Litherland and Bloomfield (1981)

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Precambr ian R esearch ,

15 (1981) 157--179 157

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

T H E P R O T E R O Z O I C H I S T O R Y O F E A S T E R N B O L I V I A

M. LITHERLAND* and K. BLOOMFIELD

In s t i t u t e o f Geo log i ca l Sc i ences, Over seas D iv i sion , Ke yw or th , No t t i ngh am NG 12 5G G

Grea t Br i ta in)

( Received December 9, 1980; revision accepted March 24, 1981 )

ABSTRACT

Litherland, M. and Bloomfield, K., 1981. The Proterozoic history of eastern Bolivia. Pre-

cambrian Res., 15 : 157--179.

About 100000 km 2 of the previously unmapped Bolivian sector of the Central Brazil

shield has been studied by Proyecto Prec~mbrico , an Anglo-Bolivian technical coopera-

tion programme. The Lower Proterozoic is represented by the Lomas Maneches Granulite

Group and the bulk of the Chiquitania Paragneiss Complex, which were formed during the

Trans-Amazonic orogenic cycle (± 2000 Ma). The Middle Proterozoic spans the orogenic

cycles of San Ignacio (± 2000--1300 Ma) and Sunsas (<1300--950 Ma). The San Ignacio

cycle included the deposition of the San Ignacio Schist Group, now belts of pelitic schists

with basic/ultrabasic sills, and the subsequent mobilisation of these and older rocks within

a north-trending orogenic belt, accompanied by granitoid development. The Sunsas cycle

began with the deposition of the molassic Sunsas Group and closed with the growth of a

westnorthwest-trending orogenic belt, bordered to the north by a marginal zone and a

stable craton, which was accompanied by granitoid phases and major basic/ultrabasic

igneous activity. The close of the Sunsas orogeny marked the cratonization o f the shield

at about 950 Ma.

Unmetamorphosed Upper Proterozoic and possibly Cambrian sediments on the south-

ern and eastern flanks of the shield represent marine transgressions related to the intra-

continental Braziliano orogenic cycle. East-trending dolerite dykes were probably intruded

during this period within the shield.

INTRODUCTION

This paper is a prel iminary geological synthesis and analysis of the work

accompl i shed dur ing the f irst phase of Pro ye cto Prech'a br ico (Fig. 1), a

technical cooperat ion programme involving regional , geochemical and geo-

chronological s tudies of the Precambrian shield of eastern Bolivia, being

carried out by the U.K. Inst i tute of Geological Sciences (IGS) and the Ser-

vicio Geol6gico de Bolivia (GEOBOL). I t is compiled from the results of re-

connaissance f ield mapping by R.N. Annells , C.J .N. Fletcher, B.A. Klinck,

M. Litherland, W.I. Mitchell, E.A. O'Connor, P.E.J. Pitfield and B.C. Webb, as-

sis ted by GEOBOL geologists , notably A. Aguilera, A. Aponte, E. Barrientos,

J .C. Nallar, G. Monte mur ro, A. Soliz and C. Terrones. Manuscr ipt maps were

*Present address: Mision GeolJgica Brit~inica, Casilla 3045, Santa Cruz, Bolivia.

0301-9268/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company



5 8

l

Phanerozoic

c o v e r

Bras i l i ano f o l d

b e l t s ( (1 O 00M a)

P recam br i an

cratons (:>1O00M

Amazonic Craton:

1 G u i ana h i e l d

Amazonic Craton:

2 C en t ra l B raz i l

h i e l d

S a o F r a n c i s c o

3 C ra ton : A t l an t i c

h i e l d

4 Paraguay -

A r a g u a i a B e l t

C ra ton l i m i t

" P R O Y E C T O P R E C A M B R I C O "

PHASE I

A (1976 -79 )

Th i s p a p e r

B P H A S E 2

( 1 9 7 9 " )

F i g. 1 . L o c a t i o n o f P r o j e c t a r ea an d s u b d i v i s i o n s o f t h e P r e c a m b r i an o f S o u t h A m e r i c a ,

a f t e r A l m e i d a e t a l. 1 9 7 6 ) .

p r e p a r e d a t a s ca le o f 1 / 1 0 0 0 0 0 , f r o m w h i c h 1 / 2 5 0 0 0 0 m a p s w i t h a c c o m p a n y -

i n g r e p o r t s w e r e c o m p i l e d ; t h e s e a r e r e f e r r e d t o i n t h e t e x t a n d w i ll b e p u b l i s h e d

in t h e n e a r fu t u r e . T h e g e o c h r o n o l o g i c a l p r o g r a m m e w a s u n d e r t a k e n b y

D . P . F . D a r b y s h i r e o f th e I s o t o p e G e o l o g y U n i t , I G S , L o n d o n .

A b o u t 1 0 0 0 0 0 k m 2 o f t h e P r e c a m b r i a n s h ie l d a n d m a r g in a l a r e a s h a v e

b e e n m a p p e d , c o r r e s p o n d i n g t o t h e S o u t h e r n Z o n e o f t h e P r o j e c t a re a (F ig . 1 ).

G e o m o r p h o l o g i c a l l y t h e r e g i o n o f t h e s h i e ld is a l a t e ri t is e d M i o c e n e p e n e p l a i n

w h i c h w a s w a r p e d i n P l i o c e n e - P l e i s t o c e n e t i m e s s o t h a t l a t e r it e l e v e ls r a n g e

f r o m 1 5 0 t o 5 0 0 m a .m . s.1 . ( B o u l a n g a n d L i t h e r l a n d , 1 9 7 8 ) . T h e r e a r e a l s o i ns el -

b e r gs o f m o r e r e s i s t a n t r o c k s r e a c h i n g 1 3 0 0 m i n a l t i t u d e , t h e l e v e ls o f w h i c h

c a n b e c o r r e l a t e d w i t h O l i g o c e n e a n d C r e t a c e o u s s u r f a c e s i n B r a z il ( K i ng ,

1 9 6 2 ) . T h e a re a , w h i c h s t ra d d l e s t h e m a i n A m a z o n - - P l a t e w a t e r s h e d , is

d r a i n e d b y e p h e m e r a l r iv e rs a n d s t re a m s . T h e r e a r e o n l y a fe w s m a l l t o w n s

l i n k e d ' b y m o t o r a b l e t r a c k s a n d t h e b u l k o f th e m a p p i n g w a s c ar ri ed o u t b y

f o o t tr a v er se s t h r o u g h t h i c k , o f t e n w a t e r l e ss , fo r e s t . T h e c o m b i n a t i o n o f t h e

f o r e s t ~ o v e r , t h e M i o c e n e la t e ri t e m a n t l e , a n d a C r e t a c e o u s f ra c t u r e s y s t e m

l im i t s t h e p h o t o g e o l o g i c a l e x p r e s s i o n o f P r e c a m b r i a n u n i ts .



1 5 9

T h e a r e a is s i tu a t e d o n t h e s o u t h w e s t e r n m a r g i n o f t h e A m a z o n i c c r a t o n

s e e F i g. 1 ) w h i c h f o r m s t h e b u l k o f t h e c e n t r a l B r a z il s h i e l d A l m e i d a e t a l. ,

1 9 7 6 ) , o n e o f th e l e a s t ~ d o c u m e n t e d a r e a s o f P r e c a m b r i a n r o c k s in t h e w o r l d .

T h e A m a z o n i c c r a t o n is l ar g el y c o m p o s e d o f v ar ia b ly r e w o r k e d T r a n s - A m a z o n i c

b a s e m e n t r o c k s w h i c h s t a b il is e d a t a b o u t 1 0 0 0 M a . W i t h in t h e C e n t r a l B ra z il

s h ie l d, t h is c r a t o n i s c u t b y t h e P a r a g u a y - - A r a g u a i a b e lt o f B r a si li a no P a n -

A f r i c a n ) a g e F i g . 1 ) .

T h e P r o t e r o z o i c h i s t o r y o f t h e a r ea c a n b e r e f e r r e d to f o u r o r o g e n i c c y cl e s

T a b l e I) : t h e T r a n s - A m a z o n i c c y c l e o f L o w e r P r o t e r o z o i c a g e + 2 0 0 0 M a ) ;

T A B L E I

St ra t igraphic t ab le . Sol id l ines a re unconformi t ies . Other symbols as for Fig . 2

O r ogen i c Ep och / S t r a t ig r aph i c un i t G r an i t o i ds Bas ic /

cyc le Ma) eo n u l t rabas ic

r o c k s

- - ? 5 2 0

l ?Cam br i an

Brasi l iano)a

U p p e r

P r o t e r o z o i c

- - - + 9 5 0

1

Sunsas

Middle

_.1300

/ ~ P r o t e r o z o i c

San Ignac io

- ~ Mur c i e lago G r o up

6 T u c a v a c a G r o u p

5 B o q u i G r o u p

Sunsas / V i bos i G r oups

3 San Ignac io Schis t Gp .

2B Ch iqui tan ia Paragneiss

c o m p l e x

- - ?

Trans-A ma zonic L ow er A Ch iqui tan ia Paragneiss

P r o te r o z o i~ 1 1 c o m p l e x

L o m a s M a n e c h e s

• G r a n u l i te G r o u p

D y k e s

G 3

G 2 B 2

G 1

B

N ori t e s i lls

aThe Br a s il iano o r og eny had o n l y m i nor e f f ec t s in t he a r ea s t ud i ed , m os t o f w h i ch w as

cra to nize d a t -+ 950 Ma.



1 6 0

the San Ignacio and Sunsas cycles which are essentially of Middle Proterozoic

age (+ 2000--950 Ma); and the Brasiliano cycle, the main development of

which was confined to the Upper Proterozoic (950--570 Ma). The divisions

of the Proterozoic follow the provisional Brazilian classification (F.F.M. A1-

meida, pers. comm., 1978). Local Bolivian nameshave been introduced for the

Middle Proterozoic cycles since there are uncertain ties of status and correla-

tion within this period in Brazil (Almeida et al., 1976).

T H E T R A N S - A M A Z O N I C O R O G E N I C C Y C L E ± 2 0 00 M a )

A var iably-defined zone of rocks containing granulite-facies assemblages in

the nor th of the area is collectively referred to as the Lomas Maneches Granul-

ite Group* (Litherland, 1979; Pitfield, 1979; Pitfield et al., in preparation).

Rb/Sr data give a best-estimate age of 1961 Ma (Darbyshire, 1979), probably

correlatable with the Trans-Amazonic cycle (2500--1800 Ma) of the Brazilian

shield (Cordani et al., 1973; Almeida et al., 1976). The out crop patte rn of the

Group is, however, controlled by s tructures related to the later San Ignacio

orogeny.

The Lomas Maneches Group is well banded on both major and minor

scales and contains a basal unit composed essentially of K-feldspar leptites

(quartzofeldspathic granulites) and charnockitic hypersthene granutites.

Higher structural levels are fo rme d of garne t--bio tite paragneisses with well-

defined layers of enderbitic hypersthene granulites, basic hypersthene granul-

ites, cordierite --si llimanite granulites, K-feldspar leptites, plagioclase leptites,

and calc-silicate granulites, with minor horizons of garnet--siUimanite schist,

diopside--hornblende--biotite granulite, amphibolite and meta-anorthosite.

The limits of the Group are defined by the disappearance of vestiges of

hypersthene- or cordierite-bearing assemblages along a transitional contac t

with the Chiquitania Paragneiss Complex. That par t of the Chiquitania Com-

plex regarded as an extension of the Lomas Maneches Group is labelled 2A

in Fig. 2. It is a fairly homogeneous paragneiss sequence, migmatised during

the later orogenic cycles, containing variably defined belts of cafemic gneis-

ses and minor bands of quartzi te, amphibolite and calc-silicate granulite. The

leptite phase is poor ly developed, in cont rast to the Lomas Maneches Group.

These two sequences are regarded, in the main, as sedimentary in origin.

Only the basic hype rsthene granulites can be proved to be derived from ig-

neous rocks, since at one locality original tex tures and minerals are preserved.

The enderb itic granulites are also though t to be igneous in origin but the

leptites and more leucocrat ic granulites are tentatively regarded as metasedi-

mentary.

T h e c l a s s i f ic a t i o n a d o p t e d f o l l o w s t h e g u i d e l i n e s r e c o m m e n d e d i n C ir c u la r

5 6 o f

t h e

I n t e r n a t i o n a l S u b c o m m i s s i o n o n S t r a t i g r a p h i c N o m e n c l a t u r e

1 9 7 7 ) .



~.0

N

\

)

f . r

~

f

f

° i

E

~ z ~ ,,~ ,,. , ,, ,~ , . ~ ~ j . [ . ~

i_~ ~ ~ . ~ : ~

' ~ = Z . ~ . , , . l Z ~ .

E ~

0

0

o

~J

o



1 6 5

T H E S A N I G N A C I O O R O G E N I C C Y C L E (+- 2 0 0 0 - - 1 3 0 0 M a )

The San Ignacio cycle began with the deposition of the San Ignacio Schist

Group, afte r the close of the Trans-Amazonic cycle, and ended with an oro-

genic phase which has been reasonably well-dated isotopically (Darbyshire,

1979). Meta-arkoses and paragneisses of the San Ignacio Group sequence at

AscensiOn (Fig. 3) yield a three -point Rb--Sr isochron of + 1346 Ma, whilst

the San Javier Granitoid ( C on Fig. 6), which intrudes the San Ignacio

Group, gives a five-point Rb-Sr isochron of + 1317 Ma. Moreover, K--Ar

dates on gneisses from the region cratonized after the San Ignacio orogeny

(Fig. 7) give + 1336 Ma. The San Ignacio Group can thus be regarded as a

Middle Proterozoic sequence deposited within the (?) 1800--1350 Ma time-

span. However, with the exception of the San Ignacio area (Figs. 2 and 5),

where the San Ignacio Group appears to overstep the o lder paragneisses,

evidence of an unconfo rmable relationship with Trans-Amazonic rocks is

lacking; this can be at tributed to widespread migmatisation during the San

Ignacio and Sunsas orogenies, which effectively blurred basal contacts.

El

q

> d '

o \

- - N San I gnac io - / ~ :

~ , ~ ~

)

0 50 10 0K m T=rum = ~ . ~

F i g . 3 . D i s t r i b u t i o n a n d n a m e s o f

the San Ignacio roup schi st

b e l ts ( b l ac k ) . O r n a m e n t e d

a r e a i n d ic a t e s r o c k s o f equivalent age o r o l d e r .

T h e S a n I g n a c i o S c h i s t r o u p

The San Ignacio Group is represented , in the main, by scattered schist

belts (Fig. 3), the shape, structure, and, with the exception of the San Igna-

cio schist belt, the metamorphism of which are due to overprinting of the

San Ignacio orogeny by the Sunsas orogeny. Most of the schist belts appear

to contain right-way-up sequences which can be in terpreted as synclinal

plunge-depressions of local facies variations within a regional supmcrustal

sequence rathe r than as the deposits of isolated sedimentary bas{ns. In certain



1 6 6

c a s es i t c a n b e d e m o n s t r a t e d t h a t t h e s c h i st b e lt s a re i n f a c t s c h i s t r e l ic s

( L i t h e r l a n d , 1 9 7 3 ) ; i .e . u n m i g m a t i s e d l o w e r - g r a d e m e g a x e n o l i t h s in a m i g-

m a t i c h o s t t e r r a i n .

T h e s c h i s t b e lt s a r e c o m p o s e d o f p e li t ic s c h i s ts , w i t h a v a r i a b le p s a m m i t i c

f r a c t i o n , a n d t h i n l a y e r s o f g r a p h i t i c s c h is t s, c a lc - s il ic a t es , m e t a - i r o n s t o n e s

a n d m e t a - v o l c a n i c s . T h e r e i s a l so a m a j o r b a s i c / u lt r a b a s i c s i ll - f o rm i n g p h a s e .

L o c a l g r o u p t h i c k n e s s e s a r e in t h e r e g i o n o f 5 0 0 0 m .

M u s c o v i t e is th e p r e d o m i n a n t m i c a i n t h e p e l it ic s c hi st s w h i c h g e n e r a l l y

c o n t a i n s u b o r d i n a t e b i o t it e a n d a l m a n d i n e . T h e r e a r e s p o r a d ic d e v e l o p m e n t s

o f s t a u r o l i te , b u t k y a n i t e a n d s i ll im a n i t e a re r a re . A t S a n I g n a c i o ( L i t h e r l a n d ,

1 9 7 9 ) m e g a c r y s t s o f g a r n e t, u p t o 4 0 m m i n d i a m e t e r , a n d s t a u r o l it e , u p t o

1 0 0 m m l o n g , a r e l o c a l ly d e v e l o p e d . M e t a . a r k o s e a n d f e l d s p a t h i c q u a r t z i te

m a k e u p t h e b a sa l fo r m a t i o n s a t C o n c e p c i g n ( F l e t c h e r , 1 9 7 9 ) a n d A s c e n si 6 n

( P i t f ie l d , 1 9 7 9 ) { F ig . 4 ) , w h i l e o r t h o q u a r t z i t e s a r e d e v e l o p e d a t S a n I g n a c i o ,

C h a q u i p o c { M i tc h e ll , 1 9 7 9 ) a n d S a n D i a b lo { O C o n n o r , 1 9 7 9 ) w h e r e c r o ss -

b e d d i n g s t r u c t u r e s a r e p re s e r v e d . I r r e f u t a b l e m e t a v o l c a n i c s h a v e o n l y b e e n

n o t e d a t C o n c e p c i 6 n i n t h e f o r m o f m e t a m o r p h o s e d b as ic t u ff s a n d a m y g -

d a l o i d a l b a s a l t s i n t e r c a l a t e d i n t h e s c h i s t s a n d p h y l l i t e s . C a l c - s il i c a te l i t h o l o -

g ie s i n c l u d e g r a n o b l a s t i c h o r n b l e n d e - - q u a r t z a n d d i o p s i d e - - g ro s s u l a r it e a s-

s e m b l a ge s a n d a r e f r e q u e n t l y a s s o c i a t e d w i t h p a r a - a m p h i b o l i te s . M e t a -i r on -

s t o n e s l o c a l ly c o n t a i n a n d r a d i t e a n d g r u n e r i t e .

I n m o s t o f t h e s c h is t b e lt s th e r e a re o u t c r o p s o f v a ri a bl y m e t a m o r p h o s e d

b a si c a n d u l t r a b a s i c r o c k s , g e n e r a l l y i n t e r p r e t e d a s i n t r u s iv e s ills . T h e r e a r e

t h i n s e r p e n t i n i t e h o r i z o n s i n th e E1 P u e n t e a n d C o n c e p c i 6 n s c h is t b e lt s a n d

i n t h e C h a q u i p o c s c h i st b e lt ( F i g . 2) , a m a j o r i n t r u s i o n c o m p o s e d o f a d i f-

f e r e n t i a t e d s e q u e n c e o f m e t a p y r o x e n i t e , o l iv in e g a b b r o, p y r o x e n e - m i c a m e -

l a d i o ri t e a n d q u a r t z - m i c a d i o r it e . A t S a n I g n a c io ( F i g. 5 ) , w h e r e t h e S u p o -

n e m a M e t a b a si c F o r m a t i o n i s t h e o n l y m a r k e r h o r i z o n i n t h e s c h i st b e lt , t h e

S u p o n e m a C o m p l e x is a 1 0 0 0 - m t h i c k d i f f e r e n t ia t e d l o p o li th c o m p o s e d o f

s e r p e n t i n i te a n d m e t a m o r p h o s e d g a b b ro , p y r o x e n i t e , m e l a t r o c t o l it e , le u c o-

t r o c t o l i t e a n d g a b b r o . I n a n o t h e r l o p o l i t h , a t P a c h o r r i { F ig . 5) , a l e u c o t r o c t o -

l it e l a y e r w i t h i n m e t a g a b b r o is b o t h g r a d e d - a n d c r o s s - b e d d e d , i n d i c a ti n g d e-

p o s i t io n a l m a g m a t i c c u r r e n t s a n d a r ig h t . w a y - u p s e q u e n c e w i t h i n t h e s c h i s t

b e l t a s a w h o l e .

CONCE~CION-ELPUENTE SAN IGNAC I0 C RISTAL- ASCENSION ~ I:~UT[C SCHISTS

CNAOUIPOC

~btlPELITIC SCHISTS

~ ORT OQU,~RT ITES

B; ,SlC/ULT AB &S ; SILL S

METAVOLCANICS

C~. ~HmC HISTS

E : : . :

o p ~ ~ o o o o ] F e M E T A - IR O N S T O N E S

Fig . 4 . c h e m a t i c s t ra t i t~ a p h i e c o l u m n s f o r i n d i v i d u a l s ch iA t b e lt s . H e i g h t o f c o l u m n

represents pproxim tely

5 0 0 0 m



. . . . . . . . . .

A x i a l t r a c e s o f f o l d s w i t h

p l u n g e ) : S u n s o s O r o g e n y

• • S y n c l i n a l t r a c e : S o n I g n a c i o

O r o g e n y

- t - q -

Gran i te Sunsos Cyc le )

L o s P a t o s S c h i s t F o r m a t i o n

Suponem a Me tabas ic Fm .

M a t a c u S c h i s t F o r m a t i o n

S o n R a fa e l M e t a g r a n i t e F m

Chiqu i tan io Para gne iss Cx.

L a m a s M o n e c h e s G r c m u l i t e

G r o u p

- L

- I S A N I G N A C I O

i ~ + + + + + +

+ + q- + + -b -

÷ - I - ÷ 4 - + +

-I--I- . 4. + ~

÷ + ÷ + -I-~

t - + - I - ~ : - : = - = ~

G 2

. . ÷ + . 4 4 ~

~ ....{.+

÷ ~--......

. .'-..

I

5

- - N - -

. + ~

+ ÷

+

F i g. 5 . T h e S a n I g n a c i o G r o u p a t S a n I g n a c i o , a d a p t e d f r o m L i t h e r l a n d ( 1 9 7 9 ) . L a t e r i t e

c o v e r a n d m i n o r u n i t s o m i t t e d .



168

In the Concepci6n area the psammitic element of the San Ignacio Group

can be traced laterally into the paragneisses of the Chiquitania Complex

(2-B in Fig. 6), while in the CrisP--San Diabio regions the paragneisses are

frequen tly muscovite,rich and carry enclaves and resisters of the San Ignacio

Group. West of the Ascensi6n schist belt there appear to be downward ex-

tensions of the San Ignacio Group composed of schists, biotite--plagioclase

gneisses and semi-pelitic gneisses which have a contac t with the Lomas Mane-

ches Granulite Group distinguished by hor izons of arfvedsonitic alkaline

gneiss. Thus the paragneisses labelled 2B in Fig. 2 are thought to include

elements of metasomatised San Ignacio Group rocks in areas of geothermal

domes, a possible basal San Ignacio Group sequence of semipelitic charac ter,

and relics of the Trans-Amazonic paragneissic basement. Only by more de-

tailed Rb--Sr studies will it be possible to distinguish these groups.

Within the lower grade, well-defined, sequences of the schist belts there

appear to be two sedimentary associations: arkose--pelite and pelite--sandstone.

The former (Concepci6n, Ascensi6n) is characterised by a high psammitic

fract ion with associated tufts and lavas (Fig. 4) and a weak development of

the basic--ultrabasic phase. The pelite--sandstone envi ronment (San Ignacio,

Chaquipoc) has a lower psammitic fraction and a major basic--ultrabasic phase.

It is thus possible tha t Concepcidn and Ascensi6n represent marginal shallow-

water facies of a deeper water basinal sequence.

The San gnacio orogeny

Deformation sequences for the San Ignacio orogeny can be identified in the

northern region, which remained stable or almost so after the close of the

San Ignacio Cycle (Fig. 7). Near Concepcidn, tight to isoclinai northeast-

trending structures are refolded by north-trending structures whilst the out-

crop pattern of the Lomas Maneches Group (Fig. 2) is controlled by original-

ly north- trending anticlinal saddles with wavelengths of abo ut 50 km. The

San Ignacio schist belt (Fig. 5) is traversed by an early pinched syncline,

the axial plane of which is transected but not refolded by the low-dipping

schistosity related to the second phase of deformation. Because this same

deformation produces a subvertical fabric in the paragneisses, defined by the

alignment of K-feldspar augen, schistosity refraction within the supracrnstal

San Ignacio Group can be invoked. At San Diablo the supracrustal sequence

contains recumbent folds, the axial planes of which can be picked out by

cross-bedding in quartzites.

Petrological studies indicate that the higher level granulites recrystallised

during the San Ignacio orogeny and the sequence of basal granulites over-

lain by paragneisses with granulite lenses is regarded as due to a transition

from anhydrous to hydrous P--T conditions. The paragneisses were mig-

matised in the upper arnphibolite facies and the San Ignacio Group meta-

morphosed in the lower amphibolite facies as seen in the San tgnacio schist

belt: the only schist sequence which preserves metamorphic minerals and



÷ + + t + ± + +

+ + + + + + + +

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+ 1

+

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+ + + ' + ~ . ~+ + ' . . . . . . . . . . . . . .

+ ± + + . . . . . . . . . . . . . . . . . . . . . . .

+ + + + . . . . . . . . . . . . . . . . . . . . .

+ + . . . . . . . . . . . . . .

. . . . . . . .

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+ + + + + + + + + +

4

. . . . . . i B . . . . i i i - i i i i i i

z- + + + + ÷ O ' ' ' . . . . . . .

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+ + + + + + + + + +

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2 B

+ + + + + + +

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C +

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S T R A T I G R A P H I C U N I T

Q u a t e r n a r y

OROGEN ~

G R A N I T O lD

RO KS

1 T u c a v a c a G r o up B r a s i l ia n o

S u n s a s G r o up S u n s a s

J

S a n I g n a c i o G r o u p

an Ignacio

Ch iqu i tan ia Paragne iss Cx .

I

C h i q u i t a n m P a r a g n e i s s C x . Trans -Am azon ic I

Lam as M aneches G ranu l i t e G p .

Post tectonic

Syn / la te tec ton ic (G 2)

Late tectonic

Sy n / la t e tec ton ic

G1

Syntectonic

Pretectonic

1 6 9

F i g . 6 . T h e g e o l o g ic a l u n i t s o f t h e C o n c e p c i J n a r e a, a d a p t e d f r o m F l e t c h e r 1 9 7 9 ) . L a t e ri te

c o v e r a n d m i n o r u n i t s o m i t t e d .



:t7 0

: Serrania Aguap ei

, \ i

\ : ~ i '. . '.'. . . . . + .~AC_,U~I

~ L . . . ~- -k~__. . -- ' -- ~

/ / p ~ % " X 9 X ..

MARGINAL

ZONE

Q . . . . . o °

S t r u c t u r e s r e l a te d t o t h e S a n

l g n a c i o O r o g e n y

Craton ic Zone s tab le a f te r the

San tgnac io Orogeny

R } N D O N A

Q ...... 5pOKm

b

Orog en ic be l t I

Tecton ic f ron t 1 0 5 0

95

l b

Sense o f hor izonta l

movement I

F i g. 7 . T h e S u n s a s o r o g e n y i n a ) t h e m a p p e d a r ea , a n d b ) p r o j e c t e d t o i n c l u d e t h e w e s t -

e ~n s e c t o r o f t h e c e n t r a l B r a z i l s h i e ld .

textures related to the San Ignacio orogeny. At Concepci6n, the western

sector of the schist belt under went only greenschist facies metamorphism

within a postulated northeast-trending thermal syncline.

The granitoid phases related to the orogenic event, the bulk of which are

concen tra ted in the upper-level San Ignacio Group, show variations in time

of emplacement. The San Rafael Metagranite Formation (Fig. 5) formed by

the melting of the premetamorphic lower pelitic fraction of t he San Ignacio

Group along the possible unconf ormit y with Trans-Amazonic rocks. The San

Ramon Granitoids (A in Fig. 6) were affected by the second phase of deform.

ation and show gradational contacts with the San Ignacio Group. The San

Andres Granitoids (B in Fig. 6} are perthite-blastic orthogneisses which are

also found in the north of the area, in the later cratonic zone, where perthite

tablets remained disoriented. These are syntectonic granites formed during

the second phase of de format ion and generally associated with belts of K-

feldspar porphyroblastic gneiss in which aligned feldspar tablets or augen

mark the S-fabric of the second deformation . The syn/late-tectonic San

Javier Granitoids (C in Fig. 6) are a group of granitic plutons, associated with

minor phases of gabbro and hornblende-diorite, which intrude the schist

belt, whilst the late/post-tectonic Refugio Granitoids (D in Fig. 6) are finer-

grained rocks, with a higher microcline con ten t, which cut the San Javier

Grani~oids but are related to them.



171

o r r e l a t i o n

T h e S a n I g n a c io o r o g e n i c c y c l e is p o s s i b l y e q u i v a l e n t t o t h e E s p i n h a ~ o

c y c l e o f B ra z il w h i c h c o n t a i n s q u a r t z i t e s a n d p h y l l i t e s ( A l m e i d a , 1 9 7 1 ) . A

s u p r a c r u s t a l s e q u e n c e s i m il a r t o t h e S a n I g n a c io G r o u p l i t h o l o g ie s h a s b e e n

r e c o g n i s e d o v e r ly i n g T r a n s - A m a z o n i c b a s e m e n t o n t h e P a c if ic s e a b o a r d o f

P e r u ( C o b b i n g e t a l. , 1 9 7 7 ) a n d t h e R o r a i m a F o r m a t i o n ( M c C o n n e l l e t a l. ,

1 9 6 4 ) , a v o l c a n o - s e d i m e n t a r y s e q u e n c e o f s im i la r a g e , r e s ts o n T r a n s -A m a -

z o n ic b a s e m e n t o f t h e G u i a n a s h i el d .

THE SUNSAS OROGENIC CYCLE < 1300 --950 Ma)

T h e S u n s a s c y c l e c o m p r i s e s t h e e r o s i o n o f p r e -e x i s ti n g r o c k s o f t h e S a n

I g n a c io c y c l e , th e d e p o s i t i o n o f th e S u n s a s a n d V i b o s i G r o u p s , a n d t h e s u b -

s e q u e n t d e f o r m a t i o n o f t h e s e a n d o l d e r r o c k s w i t h in t h e b o u n d s o f a d e -

f i n e d S u n s a s o r o g e n i c b e l t . I t w a s a c c o m p a n i e d b y a m a j o r p h a s e o f u l tr a -

b a s ic i g n e o u s i n t r u s io n a n d g r a n i to i d d e v e l o p m e n t . O u t c r o p s o f t h e S u n s a s

a n d V i b o s i G r o u p s a r e s p o r a d i c a l ly d i s t r i b u t e d i n t h e a re a , in th e f o r m o f

p r o m i n e n t r id g e s r e p re s e n t in g m e t a m o r p h o s e d s y n cl in a l in f o ld s o r m e s a s o f

f l a t- l y in g , re l a t i v e ly u n d i s t u r b e d s e q u e n c e s o f a r e g i o n a l s u p r a c r u s t a l c o v e r .

T h e y c a n b e u s e d a s l i t h o s t ra t i g r a p h ic i n d i c a to r s in e l u c id a t i n g t h e t e c t o n o -

m e t a m o r p h i c h i s to r y o f t h e o r o g e n ic p h a s e . T h e S u n s a s c y c le ca n o n l y b e

s a id to h a v e o p e n e d s o m e t i m e a f t e r 1 3 0 0 M a b u t i ts c l o s e c a n b e c o n f i d e n t -

l y p l a c e d a t + 9 5 0 M a o n t h e e v i d e n c e o f a n u m b e r o f K - - A r d a t e s f r o m a s-

s o c i a t e d la t e g ra n i te s a n d p e g m a t i te s ( D a r b y s h i r e , 1 9 7 9 ) .

T h e S u n s a s a n d V i b o s i G r o u p s

A t t h e t y p e l o c a l i t y o f S e rr a n fa S u n s a s ( M i t ch e l l, 1 9 7 9 ) t h e S u n s a s G r o u p

is a 6 5 0 0 - m t h i c k s e q u e n c e d i v is i b le in t o f o u r u n i t s : a b a s a l c o n g l o m e r a t i c

f o r m a t i o n o v e r la in b y an a r e n a c e o u s fo r m a t i o n ( 2 6 0 0 m t h i c k ), c o m p o s e d

o f a rk o s e s , s a n d s t o n e s , f e l d s p a t h i c q u a r t z i te s a n d m i c a c e o u s q u a r t z i te s ; f o l-

l o w e d b y a n a rg i l la c e ou s f o r m a t i o n ( 1 2 5 0 m ) o f b r o w n m u d s t o n e s a n d g re y

s i l t s t o n e s a n d , f i n a l l y , a n u p p e r a r k o s i c f o r m a t i o n ( 1 4 0 0 m ) . T h e b a s a l c o n -

g l o m e r a t e is o l i g o m i c t i c w i t h c l a s t s o f v e i n - a n d s m o k y - q u a r t z i n a f i n e -

g r a in e d s er ic i t ic m a t r ix w h i c h f r e q u e n t l y c o n s t i t u t e s u p t o 5 0 o f t h e r o c k .

T h e s a m e s e q u e n c e , r e d u c e d t o 8 0 0 m , f o r m s t h e U n i d a d A g u a p e i (F ig u er e-

d o a n d O l i v a t ti , 1 9 7 4 ) i n B ra z il , o u t l ie r s o f w h i c h a r e f o u n d i n B o l i v ia a t

A s c e n c i d n a n d L a s P e t a s ( P i tf ie l d , 1 9 7 9 ) w h e r e s e q u e n c e s u p t o t h e l e v el o f

t h e a r g i ll a c e o u s f o r m a t i o n a r e e x p o s e d . A s ig n i fi c a n t f e a t u r e o f th i s b o r d e r

a r e a is t h e e a s t w a r d c h a n g e i n t h i c k n e s s o f t h e c o n g l o m e r a t e f o r m a t i o n f r o m

1 0 0 t o 6 0 0 m a s t h e l in e o f t h e A g u a p e i f r o n t ( se e F ig . 7 ) is a p p r o a c h e d , a

c h a n g e a c c o m p a n i e d b y t h e d e v e l o p m e n t o f a f e r r u g in o u s m a t r ix in th e c o n -

g l o m e r a t i c l e n s e s .

I n t h e C o n c e p c i d n a r e a ( F ig . 6 ), w h e r e t h e S u n s a s G r o u p o v e r li e s S a n I g-



72

nacio Group rocks intruded by .+- 1300 Ma granites, all fou r format ions can

be identified within a 1150-m thick sequence exposed to the basal levels of

the arkosic formation (Fletcher, 1979). In this area, where the underlying

San Ignacio Group was locally metamorphosed unde r greenschist facies con-

ditions, metamorphic minerals within the argillaceous formation include

chloritoid, garnet, staurolite, sillimanite and kyanite.

The Vibosi Group, a 2600-m thick sequence of arkoses and sandstones,

unc onformabl y overlies the Sunsas Group in the southeast of the area (Mit-

chell, 1979), but is intruded by the Rincon del Tigre Igneous Complex (+

992 Ma) and affected by the Sunsas orogeny.

T h e u n s a s o r o g e n y

Unlike the San Ignacio orogeny, the Sunsas orogeny did no t a ffec t the

whole of the region but was essentially confined to three tectono- -meta-

morphic zones: the Sunsas orogenic belt, the North Marginal zone, and the

Aguapei mobile belt, which can be distinguished from a stable cratonic

zone (Fig. 7).

T h e u n s a s o r o g e n ic b el t is confined to a west-northwest-trending region

bounded to the north by a curvilinear tectonic front . At Concepcidn

(Fig. 6) the fro nt essentially divides refolia ted and remobilised basement and

Sunsas Group rocks from relatively stable rocks metamo rphosed during the

San Ignacio cycle. At San Diablo (Fig. 2) the fro nt is marked by a local

mylonitic straightening zone (Hepworth , 1967) of intensely refoliated gneis-

ses, with a stretching direction plunging gently eastwards along the zone,

in which the granulites are downgraded for a considerable distance to the

north. Along the length of the front, deeper-level rocks composed o f the

Lomas Maneches Granulite Group and Trans-Amazonic gneisses are juxta-

posed against higher-level units formed during the San Ignacio and Sunsas

cycles, indicating that the orogenic zone moved downwards relative t o the

semi-stable North Marginal zone , while major swings in the strike of San

Ignacio structures reveal a horizonta l compo nent of sinistral moveme nt along

the San Diablo f ront (Figs. 2 and 7). Early north -trending San Ignacio struc-

tures can also be picked out within the Concepcidn area of the orogenic belt

(Fig. 7).

Local def ormat ion sequences within the belt indicate east-northeast-trend-

ing, tight to isoclinal first structures refolded and sheared by inter-penetra-

tive westnorthwest-trending structures which mark the t rend of th e orogenic

belt and are parallel to the fron t. There is evidence to suggest tha t the first

phase of defo rmati on consti tuted thin-skin tectonics and affected only the

supracrustal Sunsas Group. Metamorphism within the belt was generally

of amphibol ite facies in the basement and greenschist facies in the overlying

Sunsas Group. At Concepcidn, however, west-northwest trending isograds of

Barrovian metamorphi sm, marking the appearance of garnet, staurolite and



173

s i ll im a n i te , t r a n s e c t t h e S u n s a s G r o u p - - b a s e m e n t i n t e r fa c e o n a r e g io n a l sc a le .

M i g m a t i te s , s y n t e c t o n i c g r a n i t es a n d p o s t - t e c t o n i c g r a n it e s w e r e d e v e l o p e d

d u r i n g t h e o r o g e n i c p h a s e . T h e p o s t - t e c t o n i c g r a n it e s lie a lo n g a n o r t h w e s t -

t r e n d i n g l i n e w i t h i n t h e S u n s a s o r o g e n i c b e l t ( F i g . 2 ) a n d t h e g r a n i t e n o r t h o f

S a n J o s ~ h a s a n a u r e o l e o f a n d a l u s i te - - c o r d i e r i te h o r n f e ls . P e g m a t i te s c o n -

r a in i ng m i c a , b e r y l a n d c o l u m b i t e w e r e i n t r u d e d i n t o t h e s c h is t b e lt s d u r i n g

t h i s p e r i o d a n d t h e r e w e r e s m a l l r h y o l i t e i n t r u s i o n s i n t o t h e m e t a s e d i m e n t s

o f t h e C o n c e p c i 6 n a r e a .

The North marginal zone ( Fig . 7 ) is b o u n d e d t o t h e s o u t h b y t h e C o n c e p -

c i d n - - S a n D i a b l o f r o n t a n d t o t h e n o r t h b y t h e c r a t o n i c z o n e . I t w a s a s em i -

s t a b l e r e g i o n d u r i n g t h e S u n s a s o r o g e n y . A re g i o n a l e a s t - t re n d i n g s i n is t ra l

s h e ar , c o r r e l a t e d w i t h t h e s e c o n d p h a s e o f d e f o r m a t i o n , t h e S a n t a C a t a li n a

s t ra i g h te n i n g z o n e ( F i g. 2 ) , a f f e c t e d t h e r o c k s c o n t r o l l e d b y t h e s t a b le c r a t o n i c

b l o c k to t h e n o r t h . T h i s s he a r z o n e , c o n t e m p o r a n e o u s w i t h t h e S a n D i a b l o

f r o n t , d e f l e c t s t h e t r e n d s o f e a rl ie r n o r t h - t r e n d i n g S a n I g n a c io f o l d s , t h e

a x e s o f w h i c h p l u n g e i n t o t h e s h e a r , w h i c h i s t h u s a r e g i o n a l c r o s s - s y n f o r m .

E a s t - t r e n d i n g o p e n - t o - c l o s e f o l d s al s o a f f e c t t h e S a n I g n a c i o s c h i s t b e l t

( F ig . 5) w h e r e t h e y a r e a s s o c i a t e d w i t h n o n - p e n e t r a t i v e s tr a in - s li p s t r u c t u r e s

a n d t h e m a i n p e g m a t i t e p h a s e . T h e r e is n o e v i d e n c e o f t h e e a r l y ' th i n - sk i n '

p h a s e o f S u n s a s f o l d i n g o f p r e - S u n s a s G r o u p r o c k s in th i s r e g i o n . T h e s u r-

r o u n d i n g p a r a g n ei s se s w e r e a f f e c t e d b y c o n j u g a t e s e t s o f m i g m a t i t i c s h e a r s

a n d t h e r e a r e s m a ll s y n t e c t o n i c g r a n it e s o f p l u t o n i c a n d a n a t e c t ic t y p e , p a r -

t ic u l a r ly p r o m i n e n t a l o n g t h e S a n t a C a t a l in a s t r a ig h t e n in g z o n e , o n e o f w h i c h

y i e ld s a t h r e e - p o i n t R b - - S r i s o c h r o n o f + 1 0 2 1 M a . K - - A t m i c a d a t e s o f t h e

p e g m a t i t e s a n d g r a n it e s g iv e t h e 1 0 0 0 - - 9 5 0 M a S u n s a s s p a n a n d t h e r e a r e

w h o l e r o c k K - - A r a ge s o f 1 2 0 0 - - 1 1 0 0 M a w h i c h r e p r e s e n t p a r t i a ll y r e s e t

S a n I g n a c i o d a t e s .

I n t h e s o u t h e a s t o f t h e r e g io n t h e N o r t h - M a r g i na l z o n e c a n b e i n t e r p r e t e d

t o i n c l u d e t h e R i n c o n d e l T i g re a r e a w h i c h s u f f e r e d o n l y g e n t l e f o ld i n g a n d

t h e i n t r u s i o n o f th e R i n c o n d e l T i g re I g n e o u s C o m p l e x i n t h e f o r m o f a sill

a lo n g t h e u n c o n f o r m i t y b e t w e e n t h e S u n s a s a n d V i b o s i G r o u p s . T h e C o m -

p l e x ( A n n e l ls , 1 9 7 9 ; A n n e U s e t a l. , 1 9 8 1 ) i s a d i f f e r e n t i a t e d i g n e o u s i n t ru -

s i on w h i c h v a ri e s in t h ic k n e s s f r o m 3 0 0 0 t o 4 6 0 0 m . I t c a n b e d iv i d e d i n to

a b a s a l U l t r a m a f i c U n i t ( 1 9 0 0 - - 3 0 0 0 m ) , w h i c h c o n s i s ts o f c y c l ic u n i t s o f

d u n i t e - - o l iv i n e b r o n z i t i t e - - b r o n z i t i te a n d b r o n z i t e p i c r it e - - m e l a n o r i t e , f o l-

l o w e d b y t h e M a fi c U n i t ( 7 0 0 - - 1 5 0 0 m ) , c o m p o s e d o f a l o w e r n o r i t e a n d a n

u p p e r g a b b r o l a y e r , a n d f i n a l l y th e u p p e r F e l si c U n i t ( 3 0 0 - - 7 5 0 m ) w h i c h i s

g r a n o p h y r i c , f o r m e d l a rg e l y b y m e l t in g o f t h e o v e r l y i n g V i b o s i G r o u p . T h e

C o m p l e x h a s b e e n d a t e d a t -+ 99 2 M a ( D a r b y s h i r e , 1 9 7 9 ) a n d is r e g a r d e d a s

a m a j o r i n t ru s i v e p h a s e o f t h e N o r t h - M a r g i n a l z o n e , f o r m e d a f t e r t h e f i r s t

e p i so d e o f S u n s a s d e f o r m a t i o n .

The cratonic zone

l i es t o t h e n o r t h o f t h e r e g i o n a n d c a n b e d i s ti n g u i s h e d

b y f o u r c ri te r ia : f ir s t, th i s z o n e i n c l u d e s t h e S u n s a s G r o u p s e d i m e n t s f o r m i n g



7 4

S e rr am a A g u a p e i i n B ra z il , th e w e s t e r n s e c t o r o f w h i c h is u n d e f o r m e d ; s e c o n d

t h e n o r t h - t r e n d i n g S a n I g n a c io s t r u c t u r e s in th i s z o n e a r e n o t d e f l e c t e d b y

S u n s a s o r o g e n i c t r e n d s ( F i g . 7 ) ; t h i r d , K - - A r d a t e s o f g n e is s e s a r e c h a r a c t -

e r is t i c o f t h e S a n I g n a c io o r o g e n y ; a n d , f o u r t h , t h e r e a r e n o r e c o r d e d S u n s a s

c y c l e g ra n i t es . I t c a n t h u s b e c o n c l u d e d t h a t t h e c r a t o n i c z o n e w a s a s ta b l e

r ig id b l o c k d u r i n g t h e S u n s a s c y c l e .

T h e g u a p e i m o b i l e b e l t

is a n o r t h - n o r t h w e s t - t r e n d i n g z o n e o f f o l d e d

S u n s a s G r o u p r o c k s c u t b y m a j o r t e c t o n i c s l id e s, n o t a b l y t h e A g u a p e i f r o n t

( F ig s . 2 a n d 7 ) , w h i c h d i v i d e s t h i s b e l t f r o m t h e C r a t o n i c z o n e w h e r e t h e

S u n s a s G r o u p is e s s e n t i a ll y u n d e f o r m e d . A t L a s P e ta s , a lo n g t h e l i ne o f t h is

f r o n t , a l o ca l s y n c l i n e o f g r e e n s c h i s t f a c i e s S u n s a s G r o u p r o c k s is r o t a t e d

t h r o u g h a v e r ti c a l p o s i t i o n t o l ie w i t h a d o w n w a r d - f a c i n g a t t i tu d e . T h e A g u a -

p e i m o b i l e b e l t is w e l l e x p o s e d i n B r az il , w h e r e i t is c o m p o s e d o f f o l d e d a n d

f a u l t e d S u n s a s G r o u p r o c k s ( F i g u e r e d o a n d O l iv a t t i , 1 9 7 4 ) , b u t i ts e x t e n -

s i o n i n t o B o l i v i a i s l a r g e ly c o v e r e d b y a l l u v iu m , m a k i n g c o r r e l a t i o n w i t h t h e

m a i n S u n s a s o r o g e n y d i f f i c u l t. A l t h o u g h K - - A t d a t e s ( p h y ll i t e , p e g m a t i t e )

a r e i n t h e r a n g e 1 0 5 0 - - 9 5 0 M a ( D a r b y s h i r e , 1 9 7 9 ) , i t is t h o u g h t t h a t t h e

f o r m a t i o n o f th i s m o b i l e b e l t w a s t h e l a st e v e n t o f th e o r o g e n i c c y c l e s in c e

s t r u c t u r e s w i t h s im i l ar t r e n d s ( b e t w e e n n o r t h - n o r t h e a s t a n d n o r t h - n o r t h w e s t )

a f f e c t e a rl ie r S u n s a s s t r u c t u r e s w i t h i n t h e S a n I g n a c i o s c h i s t b e l t ( F ig . 5 )

a n d s u r r o u n d i n g p a r a g n e i s s e s a n d a r e a l s o s u p e r i m p o s e d o n t h e e a s t e r n s e c t i o n

o f t h e S u n s a s o r o g e n i c b e l t . T h e s e n o r t h - n o r t h e a s t t o n o r t h - n o r t h w e s t -

t r e n d i n g s t r u c t u r e s , w h i c h m a y a ls o h a v e b e e n r e s p o n s i b l e f o r t h e m a j o r

i n f le x i o n s in t h e c u r v il in e a r t r e n d o f t h e C o n c e p c i 6 n - - S a n D i a b l o f r o n t , a r e

t h u s t e n t a t i v e l y c o r r e l a t e d w i t h a l at e -S u n s a s A g u a p e i m o b i l e b e l t.

T h e d e p o s i t i o n o f t h e S u n s a s G r o u p

T h e m o l a s s ic S u n s a s G r o u p v a r ie s in t h ic k n e s s f r o m 6 5 0 0 m a t S er ra n~ a

S u n s a s t o 8 0 0 m a t S e r ra m a A g u a p e i . S i n c e t h e f o r m a t i o n a l l i t h o l o g i e s , in -

c l u d in g th e s u p e r - m a t u r e b a s al c o n g l o m e r a t e ( F i g u e r e d o a n d O l iv a t ti , 1 9 7 4 ) ,

e x h i b i t n o f a c ie s v a r i a t i o n s , i t c a n b e a s s u m e d t h a t s u b s i d e n c e a n d s e d i m e n t a -

t i o n w e r e in e q u i li b r iu m , T h e t h i n n e s t s e q u e n c e a t A g u a p e i f o r m e d o v e r t h e

r e g i o n w h i c h w a s l a t e r to b e c o m e t h e s t a b le c r a t o n w h i l s t t h e t h i c k e n i n g a t

L a s P e t a s c o i n c i d e d w i t h t h e l in e o f t h e la t e r A g u a p e i f r o n t . P a l a e o c u r r e n t

d i r e c t io n s in t h is r e g io n a re f r o m t h e w e s t , i .e . f r o m c r a t o n t o m o b i l e

b e l t . D e p o s i t i o n o f t h e V i b o s i G r o u p a l so t o o k p l a c e in t h e S u n s a s r e g io n

o f m a x i m u m s u b s id e n c e . T h e s e fa c t o r s s u gg e st t h a t t e c t o n o - s e d i m e n t a r y

c o n t r o l o f S u n s a s G r o u p d e p o s i t i o n w a s r e l a te d t o l in e a r z o n e s o f c ru s t a l

w e a k n e s s w h i c h w e r e l a t e r r e j u v e n a t e d d u r i n g th e s u b s e q u e n t S u n s a s o ro -

g e n y i n t h e f o r m o f o r o g e n i c o r m o b i l e b e l ts .



175

orrelation

T h e S u n s a s c y c l e c an b e c o r re l a t e d w i th t h e U r u ~ u a n o c y c l e o f t h e A t -

l a n ti c s h i el d a re a (F i g. 1 ) w h i c h s p a n s t h e p e r i o d 1 3 0 0 - - 1 0 0 0 M a ( A l m e i d a

e t a l. , 1 9 7 6 ) . I n t h e w e s t e r n p a r t o f t h e C e n t r a l B r a zi l s h ie l d , t h e R o n d 6 n i a

b e l t , f o r m e d d u r i n g t h e R o n d 6 n i e n s e e v e n t ( A l m e i d a e t al . , 1 9 7 6 ) , i n c lu d e s

m o l a s s e s e d i m e n t s w h i c h a r e c u t b y 9 5 0 M a g r a n it e s ( P ri e m e t a l ., 1 9 6 6 ) .

O n t h i s b a s is i t c o u l d b e s u g g e s t e d t h a t t h e C r a t o n i c z o n e i d e n t i f i e d i n th e

n o r t h o f t h e p r e s e n t a r e a is b o u n d e d o n a ll s id e s b y (+ 1 0 0 0 M a ) o r o g e n i c

o r m o b i l e b e l t s ( F i g . 7 b ) .

THE BRASILIANO OROGENIC CYCLE 950--500 Ma)

I n e a s t e rn B o l iv i a t h e c l o s e o f th e S u n s a s c y c l e m a r k e d t h e c r a t o n i z a t i o n o f

t h e s h ie ld a n d t h e d e p o s i t s o f t h e U p p e r P r o t e r o z o i c - - L o w e r P a l a e o z o ic B r a-

s il ia n o (P a n - A f r i c an ) c y c l e a r e o n l y r e p r e s e n t e d b y u n m e t a m o r p h o s e d s e-

q u e n c e s o n i ts e a s te r n a n d s o u t h e r n f la n k s . In t h e e a s t , t h e s e d i m e n t s o f t h e

B o q u i G r o u p ( M i t c he l l, 1 9 7 9 ) , p a r t o f th e P a r a g u a y - - A r a g u a i a b e l t o f t h e

B r a s i li a n o c y c l e ( A l m e i d a e t a l. , 1 9 7 6 ) ( F ig . 1 ) , a r e m i o g e o s y n c l i n a l c o n g l o -

m e r a t e s , a r k o s e s, s h al e s a n d l i m e s t o n e s w i t h a t o t a l t h i c k n e s s o f a b o u t 1 0 0 0

m w h i c h u n c o n f o r m a b l y o v e r li e r o c k s o f t h e S u n s a s c y c l e . T h e s e p a s s l a te r al -

l y e a s t w a r d i n t o a t h i c k e r e u g e o s y n c l i n a l s e q u e n c e o f c o n g l o m e r a t e s ,

g r e y w a c k e s , b a n d e d i r o n s t o n e s , c h e r t s , l i m e s t o n e s a n d c a l c i lu t it e s. T h e r e

a re m a j o r fa c ie s c h a n g es c o n t r o l le d b y n o r t h - tr e n d i n g c o n t e m p o r a n e o u s r i f t

f a u lt s . T h e B o q u i G r o u p w a s fo l d e d i n t h e e a s t a b o u t n o r t h - tr e n d i n g a x e s

( a t t h e w e s t e r n e d g e o f th e P a r a g u a y - - A r a g u a ia m o b i l e b e l t) a n d u n c o n f o r m -

a b l y o v e rl a in b y t h e M u r c i ~l a go g r o u p o f r e e f a n d n o n - r e e f l i m e s t o n e s w i t h

d e v e l o p m e n t o f re e f fa c ie s a ls o c o n t r o l le d b y c o n t e m p o r a n e o u s n o r t h - t re n d -

i n g f a u l t s . T h e M u r c i d la g o G r o u p i s t e n t a t i v e l y r e g a r d e d a s C a m b r i a n i n a g e

s i n c e i t c o n t a i n s C a m b r i a n s t r o m a t o l i t e s i n i ts B r a z i li a n c o r r e l a t i v e , t h e C o -

r u m b ~ L i m e s t o n e ( B e u r l e n a n d S o m m e r , 1 9 6 7 ) , a n d i t a ls o r e p r e s e n t s a m a j o r

m a r i n e t ra n s g r e s si o n o v e r a f a u l t - c o n t r o l le d U p p e r P r o t e r o z o i c b a s i n ( M a t -

t h e w s a n d C o w i e , 1 9 7 9 ) .

O n t h e s o u t h e r n f l a n k o f t h e s h ie l d, U p p e r P r o t e r o z o i c s e d i m e n t a t i o n is

r e p r e s e n t e d b y t h e m i o g e o s y n c li n a l T u c a v a c a G r o u p ( M i tc h e ll , 1 9 7 9 ; O C o n -

n o r , 1 9 7 9 ) c o n f i n e d t o a w e s t - n o r t h w e s t - t r e n d i n g f a u l t - c o n t r o l le d b a s i n , a t

l e a s t 4 0 0 k m i n l e n g t h , w h i c h f o l l o w s t h e t r e n d o f t h e e a r li e r S u n s a s b e l t.

T h e G r o u p u n c o n f o r m a b l y o v e rl i es r o c k s o f t h e S u n s a s c y c le t o t h e w e s t a nd

t h e B o q u i G r o u p t o t h e e a s t . I t is c o m p o s e d o f a b a s a l d o l o m i t i c l i m e s t o n e

o v e r la i n b y s a n d s t o n e s a n d s h al es w i t h a t o t a l t h ic k n e s s o f a b o u t 2 0 0 0 m .

S l ig h t f o l d in g o n w e s t n o r t h w e s t - t r e n d i n g a x e s l o c a l l y re f o l i a t e d a d j a c e n t

b a s e m e n t r o c k s n e a r S a n J o s d ( F ig . 2 ) w h i c h y i e ld r e s e t K - - A t a ge s o f

-+ 5 4 5 M a . T h e T u c a v a c a b a s i n t h u s r e p r e s e n t s a m a j o r f a u l t - c o n t r o l l e d o f f -

s h o o t o f t h e P a r a g u a y - - A r a g u a i a b e l t . F u r t h e r m a r i n e t r a n sg r e s s io n s o c c u r -

r e d i n t h e P h a n e r o z o i c a l o n g t h is u n s t a b l e l in e w h i c h r e m a i n s a z o n e o f

s e i sm i c a c t i v i ty ( C h i q u i t o s f a u l t o f G a n s s e r , 1 9 7 3 ) .



76

It should be noted that the Brasiliano orogeny produced only local folding

within the rocks of the Boqui and Tucavaca groups; most of the area studied

remained stable during the Brasiliano cycle.

During the Upper Proterozoic, the shield was cut by minor swarms of

east-trending dykes o f dolerite--gabbro composition with minor olivine gab-

bros and norites. K--Ar dates are widely scattered and range from Lower

Proterozoic to uppermost Proterozoic, but the dykes cut structures related

to the 1000 Ma Sunsas orogeny (although they have not been noted intruding

the Sunsas Group). The dykes are particularly concen tra ted along the east-

trending Rio Mercedes fracture system, ano ther line of Phanerozoic crustal

weakness, which truncates the Paraguay--Araguaia belt in Brazil and is as-

sociated with alkaline plutonic activity of Jurassic--Cretaceous age in Bolivia

(Fig. 2).

DISCUSSION

Precambrian studies elsewhere in South America have been able to delimit,

on a regional basis, the Trans-Amazonic cratons from rocks of the Brasiliano

cycle (Almeida et al., 1976), but , due to a lack of detailed mapping and un-

reliable dates, the tectono- -metamorphic history of the intervening (1800--

1000 Ma) period is not fully unders tood. The identification of tw o orogenic

cycles in eastern Bolivia within this per iod is thus of ma jor significance and

warrants fur the r discussion.

The San Ignacio orogeny affected the Trans-Amazonic basement and the

supracrustal San Ignacio Group. Field evidence suggests that the older gra-

nulites may have been metamorphosed (+ 2000 Ma) within a horizontal

tectonic regime (Bridgewater et al., 1974) with a primary strat iform folia-

tion subparallel to t he la ter supracrustal sequence. The San Ignacio Group is

composed of pelite--psamrnite sequences which must have been deposited in

deep water since the organisms which yielded graphitic schists could not

otherwise have survived (Cloud, 1971). A possible near-shore facies has

been suggested in the text .

During the ensialic San Ignacio cycle the anticlinal saddles of granulite

may have been the crests of fault-controlled massifs during a proto.Cordil-

leran orogeny (Cobbing, 1978). The facies changes in the supracrustal se-

quence may have been controlled by these faults with the volcanic sequences

formed along them and the basic intrusive sills the result of deep-seated

dyking in the basin (pelitic) regions. During the orogenic stage an upper level

of ductile supracrustal rocks, migmatites, granites and retrograded gmnulites,

and a lower-le~eel zone of anhydrous granulite facies assemblages which re-

sisted regeneration may be envisaged (see Watson, 1973).

The Sunsas cycle included the deposition of a supracmstal sequence which

exhibits thickening, unaccompanied by facies changes, from stable platform

to mobile belt, a phe nom enon in character with a molassic ensialic basin

rather than an aulacogen (Hoffman et al., 1974). During the orogenic stage,



77

d e f o r m a t i o n w i t h in t h e b e l t w a s a c c o m p a n i e d b y s in is tr al m o v e m e n t s b e -

t w e e n c r a t o n i c b l o c k s ( s ee C a n a d i a n s h ie ld , W a t s o n , 1 9 7 3 ) b u t , a l t h o u g h t h e

e a r li e r t r e n d s a r e sl ig h t l y o f f s e t w i t h i n t h e s e m i - s ta b l e N o r t h - M a r g i n a l z o n e ,

t h e y a r e p r e s e r v e d w i t h i n t h e o r o g e n i c b e l t i n t h e C o n c e p c i d n a re a . T h u s t h e

S u n s a s b e l t is a n a l o g o u s i n a g e an d f o r m t o t h e + 1 0 0 0 M a K i b a r a n - - I r u m i d e

b e l t o f A f r i c a ( S h a c k l e t o n , 1 9 7 3 ) w h i c h o v e r p r in t s , w i t h o u t d i s p l a c e m e n t ,

t h e o l d e r U b e n d i a n - R u z i z i b e l ts a n d e n c lo s e s t h e o l d e r u n i ts i n t h e m a n n e r

o f F ig . 7 b . T h e s e p r e - P a n - A f r i c a n ( p r e - B r a s i li a n o ) m o b i l e b e l t s o f s o u t h e r n

A f r i c a a r e e n s ia l ic ( n o m a j o r d i s p l a c e m e n t s o f e a rl ie r l i n e a m e n t s ) a n d c h a -

r a c t e ri s e d b y p l a te d e s t r u c t i o n r a t h e r t h a n p l a t e a c c r e t i o n ( K r S n e r , 1 9 7 6 ) .

T h e z o n a l s t r u c t u re o f th e S u n s a s b e l t c a n n o t , h o w e v e r , b e c o m p a r e d t o t h e

t e c t o n i c z o n a t io n s o f t h e 2 6 0 0 M a L i m p o p o m o b i l e b e lt w h e r e c r a t o n a n d

m o b i le b e l t f o r m e d a t t h e sa m e t im e ( K e y a n d H u t t o n , 1 9 7 6 ) . T h e S u n s a s

o r o g e n y w i th i ts w e l l -d e f in e d t e c t o n o - - m e t a m o r p h i c z o n e s a n d p o s t - t e c t o n i c

g r a n i te s is n o t m a r k e d b y h i g h - g ra d e m i n e r a l a s s e m b l a g e s a n d , a l t h o u g h i ts

t e c t o n i c f r o n t s f i t i n t o t h e c a t e g o r y o f s t ra i g h te n i n g z o n e s , t h e s e l ie a t

r i g h t a n g l e s t o t h e w o r l d - w i d e t r e n d o f P r o t e r o z o i c h i g h - g ra d e l i n e a r b e l ts

( D a v ie s a n d W i n d l e y , 1 9 7 6 ) .

T h e A g u a p e i m o b i l e b e l t m u s t b e r e g a rd e d a s a l a te o f f s h o o t o f th e S u n s a s

b e l t . I t d o e s n o t a p p e a r t o p o s s e s s a g r a n i t i c p h a s e b u t c o n t a i n s h i g h - l e v e l

d i f f e r e n t i a t e d b a s i c i n t r u s i o n s ( F i g u e r e d o a n d O l i v a t t i, 1 9 7 4 } s i m i la r t o t h e

R i n c o n d e l T i g re I g n e o u s C o m p l e x . I t th u s s h o w s a ff in i ti e s t o t h e m a r g i n a l

z o n e r a t h e r t h a n t o t h e m a i n b e l t o f t h e S u n s a s o r o g e n y .

O N L U S I O N S

F o u r P r o t e r o z o i c o r o g e n i c c y c l e s h a v e b e e n r e c o g n i s e d i n e a s te r n B o l iv ia .

T h e i d e n t if i c a t i o n o f t h e L o w e r P r o te r o z o i c T r a n s . A m a z o n i c c y c l e a n d t h e

U p p e r P r o t e r o z o i c B r a s il ia n o c y c l e w a s a n t i c i p a t e d b e f o r e t h e f i e l d w o r k i n

t h e li g h t o f B r a z il ia n w o r k . T h e r e c o g n i t i o n o f t w o M i d d le P r o t e r o z o i c o r o -

g e n ic c y c l e s is , h o w e v e r , o f m a j o r s i gn i fi c a n c e s i n ce t h is p e r i o d o f r e w o r k i n g

o f t h e T r a n s - A m a z o n i c n u c l e u s p r i o r t o t h e B r a s il ia n o ( P a n - A f r ic a n ) e v e n t

is n o t w e l l d o c u m e n t e d in S o u t h A m e r ic a . D u r i n g t h e M i d d l e P r o t e r o z o i c ,

t h e S a n I g n a c io c y c l e (+ 2 0 0 0 - - 1 3 0 0 M a ) b e g a n w i t h t h e d e p o s i t i o n o f a w i d e -

s p r e a d s c h is t b e l t s e q u e n c e a n d c u l m i n a t e d i n t h e S a n I g n a c io o r o g e n y , a

r e gi on a l n o r t h - t re n d i n g t e c t o n o - - m e t a m o r p h i c e v e n t w h i c h r e w o r k e d t h e

o l d e r g r a nu l it e b a s e m e n t . T h e S u n s as c y c le ( < 1 3 0 0 - - 9 5 0 M a ) i n cl u d e d t h e

d e p o s i t i o n o f m o l a s s ic s e d i m e n t s a n d a n o r o g e n i c p h a s e a n a l o g o u s t o e n si al ic

m o b i l e b e l ts o f t h e s a m e a g e in s o u t h e r n A f r i c a a n d e x h i b i ts a st a b l e cr a-

t o n i c z o n e , a s em i - s t a b le m a r g in a l z o n e a n d a n u n s t a b l e o r o g e n i c z o n e , d i v i d e d

b y w e s t n o r t h w e s t - t r e n d i n g c u r v i l in e a r si n is tr a l s h e a r s.

I t is s u g g e s t e d t h a t t h e + 1 3 0 0 M a c r a t o n i c n u c l e u s i s e n v e l o p e d b y -+ 1 0 0 0

M a m o b i l e b e l t s , a h y p o t h e s i s w h i c h w i ll b e e x a m i n e d d u r i n g t h e s e c o n d

p h a s e o f P r o y e c t o P r e c ~ n b r i c o ( F ig . 1 ).



1 7 8

A C K N O W L E D G E M E N T S

Phase I of the Bolivian Precambrian Project 1976--1979) was essentially

a team effort and its successful conclusion was due en tirely to the sustained

drive and great enthus iasm of all geologists taking part, who often worked

under very primitive and dangerous conditions. Thanks are also due to the

laboratory staff of IGS in England and GEOBOL in Bolivia for essential

back-up services.

The work described results from an on-going, joint ly-funded technical

cooperation programme between the governments of Bolivia and the United

Kingdom. This is being executed by the Servicio Geoldgico de Bolivia, and the

Institute of Geological Sciences Natural Environment Research Council)

on behalf of the Overseas Development Administ ration , Foreign and Com-

monwealth Office.

The manuscript was reviewed by Drs. J.P. Berrang~, R.M. Key and F.F.M.

de Almeida, and the paper is published with the approval of the Director,

Insti tute of Geological Sciences, and the Direc tor of the Servicio Geoldgico

de Bolivia.

R E F E R E N C E S

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A l m e i d a , F . F . M . d e , H a s u i, Y . a n d B r i t o N e v e s , B .B . , 1 9 7 6 . T h e U p p e r P r e c a m b r i a n o f

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B o l iv i a. P r o c . 4 t h L a t i n A m e r i c a n G e o l . C o n g r. , P o r t o f S p a i n , T r i n i d a d a n d T o b a g o

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1 6 8 , 1 8 p p .

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B o l i vi e ( r e g io n d e S a n I g n a c i o d e V e l a s c o - S a n t a C r u z ). C a h . O R S T O M , S e r. G e o l . , 1 0

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t h e A r c h e a n o f G r e e n l a n d a n d i t s i m p l i c a t i o n s f o r e a r ly c r u s t a l t h i c k e n i n g , P r e c a m b r i a n

R e s . , 1 : 1 7 9 - - 1 9 8 .

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c r y s t a l l i n e b a s e m e n t r o c k s o f t h e C o a s t a l C o r d i l l e r a o f s o u t h e r n P e r u . G e o l . S o c . A m .

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