Collision tectonics in the Himalaya as evidenced by the Indus and Shyok rock assemblages

Tectcmophystcs, 134 (1987) 1-16 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Collision tectonics in the Himalaya as evidenced by the lndus and Shyok rock assemblages T. RADHAKRISHNA Centre for Earth Science Studies, Trivandrum

69.5 010 (India}

(Received August 9,198s; accepted August 2,1986)

Abstract Radhakrishna, T., 198’7.Collision tectonics in the Himalaya as evidenced by the Indus and Shyok rock assemblages. In: H.K. Gupta (Editor), Deep Seated Processes in Collision Zones. Tecto~oph~~~c~, 134: l-16. The Himalayan orogeny is closely associated with the tectonic developments which occurred along the Indus Suture Zone. During the last decade many new data on sediment stratigraphy and volcanic-plutonic suites have been obtained. These data have been interpreted in terms of plate tectonics. The Meso-Tethys was generated following the Permo-Triassic rifting and spreading during the Jurassic. The Tethys Ocean is postulated, in accordance with geophysical data, to have been consumed during northward subduction along the Indus Suture, resulting in the Himalayan orogeny through collision between India and Eurasia. However, the nature of tectonic processes and their chronology appear to be different in the Western and Eastern Himalayan segments. In the Western Himalaya, subduction in an island-arc setting and Eocene collision are evident mainly by the occurrence of the island-arc Dras volcanics, the Sangeluma Group (Srikantia and Rajdhan, 1980) trench deposits, ophiolitic blocks, older ( > 60 m.y.) talc-alkaline plutons and blueschist facies rocks. The Indus Group (S~k~tia and Rajdhan, 1980) continental deposits and the younger ( < 49 m.y.) ages from the Ladakh piutons are interpreted as being due to southward subduction of the back-arc (?) crust along the Shyok Suture Zone. The chronology of these events is discussed. In the Eastern and Central Himalaya, the lack of island-arc volcanism, the dominance of talc-alkaline plutonism and the epicontinental deposits to the north of the Indus Suture Zone are in favour of subduction in a Cordillarau type tectonic setting. The younger granite ages (ca 40 m.y.) indicate that continental collision occurred in the Eastern Himalaya at least lo-12 m.y. after the colhsion in the Western Himalaya.

Introduction It has been widely accepted that the Himalaya represents the largest Moulton range of the world formed due to collision tectonics. However, many problems concerning the generation of the oceans once covering the present area of the Himalaya, the nature of their destruction, the possible zones of collision and the post-collision effects remain unanswered. Research carried out during the early seventies (and probably late sixties) has outlined several tectonic models explaining these aspects. However, these models suffer a set back due to

lack of sufficient data and the intricate complexities of the region. The efforts of many Indian and foreign scientists have generated much new geological and geophysical data during the last decade and the geophysical studies have been reviewed by Gupta et al. (1982). According to them, the Indus Suture Zone (ISZ) (Fig. 1) represents the collision zone of India and Eurasia. This paper reviews the present status of geological (including sedimentation, petrology, geochemistry and geochronology) studies on all important rock units related to continental collision along the ISZ. Further, a tectonic model for the continental collision

2

Shyok

-

Gronitoid vv

Volcanic

Suture

Zone

rocks rocks

Fig. 1. Geological-tectonic

in the Himalaya following the subduction of the Tethys crust in an island-arc/Cordilleran type subduction is suggested. Sediments

Fossiliferous marine sedimentary sequences of the Palaeozoic and Mesozoic have been deposited with a facies continuity to the north of the central crystalline axis, over the northern Indian continental margin. The platform type marine sediments grade into pelagic deposits of flysch facies

Fig. 2. Detailed region

geological

in Tibet (based

map of the Dras in (A) the Ladakh

on Institute

of Geology,

China).

sketch map of the Himalaya.

in the Indus Suture Zone (Srikantia and Rajdhan, 1980; Tapponnier et al., 1981). Flysch type sediments occur again to the north along the Shyok Suture Zone (SSZ) (Thakur et al., 1981). The extent of Shyok sediments to the east in the Central and Eastern Himalaya is not yet known. The Indus Suture Zone (ISZ) Detailed litho-biostratigraphy of the ISZ (Fig. 2A) in the Western Himalaya has been given by Srikantia and Rajdhan (1980). They described the

region (based

on Srikantia

and Rajdhan,

1980) and (B) of the Xigaze

..,..,...,...,...___......_.

,.......I....._....I....,t.*.,‘*.._.

Kuksha

Skinding

mid

Middle Cretaceous

Palaeogene-Upper Cretaceous

Maklishan

Ladakh granite complex

~ncon~orrni?~

Diamictite with local sandstone and siltstone

Grey fine grained sandstone, siltstone, minor silty diamictite with shale, turbidite textures

Grey siltstone and splintery shale, carbonaceous shale, coarse sandstone, grit and diamictite

Source; Srikantia and Rajdhan, (1980); ages were assigned based on Srikantia and Rajdhan (1980) and Sahni et al. (19841.

Tecionic base

Volcanogenic shale, orbitohna limestone, local diamictite

Khaki

‘..‘.~.‘.........,..~...,..,

Volcanic-chert association, local limestones

Dras voicanics

gene(?)

Eocene to Upper Palaeo

Purple and grey cross bedded sandstone, siltstone and shale

Karit/Nurla

Shale, sandstone, siltstone, greywacke local diamictite, calcilutite, chert beds

Purple, grey and green diamictile, sandstone and shale

post-Eocene (Oligocene Miocene)

Dark grey shale, siltstone quartz aretrite, limestone, jaspary chert, sandstone, diamictite, sandstone

Shergal

sediment lithology

Karit/Hagnis

formation/ member

sediment lithology

formation assigned age

Indus group

Sangeluma group

Litho-stratigraphy of the Sangeluma and Indus group rocks

TABLE 1

to

Oligo-

. ..I...................__..r

Eocene

Younger than Eocene

Miocene cene

Lower Miocene

assigned age

w

4

ISZ in the Western

Himalaya

distinctly

stratigraphic

different

sequences Group

represented

by

and the southern

as two parallel,

but

and sedimentary

the

northern

Sangeluma

Group

Zanskar Group.

Zone Based

and

partly

on the fossil

Indus

Group

ranges between

(Table

(Sahni

et al., 1984).

from

the

evidence,

Sangeluma the Indus

Eocene and Miocene

in age

1). These two groups include the Indus Group and ..,.,17” “I ,.P *I., T ,.-_....,,-,1,.. ,.F JGQ116 C,..,ld {I /1003\ 1”LK3 111e ~&llayul c”lul-“e” “I 7.32,

Sediment stratigraphy along the ISZ in the _~‘.,0.... ,,+,3..,:,3, ,F thP 1 0rln17h K,lt ;, Irn.-.l p”“lly ..,,..l.. ~;LIJLc;III GACGLIJIVII “I LUG LLIUMII “GAL IJ “Gly

respectively. The Sangeluma

known except for a few results by Xiao Xuchang et al. (1980) and by the China-French expedition

(Pashkyum

Group

fault;

Fig.

to the north

1) over the Indus

and to the western

end over the Ladakh

The Spiti-Zanskar

sediments

Sangeluma

Group

is thrust

are thrust

to the south.

Group granite. over the

The Khalsi

For-

to the Xigage area (Tapponnier studies report ophiolites of the Yalu Zangbu tary sequences

et al., 1981). These

along the southern

(Tsangpo)

border

river with sedimen-

on either side. The sediments

of the

mation, representing the early sedimentary phase of the Sangeluma Group, comprises Orbitolina limestone together with volcanogenic sediments and sporadic diamictite of quiet shelf type de-

northern part are Late sub-flysch (greywake and deposits (Xia Xuchang et lar to the Indus Group

posits

According to Xia Xuchang et al. (1980) in the southern margin, Middle-Late Cretaceous reddish siliceous rocks and Triassic rocks of flysch affinity

of the Albian-Cenomanian

age. Unstable

conditions set in with the commencement of submarine (Dras) volcanic activity. Radiolarian cherts are

associated

with

Dras

volcanics,

indicating

deepening of the sea (Srikantia and Rajdhan, 1980). The Nindam Formation overlying the Dras volcanics comprises shale, siltstone and greywacke with turbidite textures indicating flysch sediment deposition during the Eocene age. The uppermost Shergal Formation of the Sangeluma Group is represented by argillaceous rocks, purple jaspery chert, limestone and diamictite and is reported to indicate a post-Eocene (up to Oligocene-Miocene) age. Some ultramafic and mafic blocks and a few volcanic units of ophiolitic occur within the Sangeluma

character (see below) Group with tectonic

contacts. The Indus Group represents sedimentation in a basin on continental crust without ophiolites and marine elements. The sediments overlie the Ladakh granite complex with an unconformity. The early _1____ _P __l:-_._,_r:_I_ ________r_l 1__. --pnase 01 S~~II~~~IL~LIUI~ IS represemeu uy amtinental type rocks of the Skinding Formation. This is followed by a flysch facies comprising siltstone, sandstone and shale displaying flute casts, graded bedding, load casts etc. of the Kukshu Formation. The Maklishun Formation witnesses shallowing of the basin with carbonaceous shale, diamictite (with granite clasts) and sandstone. The Karit Formation, at the culmination of the Indus Group sedimentation, constitutes molassic deposits and derived its clasts from the Spity-

Cretaceous-Palaeogene molasse) epicontinental al., 1980) probably simiof the Ladakh region.

with radiolarites occur. Probably these two formations are designated as Cretaceous wild-flysch (or melange) and Triassic flysch by Tapponnier et al. (1981). According to Tapponnier et al. (1981) the Triassic flysch corresponds to the accumulation of a prism along the passive northern margin of India, but the Cretaceous wild-flysch undoubtedly formed in a more active tectonic environment.

ssz The lithostratigraphy of the SSZ, as worked out by Thakur et al. (1981) and Thakur and Mishra (1984), constitutes ultramafic and mafic assemblages (dismembered ophiolites) along with sandstone, quart&e, limestone and a molasse deposit comprising sandstone, grit and conglomerate. Thakur (1981) has assigned a Cretaceous to Miocene-Pliocene age for the deposition of these sediments. Srikantia et al. (1982) reports a lower to middle Cretaceous age for the volcanic-stratigraphic succession. However, a satisfactory lithostratigraphic chronology is not available due to the intricate tectonic nature of the region. Volcanism The volcanic assemblages can be broadly gorised into three tectonic suites: the Panjal

catetraps

5

and other coeval volcanic rocks in the Tethys zone, Dras volcanics along the Indus Suture Zone and Khardung volcanics along the Shyok Valley. The first two volcanic suites have been studied by several workers, but the information on the Khardung volcanics is limited. Panjal traps and other coeval volcanism in the Tethys zone

The Panjal traps cover an area of approximately I2,OOO km2 in the Kashmir basin. They include basaltic lava flows with intermediate to acid pyroclastics, welded tuffs and agglomerate slates deposited under subaerial marginal marine to terrestrial conditions (Bhat and Zainuddin, 1978). The Panjal volcanics were assigned an upper-Middle Carboniferous-Triassic age based on their stratigraphic position (Bhat et al., 1981). In the Zanskar region, the corresponding volcanic sequence is represented by the Phe Volcanic Formation of Permian age (Srikantia et al., 1978). Similarly the volcanism is reported in the eastern parts of the Tethys Himalaya near Darjeeling in Sikkim Himalaya (Sinha Roy and Fumes, 1978) and as the Abor volcanics in Anmachal Pradesh (Bhat, 1984). Furthermore, in the melange zone of the Indus Suture basaltic sills and flows are intercalated with the Permian (?) and Triassic unit in the Ladakh district (Honegger et al., 1982). All these volcanic units show variable degrees of post magmatic alterations. The geochemical data on all these upper Palaeozoic-Triassic volcanics have been used (Nakazawa and Kapoor, 1973; Sinha Roy and Fumes, 1978; Bhat et al., 1981; Honegger et al., 1982; Bhat, 1984; Radhakrishna and Divakara Rao, 1987) to identify the tectonic settings of these isolated volcanic suites adopting conventional geochemical discriminatory characteristics. In summary the volcanics of Permian age were grouped into either tholeiitic or alkaline suites and are comparable to volcanics from rift zones (e.g. Aden Afar region: Cox et al., 1970; Barberi et al., 1975; and Ethiopian Plateau: Zenettin et al., 1974; Anonymous, 1981). Dras volcanics

The Dras volcanics constitute a major geological unit of the Sangeluma Group in the Indus

Suture Zone and outcrop over a distance of 400 km through Dras where they are best exposed. Equivalent units are not yet known in the central and eastern parts of the Himalaya. These volcanics are composed of basalt, basaltic andesite and minor decite (Honegger et al., 1982; Dietrich et al., 1983; Radhakrishna et al., 1984), intercalated with pyroclastics, volcanic elastic sediments and radiolarian cherts. The Dras volcanics have been assigned an Upper-mid Cretaceous to Paleocene age, based on their stratigraphic position between the Khalsi and Nindam formations of the Sangeluma Group (Srikantia and Rajdhan, 1980). Honegger et al. (1982) and Dietrich et al. (1983) have suggested an Upper Callovian-Tithonian age because the intercalated radiolarian cherts were assigned to this age. According to them the volcanism must have been active well into the Upper Cretaceous based on evidence from inclusions of Albian to Cenomanian orbitolina limestones within the Dras volcanics. Radiometric ages on this formation are very sparse. Sharma et al. (1978) has obtained a whole rock K-Ar age of 77.5 + 1 m.y. Frost et al. (1984) have reported K-Ar ages between Upper Jurassic and Palaeogene (188.6 + 11.3; 157.3 f 12; 57.9 + 2.3 m.y.) on a relatively Ar retentive mineral hornblende from the Dras volcanics. Honegger et al. (1982) have obtained a geologically unrealistic age of 264 m.y. from the slope of a Rb-Sr pseudo isochron. The older ages (Jurassic) probably relate to the volcanics associated with the Tethys ophiolite sequence which are also intimately intercalated with radiolarites. The difficulty in discriminating between the Dras volcanics and volcanic units of the Tethys ophiolite sequence might have caused the discrepancies in the reported ages. However, the Upper Cretaceous to Palaeogene age based on lithostratigraphic succession appears to be justified. The individual elemental data of the Dras volcanics suggest that a few rocks were subjected to alteration (Radhakrishna and Divakara Rao, 1983; Dietrich et al., 1983). Severely altered samples have been omitted from the interpretations of tectonic settings of the rocks. The basalt and andesite units of the Dras volcanics are either quartz or olivine normative. They are char-

i_aCe

Nd

Sm Eu Gd

Tb

Tm Y!J Lu

Fig. 3. REE patterns of Dras basalts (data source: Honegger et al., 1982; Radhakrishna et al., 1984).

acterised by lower K,O (0.57~), TiO, (0.74%); PzOs (0.20%), Rb (5 p.p_m), Pb (26 p.p.m.), Ba (89 p.p.m.), Sr (198 p.p.m.) and Zr (48 p.p.m.) concentrations and lower La/Yb (< 3.5) ratios than most intraplate tholeiites. These rocks show an increase in SiO,, FeO’ and TiO, with fractionation. Such ch~acte~stics were att~buted to their typical island arc tholeiitic nature (Honegger et al., 1982; Radhakrishna et al., 1984). The RE plots of Dras volcanic rocks (Fig. 3) show en-

riched to depleted LREE patterns. Even though the LREE enrichment is attributed to alteration, the depleted LREE pattern with high La,/Sm, ratios is not in favour of MORB characters (Sun et al., 1979). The Dras rocks plot in the island arc tholeiitic field in the immobile trace element disc~~nato~ diagrams (Fig. 4). The minor dacitic units, according to Dietrich et al. (1983) are quartz and hypersthene normative and contain low concentrations of Ba (average 336 p.p.m.), Sr (average

(b)

Zt

I

Y

.10/Z-

t-

/ Zr

Y.3

I

1

WlfHlN /-/ PLNTE BdSALT

40

100

/

1

500

Zr

Fig. 4. Plots of Dras volcanics in (a) Ti-Zr-Y diagrams. (Data source: same as Fig. 3.)

(after Pearce and Cann, 1973) and (b) Zr/Y-Zr

(after Pearce and Norry, 1979)

7

Rb (average

267 p.p.m.),

and Y (average

132 p.p.m.)

in dacites

to those

Southwest Lesser

Pacific,

Antilles

Zr (average

9 p.p.m.).

these compositions

to these workers lar

45 p.p.m.),

from

According

island

arcs in the

and are two to three

and

range

initial

as the

Honegger

ratio

mean

of 0.7035 of modern

basalt and andesitic

margins.

island

olivine,

plagioclase (Radhakrishna nocryst phases indicate

arcs

data

rocks are compatible

of

A

is in the same

et al., 1982). Geochemical

fractionation

the

times lower

than those in dacites along continental s7Sr/X”Sr

that

are very simi-

Tonga-Karmadec

(cf.

on the with the

clinopyroxene

and

et al., 1984). The phethat amphibole and

tit~omagnetite are crucial fractionation phases for the development of dacite phases from a primitive tholeiitic melt (Dietrich et al., 1983). This fractionation

history

the island arc tectonic

is also compatible

gabbros,

with

diorites

are

affected

greenschist

(lo-20%)

pumphellyte-prehinite

to

(Honegger

et al.,

1982). The age of the magma&m sial. Eocene-Miocene 1984) sediments granite The

Group

near

and

Kargil

(Honegger

observations

controver-

(Sahni

et al,

overlie the Ladakh

and are discordantly

cene molasse logical

Indus

(Srikantia

granitoids

volcanics

remains

unconformably

complex

Rajdhan,

1980).

intrude

the

overlain

by Paleo-

Dras

et al., 1982). These geo-

place

the plutonism

in the

time span between upper Middle Cretaceous Paleocene. The K-Ar, 40Ar/39Ar and Rb/Sr chronological data on these plutons in Table 2. The ages fall broadly

and geo-

are compiled into two age

brackets between 100-60 m.y. and less than 50 m.y. However, these dates have limitations since K-Ar

Khardurg volcanics

by

and granites

facies metamorphism

low-grade

setting.

(20-30%)

metamorphism

system

erogeneity

and

within

may have affected

there

is

the plutonic

considerable suite.

the het-

Preliminary

belt

results on zircon dating have been obtained on the Ladakh and Xigaze plutons (Table 2). These ages

of variable widths (3-10 km) over a distance of about 400 km through Khardung along the northern border of the Ladakh batholith. They generally overlie the Ladakh batholith, but occasionally the latter has intrusive an relation with the

support the occurrence of two age groups. However, the absence of field evidence for these groups poses the question whether the plutonic activity is continuous or whether it occurred in different phases. In the Western Himalaya, the pronounced

The Khardurg

volcanics

occur in a linear

volcanics (Thakur, 1981). The volcanics include acid and intermediate flows, volcanoclasts and tuffs with limestones).

interbedded sediments (shales and Fossil evidence indicates a Lower

Cretaceous age (Thakur et al., 1981). A few K-Ar ages of 26-38 m.y. were reported by Sharma et al. (1978) and Frost et al. (1984). However, more stratigraphic and radiometric data is required to confirm the age. There are as yet no geochemical data on the Khardurg

volcanics.

Plutonism The plutonic

rocks occur in a 30-50

km wide

batholithic belt along the Himalaya immediately to the north of the indus Suture Zone. The studies have been concentrated so far on the plutons of the Kargil region of Ladakh and the Xigaze region of central Tibet. The belt comprises predominantly granodiorites (40-60%) with less abundant

age gaps between

60 and 50 m.y. and the Paleo-

cene or Eocene age of the rocks overlying the granites throughout suggest that the earlier granitic activity

culminated

ca. 60 m.y. Such an interpreta-

tion is also evident

from the palaeomagnetic

on Ladakh

(Klootwijk

indicate

plutons

an attachment

of India

data

et al., 1979) which to Eurasia

m.y. age. The younger (ca. 50 m.y.) be an intrusive phase into the older though the lower structural and levels of these intrusives in Ladakh possible to distinguish the different

ca 60

granites must granites. Almetamorphic makes it imphases in the

field, such differences were brought out in the high level int~sions of the Kohisthan arc (cf. Reynolds et al., 1983). In the Xigaze region, the ages range between 94 and 41 m.y. with an intermittent break at every 15 m.y. (Scharer et al., 1984b). Whether these findings are real or an artifact of sampling remains to be proved. Rigorous field checks and more geochronology are nec-

8

TABLE

2

Geochronology

of the plutonic

Approximate

suite of Trans~rn~ay~

Rock type

location

region

Whole

Age (m.y.)

Reference

rock,’ mineral

K/Ar Kargil

Gneiss

Bi

49.6 + 1.7

Hemiyu

Granite

WR

26

60 km N of

Coarse granite

Bi

35.7 _+ 1.4

Honegger

et al. (1982)

Shey

Granite

Bi

48.7 I

1.6

Honegger

et af. (1982)

30 km E of Shyok

Granite

Bi

47.3 F 1.6

Honegger

et al. (1982)

Kargil

Syeni te

Hb

82

Brookfield

and Reynolds

(1981)

Khaplu

Granodiorite

Bi

39

Brookfield

and Reynolds

(1981)

Khaplu

Pegmatite

Bi

44

Khaplu

Metadiorite

Hb

NE of Xigaze

Diorite

Bi

NE of Xigaze

Diorite

Hb

101

& 2

Maluski

et ai. (1982)

NE of Xigaze

Diorite

Bi

111

i

2

Maluski

et al. (1982)

NE of Xigaze

Diorite

Hb

113

+ 2

Maluski

et al. (1982)

Somau

Granodiorite

WR

80

rt 3

Somau

Granodiorite

Muscovite/

74.4&

Satpure

Granodiorite

White mica

Ha&

Granodiorite

rt 0.6

Saxena and Miller (1972) Sharma

et al. (1978)

Kargil

39Ar/40Ar f

6

Reynolds

et al. (1983)

39.7 + 0.2

Reynolds

et al. (1983)

93

Mahrski et al. (1982)

rt 0.5 i

2

Rb/Sr Honegger

et al. (1982)

2.5

Honegger

et al. (1982)

69.8 _t 3.2

Honegger

et al. (1982)

39

Desio and Zenettin

Biotite

(pebble

from

(1970)

molasse) 60

110

Granite

WR

Kargil

Granodiorite

Zircon

103

F 3

Honegger

Kargil

Granodiorite

Zircon

101

i

Scharer

et al. (1984)

Kargil

Granite

Zircon

60.7 + 0.4

Scharer

et al. (1984)

E of Xigaze

Diorite

Zircon

93.4 + 1

Scharer

et al. (1984)

E of Xigaze

Diorite

Zircon

94.2 rL: 1

Scharer et al. (1984)

S of Qushui

Granodiorite

Zircon

41.7 -+ 0.4

Scharer

et al. (1984)

S of Qushui

Granodiorite

Zircon

41.7 + 0.4

Scharer

et al. (1984)

Shey

Honegger

et al. (1982)

U/Pb

Many

2

et al. (1982)

fission track dates are not given here.

essary to establish the different magmatic phases. Geochemical results on the plutonic rocks indicate their talc-alkaline character as in many Cordilleran batholiths (Honegger et al., 1982; Divakara Rao, 1983). The low 87Sr/86Sr ratios between 0.7036 and 0.706 on various granite intrusives (Honegger et al., 1982) point to the generation of melts from mantle reservoirs without involvement of much older continental crust. On the

contrary Scharer et al. (1984a,b) and Gariepy et al. (1985) suggest the involvement of continental crust in magma genesis on the basis of inherited radiogenic lead in the granite zircons. However, conclusive interpretations based on Rb-Sr isotopic ratios require caution unless equilibrated systems in the Trans-Himalaya plutons are established.

Ophiolitic

rocks

Ophiolitic

rocks are reported

as isolated

pockets

all along the IS2 from west to east. In the northwestern

Himalaya,

batholith, almost

to the north

ophiolitic subparallel

(Thakur

rocks

occur

along

the SSZ

to the ISZ for about

et al., 1981). The extension

the east in the Central not yet known.

is very

much

Himalaya

is

data is available

knowledge

scanty.

400 km

of this SSZ to

and Eastern

Although

on the ISZ ophiolites, ophiolites

of the Ladakh

of the Shyok

Along

the

ultramafic and mafic rock (ophiolitic rences are reported in association

SSZ,

only

rocks) occurwith tectonic

melange and radiolarites (Thakur et al., 1981). Hence the description is mainly devoted to the Indus ophiolites. Studies on the ISZ ophiolites have been concentrated the Western Himalaya.

in the Ladakh region of However, following the

China-French expedition in Tibet, a few results are available on the Xigaze block of Tibet on the eastern side (Nicolas et al., 1981; Gopel et al., 1984; Girardeau et al., 1985) (Fig. 2B). In Ladakh, ophiolitic rocks occur as tectonic slabs within the sediments

and (Dras)

volcanics

ce

LO

Sm

-lb

Er

Eu

LU

in the last 10 yrs

of the Sangeluma

Fig. 5. REE plots of Dras lherzolite and hornblende

gabbro

(DR 26), dunite

(DR

5)

(DR 55).

fabric, the Dras lherzolite has been interpreted as depleted mantle peridotite after incomplete extraction of basaltic liquid (Radhakrishna et al., 1987). The

dunite-pyroxenite

gabbro-hornblende

Group. The plutonic and hypabyssal units with minor volcanic counterparts near Dras (Fig. 2A)

gabbro-diabase rocks around Dras have relict primary igneous textures. The cumulus dunite

include

rocks have high 100 Mg/(Mg

all the petrological

suite (Anonymous, lithostratigraphy

units

of the ophiolite

1972); but a regular sequential of the ophiolite is not en-

countered in the Dras area. Therefore, the rocks are considered to represent a dismembered ophiolite.

+ Fe) ratios varying

between 91 and 93 with high Cr (1757 p.p.m.) and Ni (1371 p.p.m.) contents. The pyroxenites contain

more

diminished

MgO

(17.27%),

FeO’

(5.28%), Ni (339 p.p.m.) and Cr (745 p.p.m.) values than the dunites. A decrease in SiO,, marginal

Harzburgites and lherzolites have a tectonic fabric. Geochemically harzburgites exhibit a mantle refractory chemistry with 100 Mg/(Mg + Fe) ratios always 90 and lower CaO (0.75%), Al,O,

increase in Al,O, and Fe0 CaO to the more evolved

and little change in gabbros is noticed

(0.53%) and TiO, contents than many noncumulus peridotites of other ophiolites (Coleman, 1977).

clinopyroxene and plagioclase as cumulus phases and a minor hornblende control in the later stages.

The lherzolites exhibit variable 100 Mg/(Mg + Fe) (84-87) with complementary variations in other major and minor elements (CaO = 6.6-1.5%, Fe0 = 14-9%; NiO = 700-1300 p.p.m.)_ The REE pattern of this lherzolite depicts a general depleted LREE to enriched Lu trend with no Eu anomaly (Fig. 5). Thus, with respect to REE and major element distributions, in the light of their tectonic

A noticeable point here is the absence of orthopyroxene as a cumulus phase and in this respect the Dras sequence resembles the Vourinos ophiolite suite and differs from the Marum, Betts Cove and Troodos ophiolites where orthopyroxene is a major cumulus phase.

(Radhakrishna et al., 1987). Such chemical changes reflect the differentiation of olivine, chromite,

The Dras dunite shows essentially flat HREE and a LREE depleted pattern from Eu to La (Fig.

IO

5) contrasting

with the “U”

shaped

Newfoundland

dunites

et al., 1979). How-

(Suen

ever, the Dras hornblende LREE depleted anomaly other

ments

gabbro

volcanics. CaO/TiO, TiO,/P,O, foundland

cumulus

and ocean-floor

positive

Eu

gabbros

of

gabbros

(Dostal

1978; Suen et al., 1979). The diabase

of Dras but

of

shows a strongly

with a marked

(Fig. 5) as in typical

ophiolites

and Mucke, dykes

pattern

patterns

occur

within

are nowhere They

have

(10.7),

the Sangeluma

connected high

TiO,

Al,O,/TiO,

to the

Dras

(0.82%),

and

(18)

(3.3) ratios as in the diabases (Suen

sedi-

and

of New-

et al., 1979) and of MORB

and

in the Indian

Ocean (Frey et al., 1977) and of site

236 in the Somali

basin

in the northwest

Ocean (Frey et al., 1980). Compositions these rocks are derived

Indian

similar

to

by second stage melting

of

a refractory peridotite (Langmuir et al., 1977; Duncon and Green, 1980; details are in Radhakrishna

et al., 1987).

On the Tibetan

side, near Xigaze the ophiolites

are well exposed in an east-west length and a maximum al., 1981) According of ultramafic

belt of - 170 km

width of 20 km (Nicolas to these authors

et

large masses

and mafic rocks have been preserved

(Fig. 2) from the intense

deformation

contrary

to

are distinct from the island arc fields of Sun and Nesbit (1978). The volcanic rocks of the ophiolite melange are

the highly deformed and dismembered thrust slabs of the Ladakh region to the west. The cross-sec-

characterised by low contents of immobile elements such as Ti. P, Zr and Y and have almost

Cr diopside-bearing harzburgite at the base and a sedimentary cover over volcanics with other ultramafic-mafic units at the central portion. The intermediate zone has a few dissimilarities with the standard ophiolitic sections reported elsewhere: a (- 3.5 km) crust compared to the normal crust (- 5 km), an absence or paucity of cumulate gabbros which are abundant in other ophiolites and the occurrence of diabase swarm as sills rather than dyke swarms as in other ophiolites (Nicolas et al., 1981). The occurrence of Cr diopside-bearing

flat REE patterns (Honegger et al., 1982). These authors attributed these characters to their MORB origin contrary to the island arc characteristics of Dras volcanics. Based on experimental melting data (Mysen and Holloway, 1977), the Dras LREE depleted melts could be derived from a mantle source that has already undergone an earlier phase of LREE depletion. The distribution coefficients of olivine/liquid suggests that the liquid in equilibrium

with the olivine

(Mg 92) of Dras

dunite

would have had a 100 Mg/(Mg + Fe) ratio of at least 77-79. Similarly the partition coefficients of Ni between olivine and basaltic liquid at high temperatures and the Ni contents in the olivines of Dras dunite indicate at least 250 p.p.m. and possibly up to 500 p.p.m. Ni in the parent magmas. High Cr,O, (52.1-53.1%) and lower TiO, (0.2-0.3%) and Al,O, (14-17%) in the early formed chrome spinels, the calcic nature of plagioclase (An > 70), low Na,O (av. 2.68%) and high CaO/Na,O ratios in fresh Dras cumulus gabbros (Radhakrishna et al., 1987) and the absence of ilmenite as a primary phase with magnetite in the Dras cumulus units suggests tita~um poor and calcic-rich, chrome-rich parent magmas. These chemical characters and the olivine, plagioclase and clinopyroxene dominated crystallisation are likely to be related to the low pressure accumulates of MOR magmas, especially site 212

tion constructed

harzburgite

for the Xigaze ophiolite

is also a unique

complex. Petrologically Xigaze ophiolite indicate characters

(Girardeau

character

contains

of the Xigaze

the mafic rocks of the oceanic ridge assemblage

et al., 1985).

Pb isotopic results were pub~shed recently on the Xigaze harzburgite tectonites and magmatites (gabbros, dolerites and lavas) (Gopel et al., 1984). The magmatites yielded a Pb-Pb isochron age of 120 m.y. This is in agreement with the ca. 110 m.y. age of the radiolarian cherts overlying the magmatites (Marcoux et al., 1982). On the basis of this 120 m.y. crystallising age, an extrapolated 20’Pb/ 204Pb initial value of 15.406 has been calculated. compatible

These initial Pb isotopic ratios are with the values of oceanic basalts

especially in the Carlsberg ridge in the Indian Ocean (Dupre and Allegre, 2983) and the Inzecca and Bay of island ophiolites (Hamelin et al., 1984). The harzburgite tectonites and magmatites were interpreted to be the products of discrete magmas based on Pb isotope ratios of whole rock and

11

clinopyroxene sidered

separates.

The harzburgites

to be ca 280 m.y.

older

than

are conthe mag-

sion in the Himalaya.

Gansser

that

ophiolite

the

marcates

matites.

ISZ the

with northern

Plate. However, Metamorphic

belts

units

Studies

on this aspect in the Himalaya

limited. Even the existing restricted confined to the Western Himalaya. has reported

the occurrence

metamorphic

belts of contrasting

tions between

the Karakoram

are very

literature is Virdi (1981)

of two characteristic P and T condi-

block and the Tethys

the ultramafic

active

bile belt to the north fied (Kaila

belt (Varadarajan

deIndian

and mafic plutonic ophiolites

1977). Following

Tian-Shan,

Nan-Shan

this, mo-

of the ISZ has been identi-

and Narain,

Plate was postulated

of the

to represent

and Jhingran,

the seismically

occurrences

boundary

were not considered

(Varadarajan

(1966) considered

1976). Hence

the Indian

to extend up to the Tian-Shan and Jhingran,

1974; Kaila and Narain,

1972; Crawford,

1976; Thakur,

1981). The

zone. However, the various zones of metamorphism could not be mapped due to intricate

observations

thrusting and tectonic boundaries. The high pressure-low temperature metamorphic assemblages occur along the southern contacts of Indus Suture rocks. The metamorphosed basic units contain

and part of the volcanic units with other ophiolites elsewhere and present day oceanic crust, (b) an island arc origin for the Dras volcanics, (c) calc-alkaline plutonism similar to that at continental margins for the Trans-Himalayan plutons and (d) the marine flysch character of the ISZ sediments associated with ophiolitic rocks-undisputedly

glaucophane-bearing mineral assemblages (Frank et al., 1977; Virdi 1981). The high temperature-low pressure assemblages are formed to the north of the Khardung volcanics. This zone is characterised by the occurrence of higher grade amphibolite facies rocks such as garnet-mica schist, kyanite schist and garnet amphibolite (Thakur et al., 1981). The extension of these two belts to the Central and Eastern Himalaya is not yet known.

separated from the southblocks following the Per-

mian-Triassic rift volcanism which was widespread in the Tethys Himalaya. The validity of this interpretation can following observations:

of the ultramafic

study:

and mafic plutonic

(a) similarities assemblages

confirm that the ISZ demarcates the northern border of the Indian plate along which the MesoTethys

crust was destroyed

resulting

in the colli-

sion between India and Eurasia. This interpretation has support from recent palaeomagnetic data from both sides of the ISZ (Klootwijk

et al., 1979;

Pozzi et al., 1982; Achache et al., 1984; Besse et al., 1984). The seismically active Tian-Shan, NanShan and other mobile belts to the north of the

Discussion The Indian continent ern Tibet/Karakoram

of the present

be substantiated (a) sedimentary

by the facies

change from platform type to marine type after the volcanism (Srikantia et al., 1978) (b) identification of Permian-Triassic platform sediments in the exotic limestones of the ISZ flysch; (c) presence of relict Jurassic-Cretaceous oceanic crust as ophiolites along the ISZ and (d) differences in the sedimentary environment north and south of the Zangbo Suture developed from the Triassic onwards (Tapponnier et al., 1981). Gupta et al. (1982) have suggested from seismic data that the horizontal compression is responsible for the consumption of Tethys crust and colli-

ISZ also contain older ophiolites (Chang and Cheng, 1973) and might represent the earlier collision zones of continents. This implies the occurrences of several microplates to the north of the ISZ which are not related to the collision responsible for the Himalayan orogeny. On the contrary, the present seismic activity might be due to reactivation strike-slip

of the old mobile zones as a result of movements that occurred due to colli-

sion along the ISZ as suggested by Radhakrishna et al. (1984). Radhakrishna et al. (1984) indicated subduction of the Tethys Crust along the ISZ in an island arc setting prior to collision in the Western Himalaya contrary to the cordilleran model of Dewey and Bird (1970). A similar island arc model was suggested earlier by Honegger et al. (1982). Subduction along the ISZ must have been ini-

12

2 g PERMIAN N

Rifting

$RIASSICgJURASSIC

$XETACEOUS

$PALEOCEP(S

~SOCSNS~OLISOCSNE

$htIOCSNE

,$

and opening

of Tethys

Sea

Tethys expansion N-ward subduction along ISZ

I

Shyok Sea formation

1

Collision along ISZ (India and island arc)

0

S-ward subduction along SSZ

0

Collision along SSZ

Fig. 6. Time-space

relationships

are shown by solid column

tiated

of the tectonic

and the probable

in Albian-Cenomanian

events of the Himalayan timing of tectonic

(middle

of Middle

Cretaceous 100 m.y.) times giving way to fore-arc basin deposition represented by the early Khalsi Formation. Subduction and peak magmatic activity (Dras volcanism and plutons older than 60 m.y.) and the deposition of Flysch (Nindam Formation) must have occurred during the Late Cretaceous to Palaeogene or early Eocene (100-50 m.y.). The early stage of this activity could have resulted in the formation of a back-arc sea along Shyok. However, a possibility of the Shyok as a separate, independent, narrow sea contemporary to the Indus Tethys cannot be completely ignored until further data on Shyok sediments and volcanics

are available.

The post-Eocene

Shergal

Formation with olistostroms (marbles) and numulitic limestone clasts indicate closure of the Tethys during the Eocene (- 50 m.y.). Reduction in the rate of plate movement (20-15 cm to 10 cm/yr-‘) around 52 m.y. ago (Patriat and Achache, 1984), a lack of radiometric ages between 50-60 m.y. (Table l), a stratigraphic upper age limit of the Ladakh pluton in the Palaeogene constitute the additional evidence for the closure of the Tethys Ocean during the Eocene. The emplacement of ophiolitic rocks into all the stratigraphic units of the Sangeluma Group is a possible indication of their post-Eocene emplacement due to collision. The younger granites (< 50 m.y.) do not reflect

Orogeny.

The timing of tectonic

events where not fully established

processes

where established

are shown in open column.

subduction along the ISZ and probably relate to the tectonic activity along the SSZ. The southward subduction of Shyok oceanic crust during the Eocene-Oligocene and a later shift of the subduction to the Karakoram side has been suggested based on the corresponding radiometric dates from the Ladakh and Karakoram regions respectively (Reynolds et al., 1983). However, the S-type nature of these younger granites in the Karakoram indicates crustal anatexis and hence cannot be directly related to subduction processes. Altematively

the younger

ages (< 50 m.y.) from Ladakh

granites can be explained by the subduction and consumption of Shyok oceanic crust to the south underneath India in a Cordilleran tectonic setting. Such conclusion seems to be valid from the Indus Group (Eocene-lower Miocene) sediments. These sediments were deposited in a depression on Ladakh granites and include continental clasts (of granitic material); they are characteristic of marginal basins behind Cordilleran-type subduction. Brookfield and Reynolds (1981) gave a similar interpretation for the sediments overlying the Ladakh granite batholiths. The Miocene molasse deposits which include rock fragments of the Sangeluma Group and Zanskar ranges (Srikantia and Rajdhan, 1980) suggest uplift and erosion prior to the Miocene which resulted in thrusting of the Sangeluma sediments over the Indus

13

Group/Ladakh granite. The lack of knowledge about the occurrence of Shyok equivalent rocks in the Eastern Himalaya and the restricted occurrence of Dras island arc volcanics in the Western Himalaya only may suggest that the island arc model cannot explain the collision tectonics throughout the Himalaya. Alternatively, the dominant talc-alkaline pluto~sm and epi~ntinental sedimentation to the north of the IS2 in the Zanghu are in favour of Cordilleran type setting along the ISZ in the Eastern Himalaya. If this inte~retation is valid, the ages of about 100 m.y. and 40 m.y. of the plutons (irrespective of their periodic int~sion~ have to be related to the IS2 tectonic activity. In this case subduction was active in the eastern regions at least until 40 m.y. ago which implies that collision in the Western Himalaya had occurred at least lo-12 m.y. prior to the collision in the Eastern Himalaya. Conclusions

The debate on collision tectonics in the Himalaya during the last decade aimed at identifying the actual zone of collision and the processes responsible for collision. The proposed collision along the Tian-Shan mobile belt and the ISZ have become popular. However, the island-arc and/or talc-alkaline volcanism and pluto~sm, op~o~tic blocks, trench ocean basin sediments and blue schist facies rocks along the fSZ all favour the collision of India and Eurasia along the ISZ. The seismic activity along the Tian-Shan mobile belt (which is the main basis for the extended Indian Plate beyond the ISZ) could be the result of reactivation of this old erogenic belt consequent to the collision along the ISZ, Several models have suggested different processes which led to the final collision along the ISZ. These models can be summarised as follows: (a) Cordilleran activity along the ISZ has led to the final collision (Dewey and Bird, 1970); (bf pre-Cretaceous subduction along the Shyok was followed by upper Cretaceous-Tertiary subduction along the ISZ (Frank et al., 1977); (c) marginal basin crust formed along the ISZ due to subduction along the central crystalline axis; it

was subsequently consumed along the IS2 and caused the collision (Srikantia and Rajdhan, 1979); (d) the Indus and Shyok represent a single suture but are now separated due to erosion and removal of upper crustal layers (Rai, 1982); (e) a northward under-testing along ISZ was followed by the closure of back-arc crust along Shyok due to crustal shortening mecha~sms other than subduction processes (Thakur and Mishra, 1984) and (f) northward did-Cretaceous and southward Late Cretaceous subduction along the ISZ and on the Karakoram side respectively and later EoceneOligocene northward and lower Miocene southward subduction along the SSZ (Reynold et al., f983). These models have attained only a partial success and can not explain satisfacto~ly the complete tectonic processes along the ISZ. Therefore the follo~ng events are suggested here. The Tethys Oceanic Crust that was generated consequent to Permo-Triassic rifting all along the Himalaya, has been consumed in an island arc setting (Honegger et al., 1982; Trommsdorff et al., 1982; Reynolds et al., 1983; Radhakrishna et al., 1984). However, a Cordilleran model as suggested by Dewey and Bird (1970) is more plausible in the Central and Eastern Himalaya. In the Western Himalaya, the subduction in an island arc setting was active when India collided with the island arc during the late Palaeogene. Whether the Shyok ephiolitic rocks represent back-arc crust (Thakur and Mishra, 1984) or a separate narrow oceanic basin remains to be answered until the ~thostratigraphy of Shyok is critically evaluated. In any case the Shyok Oceanic Crust has been consumed by southward subduction along the SSZ in a Cordilleran setting without any shift of subduction to the north as suggested by Reynolds et al. (1983). The geochronological data implies that collision in the Central and Eastern Himalaya occurred at least lo-12 m,y. later than that in the Western Himalaya. Acknowledgements

The author is grateful to Dr. Harsh K. Gupta, Director, Centre for Earth Science Studies for his encouragement and permission to publish the paper. He is thankful to Prof. A. Kronner (GDR) and Dr. V. Divakara Rao (India) who have

14

reviewed

the manuscript

and suggested

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