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The proterozoic history of eastern Bolivia
Precambrian Research, 15 (1981) 157--179 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
157
THE PROTEROZOIC HISTORY OF EASTERN BOLIVIA
M. LITHERLAND* and K. BLOOMFIELD
Institute of Geological Sciences, Overseas Division, Keyworth, Nottingham NG12 5GG (Great Britain) ( Received December 9, 1980; revision accepted March 24, 1981 )
ABSTRACT Litherland, M. and Bloomfield, K., 1981. The Proterozoic history of eastern Bolivia. Precambrian 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 cooperation 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 of the shield at about 950 Ma. Unmetamorphosed Upper Proterozoic and possibly Cambrian sediments on the southern and eastern flanks of the shield represent marine transgressions related to the intracontinental Braziliano orogenic cycle. East-trending dolerite dykes were probably intruded during this period within the shield.
INTRODUCTION This p a p e r is a p r e l i m i n a r y geological synthesis a n d analysis o f t h e w o r k a c c o m p l i s h e d d u r i n g t h e first phase o f " P r o y e c t o P r e c h ' a b r i c o " (Fig. 1), a technical c o o p e r a t i o n p r o g r a m m e involving regional, g e o c h e m i c a l and geoc h r o n o l o g i c a l studies o f t h e P r e c a m b r i a n shield o f eastern Bolivia, being carried o u t b y t h e U.K. I n s t i t u t e o f Geological Sciences (IGS) a n d t h e Servicio G e o l 6 g i c o de Bolivia ( G E O B O L ) . I t is c o m p i l e d f r o m the results o f rec o n n a i s s a n c e field m a p p i n g b y R.N. Annells, C.J.N. F l e t c h e r , B.A. Klinck, M. Litherland, W.I. Mitchell, E.A. O ' C o n n o r , P.E.J. Pitfield a n d B.C. Webb, assisted b y G E O B O L geologists, n o t a b l y A. Aguilera, A. A p o n t e , E. Barrientos, J.C. Nallar, G. M o n t e m u r r o , A. Soliz a n d C. Terrones. M a n u s c r i p t m a p s were *Present address: Mision GeolJgica Brit~inica, Casilla 3045, Santa Cruz, Bolivia. 0301-9268/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
158
Phanerozoic cover
Brasiliano fold belts ((1 O00Ma)
l
Precambrian cratons (:>1O00M!
1
Amazonic Craton: Guiana Shield
2
Amazonic Craton: Central Brazil
3
Sao Francisco Craton: Atlantic
Shield
Shield
4
Paraguay Araguaia Belt Craton limit
"PROYECTO PRECAMBRICO" A B
PHASE I (1976 -79) This paper PHASE 2 (1979")
Fig. 1. Location of Project area and subdivisions of the Precambrian of South America, after Almeida et al. (1976).
prepared at a scale of 1 / 1 0 0 000, from which 1/250 000 maps with accompanying reports were compiled; these are referred to in the text and will be published in the near future. T h e geochronological programme was undertaken by D.P.F. Darbyshire of the Isotope Geology Unit, IGS, London. A b o u t 1 0 0 0 0 0 km 2 of the Precambrian shield and marginal areas have been mapped, corresponding to the Southern Zone of the Project area (Fig. 1). Geomorphologically the region o f the shield is a lateritised Miocene peneplain which was warped in Pliocene -Pleistocene times so that laterite levels range from 150 to 500 m a.m.s.1. (Boulang$ and Litherland, 1978). There are also inselbergs of more resistant rocks reaching 1300 m in altitude, the levels of which can be correlated with Oligocene and Cretaceous surfaces in Brazil (King, 1962). The area, which straddles the main Amazon--Plate watershed, is drained b y ephemeral rivers and streams. There are only a few small towns linked'by motorable tracks and the bulk o f the mapping was carried o u t by f o o t traverses through thick, o f t e n waterless, forest. The combination of the forest~over, the Miocene laterite mantle, and a Cretaceous fracture system limits the photogeological expression of Precambrian units.
159
The area is situated on the southwestern margin of the Amazonic craton (see Fig. 1) which forms the bulk of the central Brazil shield (Almeida et al., 1976), one of the least~documented areas of Precambrian rocks in the world. The Amazonic craton is largely composed of variably reworked Trans-Amazonic basement rocks which stabilised at about 1000 Ma. Within the Central Brazil shield, this craton is cut by the Paraguay--Araguaia belt of Brasiliano (PanAfrican) age (Fig. 1). The Proterozoic history of the area can be referred to four orogenic cycles (Table I): the Trans-Amazonic cycle of Lower Proterozoic age (+ 2000 Ma); TABLE I Stratigraphic table. Solid lines are unconformities. Other symbols as for Fig. 2 Orogenic cycle (Ma) --
Epoch/ eon
Stratigraphic unit
?Cambrian
-~
Murcielago Group
6
Tucavaca Group
5
Boqui Group
Granitoids Basic/ ultrabasic rocks
?520 l
(Brasiliano)a Upper Proterozoic
Dykes
---+950
1"
D
G3 G2
m
Sunsas
B2
Sunsas/Vibosi Groups Middle _.1300 /~
3 2B
San Ignacio Schist Gp. Chiquitania Paragneiss complex - - ?
Lower A Proterozoi~ 1 1
Chiquitania Paragneiss complex Lomas Maneches Granulite Group
San Ignacio
Trans-Amazonic
G1
Proterozoic
•
B1
Norite sills
aThe Brasiliano orogeny had only minor effects in the area studied, most of which was cratonized at -+ 950 Ma.
160
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. A1meida, pers. comm., 1978). Local Bolivian nameshave been introduced for the Middle Proterozoic cycles since there are uncertainties of status and correlation within this period in Brazil (Almeida et al., 1976). THE TRANS-AMAZONIC OROGENIC CYCLE (± 2000 Ma)
A variably-defined zone of rocks containing granulite-facies assemblages in the north of the area is collectively referred to as the Lomas Maneches Granulite 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 outcrop pattern of the Group is, however, controlled by structures 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 formed of garnet--biotite paragneisses with welldefined layers of enderbitic hypersthene granulites, basic hypersthene granulites, cordierite--sillimanite 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 contact with the Chiquitania Paragneiss Complex. That part of the Chiquitania Complex 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 gneisses and minor bands of quartzite, amphibolite and calc-silicate granulite. The leptite phase is poorly developed, in contrast to the Lomas Maneches Group. These two sequences are regarded, in the main, as sedimentary in origin. Only the basic hypersthene granulites can be proved to be derived from igneous rocks, since at one locality original textures and minerals are preserved. The enderbitic granulites are also thought to be igneous in origin but the leptites and more leucocratic granulites are tentatively regarded as metasedimentary.
*The classification adopted follows the guidelines recommended in Circular 56 of the International Subcommission on Stratigraphic Nomenclature (1977).
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165 T H E SAN 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 Ma)
The San Ignacio cycle began with the deposition of the San Ignacio Schist Group, after the close of the Trans-Amazonic cycle, and ended with an orogenic 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 R b - S r 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 timespan. However, with the exception of the San Ignacio area (Figs. 2 and 5), where the San Ignacio Group appears to overstep the older paragneisses, evidence of an unconformable relationship with Trans-Amazonic rocks is lacking; this can be attributed to widespread migmatisation during the San Ignacio and Sunsas orogenies, which effectively 'blurred' basal contacts.
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The San Ignacio Schist Group 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 Ignacio 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 interpreted as synclinal plunge-depressions of local facies variations within a regional supmcrustal sequence rather than as the deposits of isolated sedimentary bas{ns. In certain
166
cases it can be demonstrated that the schist belts are in fact 'schist relics' (Litherland, 1973); i.e. unmigmatised lower-grade megaxenoliths in a migmatic host terrain. The schist belts are composed of pelitic schists, with a variable psammitic fraction, and thin layers of graphitic schists, calc-silicates, meta-ironstones and meta-volcanics. There is also a major basic/ultrabasic sill-forming phase. Local group thicknesses are in the region of 5000 m. Muscovite is the predominant mica in the pelitic schists which generally contain subordinate biotite and almandine. There are sporadic developments of staurolite, but kyanite and sillimanite are rare. At San Ignacio (Litherland, 1979) megacrysts of garnet, up to 40 mm in diameter, and staurolite, up to 100 mm long, are locally developed. Meta.arkose and feldspathic quartzite make up the basal formations at Concepcign (Fletcher, 1979) and Ascensi6n (Pitfield, 1979) {Fig. 4), while orthoquartzites are developed at San Ignacio, Chaquipoc {Mitchell, 1979) and San Diablo {O'Connor, 1979) where crossbedding structures are preserved. Irrefutable metavolcanics have only been noted at Concepci6n in the form of metamorphosed basic tuffs and amygdaloidal basalts intercalated in the schists and phyllites. Calc-silicate lithologies include granoblastic hornblende--quartz and diopside--grossularite assemblages and are frequently associated with para-amphibolites. Meta-ironstones locally contain andradite and grunerite. In most of the schist belts there are outcrops of variably metamorphosed basic and ultrabasic rocks, generally interpreted as intrusive sills. There are thin serpentinite horizons in the E1 Puente and Concepci6n schist belts and in the Chaquipoc schist belt (Fig. 2), a major intrusion composed of a differentiated sequence of metapyroxenite, olivine gabbro, pyroxene-mica meladiorite and quartz-mica diorite. At San Ignacio (Fig. 5), where the Suponema Metabasic Formation is the only marker horizon in the schist belt, the Suponema Complex is a 1000-m thick differentiated lopolith composed of serpentinite and metamorphosed gabbro, pyroxenite, melatroctolite, leucotroctolite and gabbro. In another lopolith, at Pachorri {Fig. 5), a leucotroctolite layer within metagabbro is both graded- and cross-bedded, indicating depositional magmatic currents and a right.way-up sequence within the schist belt as a whole. CONCE~CION-ELPUENTE
SAN IGNACI0
C,RISTALCNAOUIPOC
ASCENSION
~
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Axial traces of folds (with plunge) : Sunsos Orogeny
Granite (Sunsos Cycle) Los Patos Schist Formation
•
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Synclinal trace : Son Ignacio Orogeny
Suponema Metabasic Fm. Matacu Schist Formation Son Rafael Metagranite F m Chiquitanio Paragneiss Cx. Lamas Moneches Grcmulite Group
-t-q-+ -L -ISAN IGN'ACIO
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+ Fig. 5. The San Ignacio Group at San Ignacio, adapted from Litherland (1979). Laterite cover and minor units omitted.
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 frequently muscovite,rich and carry enclaves and resisters of the San Ignacio Group. West of the Ascensi6n schist belt there appear to be downward extensions of the San Ignacio Group composed of schists, biotite--plagioclase gneisses and semi-pelitic gneisses which have a contact with the Lomas Maneches Granulite Group distinguished by horizons 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 character, and relics of the Trans-Amazonic paragneissic basement. Only by more detailed 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 fraction with associated tufts and lavas (Fig. 4) and a weak development of the basic--ultrabasic phase. The pelite--sandstone environment (San Ignacio, Chaquipoc) has a lower psammitic fraction and a major basic--ultrabasic phase. It is'thus possible that Concepcidn and Ascensi6n represent marginal shallowwater facies of a deeper water basinal sequence. The San Ignacio 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 northeasttrending structures are refolded by north-trending structures whilst the outcrop pattern of the Lomas Maneches Group (Fig. 2) is controlled by originally north-trending anticlinal saddles with wavelengths of about 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 overlain by paragneisses with granulite lenses is regarded as due to a transition from anhydrous to hydrous P--T conditions. The paragneisses were migmatised in the upper arnphibolite facies and the San Ignacio Group metamorphosed in the lower amphibolite facies as seen in the San tgnacio schist belt: the only schist sequence which preserves metamorphic minerals and
169
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10Km
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STRATIGRAPHIC
28 2B
l
UNIT
OROGENIC C ~
GRANI TOlD ROCKS
Quaternary 1
Tucavaca Group
Brasiliano
Sunsas Group
Sunsas
Post tectonic Syn/late tectonic(G2) J
San Ignacio Group
San Ignacio
Late tectonic Syn/lat e tectonic
Chiquitania Paragneiss Cx.
G1
I Chiquitan'mParagneissCx. Trans-Amazonic I !
Syntectonic Pretectonic
Lamas Maneches Granulite Gp.
Fig. 6. The geological units of the ConcepciJn area, adapted from Fletcher (1979). Laterite cover and minor units omitted.
:t 70 : , ", \
\
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o°
Structures related to the San lgnacio Orogeny Cratonic Zone stable after the San tgnacio Orogeny
Orogenic belt Tectonic front Sense of horizontal movement
I 1 050950 l,,b I
Fig. 7. The Sunsas o r o g e n y in (a) t h e m a p p e d area, and (b) p r o j e c t e d to include the weste~n sector of t h e central Brazil shield.
textures related to the San Ignacio orogeny. At Concepci6n, the western sector of the schist belt underwent only greenschist facies metamorphism within a postulated northeast-trending thermal syncline. The granitoid phases related to the orogenic event, the bulk of which are concentrated 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 the San Ignacio Group along the possible unconformity 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 deformation and generally associated with belts of Kfeldspar 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 finergrained rocks, with a higher microcline content, which cut the San Javier Grani~oids but are related to them.
171
Correlation The San Ignacio orogenic cycle is possibly equivalent to the Espinha~o cycle of Brazil which contains quartzites and phyllites (Almeida, 1971). A supracrustal sequence similar to the San Ignacio Group lithologies has been recognised overlying Trans-Amazonic basement on the Pacific seaboard of Peru (Cobbing et al., 1977) and the Roraima Formation (McConnell et al., 1964), a volcano-sedimentary sequence of similar age, rests on Trans-Amazonic basement of the Guiana shield. THE SUNSAS OROGENIC CYCLE (< 1300--950 Ma)
The Sunsas cycle comprises the erosion of pre-existing rocks of the San Ignacio cycle, the deposition of the Sunsas and Vibosi Groups, and the subsequent deformation of these and older rocks within the bounds of a defined Sunsas orogenic belt. It was accompanied b y a major phase of ultrabasic igneous intrusion and granitoid development. Outcrops of the Sunsas and Vibosi Groups are sporadically distributed in the area, in the form of prominent ridges representing metamorphosed synclinal infolds or mesas of flat-lying, relatively undisturbed sequences of a regional supracrustal cover. They can be used as lithostratigraphic indicators in elucidating the tectonometamorphic history of the orogenic phase. The Sunsas cycle can only be said to have opened some time after 1300 Ma b u t its close can be confidently placed at + 950 Ma on the evidence of a number of K--Ar dates from associated late granites and pegmatites (Darbyshire, 1979). The Sunsas and Vibosi Groups At the t y p e locality of Serranfa Sunsas (Mitchell, 1979) the Sunsas Group is a 6500-m thick sequence divisible into four units: a basal conglomeratic formation overlain b y an arenaceous formation (2600 m thick), composed of arkoses, sandstones, feldspathic quartzites and micaceous quartzites; followed by an argillaceous formation (1250 m) of brown mudstones and grey siltstones and, finally, an upper arkosic formation (1400 m). The basal conglomerate is oligomictic with clasts of vein- and smoky-quartz in a finegrained sericitic matrix which frequently constitutes up to 50% of the rock. The same sequence, reduced to 800 m, forms the Unidad Aguapei (Figueredo and Olivatti, 1974) in Brazil, outliers of which are found in Bolivia at Ascencidn and Las Petas (Pitfield, 1979) where sequences up to the level of the argillaceous formation are exposed. A significant feature of this border area is the eastward change in thickness of the conglomerate formation from 100 to 600 m as the line of the Aguapei front (see Fig. 7) is approached, a change accompanied by the development of a ferruginous matrix in the conglomeratic lenses. In the Concepcidn area (Fig. 6), where the Sunsas Group overlies San Ig-
172
nacio Group rocks intruded by .+- 1300 Ma granites, all four formations 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 under greenschist facies conditions, 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, unconformably overlies the Sunsas Group in the southeast of the area (Mitchell, 1979), but is intruded by the Rincon del Tigre Igneous Complex (+ 992 Ma) and affected by the Sunsas orogeny. The Sunsas orogeny
Unlike the San Ignacio orogeny, the Sunsas orogeny did not affect the whole of the region but was essentially confined to three tectono--metamorphic 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). The Sunsas orogenic belt is confined to a west-northwest-trending region bounded to the north by a curvilinear tectonic 'front '. At Concepcidn (Fig. 6) the front essentially divides refoliated and remobilised basement and Sunsas Group rocks from relatively stable rocks metamorphosed during the San Ignacio cycle. At San Diablo (Fig. 2) the front is marked by a local mylonitic straightening zone (Hepworth, 1967) of intensely refoliated gneisses, 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 of the Lomas Maneches Granulite Group and Trans-Amazonic gneisses are juxtaposed against higher-level units formed during the San Ignacio and Sunsas cycles, indicating that the orogenic zone moved downwards relative t o t h e semi-stable North Marginal zone, while major swings in the strike of San Ignacio structures reveal a horizontal component of sinistral movement along the San Diablo front (Figs. 2 and 7). Early north-trending San Ignacio structures can also be picked out within the Concepcidn area of the orogenic belt (Fig. 7). Local deformation sequences within the belt indicate east-northeast-trending, tight to isoclinal first structures refolded and sheared by inter-penetrative westnorthwest-trending structures which mark the trend of the orogenic belt and are parallel to the front. There is evidence to suggest that the first phase of deformation constituted 'thin-skin' tectonics and affected only the supracrustal Sunsas Group. Metamorphism within the belt was generally of amphibolite facies in the basement and greenschist facies in the overlying Sunsas Group. At Concepcidn, however, west-northwest trending isograds of Barrovian metamorphism, marking the appearance of garnet, staurolite and
173 sillimanite, transect the Sunsas Group--basement interface on a regional scale. Migmatites, syntectonic granites and post-tectonic granites were developed during the orogenic phase. The post-tectonic granites lie along a northwesttrending line within the Sunsas orogenic belt (Fig. 2) and the granite north of San Jos~ has an aureole of andalusite--cordierite hornfels. Pegmatites conraining mica, beryl and columbite were intruded into the schist belts during this period and there were small rhyolite intrusions into the metasediments of the Concepci6n area. The N o r t h - m a r g i n a l z o n e (Fig. 7) is b o u n d e d to the south by the Concepcidn--San Diablo front and to the north b y the cratonic zone. It was a semistable region during the Sunsas orogeny. A regional east-trending sinistral shear, correlated with the second phase of deformation, the Santa Catalina straightening zone (Fig. 2), affected the rocks controlled by the stable cratonic block to the north. This shear zone, contemporaneous with the San Diablo front, deflects the trends of earlier north-trending San Ignacio folds, the axes of which plunge into the shear, which is thus a regional cross-synform. East-trending open-to-close folds also affect the San Ignacio schist belt (Fig. 5) where they are associated with non-penetrative strain-slip structures and the main pegmatite phase. There is no evidence of the early 'thin-skin' phase of Sunsas folding of pre-Sunsas Group rocks in this region. The surrounding paragneisses were affected by conjugate sets of migmatitic shears and there are small syntectonic granites of plutonic and anatectic type, particularly prominent along the Santa Catalina straightening zone, one of which yields a three-point Rb--Sr isochron of +1021 Ma. K--At mica dates of the pegmatites and granites give the 1000--950 Ma "Sunsas" span and there are wholerock K--Ar ages of 1200--1100 Ma which represent partially reset San Ignacio dates. In the southeast of the region the North-Marginal zone can be interpreted to include the Rincon del Tigre area which suffered only gentle folding and the intrusion of the Rincon del Tigre Igneous Complex in the form of a sill along the unconformity between the Sunsas and Vibosi Groups. The Complex (Annells, 1979; AnneUs et al., 1981) is a differentiated igneous intrusion which varies in thickness from 3000 to 4600 m. It can be divided into a basal Ultramafic Unit (1900--3000 m), which consists of cyclic units of dunite--olivine bronzitite--bronzitite and bronzite picrite--melanorite, followed by the Mafic Unit (700--1500 m), composed of a lower norite and an upper gabbro layer, and finally the upper Felsic Unit (300--750 m) which is granophyric, formed largely by melting of the overlying Vibosi Group. The Complex has been dated at -+992 Ma (Darbyshire, 1979) and is regarded as a major intrusive phase of the North-Marginal zone, formed after the first episode of Sunsas deformation. T h e cratonic z o n e lies to the north of the region and can be distinguished by four criteria: first, this zone includes the Sunsas Group sediments forming
174
Serram'a Aguapei in Brazil, the western sector of which is undeformed; second the north-trending San Ignacio structures in this zone are not deflected by Sunsas orogenic trends (Fig. 7); third, K--Ar dates of gneisses are characteristic of the San Ignacio orogeny; and, fourth, there are no recorded Sunsas cycle granites. It can thus be concluded that the cratonic zone was a stable rigid block during the Sunsas cycle.
The Aguapei mobile belt is a north-northwest-trending zone of folded Sunsas Group rocks cut by major tectonic slides, notably the Aguapei front (Figs. 2 and 7), which divides this belt from the Cratonic zone where the Sunsas Group is essentially undeformed. At Las Petas, along the line of this front, a local syncline of greenschist facies Sunsas Group rocks is rotated through a vertical position to lie with a downward-facing attitude. The Aguapei mobile belt is well exposed in Brazil, where it is composed of folded and faulted Sunsas Group rocks (Figueredo and Olivatti, 1974), but its extension into Bolivia is largely covered b y alluvium, making correlation with the main Sunsas orogeny difficult. Although K--At dates (phyllite, pegmatite) are in the range 1050--950 Ma (Darbyshire, 1979), it is thought that the formation of this mobile belt was the last event of the orogenic cycle since structures with similar trends (between north-northeast and north-northwest) affect earlier Sunsas structures within the San Ignacio schist belt (Fig. 5) and surrounding paragneisses and are also superimposed on the eastern section of the Sunsas orogenic belt. These north-northeast to north-northwesttrending structures, which may also have been responsible for the major inflexions in the curvilinear trend of the Concepci6n--San Diablo front, are thus tentatively correlated with a late-Sunsas Aguapei mobile belt. The deposition o f the Sunsas Group The molassic Sunsas Group varies in thickness from 6500 m at Serran~a Sunsas to 800 m at Serram'a Aguapei. Since the formational lithologies, including the super-mature basal conglomerate (Figueredo and Olivatti, 1974), exhibit no facies variations, it can be assumed that subsidence and sedimentation were in equilibrium, The thinnest sequence at Aguapei formed over the region which was later to become the stable craton whilst the thickening at Las Petas coincided with the line of the later Aguapei front. Palaeocurrent directions in this region are from the west, i.e. from 'craton' to 'mobile belt'. Deposition of the Vibosi Group also t o o k place in the Sunsas region of maximum subsidence. These factors suggest that tectono-sedimentary control of Sunsas Group deposition was related to linear zones of crustal weakness which were later rejuvenated during the subsequent Sunsas orogeny in the form of orogenic or mobile belts.
175
Correlation
The Sunsas cycle can be correlated with the Uru~uano cycle of the Atlantic shield area (Fig. 1) which spans the period 1300--1000 Ma (Almeida et al., 1976). In the western part of the Central Brazil shield, the Rond6nia belt, formed during the Rond6niense event (Almeida et al., 1976), includes molasse sediments which are cut by 950 Ma granites (Priem et al., 1966). On this basis it could be suggested that the Cratonic zone identified in the north of the present area is b o u n d e d on all sides by (+ 1000 Ma) orogenic or mobile belts (Fig. 7b). THE BRASILIANO OROGENIC CYCLE (950--500 Ma)
In eastern Bolivia the close of the Sunsas cycle marked the cratonization of the shield and the deposits of the Upper Proterozoic--Lower Palaeozoic Brasiliano (Pan-African) cycle are only represented by unmetamorphosed sequences on its eastern and southern flanks. In the east, the sediments of the Boqui Group (Mitchell, 1979), part of the Paraguay--Araguaia belt of the Brasiliano cycle (Almeida et al., 1976) (Fig. 1), are miogeosynclinal conglomerates, arkoses, shales and limestones with a total thickness of a b o u t 1000 m which u n c o n f o r m a b l y overlie rocks of the Sunsas cycle. These pass laterally eastward into a thicker 'eugeosynclinal' sequence of conglomerates, greywackes, banded ironstones, cherts, limestones and calcilutites. There are major facies changes controlled by north-trending contemporaneous rift faults. The Boqui Group was folded in the east a b o u t north-trending axes (at the western edge of the Paraguay-- Araguaia mobile belt) and unconformably overlain b y the Murci~lago group of reef and non-reef limestones with development of reef facies also controlled by contemporaneous north-trending faults. The Murcidlago Group is tentatively regarded as Cambrian in age since it contains Cambrian stromatolites in its Brazilian correlative, the Corumb~ Limestone (Beurlen and Sommer, 1967), and it also represents a major marine transgression over a fault-controlled Upper Proterozoic basin (Matthews and Cowie, 1979). On the southern flank of the shield, Upper Proterozoic sedimentation is represented by the miogeosynclinal Tucavaca Group (Mitchell, 1979; O'Connor, 1979) confined to a west-northwest-trending fault-controlled basin, at least 400 km in length, which follows the trend of the earlier Sunsas belt. The Group unconformably overlies rocks of the Sunsas cycle to the west and the Boqui Group to the east. It is composed of a basal dolomitic limestone overlain by sandstones and shales with a total thickness of a b o u t 2000 m. Slight folding on westnorthwest-trending axes locally refoliated adjacent basement rocks near San Josd (Fig. 2) which yield 'reset' K--At ages of -+ 545 Ma. The Tucavaca basin thus represents a major fault-controlled offshoot of the Paraguay--Araguaia belt. Further marine transgressions occurred in the Phanerozoic along this unstable 'line' which remains a zone of seismic activity ('Chiquitos fault' of Gansser, 1973).
176
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 of dolerite--gabbro composition with minor olivine gabbros 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 concentrated along the easttrending Rio Mercedes fracture system, another line of Phanerozoic crustal weakness, which truncates the Paraguay--Araguaia belt in Brazil and is associated 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 unreliable dates, the tectono--metamorphic history of the intervening (1800-1000 Ma) period is not fully understood. The identification of t w o orogenic cycles in eastern Bolivia within this period is thus of major significance and warrants further discussion. The San Ignacio orogeny affected the Trans-Amazonic basement and the supracrustal San Ignacio Group. Field evidence suggests that the older granulites may have been metamorphosed (+ 2000 Ma) within a horizontal tectonic regime (Bridgewater et al., 1974) with a primary stratiform foliation subparallel to the later 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.Cordilleran orogeny (Cobbing, 1978). The facies changes in the supracrustal sequence may have been controlled b y 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 resisted 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 phenomenon in character with a molassic ensialic basin rather than an aulacogen (Hoffman et al., 1974). During the orogenic stage,
177
deformation within the belt was accompanied by sinistral movements between cratonic blocks (see Canadian shield, Watson, 1973) but, although the earlier trends are slightly offset within the semi-stable North-Marginal zone, they are preserved within the orogenic belt in the Concepcidn area. Thus the Sunsas belt is analogous in age and form to the + 1000 Ma Kibaran--Irumide belt of Africa (Shackleton, 1973) which overprints, w i t h o u t displacement, the older U b e n d i a n - R u z i z i belts and encloses the older units in the manner of Fig. 7b. These pre-Pan-African (pre-Brasiliano) mobile belts of southern Africa are ensialic (no major displacements of earlier lineaments) and characterised by plate destruction rather than plate accretion (KrSner, 1976). The zonal structure of the Sunsas belt cannot, however, be compared to the tectonic zonations of the 2600 Ma L i m p o p o mobile belt where craton and mobile belt formed at the same time (Key and H u t t o n , 1976). The Sunsas orogeny with its well-defined tectono--metamorphic zones and post-tectonic granites is not marked by high-grade mineral assemblages and, although its tectonic fronts fit into the category of 'straightening zones', these lie at right angles to the world-wide trend of Proterozoic high-grade linear belts (Davies and Windley, 1976). The Aguapei mobile belt must be regarded as a late offshoot of the Sunsas belt. It does not appear to possess a granitic phase b u t contains high-level differentiated basic intrusions (Figueredo and Olivatti, 1974} similar to the Rincon del Tigre Igneous Complex. It thus shows affinities to the marginal zone rather than to the main belt of the Sunsas orogeny. CONCLUSIONS
Four Proterozoic orogenic cycles have been recognised in eastern Bolivia. The identification of the Lower Proterozoic Trans.Amazonic cycle and the Upper Proterozoic Brasiliano cycle was anticipated before the fieldwork in the light of Brazilian work. The recognition of t w o Middle Proterozoic orogenic cycles is, however, of major significance since this period of reworking of the Trans-Amazonic nucleus prior to the Brasiliano (Pan-African) event is n o t well d o c u m e n t e d in South America. During the Middle Proterozoic, the San Ignacio cycle (+ 2000--1300 Ma) began with the deposition of a widespread schist belt sequence and culminated in the San Ignacio orogeny, a regional north-trending tectono--metamorphic event which reworked the older granulite basement. The Sunsas cycle ( < 1 3 0 0 - - 9 5 0 Ma) included the deposition of molassic sediments and an orogenic phase analogous to ensialic mobile belts of the same age in southern Africa and exhibits a stable cratonic zone, a semi-stable marginal zone and an unstable orogenic zone, divided by westnorthwest-trending curvilinear sinistral shears. It is suggested that the + 1300 Ma cratonic nucleus is enveloped by -+ 1000 Ma mobile belts, a hypothesis which will be examined during the second phase of Proyecto Prec~nbrico (Fig. 1).
178 ACKNOWLEDGEMENTS
Phase I of the Bolivian Precambrian Project (1976--1979) was essentially a team effort and its successful conclusion was due entirely to the sustained drive and great enthusiasm 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, jointly-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 Administration, Foreign and Commonwealth 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, Institute of Geological Sciences, and the Director of the Servicio Geoldgico de Bolivia. REFERENCES Almeida, F.F.M. de, 1971. Geochronological divisions of the Precambrian of South America. Rev. Bras. Geocienc.,l: 13--21. Almeida, F.F.M. de, Hasui, Y. and Brito Neves, B.B., 1976. The Upper Precambrian of South America. Bol. Inst. Geocienc. Univ. Sao Paulo, 7: 45--80. Annells, R.N., 1979. The geology and mineral potential of the Rincon del Tigre Igneous complex. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~mbrico), Phase I, 7 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. Annells, R.N., Fletcher, C,J.N. and Appleton, J.D., 1981. A major ultramafic--mafic layered intrusion of Proterozoic age: the Rincon del Tigre Igneous Complexi eastern Bolivia. Proc. 4th Latin American Geol. Congr., Port of Spain, Trinidad and Tobago (1979). Beurlen, K. and Sommer, F,W., 1957. Observacoes estratigraficas e palaeontologicas sobre o Calcaria Corumb~. Serv. Graf. Inst. Bras. Geogr. Estatistica, Rio de Janeiro, 168, 18 pp. Boulangd, B. and Litherland, M., 1978. Surfaces d'aplanissements en zone amazonienne de Bolivie (region de San Ignacio de Velasco-Santa Cruz). Cah. ORSTOM, Ser. Geol., 10 (1): 145--151. Bridgewater, D., McGregor, V.R. and Myers, J,S., 1974. A horizontal tectonic regime in the Archean of Greenland and its implications for early crustal thickening, Precambrian Res., 1: 179--198. Cloud, P., 1971. The primitive Earth. In: I.G. Gase, P.J. Smith and R.V.L. Wilson (Editors), Understanding the Earth. Artemis, Horsham, pp. 1 5 1 - - I 56. Cobbing, E.J., 1978. The Andean geosyncline in Peru and its distinction from Alpine geosynclines. J. Geol. Soc. London, 135: 207--218. Cobbing, E.J., Ozard, J.M. and Snelling, N.J., 1977. Reconnaissance geochronology of the crystalline basement rocks of the Coastal Cordillera of southern Peru. Geol. Soc. Am. Bull., 88: 241--246. Cordani, U.G., Amaral, G. and Kawashita, K., 1973. The Precambrian evolution of South America. Geol. Rundsch., 62: 309--317.
179 Darbyshire, D.P.F., 1979. Results of the age determination programme. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~mbrico), Phase I, 9 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. Davies, F.B. and Windley, B.F., 1976. The significance of major Proterozoic high-grade linear belts in continental evolution. Nature (London), 263: 383--385. Figueredo, A.J. de A. and Olivatti, O., 1974. Projecto Alto Guapor~ Vol. XI. Relatorio Final integrado. Compan. Pesquisa Miner. Goiania Brasil, 173 pp. Fletcher, C.J.N., 1979. The geology and mineral potential of the Concepcign area. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~mbrico), Phase I, 2 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. Gansser, A., 1973. Facts and theories on the Andes. J. Geol. Soc. London, 129: 93--131. Hepworth, J.V., 1967. The photogeological recognition of ancient orogenic belts in Africa. Q. J. Geol. Soc. London, 123: 253--292. Hoffmann, P., Dewey, J.F. and Burke, K., 1974. Aulacogens and their genetic relations to geosynclines, with a Proterozoic example from Great Slave Lake, Canada. In: R.H. Dott and R.H. Shaver (Editors), Modern and Ancient Geosynclinal Sedimentation. Soc. Econ. Paleont. Mineral Spec. Publ., 19: 38--55. Key, R.M. and Hutton, S.M., 1976. The western extremity of the Limpopo Mobile Belt. Precambrian Res., 3: 79--90. King, L.C., 1962. Morphology of the Earth. Oliver and Boyd, Edinburgh, 699 pp. KrSner, A., 1976. Proterozoic crustal evolution in parts of southern Africa and evidence for extensive sialic crust since the end of the Archean. Philos. Trans. R. Soc. London, Set. A, 280: 541--554. Litherland, M., 1973. Uniformitarian approach to Archean 'Schist Relics'. Nature (London), 242: 395--398. Litherland, M., 1979. The geology and mineral potential of the San Ignacio de Velasco area. Rep. E. Bolivia. Miner. Explor. Proj. (Proyecto Prec~tmbrico), Phase I, 3 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. McConnell, R.B., Williams, E., Cannon, R.T. and Shelling, N.J., 1964. A new interpretation of the geology of British Guiana. Nature (London), 204: 115--118. Matthews, S.C. and Cowie, J.W., 1979. Early Cambrian transgressions. J. Geol. Soc. London, 136: 133--135. Mitchell, W.I., 1979. The geology and mineral potential of the Santo Corazon--Rincon del Tigre area. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~mbrico), Phase I, 6 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. O'Connor, E.A., 1979. The geology and mineral potential of the San Jos~ de Chiquitos area. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~mbrico), Phase I, 5 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. Pitfield, P.E.J., 1979. The geology and mineral potential of the Las Petas--San Matias area. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~lmbrico), Phase I, 4 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. Priem, H.N.A., Boelrijk, N.S.I.M., Hebeda, B.H., Verschure, R.H. and Bon, E.H., 1966. Isotopic age of the tin granites in Rondonia, NW Brazil. Geol. Mijnbouw, 45: 191-192. Shackleton, R.M., 1973. Correlation of structures across Precambrian orogenic belts in Africa. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences, Vol. 2. Academic Press, London, pp. 1091--1095. Watson, J.V., 1973. Effects of reworking on high-grade gneiss complexes. Philos. Trans. R. Soc. London, Ser A, 273: 443--456.
157
THE PROTEROZOIC HISTORY OF EASTERN BOLIVIA
M. LITHERLAND* and K. BLOOMFIELD
Institute of Geological Sciences, Overseas Division, Keyworth, Nottingham NG12 5GG (Great Britain) ( Received December 9, 1980; revision accepted March 24, 1981 )
ABSTRACT Litherland, M. and Bloomfield, K., 1981. The Proterozoic history of eastern Bolivia. Precambrian 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 cooperation 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 of the shield at about 950 Ma. Unmetamorphosed Upper Proterozoic and possibly Cambrian sediments on the southern and eastern flanks of the shield represent marine transgressions related to the intracontinental Braziliano orogenic cycle. East-trending dolerite dykes were probably intruded during this period within the shield.
INTRODUCTION This p a p e r is a p r e l i m i n a r y geological synthesis a n d analysis o f t h e w o r k a c c o m p l i s h e d d u r i n g t h e first phase o f " P r o y e c t o P r e c h ' a b r i c o " (Fig. 1), a technical c o o p e r a t i o n p r o g r a m m e involving regional, g e o c h e m i c a l and geoc h r o n o l o g i c a l studies o f t h e P r e c a m b r i a n shield o f eastern Bolivia, being carried o u t b y t h e U.K. I n s t i t u t e o f Geological Sciences (IGS) a n d t h e Servicio G e o l 6 g i c o de Bolivia ( G E O B O L ) . I t is c o m p i l e d f r o m the results o f rec o n n a i s s a n c e field m a p p i n g b y R.N. Annells, C.J.N. F l e t c h e r , B.A. Klinck, M. Litherland, W.I. Mitchell, E.A. O ' C o n n o r , P.E.J. Pitfield a n d B.C. Webb, assisted b y G E O B O L geologists, n o t a b l y A. Aguilera, A. A p o n t e , E. Barrientos, J.C. Nallar, G. M o n t e m u r r o , A. Soliz a n d C. Terrones. M a n u s c r i p t m a p s were *Present address: Mision GeolJgica Brit~inica, Casilla 3045, Santa Cruz, Bolivia. 0301-9268/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
158
Phanerozoic cover
Brasiliano fold belts ((1 O00Ma)
l
Precambrian cratons (:>1O00M!
1
Amazonic Craton: Guiana Shield
2
Amazonic Craton: Central Brazil
3
Sao Francisco Craton: Atlantic
Shield
Shield
4
Paraguay Araguaia Belt Craton limit
"PROYECTO PRECAMBRICO" A B
PHASE I (1976 -79) This paper PHASE 2 (1979")
Fig. 1. Location of Project area and subdivisions of the Precambrian of South America, after Almeida et al. (1976).
prepared at a scale of 1 / 1 0 0 000, from which 1/250 000 maps with accompanying reports were compiled; these are referred to in the text and will be published in the near future. T h e geochronological programme was undertaken by D.P.F. Darbyshire of the Isotope Geology Unit, IGS, London. A b o u t 1 0 0 0 0 0 km 2 of the Precambrian shield and marginal areas have been mapped, corresponding to the Southern Zone of the Project area (Fig. 1). Geomorphologically the region o f the shield is a lateritised Miocene peneplain which was warped in Pliocene -Pleistocene times so that laterite levels range from 150 to 500 m a.m.s.1. (Boulang$ and Litherland, 1978). There are also inselbergs of more resistant rocks reaching 1300 m in altitude, the levels of which can be correlated with Oligocene and Cretaceous surfaces in Brazil (King, 1962). The area, which straddles the main Amazon--Plate watershed, is drained b y ephemeral rivers and streams. There are only a few small towns linked'by motorable tracks and the bulk o f the mapping was carried o u t by f o o t traverses through thick, o f t e n waterless, forest. The combination of the forest~over, the Miocene laterite mantle, and a Cretaceous fracture system limits the photogeological expression of Precambrian units.
159
The area is situated on the southwestern margin of the Amazonic craton (see Fig. 1) which forms the bulk of the central Brazil shield (Almeida et al., 1976), one of the least~documented areas of Precambrian rocks in the world. The Amazonic craton is largely composed of variably reworked Trans-Amazonic basement rocks which stabilised at about 1000 Ma. Within the Central Brazil shield, this craton is cut by the Paraguay--Araguaia belt of Brasiliano (PanAfrican) age (Fig. 1). The Proterozoic history of the area can be referred to four orogenic cycles (Table I): the Trans-Amazonic cycle of Lower Proterozoic age (+ 2000 Ma); TABLE I Stratigraphic table. Solid lines are unconformities. Other symbols as for Fig. 2 Orogenic cycle (Ma) --
Epoch/ eon
Stratigraphic unit
?Cambrian
-~
Murcielago Group
6
Tucavaca Group
5
Boqui Group
Granitoids Basic/ ultrabasic rocks
?520 l
(Brasiliano)a Upper Proterozoic
Dykes
---+950
1"
D
G3 G2
m
Sunsas
B2
Sunsas/Vibosi Groups Middle _.1300 /~
3 2B
San Ignacio Schist Gp. Chiquitania Paragneiss complex - - ?
Lower A Proterozoi~ 1 1
Chiquitania Paragneiss complex Lomas Maneches Granulite Group
San Ignacio
Trans-Amazonic
G1
Proterozoic
•
B1
Norite sills
aThe Brasiliano orogeny had only minor effects in the area studied, most of which was cratonized at -+ 950 Ma.
160
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. A1meida, pers. comm., 1978). Local Bolivian nameshave been introduced for the Middle Proterozoic cycles since there are uncertainties of status and correlation within this period in Brazil (Almeida et al., 1976). THE TRANS-AMAZONIC OROGENIC CYCLE (± 2000 Ma)
A variably-defined zone of rocks containing granulite-facies assemblages in the north of the area is collectively referred to as the Lomas Maneches Granulite 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 outcrop pattern of the Group is, however, controlled by structures 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 formed of garnet--biotite paragneisses with welldefined layers of enderbitic hypersthene granulites, basic hypersthene granulites, cordierite--sillimanite 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 contact with the Chiquitania Paragneiss Complex. That part of the Chiquitania Complex 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 gneisses and minor bands of quartzite, amphibolite and calc-silicate granulite. The leptite phase is poorly developed, in contrast to the Lomas Maneches Group. These two sequences are regarded, in the main, as sedimentary in origin. Only the basic hypersthene granulites can be proved to be derived from igneous rocks, since at one locality original textures and minerals are preserved. The enderbitic granulites are also thought to be igneous in origin but the leptites and more leucocratic granulites are tentatively regarded as metasedimentary.
*The classification adopted follows the guidelines recommended in Circular 56 of the International Subcommission on Stratigraphic Nomenclature (1977).
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165 T H E SAN 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 Ma)
The San Ignacio cycle began with the deposition of the San Ignacio Schist Group, after the close of the Trans-Amazonic cycle, and ended with an orogenic 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 R b - S r 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 timespan. However, with the exception of the San Ignacio area (Figs. 2 and 5), where the San Ignacio Group appears to overstep the older paragneisses, evidence of an unconformable relationship with Trans-Amazonic rocks is lacking; this can be attributed to widespread migmatisation during the San Ignacio and Sunsas orogenies, which effectively 'blurred' basal contacts.
q
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Co
-- N
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50
-
San Ignacio
-
100Km
d ~ , ~
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Fig. 3. Distribution and names of the San Ignacio Group schist belts (black). O r n a m e n t e d area indicates rocks o f equivalent age or older.
The San Ignacio Schist Group 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 Ignacio 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 interpreted as synclinal plunge-depressions of local facies variations within a regional supmcrustal sequence rather than as the deposits of isolated sedimentary bas{ns. In certain
166
cases it can be demonstrated that the schist belts are in fact 'schist relics' (Litherland, 1973); i.e. unmigmatised lower-grade megaxenoliths in a migmatic host terrain. The schist belts are composed of pelitic schists, with a variable psammitic fraction, and thin layers of graphitic schists, calc-silicates, meta-ironstones and meta-volcanics. There is also a major basic/ultrabasic sill-forming phase. Local group thicknesses are in the region of 5000 m. Muscovite is the predominant mica in the pelitic schists which generally contain subordinate biotite and almandine. There are sporadic developments of staurolite, but kyanite and sillimanite are rare. At San Ignacio (Litherland, 1979) megacrysts of garnet, up to 40 mm in diameter, and staurolite, up to 100 mm long, are locally developed. Meta.arkose and feldspathic quartzite make up the basal formations at Concepcign (Fletcher, 1979) and Ascensi6n (Pitfield, 1979) {Fig. 4), while orthoquartzites are developed at San Ignacio, Chaquipoc {Mitchell, 1979) and San Diablo {O'Connor, 1979) where crossbedding structures are preserved. Irrefutable metavolcanics have only been noted at Concepci6n in the form of metamorphosed basic tuffs and amygdaloidal basalts intercalated in the schists and phyllites. Calc-silicate lithologies include granoblastic hornblende--quartz and diopside--grossularite assemblages and are frequently associated with para-amphibolites. Meta-ironstones locally contain andradite and grunerite. In most of the schist belts there are outcrops of variably metamorphosed basic and ultrabasic rocks, generally interpreted as intrusive sills. There are thin serpentinite horizons in the E1 Puente and Concepci6n schist belts and in the Chaquipoc schist belt (Fig. 2), a major intrusion composed of a differentiated sequence of metapyroxenite, olivine gabbro, pyroxene-mica meladiorite and quartz-mica diorite. At San Ignacio (Fig. 5), where the Suponema Metabasic Formation is the only marker horizon in the schist belt, the Suponema Complex is a 1000-m thick differentiated lopolith composed of serpentinite and metamorphosed gabbro, pyroxenite, melatroctolite, leucotroctolite and gabbro. In another lopolith, at Pachorri {Fig. 5), a leucotroctolite layer within metagabbro is both graded- and cross-bedded, indicating depositional magmatic currents and a right.way-up sequence within the schist belt as a whole. CONCE~CION-ELPUENTE
SAN IGNACI0
C,RISTALCNAOUIPOC
ASCENSION
~
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META-IRONSTONES
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represents approximately 5 0 0 0
m.
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Axial traces of folds (with plunge) : Sunsos Orogeny
Granite (Sunsos Cycle) Los Patos Schist Formation
•
•
Synclinal trace : Son Ignacio Orogeny
Suponema Metabasic Fm. Matacu Schist Formation Son Rafael Metagranite F m Chiquitanio Paragneiss Cx. Lamas Moneches Grcmulite Group
-t-q-+ -L -ISAN IGN'ACIO
+ + q- + +-b÷ -I-÷4-+ + i ~ + + + -I--I+ +4.+4. + ~ ÷ + ÷ + -I-~ t- +
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+ Fig. 5. The San Ignacio Group at San Ignacio, adapted from Litherland (1979). Laterite cover and minor units omitted.
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 frequently muscovite,rich and carry enclaves and resisters of the San Ignacio Group. West of the Ascensi6n schist belt there appear to be downward extensions of the San Ignacio Group composed of schists, biotite--plagioclase gneisses and semi-pelitic gneisses which have a contact with the Lomas Maneches Granulite Group distinguished by horizons 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 character, and relics of the Trans-Amazonic paragneissic basement. Only by more detailed 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 fraction with associated tufts and lavas (Fig. 4) and a weak development of the basic--ultrabasic phase. The pelite--sandstone environment (San Ignacio, Chaquipoc) has a lower psammitic fraction and a major basic--ultrabasic phase. It is'thus possible that Concepcidn and Ascensi6n represent marginal shallowwater facies of a deeper water basinal sequence. The San Ignacio 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 northeasttrending structures are refolded by north-trending structures whilst the outcrop pattern of the Lomas Maneches Group (Fig. 2) is controlled by originally north-trending anticlinal saddles with wavelengths of about 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 overlain by paragneisses with granulite lenses is regarded as due to a transition from anhydrous to hydrous P--T conditions. The paragneisses were migmatised in the upper arnphibolite facies and the San Ignacio Group metamorphosed in the lower amphibolite facies as seen in the San tgnacio schist belt: the only schist sequence which preserves metamorphic minerals and
169
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STRATIGRAPHIC
28 2B
l
UNIT
OROGENIC C ~
GRANI TOlD ROCKS
Quaternary 1
Tucavaca Group
Brasiliano
Sunsas Group
Sunsas
Post tectonic Syn/late tectonic(G2) J
San Ignacio Group
San Ignacio
Late tectonic Syn/lat e tectonic
Chiquitania Paragneiss Cx.
G1
I Chiquitan'mParagneissCx. Trans-Amazonic I !
Syntectonic Pretectonic
Lamas Maneches Granulite Gp.
Fig. 6. The geological units of the ConcepciJn area, adapted from Fletcher (1979). Laterite cover and minor units omitted.
:t 70 : , ", \
\
" : ~i
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~
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Serrania Aguapei
i
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5pOKm b
,Q
.....
o°
Structures related to the San lgnacio Orogeny Cratonic Zone stable after the San tgnacio Orogeny
Orogenic belt Tectonic front Sense of horizontal movement
I 1 050950 l,,b I
Fig. 7. The Sunsas o r o g e n y in (a) t h e m a p p e d area, and (b) p r o j e c t e d to include the weste~n sector of t h e central Brazil shield.
textures related to the San Ignacio orogeny. At Concepci6n, the western sector of the schist belt underwent only greenschist facies metamorphism within a postulated northeast-trending thermal syncline. The granitoid phases related to the orogenic event, the bulk of which are concentrated 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 the San Ignacio Group along the possible unconformity 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 deformation and generally associated with belts of Kfeldspar 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 finergrained rocks, with a higher microcline content, which cut the San Javier Grani~oids but are related to them.
171
Correlation The San Ignacio orogenic cycle is possibly equivalent to the Espinha~o cycle of Brazil which contains quartzites and phyllites (Almeida, 1971). A supracrustal sequence similar to the San Ignacio Group lithologies has been recognised overlying Trans-Amazonic basement on the Pacific seaboard of Peru (Cobbing et al., 1977) and the Roraima Formation (McConnell et al., 1964), a volcano-sedimentary sequence of similar age, rests on Trans-Amazonic basement of the Guiana shield. THE SUNSAS OROGENIC CYCLE (< 1300--950 Ma)
The Sunsas cycle comprises the erosion of pre-existing rocks of the San Ignacio cycle, the deposition of the Sunsas and Vibosi Groups, and the subsequent deformation of these and older rocks within the bounds of a defined Sunsas orogenic belt. It was accompanied b y a major phase of ultrabasic igneous intrusion and granitoid development. Outcrops of the Sunsas and Vibosi Groups are sporadically distributed in the area, in the form of prominent ridges representing metamorphosed synclinal infolds or mesas of flat-lying, relatively undisturbed sequences of a regional supracrustal cover. They can be used as lithostratigraphic indicators in elucidating the tectonometamorphic history of the orogenic phase. The Sunsas cycle can only be said to have opened some time after 1300 Ma b u t its close can be confidently placed at + 950 Ma on the evidence of a number of K--Ar dates from associated late granites and pegmatites (Darbyshire, 1979). The Sunsas and Vibosi Groups At the t y p e locality of Serranfa Sunsas (Mitchell, 1979) the Sunsas Group is a 6500-m thick sequence divisible into four units: a basal conglomeratic formation overlain b y an arenaceous formation (2600 m thick), composed of arkoses, sandstones, feldspathic quartzites and micaceous quartzites; followed by an argillaceous formation (1250 m) of brown mudstones and grey siltstones and, finally, an upper arkosic formation (1400 m). The basal conglomerate is oligomictic with clasts of vein- and smoky-quartz in a finegrained sericitic matrix which frequently constitutes up to 50% of the rock. The same sequence, reduced to 800 m, forms the Unidad Aguapei (Figueredo and Olivatti, 1974) in Brazil, outliers of which are found in Bolivia at Ascencidn and Las Petas (Pitfield, 1979) where sequences up to the level of the argillaceous formation are exposed. A significant feature of this border area is the eastward change in thickness of the conglomerate formation from 100 to 600 m as the line of the Aguapei front (see Fig. 7) is approached, a change accompanied by the development of a ferruginous matrix in the conglomeratic lenses. In the Concepcidn area (Fig. 6), where the Sunsas Group overlies San Ig-
172
nacio Group rocks intruded by .+- 1300 Ma granites, all four formations 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 under greenschist facies conditions, 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, unconformably overlies the Sunsas Group in the southeast of the area (Mitchell, 1979), but is intruded by the Rincon del Tigre Igneous Complex (+ 992 Ma) and affected by the Sunsas orogeny. The Sunsas orogeny
Unlike the San Ignacio orogeny, the Sunsas orogeny did not affect the whole of the region but was essentially confined to three tectono--metamorphic 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). The Sunsas orogenic belt is confined to a west-northwest-trending region bounded to the north by a curvilinear tectonic 'front '. At Concepcidn (Fig. 6) the front essentially divides refoliated and remobilised basement and Sunsas Group rocks from relatively stable rocks metamorphosed during the San Ignacio cycle. At San Diablo (Fig. 2) the front is marked by a local mylonitic straightening zone (Hepworth, 1967) of intensely refoliated gneisses, 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 of the Lomas Maneches Granulite Group and Trans-Amazonic gneisses are juxtaposed against higher-level units formed during the San Ignacio and Sunsas cycles, indicating that the orogenic zone moved downwards relative t o t h e semi-stable North Marginal zone, while major swings in the strike of San Ignacio structures reveal a horizontal component of sinistral movement along the San Diablo front (Figs. 2 and 7). Early north-trending San Ignacio structures can also be picked out within the Concepcidn area of the orogenic belt (Fig. 7). Local deformation sequences within the belt indicate east-northeast-trending, tight to isoclinal first structures refolded and sheared by inter-penetrative westnorthwest-trending structures which mark the trend of the orogenic belt and are parallel to the front. There is evidence to suggest that the first phase of deformation constituted 'thin-skin' tectonics and affected only the supracrustal Sunsas Group. Metamorphism within the belt was generally of amphibolite facies in the basement and greenschist facies in the overlying Sunsas Group. At Concepcidn, however, west-northwest trending isograds of Barrovian metamorphism, marking the appearance of garnet, staurolite and
173 sillimanite, transect the Sunsas Group--basement interface on a regional scale. Migmatites, syntectonic granites and post-tectonic granites were developed during the orogenic phase. The post-tectonic granites lie along a northwesttrending line within the Sunsas orogenic belt (Fig. 2) and the granite north of San Jos~ has an aureole of andalusite--cordierite hornfels. Pegmatites conraining mica, beryl and columbite were intruded into the schist belts during this period and there were small rhyolite intrusions into the metasediments of the Concepci6n area. The N o r t h - m a r g i n a l z o n e (Fig. 7) is b o u n d e d to the south by the Concepcidn--San Diablo front and to the north b y the cratonic zone. It was a semistable region during the Sunsas orogeny. A regional east-trending sinistral shear, correlated with the second phase of deformation, the Santa Catalina straightening zone (Fig. 2), affected the rocks controlled by the stable cratonic block to the north. This shear zone, contemporaneous with the San Diablo front, deflects the trends of earlier north-trending San Ignacio folds, the axes of which plunge into the shear, which is thus a regional cross-synform. East-trending open-to-close folds also affect the San Ignacio schist belt (Fig. 5) where they are associated with non-penetrative strain-slip structures and the main pegmatite phase. There is no evidence of the early 'thin-skin' phase of Sunsas folding of pre-Sunsas Group rocks in this region. The surrounding paragneisses were affected by conjugate sets of migmatitic shears and there are small syntectonic granites of plutonic and anatectic type, particularly prominent along the Santa Catalina straightening zone, one of which yields a three-point Rb--Sr isochron of +1021 Ma. K--At mica dates of the pegmatites and granites give the 1000--950 Ma "Sunsas" span and there are wholerock K--Ar ages of 1200--1100 Ma which represent partially reset San Ignacio dates. In the southeast of the region the North-Marginal zone can be interpreted to include the Rincon del Tigre area which suffered only gentle folding and the intrusion of the Rincon del Tigre Igneous Complex in the form of a sill along the unconformity between the Sunsas and Vibosi Groups. The Complex (Annells, 1979; AnneUs et al., 1981) is a differentiated igneous intrusion which varies in thickness from 3000 to 4600 m. It can be divided into a basal Ultramafic Unit (1900--3000 m), which consists of cyclic units of dunite--olivine bronzitite--bronzitite and bronzite picrite--melanorite, followed by the Mafic Unit (700--1500 m), composed of a lower norite and an upper gabbro layer, and finally the upper Felsic Unit (300--750 m) which is granophyric, formed largely by melting of the overlying Vibosi Group. The Complex has been dated at -+992 Ma (Darbyshire, 1979) and is regarded as a major intrusive phase of the North-Marginal zone, formed after the first episode of Sunsas deformation. T h e cratonic z o n e lies to the north of the region and can be distinguished by four criteria: first, this zone includes the Sunsas Group sediments forming
174
Serram'a Aguapei in Brazil, the western sector of which is undeformed; second the north-trending San Ignacio structures in this zone are not deflected by Sunsas orogenic trends (Fig. 7); third, K--Ar dates of gneisses are characteristic of the San Ignacio orogeny; and, fourth, there are no recorded Sunsas cycle granites. It can thus be concluded that the cratonic zone was a stable rigid block during the Sunsas cycle.
The Aguapei mobile belt is a north-northwest-trending zone of folded Sunsas Group rocks cut by major tectonic slides, notably the Aguapei front (Figs. 2 and 7), which divides this belt from the Cratonic zone where the Sunsas Group is essentially undeformed. At Las Petas, along the line of this front, a local syncline of greenschist facies Sunsas Group rocks is rotated through a vertical position to lie with a downward-facing attitude. The Aguapei mobile belt is well exposed in Brazil, where it is composed of folded and faulted Sunsas Group rocks (Figueredo and Olivatti, 1974), but its extension into Bolivia is largely covered b y alluvium, making correlation with the main Sunsas orogeny difficult. Although K--At dates (phyllite, pegmatite) are in the range 1050--950 Ma (Darbyshire, 1979), it is thought that the formation of this mobile belt was the last event of the orogenic cycle since structures with similar trends (between north-northeast and north-northwest) affect earlier Sunsas structures within the San Ignacio schist belt (Fig. 5) and surrounding paragneisses and are also superimposed on the eastern section of the Sunsas orogenic belt. These north-northeast to north-northwesttrending structures, which may also have been responsible for the major inflexions in the curvilinear trend of the Concepci6n--San Diablo front, are thus tentatively correlated with a late-Sunsas Aguapei mobile belt. The deposition o f the Sunsas Group The molassic Sunsas Group varies in thickness from 6500 m at Serran~a Sunsas to 800 m at Serram'a Aguapei. Since the formational lithologies, including the super-mature basal conglomerate (Figueredo and Olivatti, 1974), exhibit no facies variations, it can be assumed that subsidence and sedimentation were in equilibrium, The thinnest sequence at Aguapei formed over the region which was later to become the stable craton whilst the thickening at Las Petas coincided with the line of the later Aguapei front. Palaeocurrent directions in this region are from the west, i.e. from 'craton' to 'mobile belt'. Deposition of the Vibosi Group also t o o k place in the Sunsas region of maximum subsidence. These factors suggest that tectono-sedimentary control of Sunsas Group deposition was related to linear zones of crustal weakness which were later rejuvenated during the subsequent Sunsas orogeny in the form of orogenic or mobile belts.
175
Correlation
The Sunsas cycle can be correlated with the Uru~uano cycle of the Atlantic shield area (Fig. 1) which spans the period 1300--1000 Ma (Almeida et al., 1976). In the western part of the Central Brazil shield, the Rond6nia belt, formed during the Rond6niense event (Almeida et al., 1976), includes molasse sediments which are cut by 950 Ma granites (Priem et al., 1966). On this basis it could be suggested that the Cratonic zone identified in the north of the present area is b o u n d e d on all sides by (+ 1000 Ma) orogenic or mobile belts (Fig. 7b). THE BRASILIANO OROGENIC CYCLE (950--500 Ma)
In eastern Bolivia the close of the Sunsas cycle marked the cratonization of the shield and the deposits of the Upper Proterozoic--Lower Palaeozoic Brasiliano (Pan-African) cycle are only represented by unmetamorphosed sequences on its eastern and southern flanks. In the east, the sediments of the Boqui Group (Mitchell, 1979), part of the Paraguay--Araguaia belt of the Brasiliano cycle (Almeida et al., 1976) (Fig. 1), are miogeosynclinal conglomerates, arkoses, shales and limestones with a total thickness of a b o u t 1000 m which u n c o n f o r m a b l y overlie rocks of the Sunsas cycle. These pass laterally eastward into a thicker 'eugeosynclinal' sequence of conglomerates, greywackes, banded ironstones, cherts, limestones and calcilutites. There are major facies changes controlled by north-trending contemporaneous rift faults. The Boqui Group was folded in the east a b o u t north-trending axes (at the western edge of the Paraguay-- Araguaia mobile belt) and unconformably overlain b y the Murci~lago group of reef and non-reef limestones with development of reef facies also controlled by contemporaneous north-trending faults. The Murcidlago Group is tentatively regarded as Cambrian in age since it contains Cambrian stromatolites in its Brazilian correlative, the Corumb~ Limestone (Beurlen and Sommer, 1967), and it also represents a major marine transgression over a fault-controlled Upper Proterozoic basin (Matthews and Cowie, 1979). On the southern flank of the shield, Upper Proterozoic sedimentation is represented by the miogeosynclinal Tucavaca Group (Mitchell, 1979; O'Connor, 1979) confined to a west-northwest-trending fault-controlled basin, at least 400 km in length, which follows the trend of the earlier Sunsas belt. The Group unconformably overlies rocks of the Sunsas cycle to the west and the Boqui Group to the east. It is composed of a basal dolomitic limestone overlain by sandstones and shales with a total thickness of a b o u t 2000 m. Slight folding on westnorthwest-trending axes locally refoliated adjacent basement rocks near San Josd (Fig. 2) which yield 'reset' K--At ages of -+ 545 Ma. The Tucavaca basin thus represents a major fault-controlled offshoot of the Paraguay--Araguaia belt. Further marine transgressions occurred in the Phanerozoic along this unstable 'line' which remains a zone of seismic activity ('Chiquitos fault' of Gansser, 1973).
176
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 of dolerite--gabbro composition with minor olivine gabbros 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 concentrated along the easttrending Rio Mercedes fracture system, another line of Phanerozoic crustal weakness, which truncates the Paraguay--Araguaia belt in Brazil and is associated 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 unreliable dates, the tectono--metamorphic history of the intervening (1800-1000 Ma) period is not fully understood. The identification of t w o orogenic cycles in eastern Bolivia within this period is thus of major significance and warrants further discussion. The San Ignacio orogeny affected the Trans-Amazonic basement and the supracrustal San Ignacio Group. Field evidence suggests that the older granulites may have been metamorphosed (+ 2000 Ma) within a horizontal tectonic regime (Bridgewater et al., 1974) with a primary stratiform foliation subparallel to the later 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.Cordilleran orogeny (Cobbing, 1978). The facies changes in the supracrustal sequence may have been controlled b y 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 resisted 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 phenomenon in character with a molassic ensialic basin rather than an aulacogen (Hoffman et al., 1974). During the orogenic stage,
177
deformation within the belt was accompanied by sinistral movements between cratonic blocks (see Canadian shield, Watson, 1973) but, although the earlier trends are slightly offset within the semi-stable North-Marginal zone, they are preserved within the orogenic belt in the Concepcidn area. Thus the Sunsas belt is analogous in age and form to the + 1000 Ma Kibaran--Irumide belt of Africa (Shackleton, 1973) which overprints, w i t h o u t displacement, the older U b e n d i a n - R u z i z i belts and encloses the older units in the manner of Fig. 7b. These pre-Pan-African (pre-Brasiliano) mobile belts of southern Africa are ensialic (no major displacements of earlier lineaments) and characterised by plate destruction rather than plate accretion (KrSner, 1976). The zonal structure of the Sunsas belt cannot, however, be compared to the tectonic zonations of the 2600 Ma L i m p o p o mobile belt where craton and mobile belt formed at the same time (Key and H u t t o n , 1976). The Sunsas orogeny with its well-defined tectono--metamorphic zones and post-tectonic granites is not marked by high-grade mineral assemblages and, although its tectonic fronts fit into the category of 'straightening zones', these lie at right angles to the world-wide trend of Proterozoic high-grade linear belts (Davies and Windley, 1976). The Aguapei mobile belt must be regarded as a late offshoot of the Sunsas belt. It does not appear to possess a granitic phase b u t contains high-level differentiated basic intrusions (Figueredo and Olivatti, 1974} similar to the Rincon del Tigre Igneous Complex. It thus shows affinities to the marginal zone rather than to the main belt of the Sunsas orogeny. CONCLUSIONS
Four Proterozoic orogenic cycles have been recognised in eastern Bolivia. The identification of the Lower Proterozoic Trans.Amazonic cycle and the Upper Proterozoic Brasiliano cycle was anticipated before the fieldwork in the light of Brazilian work. The recognition of t w o Middle Proterozoic orogenic cycles is, however, of major significance since this period of reworking of the Trans-Amazonic nucleus prior to the Brasiliano (Pan-African) event is n o t well d o c u m e n t e d in South America. During the Middle Proterozoic, the San Ignacio cycle (+ 2000--1300 Ma) began with the deposition of a widespread schist belt sequence and culminated in the San Ignacio orogeny, a regional north-trending tectono--metamorphic event which reworked the older granulite basement. The Sunsas cycle ( < 1 3 0 0 - - 9 5 0 Ma) included the deposition of molassic sediments and an orogenic phase analogous to ensialic mobile belts of the same age in southern Africa and exhibits a stable cratonic zone, a semi-stable marginal zone and an unstable orogenic zone, divided by westnorthwest-trending curvilinear sinistral shears. It is suggested that the + 1300 Ma cratonic nucleus is enveloped by -+ 1000 Ma mobile belts, a hypothesis which will be examined during the second phase of Proyecto Prec~nbrico (Fig. 1).
178 ACKNOWLEDGEMENTS
Phase I of the Bolivian Precambrian Project (1976--1979) was essentially a team effort and its successful conclusion was due entirely to the sustained drive and great enthusiasm 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, jointly-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 Administration, Foreign and Commonwealth 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, Institute of Geological Sciences, and the Director of the Servicio Geoldgico de Bolivia. REFERENCES Almeida, F.F.M. de, 1971. Geochronological divisions of the Precambrian of South America. Rev. Bras. Geocienc.,l: 13--21. Almeida, F.F.M. de, Hasui, Y. and Brito Neves, B.B., 1976. The Upper Precambrian of South America. Bol. Inst. Geocienc. Univ. Sao Paulo, 7: 45--80. Annells, R.N., 1979. The geology and mineral potential of the Rincon del Tigre Igneous complex. Rep. E. Bolivia Miner. Explor. Proj. (Proyecto Prec~mbrico), Phase I, 7 [unpublished interim report available on open file in Bolivia (GEOBOL) and the United Kingdom (IGS)]. Annells, R.N., Fletcher, C,J.N. and Appleton, J.D., 1981. A major ultramafic--mafic layered intrusion of Proterozoic age: the Rincon del Tigre Igneous Complexi eastern Bolivia. Proc. 4th Latin American Geol. Congr., Port of Spain, Trinidad and Tobago (1979). Beurlen, K. and Sommer, F,W., 1957. Observacoes estratigraficas e palaeontologicas sobre o Calcaria Corumb~. Serv. Graf. Inst. Bras. Geogr. Estatistica, Rio de Janeiro, 168, 18 pp. Boulangd, B. and Litherland, M., 1978. Surfaces d'aplanissements en zone amazonienne de Bolivie (region de San Ignacio de Velasco-Santa Cruz). Cah. ORSTOM, Ser. Geol., 10 (1): 145--151. Bridgewater, D., McGregor, V.R. and Myers, J,S., 1974. A horizontal tectonic regime in the Archean of Greenland and its implications for early crustal thickening, Precambrian Res., 1: 179--198. Cloud, P., 1971. The primitive Earth. In: I.G. Gase, P.J. Smith and R.V.L. Wilson (Editors), Understanding the Earth. Artemis, Horsham, pp. 1 5 1 - - I 56. Cobbing, E.J., 1978. The Andean geosyncline in Peru and its distinction from Alpine geosynclines. J. Geol. Soc. London, 135: 207--218. Cobbing, E.J., Ozard, J.M. and Snelling, N.J., 1977. Reconnaissance geochronology of the crystalline basement rocks of the Coastal Cordillera of southern Peru. Geol. Soc. Am. Bull., 88: 241--246. Cordani, U.G., Amaral, G. and Kawashita, K., 1973. The Precambrian evolution of South America. Geol. Rundsch., 62: 309--317.
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