geological development and phanerozoic crustal accretion in the western segment of the southern tien...

Geological development and Phanerozoic crustal accretion in thewestern segment of the southern Tien Shan (Kyrgyzstan,

Uzbekistan and Tajikistan)

M.E. Brook®eld

Land Resource Science, Guelph University, Guelph, Ontario, Canada N1G 2W1

Abstract

The Tien Shan form a high intracontinental mountain belt, lying north of the main India-Asia collision mountains, and consist

of re-activated Paleozoic orogens. The western segment of the southern Tien Shan lies northwest of the Pamir and west of the

Talas±Fergana fault. The stratigraphy, lithology, igneous and metamorphic petrology and geochemistry of this segment

indicate that it was formed by the assembly of Lower Paleozoic arcs which developed into microcontinents with Upper

Paleozoic mature shelf and slope clastic and carbonate sediments. Precambrian continental crust is con®ned to two small

blocks along its southern margin.

The bulk of the southern Tien Shan consists of ?Vendian to Silurian oceanic and slope clastic rocks, resting on oceanic

lithosphere, and overlain by thick passive margin Devonian to mid-Carboniferous mature shelf clastics and carbonates. These

are unconformably overlain by syn- and post-orogenic immature clastic sediments derived from mountains on the north formed

by closure of a Carboniferus southern Tajik and a northern Vendian to Carboniferous Turkestan ocean with the southern Tien

Shan microcontinent sandwiched between. Associated with these collisions are late Carboniferous to Permian intrusives, which

form three south to north (though overlapping) suites; a southern calc-alkaline granodiorite±granite suite, an intermediate

gabbro±monzodiorite±granite suite, and a northern alkaline monzodiorite±granite±alaskite suite. The gabbro±monzodiorite±

granite suite forms the earliest subduction-related magmatism of the southern Tien Shan: rare earth element patterns are

consistent with derivation from a primitive or slightly enriched mantle. The other suites show more crustal contamination.

Rb and Sr vary with depth and degree of partial melting and are consistent with progressive involvement of crustal material in

partial melts during collision. The gradual change in composition within each complex, lasting in some cases from 295 to

250 Ma (the entire Permian), may be explained by a consecutive shift in the melting sedimentary cover of the subducting plate

from oceanic crust through transitional crust to marginal continental crust. Like the Central Asian orogenic belt (the main focus

of IGCP 420), the Tien Shan represent a net addition of continental crust during the Phanerozoic. Very little of the belt has any

Precambrian precursor. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Phanerozoic crustal accretion; Southern Tien Shan

1. Introduction

The Tien Shan are a high intracontinental mountain

belt, lying north of the main Tertiary collisional

orogenic belts of the Hindu Kush, Pamir, Karakorum,

Himalaya, and Tibet, and south of the Paleozoic

orogens of Kazakhastan and Mongolia (Khain,

1985). They stretch in a straight line for about

2500 km from central Uzbekistan to eastern Xinjiang,

and are part of a dominantly Paleozoic orogenic

system which was formed by successive arc and

micro-continental collisions from Devonian to

Tectonophysics 328 (2000) 1±14

0040-1951/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.

PII: S0040-1951(00)00175-X

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Triassic times (Fig. 1). This system is unconformably

overlain by Jurassic to Recent sedimentary deposits

deformed mainly in Miocene to Recent times

(Sadibakasov, 1990). The present height and orienta-

tion of the Tien Shan is almost entirely due to defor-

mation associated with the late Cenozoic collision of

India with Asia (Abdrakhmatov et al., 1996).

The Southern Tien Shan are separated from the

Northern Tien Shan by Cenozoic intermontane basins

and are further divided into geologically rather

different western and eastern segments by the

Talas±Fergana fault (Fig. 1). The position of this

strike±slip fault is controlled by the rheological

contrasts between the Tarim platform and the Tadjik

basin during Cenozoic deformation (Burtman and

Molnar, 1993). Most of the east±west trending faults

in the southern Tien Shan are south-dipping reverse

faults related to deformation by the northward move-

ment of the Pamir (Shcherba, 1990). However, these

frequently re-activate earlier thrust sheets inherited

from Paleozoic deformation (Allen et al., 1999;

Yablonskaya, 1989). Recent earthquakes along these

faults give thrust solutions, as do all earthquakes

along the southern Tien Shan margin (Nelson et al.,

1987). Measurements show that the Pamirs and

Central Tien Shan presently converge at 0.02 m/yr.

And most of the Miocene to Recent shortening has

taken place at the front of the North Pamir (Zonen-

shain et al., 1990).

The western segment of the Southern Tien Shan,

forms a great mountain arc stretching westwards from

the Talas±Fergana fault (Fig. 2). Its eastern part is

disrupted and obscured by deformation related to the

Neogene deformation of the Pamir indenter. On the

west, Neogene deformation decreases, and the various

tectonic zones are more complete. The geological

maps of Central Asia show relatively little proved

Precambrian crust anywhere. The aim of this paper

is to provide stratigraphic, structural and magmatic

evidence that the southern Tien Shan is almost

entirely a Paleozoic±Triassic addition to continental

crust.

2. Units and stratigraphy

The western segment of the Southern Tien Shan has

several distinct zones whose nature was formerly

obscured by the concept of ®xist tectonics (e.g. Kukh-

tikov, 1968; Tulyaganov, 1972). Three Paleozoic

M.E. Brook®eld / Tectonophysics 328 (2000) 1±142

Fig. 1. Distribution of Paleozoic sedimentary rocks, known Precambrian basement, and plutons in the Tien Shan and Pamir (inset-location of

Tien Shan).

sutures separate distinct crustal blocks (Fig. 2). All the

zones can be seen in a representative cross-section at

the longitude of Dushanbe (Fig. 3), though southern-

most and northernmost zones required some further

information from adjacent areas; the Baysunta block

to the west, and the South Fergana unit to the east

(Fig. 4, Table 1). From south to north, Russian geol-

ogists distinguish the following major units (Rogoz-

hin, 1993), whose names I have simpli®ed, since each

unit tends to encompass a particular mountain range.

The stratigraphy and inferred environments in each

unit of the representative cross-section is summarized

in Fig. 5. The present trend of the western segment of

the southern Tien Shan is slightly oblique to the

Paleozoic structures. Thus, the Baysunta to Turkestan

zones are best seen in a transect from west of Dush-

anbe to Khodzhent, but thin out or are obscured by

younger deposits towards the east. The Turkestan-

Alai and South Fergana units are best seen south of

Osh to the east; but here, the Gissar to Turkestan

zones die out or perhaps reappear in part in the north-

ward trending units of the Fergana sigmoid structure

(Burtman, 1975). West of the Dushanbe±Khodzent

transect the Tien Shan mountains run into the Kara-

kum depression where the various zones are mostly

obscured by Jurassic to Recent sediments. The zones,

however, do appear in places in various inliers which

allow them to be traced as far as the (obscure) junction

with the Ural orogenic system (Brezhnev and Ilin,

1989).

(1) The Baysunta unit consists of a Proterozoic meta-

morphic core unconformably overlain by Lower

Carboniferous volcanics and Upper Carboniferous to

M.E. Brook®eld / Tectonophysics 328 (2000) 1±14 3

Fig. 3. Structural cross-section from just west of Dushanbe (location

shown in Fig. 2.).

Fig. 2. Structural units of western segment of southern Tien Shan.

M.E

.B

roo

k®eld

/T

ectonophysics

328

(2000)

1±

14

4

Fig. 4. Stratigraphic sections and interpretations for units (legend in Table 1).

Permian continental clastics. It is an exposed part of

the thinned, partly Precambrian, continental crust

underlying the Turan platform. The Proterozoic

consists of metapelites (plagioclase-bearing gneiss,

schists and migmatites) about 9 km thick, which

give K/Ar cooling ages of 650±600 Ma (Mirkhodz-

hayev et al., 1972). The overlying Lower Carbonifer-

ous (Tournaisian±Visean) is mostly continental and

consists of about 3 km of conglomerates, sandstones,

acid volcanics (quartz porphyries and dacites) and

tuffs, with some marine limestones in the middle.

The overlying Upper Carboniferous (Bashkirian±

Kasimovian) is mostly marine and begins with

submarine spilitic basalts which underlie a coarsening

upwards section of interbedded sandstones, siltstone,

and shales with occasional conglomerates and

limestones. Unconformably on these are latest Carbo-

niferous (Gzelian) conglomerates with limestone

lenses? Upper Permian non-marine conglomerates

unconformably cap these. I interpret the Carbonifer-

ous as forming in a continental rift basin.

Along its northern edge, the Baysunta zone appears

to grade into the South Gissar suture, although the

intricate Cenozoic faulting makes this dif®cult to

prove. A belt of disconnected ultrabasic masses asso-

ciated with the Lower Carboniferous acid volcanics

may be ophiolitic nappes.

(2) The South Gissar unit is an ophiolitic suture,

dominated by greenschist metamorphics, ophiolites,

and melanges, which separates the Baysunta unit

from the Gissar unit to the north (Pomaskov, 1972).

The younger Bogainskiy fault follows and disrupts

the suture (Portnyagin et al., 1976). South of the

Bogainskiy fault a belt of greenschist facies meta-

morphic rocks with occasional glaucophane-bearing

blueschists is at least 5 km thick. These derive from

pelitic carbonates, siltstones and shales with minor

basic rocks and give K/Ar cooling ages of 420±

320 Ma. Gao et al. (1995) noted similar cooling

ages of 345±350 Ma occur in the southern Tien

Shan high pressure belt of Xinjiang. North of the

Bogainskiy fault (lined with ultrabasic lenses), these

metamorphics are intruded by a north-westerly trend-

ing parallel basaltic dike swarms. The dikes appar-

ently feed Visean basaltic volcaniclastics overlain

by Namurian spilitic pillow lavas, hyaloclastics,


Table 1

Legend for ®gures

tuffs and cherty limestones (Portnyagin, 1974).

Northwards, the dikes increase in number, eventually

forming a homogeneous group of dikes with chilled

margins mostly on their northern sides (Pospelov,

1975). Despite alteration, plotting Pospelov (1975)

chemical analyses show that the dikes are low-

potassium tholeiites.

The South Gissar suture contains only Carbonifer-

ous and younger oceanic material and is too young in

relation to the Late Carboniferous collisions within

the Tien Shan to be anything other than a marginal

basin or narrow rift basin.

(3) The Gissar unit is dominated by huge late Paleo-

zoic batholiths which intrudes thin Ordovician to

Lower Carboniferous shelf sediments and Upper

Carboniferous to Permian volcanics. It passes into

and is faulted against the related Zeravshan unit to

the north. Between the two (and partially replacing

them) just north of the Pamir, is the Garm meta-

morphic block. east of Dushanbe, the Gissar range

batholith pass below the Vakhsh and related thrusts

marking the northern edge of Mesozoic-Tertiary sedi-

mentary nappes. This thrust zone is currently active

and marked by a zone of shallow earthquakes (Pavlis

and Hamburger, 1991).The Ordovician to Devonian

sections consist of mature deep shelf or slope

turbidites passing up into massive shelf limestones

(Tulyagonov, 1972; personal observations in 1990).

Unconformably on these are Carboniferous (Visean

to Bashkirian) andesite and dacite volcanics with

associated marine volcaniclastics and thin limestones

which pass up into early Moscovian clastic sediments

include marine intercalations. These are overlain by

coarse non-marine late Moscovian clastics which

appear to correspond with intense folding at the end

of the Middle Carboniferous, accompanied by numer-

ous granitoid intrusions and regional metamorphism

(Baratov, 1966). The intrusions, despite marked

differences in morphology, composition and texture,

form a coherent calc-alkaline suite with latest


Fig. 5. Location of plutons in Tien Shan and plutons analyzed by Solov'ev (1998). I±V gabbro±monzodiorite±granite complexes; VI and VII

granodiorite±granite complexes; VIII and IX monzodiorite±granite±alaskite complexes.

Carboniferous K/Ar cooling ages of 300±290 Ma

(Khasanov, 1975). Younger alkaline granite intrusion

give K/Ar cooling ages of 270 Ma (Baratov, 1966).

These last granites cut the South Gissar suture.

Permian calc-alkaline extrusive dacites and rhyolites

are still preserved in places along the southern edge of

the range (Narizhnev and Steblova, 1992).

A simple explanation of the Baysunta, South Gissar

suture and Gissar unit relationships is that the

Baysunta block collided with a northward dipping

subduction zone in the Middle Carboniferous, marked

by the South Gissar suture and Gissar arc. The young-

est K/Ar cooling dates for the blueschists are 320 Ma

(Visean). However, arc volcanism did not start until

the Middle Carboniferous (Bashkirian) in the Gissar

unit and the `ocean ¯oor' of the South Gissar suture

appears to be entirely Lower Carboniferous in age.

This is hardly time enough to form an ocean and the

South Gissar suture was probably a relatively small

ocean between the Baysunta and Gissar blocks. There

is insuf®cient evidence to decide between a narrow rift

ocean (like the present Red Sea) or a marginal basin

(like the present Sea of Japan). But the evidence of

northward subduction and Middle Carboniferous

collision of the north-facing passive margin of the

southern Tien Shan microcontinent (of which this

area is the southern part) suggests a rift.

The main problem with this interpretation is how

the huge Carboniferous±Permian granitoid intrusions

of the Gissar range were generated. They do not exist

in the Zeravshan and Turkestan units to the north, so it

seems unlikely that they can be related to the closure

of the Turkestan Ocean along the northern edge of the

Tien Shan. Yet the Gissar suture is too short-lived

(and too narrow) to generate such batholiths. The

only alternative is that the South Gissar `rift' localized

intracontinental thrusting of the Baysunta block below

the Gissar range causing massive partial melting of

continental crust and the intrusion and eruption of the

Moscovian to Permian acid volcanics and granitoids

(Savchuk et al., 1991).

(4) The Zeravshan unit consists of thin, mature

Cambrian to Ordovician passive margin clastics,

overlain by thick Silurian turbidite clastics and thick

Lower to Middle Devonian carbonates. These are

unconformably overlain by Lower Carboniferous

cherts, then turbidites interbedded with sedimentary

bloc melanges, derived from the Turkestan±Alai zone

to the north. To the west the Zeravshan zone runs

under unconformable Neogene sediments, but reap-

pears in the Zirabulak mountains, where the ophiolitic

melange between the Zeravshan and Turkestan units

is well exposed. To the east the Zeravshan unit seems

to be sheared out along the northwestern edge of the

Garm Block. Along the northern edge of the zone

ma®c greenschists (locally blueschists) up to 1.5 km

thick developed from Vendian to Lower Paleozoic

ma®c and rarer acid volcanics, quartzites, shales and

shaly limestones (Volkova and Budanov, 1999).

Ma®c volcanics decrease and carbonates increase up

the section. The greenschists derive from normal and

subalkalic ocean ridge basalts and represents the at

least partly oceanic basement on which the overlying

succession was deposited (Leonov, 1985). Upwards,

with decreasing metamorphism these pass into Ordo-

vician clastics and volcaniclastics overlain by Silurian

to Lower Devonian carbonates. The latter pass rapidly

up into a thick (2.5 km) deeper-water Middle to Upper

Devonian shales, limestones and cherts. This passive

margin succession is unconformably overlain by;

Lower Carboniferous (Tournaisan) coarse ¯uviatile

clastic sediments with pebbles of plutonic and volca-

nic rocks and coals, Visean to Lower Moscovian

shallow marine limestones, and Upper Carboniferous

to ?Permian ¯uviatile conglomerates, sandstones and

shales (Sokolov and Garkovets, 1984; Tulyaganov,

1972). The Zirabulak Range to the west has a compar-

able Paleozoic succession (Pavlov, 1971).

The Zeravshan unit thus shows orogeny involving

deformation and granitoid intrusion in both Lower

Devonian and Upper Carboniferous to Permian

times. It has since been cut by numerous north-west

trending Neogene faults.

(5) The Zirabulak unit is a highly deformed and

impersistent ophiolitic belt between the Zeravshan

and Turkestan-Alai units, and coincides roughly

with the Zeravshan fault zone (Kukhitov, 1969). The

fault is a zone of strong shearing and plastic deforma-

tion and (like the North Gissar fault) is a Middle

Carboniferous thrust zone reactivated in the Ceno-

zoic. The Zeravshan valley is predominantly formed

of thick, highly deformed metamorphosed Silurian

volcaniclastic turbidite sandstones. These are suppo-

sedly overlain by Devonian reef limestones, but these

may simply be tectonically incorporated blocks from

the Zeravshan unit (Volkova and Budanov, 1999).


Above are Carboniferous cherts and exotic limestone

block melanges (derived from the Turkestan zone to

the north) which apparently rest on Middle Carboni-

ferous shelf limestones (Rogozhin, 1977). Despite

evidence that the northern Zeravshan and Zirabulak

formed a subduction-accretionary in the Vendian±

Silurian, it appears that actual ocean closure was

delayed until the Carboniferous. The similarity of

Upper Permian sediments and faunas across this

suture implies actual collision in early Permian times.

(6) The Turkestan-Alai unit has similar Paleozoic

sections to those of the Zeravshan unit. No basement

is known, and alternating shales and quartz±sand-

stones with Ordovician to Silurian graptolites pass

up into a dominantly carbonate section of Upper

Silurian (Ludlovian) to Devonian (300 m) and

Lower to early Middle Carboniferous (400 m) carbo-

nates. A thick Middle to Upper Carboniferous clastic

succession (3 km), in part volcanogenic, overlies

these (Rogozhin, 1977). The northern part of the

Turkestan unit (Alai) is overthrust from the north by

the oceanic and arc units of the South Fergana unit

and the details of foredeep development during colli-

sion can be worked out. Middle Carboniferous

(Namurian) coarse marine fan deposits and sedimen-

tary olistostromes derived from southwardly moving

thrusts mark the onset of collision with the Kazakh-

stan continent and the development, migration and

diachronous ®lling of a foreland basin. The ®rst

marine clastic in¯ux onto the shelf changes from

Visean in the north to late Moscovian in the south.

Likewise the upward passage from marine to coarse

continental clastics ranges (north to south) from early

Moscovian to Gzelian and by Middle Permian times,

the ®lling was complete (Volochkovich et al., 1981).

The unit re-appears again from below Cenozoic cover

in the Nuratau and Tamdytau ranges to the west. In the

Tamdytau range, the contact with the extension of the

South Fergana oceanic lithosphere is well-exposed,

and the whole is unconformably overlain by Lower

Permian conglomerates (Mukhin et al., 1988).

(7) The South Fergana unit is thrust over the

Turkestan-Alai zone to the south and consists of

Paleozoic oceanic and arc rocks. It is poorly exposed

in the southern Tien Shan, lying mostly buried

beneath the Fergana and Kyzyl Kum basins. In the

Dushanbe±Khodzent transect, only a very small

segment lies along the northern side of the Malguzar

range. The more complete and more thoroughly

studied segment along the southern edge of the

Fergana basin will have to form the basis for descrip-

tion (Burtman, 1975; Yablonskaya, 1989). Here, the

unit consists of several allochthonous thrust packets

associated with the Turkestan fault zone (also called

the Kurganak tectonic zone) which extends for more

than 400 km along the northern margin edge of the

Tien Shan. These allochthons were thrust southward

during the collision noted above and consists of a

series of continental margin, oceanic and magmatic

arc units thrust over the Alai zone. However, the unit

is mostly buried beneath the Mesozoic±Cenozoic of

the Fergana basin. So to complete the traverse I will

have to dog-leg to the east to the Karachatyr area,

which fortunately is one of the most thoroughly

studied areas of the southern Tien Shan (Burtman,

1975). Bakirov et al. (1984) divided the South

Fergana allochthons thrust over the Alai zone into

three main units.

The lower allochthons (Lower 1 and 2) have

Silurian clastics passing up into Devonian and Carbo-

niferous shelf carbonates like those of the Turkestan

unit and represents a deeper, partly starved, passive

margin succession. These sections obviously form

slices from the northern passive margin of the Alai

microcontinent and adjacent ocean. They are over-

thrust by much thinner deep-water oceanic basalt±

chert sections of the middle allochtons.

The middle allochthons consist of ophiolitic

melanges with Ordovician to Lower Carboniferous

oceanic rocks, dominated by pillow lavas and

cherts.The base of the thrust units are frequently

formed of serpentinite and tectonic block melanges.

Above, oceanic lithosphere is sometimes represented

by complete ophiolite suites. These sections are

unconformably overlain by Middle Carboniferous

(Namurian±Bashkirian) coarse marine clastics and

olistostromes (Bakirov et al., 1984). Overlying thick

marine Carboniferous turbidite sandstones pass up

into thick Upper Carboniferous to Permian conglom-

erates with igneous and metamorphic pebbles, which

®ll in fault-bounded basins and are differentially

sheared and compressed along left-lateral Neogene

shears (Leonov et al., 1995). The whole is intruded

by late-orogenic Permian granitoids.

The upper allochthons have greenschists uncon-

formably overlain by Lower Devonian to Lower


Carboniferous shelf deposits and Middle Carbonifer-

ous (Bashkirian) and younger marine clastics

(Burtman and Klishevich, 1972).

The Turkestan ocean suture consists of massive

olistostromes (associated with ophiolitic melanges

and ophiolite) which now form slabs dipping steeply

southward to considerable depths. The olistostromes

contain a wide variety of exotic blocks from the south-

ern Tien Shan microcontinent shelf and slope to the

Turkestan ocean ¯oor. Some blocks contain the oldest

oceanic deposits recorded from the southern Tien

Shan with limestones as old as Vendian and Cambrian

overlying pillow basalts and hyaloclastics (Kurenkov,

1978). The youngest blocks have Lower Carbonifer-

ous cherts (Loshmanov, 1991). To the north, in the

Kuramin unit of the central Tien Shan (Fig. 3), thick

Devonian intermediate and acid lavas and intrusions

mark the arc behind this subduction zone.

Thick syn- and post-orogenic coarse Upper

Carboniferous to Permian clastics lie transgressively

across all the thrusted units and the suture. The marine

lower parts consist of conglomerates, sandstones and

shales up to 3 km thick with late Moscovian to early

Permian foraminifera, corals and brachiopods. The

continental upper parts consist of coarse conglomer-

ates and sandstones up to 1 km thick with interbedded

acid tuffs, freshwater limestones and gypsum with a

late Permian ¯ora.

The South Fergana unit has been interpreted for

many years as the suture zone between the southern

and northern Tien Shan marking the site of a

`Turkestan' ocean (Burtman, 1975) Paleomagnetic

results indicate that this paleocean was at least

1800 km wide between the Alai microcontinent and

the northern Tien Shan continent during the Lower

Devonian (Klishevich and Khramov, 1994). The

paleocean supposedly closed when the southern

Tien Shan microcontinent with its northward facing

passive margin collided with the oceanic islands,

fore-arc slabs and melanges and ®nally with the

magmatic arcs along the southern edge of Asia

(Kurenko and Aristov, 1996). The start of deformation

of the Alai passive margin in the northern subduction

zone is precisely de®ned as early Namurian by the age

of blocks and matrix fossils of the olistostromes

covering shelf sediments. Actual continent±continent

collision is de®ned by the Bashkirian-early Moscov-

ian coarsening upwards marine clastic deposits culmi-

nating in late Moscovian and younger coarse

continental clastics which ®ll in the foredeeps

between the approaching continents (Kurenkov,

1978).

Eastwards from the Dushanbe traverse, the

Baysunta to Turkestan zones progressively disappear

eastwards below the Mesozoic deposits of the Tadjik

basin (Fig. 3). Before they do, the Gissar and Zerav-

shan units are partially replaced by the Garm block.

The Garm block is dominated by metamorphic

rocks, is faulted against the Gissar and Zeravshan

zones on the west and north, and is unconformably

overlain by Mesozoic sediments on the south. The age

of the metamorphic rocks is disputed. Gneisses give a

U/Pb isochron on zircons of 2600 ^ 10 Ma which

either dates intrusions or detrital zircons. These

gneisses are overlain by a diverse assemblage of

amphibolite facies metamorphics which contain

Paleozoic fossils in places (Khoreva et al., 1975).

Scattered within the Garm block are disconnected

ultrabasic masses. The Garm block contains the only

proved Precambrian basement in the southern Tien

Shan, apart from the Baysunta unit to the west; and

possibly represents a highly deformed eastward

extension of the Baysunta unit, plus elements of the

Gissar and Zeravshan units, squashed northwards

during the formation of the Pamir.

3. Igneous and metamorphic petrology

Detailed descriptions and chemical analyses are in

Lutkov and Mogarovskii (1999); Pomazkov (1972);

Savchuk et al. (1991); Solov'ev (1998); and Tulyaga-

nov (1972).

The Baysunta and Garm blocks are the only units

which appear to have Precambrian basement (Lutkov

and Mogarovskii, 1999). The Baysunta block is domi-

nated by amphibolite facies metamorphic rocks,

consisting of metapeltic gneisses, schists and migma-

tites (75%) and subordinate amphibolites (6%), which

give K/Ar cooling ages of 600±650 Ma (Mirkhodz-

hayev et al., 1972); and aluminous late Paleozoic

granites (15%). The average rock composition

approximates to potassic leucogranite of S-type. The

Garm block consists of amphibolite and granulite

facies metamorphic rocks, dominated by garnet±

cordierite±biotite gneisses and migmatites (54%),


aluminous schists (15%), gneissic granitoids (23%)

and minor marble, quartzite, and amphibolites (8%).

These give Pb isochron ages of 3000±2600 Ma

(Budanov, 1993). The average composition approxi-

mates to granitoids of S-type.

The Zeravshan and Turkestan±Alai units are domi-

nated by lower Paleozoic oceanic and slope basement

and clastics, and Upper Paleozoic shelf carbonates, as

noted above. They contain large subalkaline granitoid

plutons only in the west, forming extensions of the

late orogenic Permian plutons of the Gissar range,

which are dominated by large granitoids. Most

plutons were intruded between 295 and 250 Ma,

spanning the Permian (Khasanov, 1975; Kostitsyn,

1991).

Solov'ev (1998) noted three main tungsten-bearing

plutonic association in the southern Tien Shan which

overlap in distribution but tend to be arranged from

south to north (Fig. 5).

The granodiorite±granite complexes dominate the

Gissar unit, though one (the large Karatyube pluton)

occurs in the Zeravshan unit south of Samarkand.

Plutons intrude highly deformed Middle Carbonifer-

ous rocks and are dominated by porphyritic two-

felspar granodiorites, which grade into marginal

quartz diorite and monzodiorite, and often have

subalkaline granite centres (Khasanov, 1975). Abun-

dant large alkali felspar phenocrysts form up to 40%

of the rock. The granodiorites have normal to slightly

high alkali metal content. Late stage rocks are

enriched in F, Rb, Be, Sn and Nb, and depleted in

Ba and Sr, with very low Ni,V, and Cr. The complexes

correspond to calc-alkaline complexes of mature

stages of arcs.

The gabbro±monzodiorite complexes tend to lie

north of the granodiorite±granite complexes in the

Zeravshan unit. Plutons tend to be small and are domi-

nated by intermediate intrusive phases (quartz monzo-

diorite±quartz monzonite±granodiorite) with less

abundant basic (olivine gabbro, diorite and quartz

diorite) and acid (subalkaline granite and leucogra-

nite) phases. The gabbro±monzodiorites have chemi-

cal compositions which ®t the high-K calc-alkaline

series. They are markedly enriched in Ba, Sr, and F,

with relatively low Rb, Li, Nb, Sn and Be. The

complexes correspond to the andesite±latite type of

the early stages of island arc evolution.

The monzodiorite±granite±alaskite complexes lie

far to the north, in isolated inliers of the Turkestan

paleocean, where they are con®ned to narrow grabens

in anticlinal domes. They have elevated alkali metal

content, with K enriched in the later phases, elevated

Ba and Sr, and are enriched in Rb, Nb, and Ta. They

correspond to alkaline granites and rare metal grani-

toids.

K, Rb, and Sr vary with depth of magmatic sources

and degree of partial melting. Plots of K/Rb versus

Rb, Sr versus Rb, and Sr versus Rb/Sr, allow an esti-

mate of magma production depth to be made. The

plots for all three complexes lie in different, but over-

lapping ®elds (Figs. 6±8). The granodiorite±granite

complexes, with two exceptions, plot in the crustal

®eld. The monzodiorite±granite±alaskite complexes,

again with two exceptions, plot in the mantle ®eld.

The gabbro±monzodiorite±granite complexes plot

across all three ®elds.

On an La/Sm plot, used to discriminate the tectonic

setting of island arc rocks, the gabbro±monzodiorite±

granite complexes plot in early arc and arc ®elds,

while the monzodiorite±granite±alaskite complexes

plot in the mature arc ®eld (Fig. 9). The compositions

of the gabbro±monzodiorite±granite complexes

suggest a primitive or slightly enriched mantle

source. The monzodiorite±granite±alaskite sources,

however, were probably metasomatically enriched


Fig. 6. K/Rb against Rb plot for plutons (from Solov'ev, 1998, Fig.

5).

before or during melting, as indicted by the high Sr,

Nb, Ta, Zr, and F contents.

The further evolution of the melts was controlled

mainly by crystal fractionation. This process deter-

mined the general basic to acid sequence in each

complex, and their respective enrichment in alkali

metals, and depletion in Mg, Ca, and Fe (Solov'ev,

1998).

Rare earth element (REE) patterns in sequential intru-

sive phases of the crustally-derived granodiorite±

granite complexes show slightly depleted Eu consistent

with melting of metapelitic or metabasic rocks enriched

in dark coloured minerals (Fig. 10, nos. VI±VII). The

subsequent relative enrichment in Eu may be related to

further melting of feldspar.

REE patterns in the mantle-derived monzodiorite±

granite±alaskite complexes show gradual depletion in

light rare earth elements (LREE) and simultaneous

enrichment in heavy rare earth elements (HREE)

(Fig. 10, nos. VIII and IX). This is consistent with

fractionation of LREE-bearing accessory minerals

like apatite and magnetite. The clear Eu minimum

in late silicic rocks testi®es to the important role of

felspar fractionation. However, the drastic differences

in REE content between the intermediate and acid

rocks suggests that the acid magmas formed from

crustal melts, and not from fractionated deep basic

magmas.

The gabbro±monzonite±granite complexes show a

gradual enrichment in LREE and partly in HREE,

with a depletion in Eu in the early and intermediate

differentiates. These are followed by a rapid decrease

in the LREE, and enrichment in the HREE, in the late

granitic phases with a pronounced Eu minimum (Fig.

10, nos. I±V). The gabbro±monzodiorite±granite

complexes seem to represent the earliest subduction-

related magmatism in the southern Tien Shan.

Presumably they are related to the northward subduc-

tion of oceanic crust below the separated Alai micro-


Fig. 7. Sr against Rb plot for plutons (from Solov'ev, 1998, Fig. 5).

Fig. 8. Sr against Rb/Sr for plutons (from Solov'ev, 1998, Fig. 5).

Fig. 9. La against Sm for: (1) gabbro±monzodiorite±diorite

complexes (nos. 1±14), and (3) monzodiorite±granite±alaskite

complexes (nos. 21±22).

fragment of deformed Lower Paleozoic arcs overlain

by late Paleozoic mature `shelf and slope' sediments.

The subsequent south to north migration and increas-

ing alkalinity of the granodiorite±granite and monzo-

diorite±granite±alaskite complexes, together with

their increasingly silicic character, has been related

by Savchuk et al. (1991) to progressive underthrusting

of the Gissar magmatic arc by a continental fragment

now represented by the separate Baysunta and Garm

blocks (Fig. 11). The gradual change in compositions

in each complex, lasting in some cases from 295 to

250 Ma (the entire Permian), is explained by a conse-

cutive shift in the melting sedimentary cover of the

subducting plate; from oceanic through transition

crust to marginal continental. Increase in alkaline

magmatism in the latest stages might be caused by

the increasing proportion of shelf limestones in the

melting sediment cover.

4. Conclusions

The western segment of the southern Tien Shan

consists of deformed Lower Paleozoic oceanic and

arc rocks overlain by Upper Paleozoic slope and

shelf clastics and intruded by various gabbroic to

granitoid plutons. The Gissar, Zeravshan and

Turkestan±Alai zones appear to represent an Upper

Paleozoic carbonate platform built on the ?latest

Precambrian to Silurian oceanic and arc substrate.

The stratigraphy, petrology and geochemistry show

that the segment is dominated by crustal material

created in the Phanerozoic. Apart from the Baysunta

and Garm blocks, there is no sign of appreciable

Precambrian continental crust. Like the areas to the

northeast, this part of Asia represents a Phanerozoic

addition to continental crust. The entire southern Tien

Shan remained a stable continental area until Miocene


Fig. 10. REE patterns in sequential intrusive phases for: gabbro±monzodiorite±diorite complexes (I±V); granodiorite±granite complexes (VI

and VII), and monzodiorite±diorite±alaskite complexes (VIII and IX). Numbers are in order from early to late phases; for example in I, 1 is

earliest phase, and 5 is youngest.

to Recent deformation associated with the collision of

India with Asia (and marked by the indentation of the

Pamir in this area).

Acknowledgements

I am grateful to A.B. Bakirov, V.S. Burtman, A.I.

Kim, E.A. Yolkin, who introduced me to the Southern

Tien Shan in 1984 more recently to the villagers,

shepherds and climbers in the area. Work in Central

Asia over the last ten years was funded by N.S.E.R.C.

(Canada).

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