O vs. Na2O ratios indicate the

evolutionary changes in the average composition of the continental crust (Schwab, 1978) and the source.

Metapelitic gneisses in central parts of the Klippe chemically show a syn-collisional tectonic setting, while the

signatures for the Ramgarh gneisses fall in both the late- and post-COLG fields . The data from different parts of the

world have been correlated with the present data on the Ramgarh Group, employing the plot of Pearce et al. (1984),

and our data show a marked correspondence with the data of Roger and Greenberg (1990), Neymark et al. (1994)

and Liu et al. (2005). Some of the data plot outside the expected field due to some aberration, such as contamination

during data processing, flexibility in the analyzed values and the somewhat non-ideal nature of the samples. Most

pelitic gneisses of the Almora Group fall in the syn-COLG field, while the Ramgarh gneisses yield mixed signatures

and plot in the late- and post-COLG fields (Harris et al., 1986). The ∑REE ranges from 152.24 to 265.53 in the

Ramgarh Group gneisses and 144.91 to 155.12 in the Almora Group gneisses, clearly ratifying the field and

petrographic observations that the two gneisses are of distinctly different origins. The negative Eu anomaly in the

Ramgarh gneisses indicates lower plagioclase fractionation.

In contrast, the Eu anomaly in the Almora gneisses is likely to have been controlled by feldspar crystallization

in the restites during anatexis and movement of the melt away to its crystallization site (Rollinson, 1993). It is

obvious that the Eu depletion in the Almora gneisses is a likely artifact of the melting of the albite-rich precursors.

The variation in the negative Eu anomaly spikes likely reflects various stages of melt evolution. The unexpected

increase in the Rb/Sr ratio and decreasing Sr content in the Almora gneisses is likely due to fractional crystallization

of the anatectic melt. The Rb/Sr ratio of ~0.54 and Nb ~10 ppm is consistent with their granodioritic composition.

The negative Sr anomaly for the Ramgarh gneisses indicates a distinctive mantle source/plagioclase fractionation.

However, a moderate negative Sr anomaly in the Almora gneisses corresponds well with a moderate negative Eu

anomaly, both of which are likely to be related to plagioclase fractionation.

The Rb/Sr vs. Sr plots (Fig. 17) again show two distinctly different trends viz. fractional crystallization for the

Almora gneisses and assimilation and fractional crystallization for the Ramgarh gneisses. The analyses for the

Debguru granites (Singh and Joshi, 2001), located close to the type area of Ramgarh (Valdiya, 1980), merge

imperceptibly with the Ramgarh gneisses from the Askot Klippe, further strengthening the hypothesis that the two

are closely related. Singh and Joshi (2001) characterized the Debguru granites as S-type, with the caveat that “it was

not possible to characterize the source rock unequivocally with the available data”.

27

Fig. 17. Sr vs. Rb/Sr plot. Almora gneisses show the vertical profile whereas Ramgarh and Debguru granites (Singh et al., 2001) show the uniform profile.

Based on Fet/Fe

t+Mg vs. Al

iv and the FeO/MgO ratio, the biotite in the rock is mainly siderophyllitic in

composition (Deer et al., 1963) and peraluminous (S-type) in character. In a ternary plot of FeO-MgO-Al2O3, the

gneisses of the Almora Group have an affinity towards the crystallized phase in the peraluminous (S-type) suite.

The Almora gneisses and the Ramgarh gneisses are also clearly distinguished in the classification diagram of

Whalen et al. (1987) (Fig.18). The Almora gneisses plot in the FG field (fractionated felsic granite), while the

Ramgarh gneisses plot in the OGT field (non-fractionated granite). Interestingly, one sample of Ulleri gneiss

(Celerier et al. 2009) from the Ramgarh Group in the Nepalese Himalaya also plots in the OGT field. The Ramgarh

gneisses correspond well with gneisses of other parts of the Himalaya, viz. gneisses from NW Himalaya and the

Nepalese Himalaya by Sharma and Rashid (2001); Islam et al. (2011); Regmi and Arita (2008) and Miller et al.

(2000). We feel that the compositional similarities between the Ramgarh gneisses from Askot Klippe and the data

on granites from different parts of the world by Rogers and Greenberg (1990), Neymark et al (1994), Liu et al

(2005), Celerier et al. (2009), are acceptable and lend further support to our identification of the Ramgarh gneisses

from the basal part of the Askot Klippe.

Fig. 18. Geotectonic classification of granitoids (after Whalen et al. 1987).

For the Almora Group, the biotites are mainly siderophyllitic in nature, and fall in the peraluminous field.

Thus, the broad tectonostratigraphic disposition of the Askot Klippe and the Almora Nappe are similar, lending

further support to their being remnants of the same thrust sheet, with its root zone in the Higher Himalaya, which

moved over the MCT to at one point cover large parts of the Kumaun Himalaya over the Lesser Himalayan

sedimentaries with a Basal Shear Zone (Joshi, 1999), equivalent to the MCT.

Unlike Rao and Sharma (2009), we found that most of our Ramgarh gneiss samples fell in the late- and post-

COLG tectonic setting. This is consistent with our hypothesis that the Ramgarh gneisses comprise the lowermost

units in the Askot Klippe, which, as elsewhere in the Lesser Himalaya (Valdiya, 1983), represent tectonically

transported basement.

28

7 Conclusions

Askot Klippe resembles a synform, with its central part comprising an asymmetrically overturned fold with a

gentle northern limb and a steep southern limb. The two tectonostratigraphic units are being distinguished for the

first time from the Askot Klippe, viz. the basal Ramgarh Group and the overlying Almora Group as the

supracrustals. The Almora Thrust has been identified for the first time from the klippe. Garnet-K-feldspar-

sillimanite-cordierite bearing gneisses in the central part of the klippe reached granulite facies conditions and record

the highest grade of metamorphism from any of the Lesser Himalayan nappes and klippes. The Ramgarh Group

rocks have been distinguished from the Almora Group rocks physically, petrochemically and tectonically. The

Almora Group has been affected by crustal anatexis and solidification of anatectic magma under fluid-present

conditions, whereas the Ramgarh Group has been affected by subduction metamorphism governed by mantle-

derived fractional crystallization, as evidenced by a negative „Sr‟ anomaly. The Almora pelitic gneisses are syn-

Collisional, whereas the Ramgarh granite gneisses are post- and late-Collisional volcanic arc granites. The Almora

Group pelitic gneisses are felsic-peraluminous, whereas the Ramgarh Group of rocks are highly peraluminous in

character. The Almora pelitic gneisses are basically S-type, whereas the Ramgarh granite gneisses are basically I-

type. Metamorphism in the Almora pelitic gneisses exceeded a temperature of 776oC and pressure of 6.6 kbar,

indicating the onset of granulite facies conditions, as demonstrated by phase section modelling and the Petrogenetic

Grid System (PGS). The Almora pelitic gneisses underwent fractional crystallization, while the Ramgarh granite

gneisses underwent assimilation and fractional crystallization. The Ramgarh Group is correlatable with many of the

Palaeoproterozoic granites of other parts of the Himalaya, including NW Himalaya and the Nepalese Himalaya, all

of which show an anorogenic and non-fractionated felsic granitic nature.

A simplified cross-section kinematics model (Fig. 19) has been prepared, modifying Celerier et al. (2009).

There is a significant increase in the intensity of deformation close due to the proximity of the Askot Klippe to the

MCT, as shown by the increase in the number and width of the bands comprising ultramylonites after the Ramgarh

gneisses at the base of the Askot Klippe, as compared to the mylonitized Ramgarh gneisses at the base of the

Almora Nappe, located farther south. Sometime during the middle part of the Indo-Asian collision, the Main Central

Thrust (MCT) is likely to have been juxtaposed over the Bhatwari Thrust (BT), and the package comprising the

basement Ramgarh gneisses overlain by the greenschist to granulite facies metapelitic cover, viz. the Almora Group,

moved southwards. The Chhiplakot and Askot klippes and the Almora Nappe are tectonostratigraphically identical,

detached tectonic outliers of this once-continuous thrust sheet. The slivers of the granitic gneisses of around

1875±90 Ma (Rb-Sr whole rock; Trivedi et al., 1984) representing the Ramgarh Group at the base of the Almora

Group in the Almora Nappe, along with their equivalents dated at 1878±19Ma (U-Pb zircon; Mandal et al., 2016)

from the Askot Klippe, are likely to representat the Precambrian basement of the Indian Craton. Although the pelitic

Almora gneisses discussed above and the Ramgarh granite gneisses were not distinguished by Mandal et al. (2016)

in the klippe, the 1865±60Ma Almora gneisses (Rb-Sr whole rock; Trivedi et al., 1984) in the Almora Nappe and the

1857±19Ma (U-Pb zircon; Mandal et al., 2016) pelitic granite gneisses from the core of the Askot Klippe belong to

the metapelitic cover of the leading edge of the Indian Plate. The edge of a Palaeocontinental arc in the Lesser

Himalaya yielded an age of 1830±50Ma based on U-Pb zircon dating after Kohn et al. (2010). The „thick-bedded

rusty quartzite‟ picked up from the „central part‟ of the Almora Nappe has been assigned a maximum depositional

age of 580 Ma by Mandal et al. (2014) and may well represent the Higher Himalayan elements sensu stricto, which

are missing in Askot, due to a deeper level of erosion.

29

Fig. 19. Simplified cross-section of the Kumaun Lesser Himalaya along X-Y from north to south (modified after Valdiya, 1980;

Celerier et al., 2009).

Based on the bulk rock and elemental geochemistry of both the pelitic gneisses of the Almora Group and the

granitic gneisses of the Ramgarh Group, it can be concluded that the Ramgarh granites are continental arc granites,

likely to be associated with the Palaeoproterozoic amalgamation of the Columbia supercontinent, based on ages

given by Mandal et al. (2016), and these are products of subduction-related midcrustal anatexis. The Almora Group

was deposited and metamorphosed as a supracrustal cover.

Acknowledgements

We thankfully acknowledge the financial support from the UGC CAS-I &II grant and DST, New Delhi for financial

assistance to MJ. BPD acknowledges a fellowship from the UGC, New Delhi. We also thank the Head, Department

of Geology, Banaras Hindu University for providing essential facilities for this work. MJ gratefully acknowledges

the discussions over a prolonged time with Prof. Ram S Sharma, Prof. K.S. Valdiya, Dr. O.N. Bhargava and Prof.

A.K. Jain. The Activation Laboratory, Canada, and the WIHG, Dehradun, are acknowledged for the chemical

analyses.

Reference

Abdel Rahman, A.M., 1994. Nature of biotites from alkaline, calcalkaline and peraluminous magmas. Journal of Petrology, 35:

525-541.

Auden, J.B., 1935. Traverses in the Himalaya. Records of the Geological Survey of India, 69: 123–167.

Baig, M.S., Lawrence, R.D. and Snee, L.W., 1988. Evidence for latePrecambrian to early Cambrian orogeny in

northwestHimalaya. Pakistan Geological Magazine, 125: 83-86.

Bhargava, O.N., 1980. The tectonic windows of the LesserHimalaya. Himalayan Geology, Wadia Institute of Himalayan

Geology, 10: 135-155.

30

Bhargava, O.N. and Bassi, U.K., 1994. The crystalline thrust sheets in the Himachal Himalaya and the age of amphibolite grade

metamorphism. Journal of Geological Society of India, 43: 343-352.

Bhargava, O.N., 2011. Early Palaeozoic paleogeography, basin configuration, palaeoclimate and tectonics in the Indian Plate.

Memoir, Geological Society of India, 78: 69-99.

Boynton, W.V., 1984.Cosmochemistry of the rare earth elements; meteorite studies. In: Rare earth element geochemistry.

Henderson, P. (Editors), Elsevier Sci. Publ. Co. Amsterdam, 63-114.

Chappell, B.W. and White, A.J.R., 1992. I- and S-type granites in the Lachlan Fold Belt. Earth and Environmental Science,

Transactions of the Royal Society of Edinburgh, 83:1-26.

Chappell, B. W., & White, J. R. (2001). Two contrasting granite types: 25 years later. Australian Journal of Earth Sciences, 489-

499.

Chinner, G.A., 1960. Pelitic gneisses with varying ferrous/ferric ratios from Glen Clova, Angus, Scotland. Journal of Petrology,

1: 178-217.

Condie, K.C., 2002. Breakup of a Paleoproterozoic supercontinent. Gondwana Research, 5: 41–43.

Connolly, J., 2005. Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its

application to subduction zone decarbonation. Earth and Planetary Science letters, 236: 524-541.

Cullers, R.L., Graf, J., 1983. Rare earth elementsin igneous rocks of the continental crust: intermediate and silicic rocks,

orepetrogenesis. Rare-Earth Geochemistry (Henderson, P., ed.), 275–312.

Dasgupta, S., 1995. Jaishidanda Formation. In: O.N. Bhargava (Ed.), The Bhutan Himalaya: a Geological

Account.GeologicalSurveyof India, Calcutta. Special Publication, 39: 79-88.

Debon, F. and Le Fort, P., 1983. A chemical mineralogical classification of common plutonic rocks and association. Transactions

of Royal Society, Edinburgh, Earth Sciences, 73: 135- 149.

DeCelles, P. G., D. M. Robinson, J. Quade, T. P. Ojha,C. N. Garzione, P. Copeland, and B. N. Upreti, 2001. Stratigraphy,

structure, and tectonic evolution of the Himalayan fold-thrust belt in western Nepal. Tectonics, 20: 487-509.

Deer et al. 1963. Rock forming minerals, Sheet Silicates. London, Longman Green and Co., 3:1-270.

Exner, C., 1949. Tektonik, Feldspatausbildungen und derengegenseitige Beziehungen in den ostlichen Hohen Tauren.

Beitragezur Kenntnis der Zentralgneisfazies. I. Teil.-TMPM, ser. 3, 1: 197-284.

Ferry, J.M., Spear, F.S., 1978. Experimental calibration of the partitioning of Fe and Mg between biotite and garnet.

Contribution to Mineralogy and Petrology, 66: 113-117.

Frost, B.R., Barnes, C.G, Collins, W.J, Arculus, R.J., Ellis, D.J. and Frost C.D., 2001. A Geochemical Classification for Granitic

Rocks, Journal of Petrology, 42 (11):2033-2048.

Fuchs, G., 1968. The geological history of the Himalayas. Proc. 23rd Geological Congress, 3:161-174.

Gansser, A., 1964.Geology of the Himalayas. Interscience Publishers, John Wiley & Sons Ltd London, 289.

Gehrels, G. E., P. G. DeCelles, A. Martin, T. P. Ojha, G. Pinhassi, B. N. Upreti, 2003. Initiation of the Himalayan orogen and an

early Paleozoic thin-skinned thrust belt. GSA, 13: 4-9.

Gehrels, G.E., DeCelles, P.G., Ojha, T.P., Upreti, B.N., 2006. Geologic and U-Pb geochronologic evidence for early Paleozoic

tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya. Journal of Asian Earth Sciences, 28: 385-408.

Heim, A. andGansser, A., 1939. Central Himalaya: Geological Observations of the Swiss Expedition 1936, Zürich. Mémoires de

la Société Helvétique des Sciences Naturelles, 73: 1, 245.

Harris, N.B.W., Pearce, J.A., Tindle, A.G., 1986. Geochemical characteristics of collision-zone magmatism. In: MP Coward and

ACRies (Eds.), Collision Tectonics. Geological Society of London Special Publication, 19: 67-81.

Heim, A. and Gansser, A., 1939. Central Himalaya – Geological observations of Swiss expedition 1936. Mem. Soc. Helv. Sci.

Nat., 73: 1–245.

Henderson,P., 1980. Rare earth element partition between sphene, apatite, and other coexisting minerals of the

Kangerdlugssuaqintrusion, E. Greenland. Contribution to Mineralogy and Petrology, 72:81-85.

Holdaway, M.J., Lee, S.M., 1977. Fe-Mg cordierite stability in high grade pelitic rocks based on experimental, theoretical and

natural observations. Contribution to Mineralogy and Petrology, 63: 175-198.

31

Holland, T., and Powell, R., 1998. An internally‐ consistent thermodynamic dataset for phases of petrological interest. Journal

of Metamorphic Geology, 16: 109–343.

Holtz, F. and Barbey, P., 1991. Genesis of peraluminous granites II. Mineralogy and chemistry of the Taurem complex (North

Portugal). Sequential melting vs. restite unmixing. Journal of Petrology, 32 (5): 959-978.

Islam, R., Ghosh, S.K., Vyshnav, S. and Sundriyal, Y.P., 2011. Petrography, geochemistry and regional significance of

crystalline klippen in the Garhwal Lesser Himalaya, India. Journal of Earth System Science, 120: 489-501.

Joshi, M., Singh, B.N. and Goel, O.P., 1994. Metamorphic conditions of the aureole rocks from Dhunaghat area, Kumaun Lesser

Himalaya. Current Science, 67: 185-188.

Joshi, M., Tiwari, A.N., 2005. Evidence of upper amphibolite facies metamorphism from Almora Nappe, Kumaun Himalaya. In

„Granulite Facies Metamorphism and Crustal Evolution‟...) Felicitation volume of Prof. R. S. Sharma, “Metamorphism

and Crustal Evolution” published by Atlantic Publishers and Distributers, 215 - 229.

Joshi, M., 1999. Evolution of the Basal Shear Zone of the Almora Nappe, Kumaun Himalaya. Memoirs of Gondwana Research

Group, 6: 69–80.

Joshi, M., Kumar, A., Ghosh, P., Das, B.P. and Devi, Ph. M., 2018. North Almora Fault: A crucial missing link in strike slip

tectonics of western Himalaya, Journal of Asian Earth Sciences, in press, https://doi.org/10.1016/j.jseaes.2018.09.002.

Joshi, M., Tiwari, A.N., 2004. Quartz C-axes and metastable phases in the metamorphic rocks of Almora Nappe: Evidence of

pre-Himalayan signatures. Current Science, 87 (7):995–999.

Joshi, M., Tiwari, A.N., 2009. Structural events and metamorphic consequences in Almora Nappe during Himalayan collision

tectonics.Journal of Asian Earth Sciences, 34: 326-335.

Kohn, M.J., Paul, S.K., Corrie, L.S., 2010. The lower Lesser Himalayan sequence: A Palaeoproterozoic arc on the northern

margin of the Indian plate. Bulletin of Geological Society of America, 122: 323-335.

Le Fort, P., 1975. Himalayas: The collided range. Present knowledge of the continental arc. American Journal of Science,

275A:1–44.

Le Fort, P., 1996. Evolution of the Himalaya: In Yin, A., and Harrison, T.M., eds., the Tectonic Evolution of Asia. New York,

Cambridge University Press, 95–106.

Le Fort, P., and Rai, S.M., 1999. Pre-Tertiary felsic magmatism of the Nepal Himalaya: recycling of continental crust. Journal of

Asian Earth Sciences, 17: 607-628.

Liu, S.W., Pan, Y.M., Xie, Q.L., Zhang, J., Li, Q.G., Yang, B., 2005. Geochemistry of the Paleoproterozoic Nanying granitic

gneisses in the Fuping Complex: implications for the tectonic evolution of the Central Zone, North China Craton. Journal

of Asian Earth Sciences, 24: 643–658.

Malisa, E.P., 2005. Petrography and mineral chemistry of the pelitic and semi-pelitic gneisses of the Merelani Tanzanite mining

area, Northeastern Tanzania. Tanzania Journal of Science, 31: 81-92.

Mandal, S., Robinson, D.M., Khanal, S. and Das, O., 2014. Redefining the tectonostratigraphic and structural architecture of the

Almora klippe and the Ramgarh–Munsiari thrust sheet in NW India, in Mukherjee, S., Carosi, R., van der Beek, P. A.,

Mukherjee, B. K. & Robinson, D. M. (eds) Tectonics of the Himalaya. Geological Society of London Special

Publications, 412: 247-269.

Mandal, S., Rabinson, D.M., Kohn, M.J., Khanal, S., Das, O. and Bose, S., 2016. Zircon U-Pb ages and Hf isotopes of the Askot

klippe, Kumaun, northwest India: Implications for Paleoproterozoictectonics, basin evolution and associatedmetallogeny

of the northern Indiancratonic margin, Tectonics, 35(4): 965-982.

Meert, J.G., Santosh, M., 2017. The Columbia supercontinent revisited. Gondwana Research, 50: 67-83.

Miller, C., Klötzli, U., Frank, W., Thöni, M., Grasemann, B., 2000. Proterozoic crustal evolution in the NW Himalaya (India) as

recorded by circa 1.80 Ga mafic and 1.84 Ga granitic magmatism. Precambrian Research, 103: 191-206.

Molnar, P., Tapponier, P., 1975. Cenozoic tectonics of Asia effects of a continental collision. Science, 189 (4201): 419-426.

Nance, R.D., Worsley, T.R., and Moody, J.B., 1988. The supercontinent cycle: Scientific American, 259: 72–79.

32

Navarro, G.R.B., Zanardo, A., Conceição, F.T. D., 2013. Metamorphic evolution and P-T path of gneisses from Goiás

Magmatic Arc in southwestern Goiás State. Brazilian Journal of Geology, 43(2):301-315.

Neymark, Leonid. &Yu,Amelin. V., & Larin, A., 1994. Pb-Nd-Sr isotopic and geochemical constraints on the origin of the 1.54–

1.56 GaSalmirapakivi granite-Anorthosite batholith (Karelia, Russia). Mineralogy and Petrology, 50:173-193.

Patel R.C., Adlakha, V., Singh, P., Kumar, Y. and Lal, N., 2011. Geology, Structural and Exhumation history of the Higher

Himalayan Crystallines in Kumaun Himalaya, India. Jour. Geol. Soc. India, 77: 47-72.

Pearce, J.A., Harris, N.B.W., Tindle, A., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic

rocks. Journal of Petrology, 25: 956-983.

Perchuk, L.L. and Lavrent‟eva, I.V., 1983. Experimental investigation of exchange equilibria in the system cordierite-garnet-

biotite; In: Kinetics and equilibrium in mineral reactions (ed.) Saxena S.K., Springer-Verlag, New York, 199–239.

Rao, D.R. and Sharma, R., 2009.Petrogenesis of the Granitoid Rocks from Askot Crystallines, Kumaun Himalaya. Journal of

Geological Society of India, 74:363-374.

Rao, D.R. and Sharma, R., 2011. Arc magmatism in eastern Kumaun Himalaya, India: A study based on geochemistry of

granitoid rocks. Island Arc, 20:500-519.

Regmi, K.R. and Arita, K., 2008. Major and trace element geochemistry if augen gneisses from Higher Himalayan crystalline

zone, Main Central Thrust zone and Lesser Himalayan dome in Tamakoshi-LikhuKhola area, east Nepal. Himalayan

Geology, 29: 95-108.

Rogers, J., Greenberg, J., 1990. Late-Orogenic, Post-Orogenic, and Anorogenic Granites: Distinction by Major-Element and

Trace-Element Chemistry and Possible Origins. The Journal of Geology, 98: 291-309.

Rogers, J.J.W., 2000. Origin and fragmentation of the possible approximately 1.5-Ga supercontinent Columbia: Geological

Society of America Abstracts with Programs, 32: A-455.

Rogers, J.J.W., Santosh, M., 2002. Configuration of Columbia, A Mesoproterozoic supercontinent. Gondwana Research, 5: 5–22.

Rollinson, H.R., 1983. The geochemistry of mafic and ultramafic rocks of the Archean greenstone belts of Sierra Leone.

Mineralogical Magazine, 47: 267-280.

Schwab, F.L., 1978. Secular trends in the composition of sedimentry rock assemblages: Archean through Phanerozoic Time.

Geology, 6: 532-536.

Sharma, I.N. and Prakash, D., 2007. Reaction textures and P-T conditions of opx - crd gneisses from karimnagar, Andhar

Pradesh, India. Mineralogy and Petrology, 90: 175-197.

Sharma, K.K. and Rashis, S.A., 2001. Geochemical Evolution of Peraluminous Paleoproterozoic Bandal Orthogneiss, NW

Himalaya, Himachal Pradesh, India: Implications for the Ancient Crustal growth in the Himalaya. Journal of Asian

EarthSciences, 19: 413-428.

Singh, BN. and Joshi, M., 2001. Geochemical characteristics of the Debguru Granites of Ramgarh Group from Dhunaghat Area

of Kumaun, Lesser Himalaya. Indian Mineralogist, 35:113-120.

Singh, R.K.B., 2010.Geochemistry and Petrogenesis of Granitoids of Lesser Himalayan Crystallines, Western

ArunachalHimalaya.Journal of Geological Society of India, 75: 618-631.

Sinha-Roy, S., 1980.Stratigraphic relation of the Lesser HimalayanFormations of the Eastern Himalaya. In: K.S. Valdiya and

S.B.Bhatia (Ed.), Stratigraphy and Correlations of LesserHimalayan Formations. Hindustan Publication Corporation

(India), Delhi, 242-254.

Spear, F.S. and Cheney, J.T., 1989. A petrogenetic grid for pelitic schists in the system SiO2 - Al2O3 - FeO - MgO - K2O - H2O.

Contributions to Mineralogy and Petrology, 101:149-164.

Spear, F.S., Kohn, M.J., and Cheney, J.T., 1999. P-T paths from anatectic pelites, Contributions to Mineralogy and Petrology,

134:17-32.

Srikantia, S.V. 1977. Sedimentary cycles in the Himalaya and their significance on theorogenic evolution of the mountain belt:

International Colloquium. InternationalCentre of National Research in Science (CNRS), 268: 395-407.

Thompson, A.B., 1974. Calculation of muscovite-paragonite-alkali feldspar phase relation. Contribution to Mineralogy and

Petrology, 44: 173–194.

33

Thompson, A.B., 1976. Mineral reaction in pelitic rocks: I. Prediction of P–T–X (Fe–Mg) phase relations. II. Calculation of

some P–T–X (Fe–Mg) phase relations. American Journal of Science, 276: 401–454.

Turner, F.J., Verhoogen, J., 1960. Igneous and Metamorphic Petrology. 2nd edn. McGraw-Hill Book Co. New York, 694.

Valdiya, K.S., 1980. Geology of the Kumaun Lesser Himalaya: Dehradun, India. WadiaInstitute of Himalayan Geology, 291.

Valdiya, K.S., 1983. Lesser Himalayan Geology: Crucial Problems and Controversies. Current Science, 52: 839-857.

Valdiya, K.S., 1995. Proterozoic sedimentation and Pan- African geodynamic development in the Himalaya. Precambrian

Research, 74, 35–55.

Valdiya, K.S., 2009. Behaviour of basement-cover decoupling in compressional deformation regime, Northern Kumaun

(Uttarakhand) Himalaya. Proc. Indian National Science Academy, 75: 27-40.

Valdiya, K.S., 2010. The Making of India: Geodynamic Evolution. Macmillan Publishers, India Ltd,816.

Villaseca, C., Barbero, L. and Herreros, V., 1998. A re-examination of the typology of peraluminous granite types in

intracontinentalorogenic belts. Earth and Environmental Science, Transactions of the Royal Society of Edinburgh, 89 (2):

113-119.

Wickham, M., 1987. Crustal Anatexis and Granite Petrogenesis during Low-pressure Regional Metamorphism: The Trois

Seigneurs Massif, Pyrenees, France.Journal of Petrology, Volume 28(1): 127–169.

Wilson, J.T., 1963. Evidence from islands on the spreading of ocean floor. Nature, 197: 536–538.

Winkler, H.G.F., 1978.Petrogenesis of metamorphic rocks. Springer Verlag, New York, 334.

Wood, D.A., Joron, J.L., Treuil, M., Norry, M.J., Tarney, J., 1979. Chemical composition and isotopic ratios of basic lavas from

Iceland and the surrounding ocean floor: Supplement to Wood, D.A et al. (1979) Elemental and Sr isotope variations in

basic lavas from Iceland and the surrounding ocean floor. Contributions to Mineralogy and Petrology, 70 (3):319-339.

Wu, C.M., Zhang, J., Ren, L.D., 2004. Empirical garnet–biotite–plagioclase–quartz (GBPQ) geobarometry in medium- to high-

grade metapelites. Journal of Petrology, 45: 1907-1921.

Yin, A., 2006. Cenozoic tectonic evolution of the Himalayanorogen as constrained by along-strike variation of structural

geometry, exhumation history, and forelandSedimentation. Earth-Science Reviews, 76: 1–131.