Terranes, Synaccretionary, and Postaccretionary Complexes of the

#### Transbaikalia and Southeastern part of Eastern Sayn Regions, Siberia

**By A.N. Bulgatov and I.V.Gordienko**

**(Geological Institute, Siberian Branch, the Russian Academy of
Sciences, Ulan-Ude, Russia)**

# INTRODUCTION

The southern margin of the North-Asian Craton in the Eastern Sayn and
Transbaikalia regions is bordered by various-age fold belts. The fold
belts consist of a collage of terranes that were successively accreted
to the craton at the end of Riphean and early Paleozoic, and in the
Permian and Early Triassic. This paper describes and interprets the map
of Transbaikalia tectonostratigraphic terranes and overlap complexes
(assemblages) that is compiled and published at a scale of 1:5,000,000
in this volume (Gordienko and Bulgatov, this volume). Previous
preliminary terrane analyses of the fold belts in this region have been
published by Parfenov and others (1995), Bozhko (1995), Gusev and Khain
(1995), and Gordienko (1997). The terranes are classified according to
the following actualistic principles (i.e., tectonic environments):
cratonal, metamorphic, ophiolite (oceanic), island-arc, turbidite basin,
and accretionary wedge with a predominance of oceanic rocks. In
addition, the passive continental margin of the North Asian Craton is
distinguished.

# PASSIVE CONTINENTAL MARGIN OF NORTH ASIAN CRATON

The passive continental margin of the North Asian Craton, named the
Patom fold-thrust belt, consists of a sequence of thick (up 15 km),
Riphean terrigenous and carbonate sedimentary rocks that accumulated on
the depressed passive margin of the pre-Riphean basement of the North
Asian Craton (Parfenov and others, 1995). These sedimentary rocks thin
towards the craton. The western, northern, and eastern boundaries of the
belt form the frontal zone of an orogenic belt. To the south, the
orogenic belt is bounded by a variable-age, late Riphean and Paleozoic
granite batholith. The formation of the Patom fold-thrust belt commenced
with the deposition of the Anangr suite of the Bodaibin series during
the Anangr stage of the late Riphean. Petrographic, petrochemical, and
geochemical studies of the terrigenous deposits in the belt indicate
that a number of Riphean terranes with different geodynamic natures
(tectonic origins) were

accreted to the craton. Subsequently, various postaccretion collisions
formed the Baikal-Patom fold and thrust belt during the Vendian, Early
Paleozoic, Middle Paleozoic, and Late Paleozoic.

# TECTONOSTRATIGRAPHIC TERRANES

## 

## Cratonal Terranes

Cratonal terranes (fragments of craton) that occur in the orogenic
collage are composed of early Precambrian gneiss and granite, and
stratified complexes that are regionally metamorphosed to granulite and
amphibolite facies. The Muya cratonal terrane, consisting of the
Kindikan series, Ileir and Lunkut suites, and a protoplatformal complex
are characteristic examples. The Kindikan series forms the lowest part
of the terrane (Bulgatov, 1983) and consists of lower and upper parts.
The lower part consists of amphibole and biotite-amphibole gneiss. The
upper part consists of garnet-amphibole and garnet-pyroxene-amphibole
gneiss with minor amounts of amphibole-pyroxene gneiss with interlayered
amphibolite, and biotite and garnet-biotite gneiss. The upper part of
the Kindikan series consists of biotite and garnet-biotite (sometimes
with hypersthene, kyanite, and sillimanite) gneiss with interlayered
garnet-pyroxene and amphibole gneiss and amphibolite. Amphibolite facies
retrogressive metamorphism resulted in hypersthene replacement by
amphiboles. On the basis of composition and metamorphism, the Kindikan
series is correlated with Kurultin series of the Aldan shield of the
North Asian Craton.

The younger Muya terrane consists of the Ileir and Lunkut suites. The
Ileir suite is composed of gneiss and schist composed of
garnet-mica-amphibole, biotite-amphibole, garnet-biotite, mica, biotite
that form interlayered amphibolite, amphibole schist, and marble. The
Lunkut suite is a gneiss-carbonate-schist unit that occurs as a slice
conformably on the Ileir suite. This slice consists of schist and gneiss
composed of mica, garnet, amphibole, and carbonate minerals that form
interlayered marble, dolomite, amphibolite, and quartzite. The Ileir and
Lunkut suites are metamorphosed to amphibolite facies and are assumed to
be Late Archean. Also part of the Muya craton terrane is an Early
Proterozoic carbonate terrigenous protoplatform complex (Param series)
that occurs unconformably on the Lunkut suite.

Geological-geophysical data indicate the Muya craton terrane forms a
large plate, about 20 km long, that dips gently eastwards (Bulgatov,
1988). The inner structure, although complicated, consists of a series
of tectonic plates separated by thrusts that contain lenticular bodies
of eclogite (Dobretsov and Bulgatov, 1991) with an age of 653 ± 21 Ma
(as determined by Sm-Nd whole-rock and mineral isochron analysis). These
data indicate a Vendian age of deformation and collision (Shatskiy and
others, 1996). Also occurring in the Muya terrane are Late Riphean and
Paleozoic granitoid massifs that indicate the terrane was subjected to
structural, thermal, and igneous events in the Riphean, Vendian, and
early Paleozoic.

**Metamorphic Terranes**

In this area, metamorphic terranes are mainly derived from cratonal
terranes that were extensively deformed in the Phanerozoic. For a
example, the Olkhon metamorphic terrane contains a banded structure with
the main bands consisting of various structural plates (Fedorovskiy and
others, 1995). Several endogenic events affected this terrane (Letnikov
and others, 1995, 1996): (1) granulite faces metamorphism at 1,900 to
1,800 Ma and formation of granite; (2) intrusion of potassium-sodic
granite massifs at 1200 Ma; (3) formation of gabbroic massifs,
pegmatites, and alkaline metasomatism associated with syenite formation
at 570-530 Ma; (4) occurrence of high-pressure, fractured, regional,
granulite facies metasomatism at 500-445 Ma; (5) formation of small
bodies of gabbro, pegmatite, and microcline granite at 410-386 Ma; and
(6) formation of a nepheline syenite massif at 397-386 Ma.

**Early and Middle Riphean Oceanic Terranes**

The Param, Ilchir, and Kulindin oceanic terranes consist of Early and
Middle Riphean ophiolites that occur in tectonic plates and wedges that
are 60 km wide by 300 km long. The ophiolites consist of peridotite,
dunite, gabbro, basalt, and deep-water chert and shale. The mafic
volcanic rock units in the ophiolites are N-, T-, P- MORB island-arc
type basalt (Parfenov and others, 1995). The basement of the ophiolites
is interpreted as a oceanic crust (Bulgatov, 1988). The ophiolite
terranes are interpreted as basements of island arcs.

**Middle and Late Riphean Island Arc Terranes**

The Kilyan island arc terrane, of Middle and Late Riphean age, consists
of tuff, tuffite, rhyolite, andesite, and basalt, and also contains
gabbro and plagiogranite. The rhyolites are dated at 923 Ma and the
plagiogranite is dated at 850-900 Ma (Konnikov and others, 1994). The
Oka island-arc terrane in the Eastern Sayn region, of Late Riphean age,
consists of the terrigenous-volcanogenic complexes of the Sarkhoij and
Oka series, and the Ospa and Barungol suites.

**Oka Island Arc Terrane**

The Sarkhoij series of the Oka island arc terrane consists mostly of
terrigenous and volcanic formations. The volcanic rocks, of mafic,
intermediate, and siliceous composition, form an uninterrupted,
differentiated sequence. The basalts are subalkalic potassium-sodic and
sodic, low- and moderate titaniferous, tholeiite, and low alkaline and
sodic, calc-alkali dacite and rhyolite (Kuzmichev, 1990). The age of the
volcanic rocks in Sarkhoij series is 718±30 Ma as determined by Rb-Sr
isochron studies (Buyakaite and others, 1989).

The lower part of the Oka series consists of volcanogenic rocks,
including siliceous to mafic tuff and flows, gabbro-diabase sills and
dikes, and the upper part consists of mostly rudaceous flysch
(Kuzmichev, 1997) composed of sandstones and siltstone with interlayered
conglomerate and breccia. The fragmental volcanic rocks in both slices
formed on steep slopes. The upper flysh part is composed of siltstone,
argillite, and sandstone up to 50 m thick. The clasts in the sandstone
are derived from volcanic rocks in the lower part. Sedimentary
structures indicate the flysch slice formed at the base and away from
the base of a volcanic uplift. This series is characterized by
glaucophane schist with ages of 825 ± 25 and 645 ± 52 Ma as determined
by Rb-Sr isochron studies. The older age corresponds to the time of
high-pressure metamorphism, and the younger age corresponds to the time
of superposed low-pressure greenschist facies metamorphism (Konnikov and
others, 1994).

Sills and gently transverse dikes occur in the Oka series and range from
a few meters to several tens of meters thick, and are mainly composed of
gabbro and diabase. Some thick bodies range from gabbro-diabase to
granophyre. Rhyolite and andesite comprise up to 10% such bodies by
volume (Kuzmichev, 1997). The gabbro and diabase are moderately
titaniferous, low-potassic and low-phosphatic. These and REE data
indicate a N-MORB type origin (Gladkochub, Sklyarov, 1996). Their age is
756 ± 43 Ma (as determined by a Sm-Nd isochron) (Kuzmichev, 1997).

The island-arc (sedimentary-volcanogenic complexes) of the Ospin and
Barungol suites, also part of the Oka island arc terrane, are
tectonically bounded units that occur adjacent to the Gargan cratonal
terrane. The volcanic rocks in these suites consist of rhyolite,
andesite, and basalts. The basalts are tholeiitic and calc-alkalic. The
sedimentary rocks are mainly terrigenous (interlayered carbonaceous
shale, siltstone, and sandstone, and conglomerate, often with graded
bedding). Rare limestones occur in these suites. The suites also
contains high titaniferous and subalkalic gabbro-diabase and diabase
sills that are abundant to the northwest near the adjacent The sills of
gabbro-diabase and diabase are abundant in the southeast part of this
terrane and also are of subalkalic high-titaniferous, high-potassium,
and high-phosphate composition, and are enriched in LREE (Gladkochub,
Sklyarov, 1996).

**Early-Middle Riphean Turbidite Basin Terranes**

Turbidite basin terranes, mainly of Early-Middle Riphean age, consist of
the Barguzin, Verkhnevitim, Bambuy, Delunuran, Shaman, Olokit, Kunaley,
Bitudzhidin and Argun terranes. These terranes are composed mainly of
metamorphosed carbonate, sandstone, and shale deposits that overlay a
basement of continental crust (Bulgatov, 1988). The sedimentary rocks in
these terranes sometimes contain volcanic rocks and subvolcanic bodies.
The Barguzin terrane contains high-titaniferous and high-phosphate
basalt, and diabase and gabbro-diabase enriched with LREE, with
compositions similar to T-MORB basalt. The volcanic rocks of the
Verkhnevitim terrane are mainly moderate titaniferous, low alkaline and
tholeiitic (Bulgatov, 1983). In the Argun terrane, the volcanic rocks
consist of basalt that is either moderately titaniferous, N-, T-MORB
type, or high-titaniferous, P-MORB type (Gusev, Peskov, 1996).

**Vendian and Cambrian Ophiolite, Island Arc, and Turbidite Basin
Terranes**

The Vendian-Cambrian oceanic and island arc terranes are the Khasurta
ophiolite terrane, the Khamsarin, Verkhnekhasurta, Eravna island-arc
terranes, and the Dzhida turbidite basin terrane.

The Khasurta ophiolite terrane consists of several tectonic plates
(Kuzmin and others, 1995) composed of hyperbasites, gabbro and
gabbro-pyroxenite, high-Cr basalt, high-titaniferous, subalkalic basalt
with lenses and interlayered limestone. The ophiolites ages range in age
from Vendian to the beginning of the Early Cambrian. These tectonic
plates dip steeply (50-80 degrees) northward (present-day coordinates).
The ophiolites exhibit no positive gravimetric anomalies. These data
indication obduction of the Khasurta oceanic terrane over the Kupchin
cratonal terrane.

The Khamsars island-arc terrane occurs in the southeastern part of the
Eastern Sayn region and is the northeastern part of the same terrane
that occurs mainly in Eastern Tuva. The Khamsars terrane consists mainly
(about 90%) of Early and Middle Paleozoic granitoids. Stratified
island-arc formations occur as large xenoliths or roof pendants in the
granitoids, and consist of metamorphosed andesite, dacite, and rhyolite
with interlayered lenses of carbonate, shale, reef limestone, and
sandstone with Early Cambrian fauna (Gordienko, 1987).

The Verkhnekhasurta island-arc terrane occurs on the right bank of the
Dzhida River. It is obducted over the Kupchin cratonal terrane. At the
confluence of the Tsakirka and the Dzhida Rivers, the
sedimentary-volcanogenic island-arc complex occurs as a great xenolith
among granitoids and forms several tectonic plates that dip northward.
This terrane contains low-Cr basalt and andesite basalt, associated
tuff, and interlayered lenses of chert, plagiorhyolite, andesite, and
associated tuff, tuffite. Also occurring is a tuff-turbidite slice
composed of volcanogenic-terrigenous turbidite and tuff with
interlayered lenses of andesite-basalt, carboniferous shale, and
limestone with Early Cambrian fauna (Kuzmin and others, 1995;
Belichenko, 1969).

The Early Cambrian Eravna island-arc terrane is composed of
differentiated volcanic rocks and various age granitoids. The Eravna
xenolith, that occupies 150 km^2^, contains the large Ozernoye
pyrite-polymetallic deposit. This xenolith is composed of
plagiorhyolite, dacite, andesite, basalt, tuff, turbidite tuff
(siltstone, mudstone, sandstone) with interlayered lenses of reef
limestone with Early Cambrian fauna. The zenolith also contains stocks
and dikes of mafic, intermediate and siliceous composition (Gordienko,
1987; Postnikov and others, 1997). The terrane also contains terrigenous
and clastic-carbonate deposits formed in back-arc and inter-arc basins.
In addition, the xenolith contains hyperbasites bodies near the
northwest margin. These bodies were intruded into the upper crustal
units and are clearly derived from the Vendian ophiolitic terranes that
underlie the Eravna island-arc terrane.

The Vendian and Cambrian Dzhida turbidite basin terrane occurs between
the Riphean Bitudzhida turbidite and Vendian and Early Cambrian oceanic
and Early Cambrian island-arc terranes. The Dzhida turbidite basin
terrane contains carbonate-terrigenous and turbidite deposits that
formed on slopes and basin floors. The terrane occurs in tectonic plates
that dip north. The margins of the terrane are olistromal zones of
serpentinized melange and thrust zones (Bulgatov, Klimuk, 1998).

## 

## Paleozoic and Early Mesozoic Ophiolite, Island Arc, Turbidite Basin, and Accretionary Wedge Terranes

The middle Paleozoic turbidite basin terranes in the region are the Daur
and Aga terranes.

The Daur terrane is composed of Late Devonian-Early Carboniferous
sandstone and shale turbidites with interlayered andesite-basalt,
jasper, siliceous shale, conglomerate, and gritstone. Small blocks of
early Precambrian metamorphic rocks and granitoids occur in the terrane
(Fomin and others, 1985). The terrane is characterized by a
gravitational minimum (Menaker, 1972), indicating a continental crust
for the Daur terrane.

The Devonian and Early Carboniferous sandstone-shale turbidites of the
Aga terrane consist of interlayered lenses of gritstone, conglomerate,
reef limestone with Devonian and Early Carboniferous fauna, jasper,
siliceous shale, basalt, dacite, and rhyolite (Gordienko, 1987). The
basalts are either (1) high-titaniferous, high-phosphate and enriched
with LREE, Ta, Nb, Zr, Hf, Y; or (2) low-titaniferous, low-phosphate,
with low concentrations of Nb, Ta, Zr, Hf, Y, REE, and especially LREE.
The first type is similar to P-MORB basalt; the second is similar to
N-MORB basalt (Gusev and Peskov, 1996). The Middle Carboniferous to
Early Triassic terrigenous turbidites in the terrane are regressive and
accumulated in relict basins separated by local uplifts (Amantov, 1975).
The terrigenous turbidites consist of conglomerate, gritstone,
argillite, siltsone, and sandstone. The Early Triassic units are more
rudaceous. The stratified units of the Aga terrane range up to 20 km
thick. The composition and thickness of Middle Carboniferous and Early
Triassic turbidites indicate rapid and thick sedimentation. Tectonic
plates and wedges of the Kulindin Riphean ophiolite terrane occur in the
Devonian through Early Triassic turbidite deposits of the Aga turbidite
basin terrane, indicating an ophiolitic (oceanic) crust for the Aga
terrane. The Devonian through Early Triassic Aga terrane is
characterized by a gravitational maximum that differs from the Daur
terrane (Menaker, 1972). The Aga terrane contains Middle Carboniferous
and Permian avalanche deposits that are interpreted as forming during
\"gentle\" collision, and Early Triassic deposits that formed during
\"hard\" collision with the final accretion of the Daur, Aga, and Argun
terranes.

The Shilka ophiolite terrane consists of accreted wedges and plates
composed mainly of Riphean ophiolites. The terrane extends along the
Mongol-Okhotsk suture zone and unites fragments of Riphean ophiolites
that are occur as tectonic plates and wedges among older and younger
terranes. To the north-east, the Shilka terrane is named
Tukuringra-Dzhagdin ophiolite terrane (Nokleberg and others, 1994,
1997). The ophiolites are composed of serpentinite, metagabbro,
metabasalt, greenschist orthoschist, sericite-siliceous and
carbonate-chlorite-sericite-siliceous schist, microquartzite, and jasper
with interlayered sandstone and siltstone. The terrane also includes
melange, containing serpentinite and blueschist derived from the basalt.
Basalts and gabbro are subalkalic, moderate-titaniferous and
high-titaniferous, and are similar to N-, and T-MORB basalt (Gusev,
Peskov, 1996). Blocks of early Precambrian metamorphic rocks and
granitoids also occur within the Shilka terrane. From older to younger,
the blocks consist of Riphean psammitic schist and granitoid complexes,
lower Paleozoic granitoids, Middle Paleozoic carbonate-terrigenous
deposits, and Late Carboniferous granitoids.

# ASSOCIATED OROGENIC BELTS

Associated orogenic belts consist of synaccretionary molasse that occurs
in intermontane troughs and basins, and stitching calc-alkali
granitoids.

## 

## Synaccretionary Molasse

Synaccretionary molasse of Late Riphean age occurs in the Padrin and
Sinnir troughs. The Padrin trough occurs along the Vitim River and
overlies eroded island-arc plagiogranite (Bulgatov, 1983). From older to
younger, the molasse, consists of a basal unit of sedimentary breccia
with angular fragments of underlying plagiogranite; interstratified,
motley sandstone and siltstone; sedimentary breccia; and gritstone and
conglomerate. The sedimentary rocks are 55 m thick. Alternating, motley
tuff, rhyolite, dacite, basalt and siltstone, sandy-gritstone, and
conglomerate layers occurs above the basal unit. The volcanogenic
molasse of the Padrin trough is 3.2 to 4.5 km thick and is interpreted
as forming under subaerial conditions associated with centralized
volcanism and sedimentation in a lake basin that was 35 km long by10 km
wide. The volcanic rocks are mainly basalt (mafic), and dacite and
rhyolite (siliceous) and constitute a differentiated bimodal series. The
basalt is high-titaniferous, high-phosphate, LREE-enriched, theoleiitic
and calc-alkalic. The dacite and rhyolite are potassium-sodic,
subalkalic, and normal. Gabbro and diabase constitute comagmatic
intrusions and occur in fractured, steeply-dipping bodies. Associated
leucogranite and granite-porphyry form large bodies up to 60 km^2^. The
age of the volcanic rocks is 765 Ma as determined by Rb-Sr isochron
methods (Mitrofanov, 1978).

The Sinnir trough is about 150 km long and 12 to 15 km wide. The basal
unit in the trough consists of green and violet basalt and tuff about
1000 m thick. Overlapping units are: an argillite and sandy dolomite
unit about 650-750 m thick; conglomerate and argillaceous sandstone
about 2,600 to 2,700 m thick; volcanoclastic andesite-basalt about
700-800 m thick; and kalirhyolitesand kalidacite. Comagmatic mafic
intrusions include dunite-peridotite-troctolite, and gabbro-norite with
an age of 700-750 Ma (Konnikov and others, 1994).

## 

## Stitching Calc-Alkalic Granitoids

The formation of calc-alkalic granites is related to the accretion of
terranes to the North-Asian Craton. Late Riphean calc-alkali granites
occur within the Argun terrane. They intrude Riphean deposits that are
metamorphosed to amphibolite facies. The granites are overlapped by
Vendian and Cambrian sandy-shale and carbonate deposits. Zircon U-Pb
isochron ages of granoblastic granite gneiss are 850 ± 20 Ma and 500 ±
20 Ma. The latter age is interpreted as a later, superposed thermal
event (Bibikova and others, 1979). This complex contains granite gneiss,
plagiogranite gneiss, and coarse porphyroblastic granite gneiss.
Migmatite occurs at the contact of granite massifs.

Late Riphean granitoids also occur in the northern part of the Barguzin
turbidite basin terrane where volcanogenic-sedimentary deposits are
regionally metamorphosed from greenschist to amphibolite facies, and
where migmatites occur along the contacts of the granite batholith.
Regional metamorphism and granite magmatism are interpreted as forming
from an intense heat flux (Bolonev and others, 1983; Bulgatov, 1983).
Vendian and Cambrian deposits, including basal conglomerate that overlap
the Riphean regionally metamorphosed complex, are non-metamorphozed.
They exhibit sericite and chlorite epigenetic recrystallization. Local
biotite and garnet-bearing schist form conglomerate clasts, indicating
that a major unconformity occurs between Riphean and Vendian and
Cambrian complexes.

The absolute age of granites intruding the Barguzin terrane is 1,014 ±
80 Ma and 326 ± 27 Ma for granite veins, as determined by Rb-Sr isochron
methods (Shergina and others, 1981). The granites are also dated by U-Pb
zircon method as 301 to 314 Ma with inherited zircon with an age 1,190
Ma (Neimark and others, 1993). Isotopic data for alkaline-earth granites
intruding other terranes to the north and northeast of the Eravna
island-arc terrane (Parfenov and others, 1995; Jarmoluk and others,
1997) indicate the granitoids incorporate continental crustal material
with ages of: 2,100 to 1,700 Ma; 1,600 to 1,560 Ma; 1,375 Ma; 1,200 Ma;
and 503 Ma . The age of granite intrusion was variable and occurred at
1,014 Ma, 1,190 Ma, 556 to 537 Ma, 470 to 426 Ma, 387 to 320 Ma, and 320
to 270 Ma . The first and the last three intervals define tectonically
active periods of the Transbaikalia region. The interval from 556 to 537
Ma is interpreted as a time of decreased tectonism during which
carbonate rocks formed with magmstism.

Collisional-origin, calc-alkali granites of the Eastern Sayn region are
dated at 463 Ma. A similar Ordovician age for the same granites
intruding the Argun terrane, and a Late Carboniferous age for granites
intruding the Daur terrane are collaborated by field geologic data.

# 

# OVERLAPPING ASSEMBLAGES

Riphean terranes of the northern regions of Transbaikalia and Eastern
Sayan region accreted to the craton at the end of Late Riphean and are
overlapped with angular unconformity by Vendian terrigenous and Cambrian
carbonate deposits. These overlapping units are similar to coeval
deposits of the Siberian platform, but differ in being greatly thicker,
and also lack evaporite deposits. They comprise isolated fields
represented by outliers of much larger sedimentary covers that were
preserved after erosion. The structure and composition of
Vendian-Cambrian slices indicate that they formed as part of the passive
margin of the Siberian continent.

The Daur and Aga terranes of Eastern Transbaikalia accreted to the
craton in Early Triassic. Overlapping these terranes are Late Triassic
to Early Jurassic terrigenous deposits. The lack of volcanic rocks in
these deposits indicates they formed along a passive margin, in the
Mongol-Okhotsk marine bay adjacent to the ancestral Pacific ocean.

# 

# ASSOCIATION OF ACTIVE CONTINENTAL MARGINS: THE SAYN-TRANSBAIKALIA AND SELENGA-VITIM VOLCANIC-PLUTONIC BELTS

The Devonian and Early Carboniferous Sayn-Transbaikalia
volcanic-plutonic belt extends for about 2,500 km from the Altai-Sayn
region and Northern Mongolia into the central and northern parts of the
Transbaikalia region (Gordienko, 1987). The belt is composed of
subalkalic and alkalic volcanic rocks, granite, and gabbroic rocks. The
occurrence of this volcanic-plutonic belt is tectonically linked to the
turbidite deposits of the Daur, Aga, and Argun terranes. The marine
turbidite basin deposits of these terranes and the Sayn-Transbaikalia
volcanic-plutonic belt are interpreted as forming in an active
continental margins.

In the Transbaikalia region, the Selenga-Vitim volcanic-plutonic belt
formed in the Middle Carboniferous to Early Triassic as a continental
arc. The belt extended for about 2,000 km from western Mongolia to the
Olekma River basin (Gordienko, 1987; Jarmoluk, Kovalenko, 1991;
Gordienko and others, 1998). The belt is composed of bimodal
trachyrhyolite and trachybasalt volcanic flows, alkalic and subalkalic
granite, nepheline and pseudo-leucite syenite, and ultramafic-alkaline
rocks. At the same time, associated turbidite deposits were accumulating
in the Aga terrane

# 

# INNERCONTINENTAL ASSOCIATIONS. 

# THE SAYN-STANOVAYA AND MONGOL-

# TRANSBAIKAL RIFT SYSTEMS

## 

## Sayn-Stanovaya Rift System

The Sayn-Stanovaya rift system consists of regionally-extensive
continental troughs that were with Late Triassic and Early and Middle
Jurassic deposits. The deposits consist of volcanogenic,
volcanogenic-rudaceous, and rudaceous units. A typical volcanogenic
trough is Malo-Khamardaban (with dimensions of 100 by 18 km) which is
bounded by the Khamardaban metamorphic terrane on the southeast. Most of
the volcanic rocks consist of trachybasalt and trachyte in a proportion
10:1. Trachyandesite-basalts comprise less than 2 percent of the
volcanic section by volume, and the terrigenous constituent, that forms
separate, thin layers, comprises about two percent of the section.
Numerous subvolcanic bodies occur and are composed of trachydolerite,
syenite-porphyries, and comendite with syenite being dominate. Isotopic
ages range 167 to 150 Ma (as determined by Rb-Sr and K-Ar methods)
(Litvinovskiy and others, 1996).

The volcanogenic-rudaceous structure is best displayed in the great
Tugnuiy trough (dimensions of 140 by 35 to 40 km) that occurs between
the Zagan metamorphic and Eravna island-arc terranes. The trough is
started to form in the Late Paleozoic (Gordienko, Klimuk, 1995). It
contains: (1) Late Triassic limnetic deposits, trachybasalt,
trachyandesite-basalt, subvolcanic bodies of trachyte and
syenite-porphyry; and (2) Early and Middle Jurassic rudaceous deposits,
trachyrhyolite and trachybasalt, and lesser trachybasalt-trachyandesite,
subvolcanic trachyte bodies, syenite-porphyrie, and trachydacite. Also
associated were small, coeval, Late Triassic to Middle Jurassic
sedimentary troughs that were filled with small- to medium-size,
angular, fragmental debris. Most of the Late Triassic to Middle Jurassic
troughs were this type. A few troughs contain rudaceous sediments. As an
example, the Naringol trough, that occurs in the Eastern Sayan region,
on the northeastern side of the Khamsara island-arc terrane, has a
triangular shape and formed between two fractures, the Main Sayan and
associated fractures. These trough deposits consist of rubble and pebble
conglomerate, gritstone, sandstone, and siltstones with interlayered
coal-clay shale and coal beds. Thin-terrigenous sediments with coal beds
occur in the upper part of the section. Flora indicate an Early-Middle
Jurassic age; the thickness ranges up to 2,000 m. Numerous bodies of
subalkalic and alkalic granites formed in a Late Triassic to Middle
Jurassic period of tectonism in the eastern Transbaikalia region. The
extensive granitoids in the Daur terrane. occupy an area of several
hundred km^2^.

## 

## Mongol-Transbaikalia Rift System

The Late Jurassic and Early Cretaceous Mongol-Transbaikalia rift system
consists of numerous continental troughs filled with sedimentary graben
and volcanogenic graben units. The volcanic rocks occur in the lower
parts of section and consist of subalkalic rhyolite-basalt and
trachybasalt. Formation of the rift system was followed by intrusion of
subalkalic and alkalic granitoids and gabbros. Most rifts are
longitudinal and are controlled by fault zones. The troughs form a
system of narrow (1 by 15 km) and elongate (up to tens and hundreds km)
grabens that are bounded by strike-slip faults on one or both sides.
Cross troughs, oriented transversely to folds and faults, are relatively
few. Unlike longitudinal troughs, cross troughs are wider, and are
sometimes isometric and syncline-shaped. Faults occur along some, but
not all of their boundaries. The longitudinal troughs consist of shift
and cross-thrust types (Bulgatov, Turunkhajev, 1996).

**CONCLUSIONS**

The terranes that make up orogenic belts in the Transbaikalia region and
in the southeastern part of the Eastern Sayan region consist of
fragments of: (1) cratonal and metamorphic terranes' (2) Early and
Middle Riphean, and Vendian and Early Cambrian ophiolite terranes; (3)
Middle-Late Riphean and Early Cambrian island-arc terranes; and (4)
Early-Middle and Riphean, Vendian and Cambrian, and Middle and Late
Paleozoic turbidite basin terranes.

In the late Riphean, collision and accretion of Riphean terranes to the
North Asian Craton occurred. Associated, synaccretionary stratificated
units consist a volcanogenic molasse complex and stitching calc-alkali
granites. The accreted terranes are overlapped by Vendian and Cambrian
terrigenous and carbonate deposits that form part of the passive
continental margin of the Siberian platform. The late Riphean collision
and accretion was followed by large-scale displacement of the accreted
units along northwest- and northeast-striking faults (present-day
coordinates), and by folding and thrusting. As a result, a single island
arc was separated into the Oka and Kilyan terranes (Parfenov and others,
1995).

An early Paleozoic collision consisted of: (1) intrusion of calc-alkali
granite massifs; (2) fault displacement of blocks, folding and
thrusting; (3) accretion of Vendian and Cambrian terranes to the North
Asian Craton; (4) greater separation of the Riphean Oka and Kilyan
island-arc terranes; and (5) fragmentation and displacement of an
interpreted single Early Cambrian island arc into the Khamsara,
Verkhnekhasurta, and Eravna island arc terranes. Kinematics of the
terranes displacements in Late Riphean and Early Paleozoic consisted of
southward movement (present-day coordinates) of the Angara-Anabar block
of the North Asian Craton (Bulgatov and Klimuk, 1998).

Various middle Paleozoic terranes were accreted at the end of Early
Triassic and were overlapped by Late Triassic to Middle Jurassic
terrigenous deposits of the Mongol-Okhotsk ocean which was part of a
passive continental margin.

In the Devonian and Early Carboniferous, and in the Middle Carboniferous
to Early Triassic, the Sayan-Transbaikalia and Selenga-Vitim
volcanic-plutonic belts formed and overlapped previously-accreted
terranes. Middle and late Paleozoic great deformations took place to the
north of the Eravna island-arc terrane and consisted of additional
movement of the mountain masses to the north (present-day coordinates)
and final formation of nappe-forming thrusts in the frontal parts of the
following orogenic belts: Sredne-Vitim (Bulgatov, 1983); Akitkan
(Bulgatov, Zaitsev, 1994); and Patom (Ivanov, Ryazanov, 1992). This
process was followed by multi-staged formation of collision granites and
high-temperature regional metamorphism.

The following continental rift systems formed in the region: (1) the
Sayan-Stanovaya rift system in the Late Triassic to Middle Jurassic; (2)
the Mongol-Transbaikalia rift system in the Late Jurassic-Early
Cretaceous; and (3) the Baikal rift system in the late Cenozoic. The
southward movement (present-day coordinates) of the Angara-Anabar
megablock of the North-Asian Craton formed the Khamardaban nappe-fold
system in the Mesozoic.

# 

# ACKNOWLEDGEMENTS

The authors have discussed the geology and tectonics of the
Transbaikalia region with L.M. Parfenov and W.J. Nokleberg. The studies
were sponsored by the Russian Fund of Fundamental Research (Grants
96-05-64361 and 97-05-96416).

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