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Earth-Science Reviews, 17 (1981) 1--29 1 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Brazilian Structural Provinces: An Introduction F.F.M. DE ALMEIDA l, y. HASUI 2, B.B. DE BRITO NEVES 3 and R.A. FUCK 4 1 Insti tuto Oeoci~ncias, Universidade de SEo Paulo, S~o Paulo, SP (Brazil) 2 Insti tuto Pesquisa Tecnoldgica de S~o Paulo, S~o Paulo, SP (Brazil) 3 Dept. Geologia, Universidade Federal de Pernambuco, Recife, PE (Brazil) 4 Dept. Geoci$ncias, Universidade de BraszTia, Brasilia, DF (Brazil) ABSTRACT De Almeida, F.F.M., Hasui, Y., De Brito Neves, B.B. and Fuck, R.A., 1981. Brazilian structural provinces: an introduction. Earth-Sci. Rev., 17: 1--29. The territory of Brazil coincides almost entirely with the South American Platform, the crystalline core of the continent. Its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the Phanerozoic. Sedimentary rocks with almost horizontal bedding cover this crystal- line basement. This latter shows ages as old as Early Precambrian, although ages between 500 and 1000 m.y. are conspicuously frequent. The cratonic areas became consolidated more than 1700 m.y. ago, whereas the fold belts formed essentially between 500 and 1700 m.y. ago. The sedimentary cover accumulated from the Early Silurian in three large intracratonic basins, until the platform became completely stabilized. A Late Jurassic to Early Cretaceous reactivation, caused by the break-up of the ancient Gondwana conti- nent, created another basin sequence chiefly along the Atlantic continental margin. Based on the nature of the crystalline basement rocks and the sedimentary cover, ten structural provinces can be distinguished: (1) Rio Branco Province, in the north of the country, occupied by the Guyana Shield and still only poorly known, with an important fold belt developed during the so-called Transamazonian Cycle (2000 ± 200 m.y.) with high-grade metamorphic rocks and only slight influence of later events. (2) Tapaj6s Province, in central Brazil, corresponding to the Amazonas or Guapor6 Craton, showing chiefly Precambrian crystalline rocks and behaving as an cratonic area during the Phanerozoie. (3) Sffo Francisco Province, located on the Atlantic Shield, with its basement covered by rocks of different ages, chiefly affected by the Brasiliano Cycle (between 1000 and 500 m.y.) and constituting another cratonic area. (4) Tocantins Province, between the Amazonas and S~'o Francisco Cratons, with the oldest rocks in its centre (ages over 2600 m.y.), and at the eastern and western borders metamorphic sequences of various fold belts, and almost no Phanerozoic deposits. (5) Mantiqueira Province, located along the southern part of the Atlantic coast, affected chiefly by the Brasiliano folding cycle. (6) Borborema Province, in the northeast Brazilian fold belt, affected by the Brasiliano Cycle in a very complex way, and with impor tan t faulted zones. It was reactivated in Phanerozoic times, when also sedimentary covers accumulated. (7) Amazonas Province, represented by the Amazonas sedimentary basin (syneclise), and subdivided into four parts by three important arcs. 0012-8252/81/0000---0000/$ 07.25 © Elsevier Scientific Publishing Company (8) Parnaiba Province, coinciding with the P i a u f - Maranh~b syneclise, and filled with a rather thick sedimentary sequence. (9) Paran~ Province, the sedimentary basin of southern Brazil, in which the well- known Late Paleozoic glaciation features are found, and which possesses an extensive cover of basaltic rocks of Late Jurassic to Early Cretaceous age. (10) Coastal Province and Continental Margin, the youngest structural unit, developed during the separation of the continent and represented by rift-valleys and coastal basins filled with Mesozoic---Cenozoic deposits of various kinds. GENERALITIES The complete Brazilian territory forms part of the South American Plat- form whose youngest geosynclinal foldings were consolidated at the beginning of the Phanerozoic. This basement, composed of metamorphic and igneous rocks, almost entirely of Precambrian age, crops out widely in the country. It occupies an area of about 4.6 × 106 km 2, subdivided in three large shields which extend into neighbouring countries: that of the Guyanas in the north, the Central Brazilian Shield inland south of the Amazon river, and the Atlan- tic Shield situated along the Atlantic border (Fig. 1). The sedimentary cover of the South American Platform accumulated from the Early Silurian on, chiefly filling three large sedimentary basins of syne- clise character: Amazonas, Piauf--Maranh~o and Paran~. Smaller areas of variously aged covers, including Precambriar~, occur scattered over the above- mentioned shields. Considering the nature of the exposed basement and its cover, it becomes possible to recognize structural provinces (Fig. 2). These are large areas which show features of stratigraphic, tectonic, metamorphic and magmatic evolution, different from those presented by the bordering provinces. The limits of these provinces as indicated on Fig. 2 are of two types: geo- logically defined and arbitrarily limited. Those of the first type include limits established following structural components of major size such as faults or fault zones, metamorphic fronts, rapid transitions of fold belts into their forelands, or erosional borders of large sedimentary basins. The conventional limits include those that are geologically undefined, either by the particular conditions of their structure, or due to lack of adequate knowledge, or because of being covered; they also include transitional limits with a certain degree of gradual passage from marginal folding zones into deformed plat- form covers. As the limit of the continental margin, the isobath of 2000 m which is almost entirely included in Brazilian territory, was adopted. In the tectonic evolution of the platform basement, two older areas are distinguished (cratons) which in times more recent than 1800 m.y. have no longer been subjected to the geosynclinal evolution. The largest of these is the Amazonian Craton, naturally subdivided into two provinces by the Area: zonas sedimentary basin. The northern province, included in the Guyana Shield, is called Rio Branco Province, the southern one, forming part of the 1 0 0 0 km i i J ~-~ I I _ , o / F / O O I I : : .: . ' .:~..'..'.'.. R A Z I L • <, qy o O y o f~ 2 a b Fig. 1. The major g e o t e c t o n i c regions o f S o u t h America . Legend: 1 = A n d e a n Chain; 2 = Patagonian Plat form; 3 = S o u t h Amer ican Plat form: a, Phanerozo ic covers; b Precambrian basement . I = G u y a n a Shie ld; H = Central Brazil ian Shield; III= Atlant ic Shield. Central Brazilian Shield, is the Tapa]ds Province. The SEo Francisco Province, in the Atlantic Shield, coincides with another cratonic area of ancient con- solidation. The other three provinces of the basement have as a c o m m o n character- istic the persistence of geosynclinal evolution up to the end of the Pre- cambrian, with final phenomena such as tectonic activity, uplift of mountain chains, molasse accumulation, subsequent volcanic plutonism, pegmatite formation, etc., already developed during the Cambro-Ordovician. Accord- 6 5 5 5 4 5 . . . . . . . ' . - ' . . . ' ' . ":".:.":::'".".:.'.:.'.:":.:':':i'.".:': :" . ' • . : . . . . " . ' . ' i;i;ii ili -- i!iiiiiiiil • / 3 lOOO k m 25- L J 35-F Fig. 2. T h e s t r u c tu r a l p rov inces o f Brazil . Legend : 1 = Rio Branco ; 2 = Tapa j6s ; 3 = S~'o F ranc i sco ;4 = T o c a n t i n s ; 5 = Man t ique i r a ; 6 = B o r b o r e m a ; 7 = A m a z o n i a n ; 8 = P a r n a ~ a ; 9 = Paran~; 10 = Coasta l Province and C o n t i n e n t a l Margin. D a she d l ines r ep re sen t conven- t ional l imi ts o f provinces . ing to their geographic situation and position in relation to the ancient cratons, one can distinguish, within the Central Brazilian Shield, the Tocan- tins Province and within the Atlantic Shield the Borborema and Mantiqueira Provinces. The three large sedimentary basins represent areas of persistent subsidence during the Phanerozoic, with a consequent accumulation of thick sedimentary sequences and basaltic rocks. They constitute other provinces named after the sedimentary basins proper: Amazonas, Parnaiba and Parand. Finally, the small sedimentary coastal basins and their submerged extension on the continental margin characterize the youngest of the Brazilian geological pro- vinces: the Coastal and Continental Margin Province. As thus defined, the provinces may be grouped according to the successive TABLE I Brazilian structural provinces Guyana Shield Central Brazilian Shield Atlantic Shield Sedimentary covers Cratons (consolidated at more than 1700 m.y.) Fold belts, formed between 1700 and 500 m.y. Basins (syneclises) of Paleozoic age Basins of Mesozoic- Cenozoic age Rio Branco TapajSs Tocantins S~'o Francisco Borborema Mantiqueira Amazonas Parnaiba Paran~ Coastal and Continental Margin stages during the structural evolution of the country (Table I); they are characterized in their order of decreasing age. Although having distinct characteristics and histories, these ten provinces present many interdependent relations in their structural evolution. The TapajSs and S~o Francisco provinces constituted the forelands of the geo- synclines in which the fold belts developed, these latter characterizing the three younger basement provinces. Together with their forelands, these folded zones show evolutionary conditions comparable to miogeosynclines with evident tectonic transport towards them. The sedimentary sequences developed in these geosynclines are chronologically equivalent to the deposits covering the nearby cratons, so that one stratigraphical set may belong to two contiguous provinces. The sedimentary basins which characterize the Paran~ and Parnaiba Pro- vinces are neatly superimposed on these basement structures, which were the last to stabilise and from which their configuration inherited many structural trends. Only the Amazonas basin appeared during the Early Silu- rian, in a crustal area of very ancient consolidation. RIO BRANCO PROVINCE The Rio Branco Province (Fig. 3) is situated in a region almost entirely covered by a dense tropical forest. It is abundantly drained by large rivers and its low relief generally does not exceed 600 m. In spite of this, in the - . . . . . • • o 6.5 5.5 50 ---051 J o/ Fig. 3. Rio Branco (1) and Tapaj6s (2) provinces. Legend: 1 = Lower and Middle Pre- cambr ian undi f fe ren t ia ted complexes ; 2 = Trans-Amazonian areas; 3 = Precambrian cratonic granites; 4 = sedimentary (b) and volcano-sedimentary (a) covers o f Upper Pre- cambrian age; 5 = sedimentary covers related to the adjacent Brasiliano fold bel t ; 6 = Phanerozoic sed imentary covers. Heavy lines represent province boundaries; l ight discon- t inuous lines represent folding trends• mountains which limit it to the north, there exist heights of over 2000 m, e.g., the Neblina Peak {2992 m), the highest mountain on the South Ameri- can continent outside the Andes. Geologically the province is constituted chiefly of metamorphic and igneous rocks of Precambrian age, locally over- lain by thin post-Paleozoic covers. The Guyana Complex is exposed in the major part of the province, con- stituting the basement upon which the younger geological units lie in angu- lar unconformity. In its still poorly known structure there mainly participate different types of gneisses in amphibolitic to granulitic facies, migmatites, amphibolites and numerous intrusive rocks with predominance of granites I and granodiorites. Intense granitization and migmatization phenomena took place in this complex, even before the development of the Transama- zonian Cycle. The structures of the Guyana Complex display trends generally oriented in the northwest quadrant, with sinuosities towards E--W. The granulitic complex of Manuku, south of Guyana, with a supposed age of more than 2600 m.y. extends into the area of the province. Also in the eastern part of Amap~i, probably very old granulitic rocks are abundant. It is supposed that the major part of the Guyana Complex is constituted of rocks with pre- Transamazonian ages; these were remobilized and isotopically rejuvenated during this cycle, as indicated by the radiometric ages determined. A significant fold belt developed in the large geosyncline of the Trans- amazonian Cycle (2000 + 200 m.y.) in the region north of the Rio Branco Province. Its ramifications extend chiefly into the eastern region where they constitute isolated strips of metasedimentary rocks oriented to the north- west and lying with angular unconformity upon the Guyana Complex. These strips contain schists, itabirites, quartzites, amphibolites and serpentinites, metamorphosed in greenschist to amphibolite facies. They are included in the Vila Nova Group which has in its gondites the proto-ore of an important lateritic manganese deposit. Radiometric age determinations indicate about 2000 m.y. for the metamorphism of this group, it thus being attributed to the Transamazonian Cycle. Isolated occurrences of metasediments appear in the extreme north (Cauani Group) and west (Tuntff Group) of the province, also tentatively attributed to the same cycle. As a late stage in the evolution of the Transamazonian Fold Belt there took place extensive volcanic--plutonic phenomena of acid to intermediate character. The non-metamorphosed and lightly deformed rocks of the belt comprise the so-called Burumfi and Iricoum~ formations, both exposed at the northern border of the province. These rocks extend into the neighbour- ing countries, where they appear folded and somewhat metamorphosed. The age of this volcanism is about 1850 m.y. It was followed by the deposition of continental molasses of the end of the cycle, represented by the Roraima Group and by a thermal basic magmatism which formed numerous gabbro and dolerite sills and dykes. The age of these eruptive rocks, about 1800 m.y., marks the end of the Transamazonian Cycle in the Guyana Shield. The time interval between 1800 and 1000 m.y. is distinguished in the Rio Branco Province by a series of proto-activation processes of the ancient craton. These processes produced an intense and diversified faulting, vol- canism and plutonism, as well as accumulation of detritic sediments in smaller local basins. The magmatic activity affected almost the whole pro- vince but was particularly intense in its eastern region, east of the Branco river and near to the Amazon Basin. It took place chiefly between 1450 and 1750 m.y., producing outflows of rhyolites, rhyodacites, dacites, andesites and the respective pyroclastics, as well as associated sediments. Numerous intrusions of granites, some of rapakivi type, of granodiorites, diorites, gab- bros, syenites and alkali-syenites have been associated with this volcanism. Between about 1300 and 1100 m.y. another phase of proto-activation occurred in the province, manifested chiefly by intensive faulting, devel- opment of wide mylonite and cataclasite belts, local metamorphism and intrusion ofalkali-syenites. This phenomenon, also known in the neigh- bouring countries, is known in Brazil as the Jari-Balsino Episode. During the Paleozoic the Guyana Shield seems to have passed through a long phase of stabilization, remaining emerged and acting as a distributive province for the sediments which accumulated in the Amazonas Basin in the south and in the Andean region in the west. During this long erosional pro- cess the youngest Precambrian cover became destroyed, but near the margins of the ~Amazonas Basin some remnants have been preserved, represented by undeformed continental detrital beds, which make up the Prosperanqa and Sete Quedas Formations existing between the Mapuera and Negro rivers. With the tectonic--magmatic reactivation suffered by the South American Platform during the Mesozoic, some of the ancient faults in the Guyana Shield were reactivated. The Takutu graben in Guyana, is the most expres- sive manifestation of this phenomenon in the shield. It has been filled by basaltic and andesitic lavas (Apoterf Formation) of Jurassic age, associated with continental sediments (Takutu Formation). This graben and its deposits extend up to the upper Branco river basin in the centre-north of the province. Also related to the Mesozoic reactivation in the Rio Branco Province are intrusions of alkali-syenites (Catrimani) and of diabases, the latter forming an important group of NNW-oriented dykes situated near the coast of Amap~ (Cassipord diabase). Tertiary and Quaternary detrital cover extends over an appreciable area of the western region of the province, near the major rivers as well as along the coast. TAPAJOS PROVINCE The TapajSs Province (Fig. 3) covers an area of about 1,670,000 km ~, corresponding to that part of the Amazonian Craton known as the Guapord Craton. Only in a rather reduced part are its Precambrian rocks covered by remnants of Phanerozoic deposits. The province presents a low relief, with altitudes generally not over 600 m. It is covered with a dense tropical forest which makes geological investigation quite difficult. The Xingu Complex, comprising the major part of the exposed Precam- brian rocks, is chiefly constituted of gneisses and migmatites of amphibolitic to granulitic facies in which there locally occurs amphibolites, quartzites, schists and other metasediments and metavolcanics as well as numerous intrusive bodies of granites, granodiorites and other igneous rocks. The domi- nating structural trends are directed generally in the northwest quadrant, but in the extreme west of the province some NNE--NE trends are present. The ages of the Xingu Complex rocks have not been definitely established. It is supposed that a great part were formed more than 2500 m.y. ago, as is suggested by a few isolated radiometric determinations; however, diverse later tectonic--thermic episodes have imprinted younger ages upon the rocks. Above all during the Transamazonian Cycle, the remobilization and isotopic rejuvenation phenomena of this basement were important, with many granites being formed. It is probable that part of the gneisses and migmatites of the complex have originated from the sediments and volcanic rocks of the Transamazonian Cycle. A fold belt with certainty belonging to this cycle occurs in the eastern region of the province. The constituting Gr~o Par~ Group is composed of quartzites, phyUites, micaschists, jaspilites, itabirites and basic metavol- canics. An important occurrence of banded iron ore (Serra dos Caraj~s) is associated with this group. The late molasses of this cycle are represented by the Rio Freco Formation, with known granitic plutons about 1850 m.y. old, in the same area, representing episodes of the end of the cycle. Between 1700 and 1450 m.y. almost the entire area of the Tapaj6s Province was affected by significant faulting, volcanism and plutonism of acid to intermediate nature, and sedimentation processes. The phenomena constitute the Par~ Event, homologous with that occurring at the same time in the Rio Branco Province. The volcanic rocks, attributed to the Uatum~ Group, are widely distributed through the province and are rhyolites, rhyo- dacites, andesites and equivalent pyroclastics. Numerous intrusions of com- monly alkalic cratonic granites, granodiorites, diorites, gabbros, syenites and diabase dykes are other magmatic manifestations of this event. Con- tinental and marine sedimentary rocks, sometimes associated with the vol- canic rocks, crop out in many places. Those belonging to the Beneficiente Group stand out due to their extent, in the centre-north of the province. The intense faulting conditioned the distribution of effusive and intrusive rocks as well as of the sedimentary basins and the deformation of their deposits. A few of these covers have been lightly metamorphosed during this event. Another tectonic--thermic event of proto-activation, called the Madeira Event, took place at about 1300--1100 m.y., especially in the centra and west of the province. Ancient faults then also became reactivated, sediment cover folded and metamorphosed, accompanied by a new phase of intrusion of cratonic granites and isotopic rejuvenation of ancient basement rocks. The intrusion of the alkali-syenite of Canama dates from this period. These processes of the Madeira Event correspond to those of the Jari-Falsino Event of the Rio Branco Province. The final proto-activation process of the Tapaj6s Province is the Ron- 10 dSnia Event, affecting the western area at 1000 m.y. It was also marked by faulting, cratonic granite and granodiorite intrusion, some with ring struc- tures, and extrusion of rhyolites, andesites, t rachytes and basalts. Associated with this event there occurred an important tin mineralization. Probably less than 1000 m.y. ago detrital continental sedimentary cover (Prainha, Secunduri and Riozinho do Afrfzio formations) developed in sub- horizontal positions. These covers are correlative with the Prosperanqa and Seta Quedas covers of the Rio Branco Province, and to the Modoquena, Araras, Alto Paraguai and other groups of the eastern border of the TapajSs Province, representing extensions over the foreland of deposits from adjacent geosynclines of the Tocantins Province. The Jacadigo Group at this border, known for its important deposits of sedimentary iron and manganese ores, was possibly also deposited at the end of the Precambrian. During the Phanerozoic the province behaved as a shield, providing detri- tus to the sedimentary basins of the Amazonas and the Devonian sediments of the Andean region. Only along its borders neo-Cretaceous (part of the Parecfs Group) and Cenozoic deposits accumulated, with the manifestation of the Jurassic volcanism of the Serra de Tapirapu~. SAO FRANCISCO PROVINCE The S~o Francisco Province (Fig. 4) is a tableland in the inner part of which elevations attain between 500 and 1000 m and it is drained by the S~o Francisco river and directly to the ocean in its eastern part. North of that river the relief is table-form with altitudes not greater than 900 m, but in the east the tableland has a mountainous aspect with local elevations of nearly 1800 m in the extreme south. The province is covered chiefly by a savannah vegetation. Structurally it differs greatly from the bordering provinces due to its role as a cratonic foreland of the geosynclinal fold belts developed in it at the end of the Precambrian. Its limits with these provinces are for a great part conventional because they developed in tectonic transition zones. The basement of the S~:o Francisco Province is for the greater part over- lain by cover of various ages, but where it is exposed there occur granitic-- gneissic complexes, greatly migmatized and withmetamorphic facies varying from high-grade amphibolite to granulite. They have been dissected by numerous intrusions, principally of granitoid, but also of mafic to ultramafic nature. These complexes have radiometric ages of 2600 m.y., apparently corresponding to the remobflization epoch of the basement, with extensive granitization and migmatization features during the so-called Jequi~ Event. These phenomena acted partly upon rocks older than 3100 m.y., locally identified in the east of the province. The sinuous structural trends of this basement are oriented about N--S in the east of the province where granu- litic rocks of various types are most abundant . In the central area, where the ancient basement became uncovered by the structures of the Serra do Espin- haqo, granitic--migmatitic complexes dominate, with trends oriented to the NNW parallel to the structures of this range. 1 1 L__ 2 0 0 k m , 44 O~ 4O i t 2 - - .:. ".. / I ' \ 5 6 1 2 Fig. 4. Sgo F ranc i sco Province . Legend: 1 = Lower an d Middle P r ecamb r i an und i f f e r en - t i a t ed complexes ; 2 = J a c o b i n a be l t ; 3 = Pre-Brasi l iano s e d i m e n t a r y and m e t a s e d i m e n t a r y covers ; 4 = s e d i m e n t a r y covers re la ted to n e i g h b o u r i n g Brasi l iano fo ld bel ts ; 5 = molasse depos i t s ; 6 = P h a n e r o z o i c s e d i m e n t a r y covers. Heavy l ines r ep re sen t ma jo r faul t s ; l ight d i s c o n t i n u o u s l ines r ep re sen t fo ld ing t rends . Supracrustal structures of limited extent, affected by the rheomorphic remobilization of the basement, locally cover the gneissic--migmatitic com- plexes. They are constituted by various types of metasediments, meta- volcanics and metamorphosed mafic and ultramafic intrusives in schist to 12 low-grade amphiboli te facies. They have been interpreted as greenstone belts, but their age of older than 2600 m.y. has only been proven up to now for the Rio das Velhas Group in the extreme south of the province, that con- tains the largest gold mine in the country. The age of the Roquira Forma- tion, in the centre of the province, is still doubtful , but it is probably very old; it contains an important occurrence of lead. The Transamazonian geotectonic cycle (2000 + 200 m.y.) acted upon restricted parts of the province, developing fold belts, remnants of which are represented by the Jacobina Group and its possible extension (Contendas) in the State of Bahia, and by the Minas Supergroup of the extreme south of the province in the State of Minas Gerais. These belts are const i tuted of very thick sequences of metasediments of clastic and chemical origin, with meta- basitic intercalations, intensively folded and metamorphosed in a chiefly greenschist facies. The Minas Supergroup is famous because of its important mines of banded iron ore of the so-called Quadril~tero Ferrffero. During the Transamazonian Cycle intrusions occurred in various places in the province, principally of granitic rocks. The Serra do Espinha~o, situated in the centre of the province, in the State of Bahia, constitutes a zone of NNW-oriented folds, tectonized and metamorphosed in greenschist facies at about 1200 -+ 100 m.y. ago. This zone developed between 1700 and 1100 m.y. ago in a region where the base- ment is cut through by ancient and extensive faults, upon which a sequence of about 4000 m of sediment accumulated. This sedimentation does not characterize a geosynclinal environment, but rather a tectonically very active paraplatform with homologues in the Tapaj6s Province. Outside of the Espinhaqo fold belt, as is also the case in the latter province, the beds become gradually thinner, with the metamorphism disappearing, changing to the character of a tabular platform cover (Chapada Diamantina Group) deformations of which are of induced nature due to local movement of the basement faults. The upper stage of the Precambrian cover on the craton represents the extension upon it of the deposits accumulated in the geosynclines developed at its borders during the Brasiliano Cycle. In this cover remnants are recog- nized of a continental glaciation which took place at the end of the Pre- cambrian (Macafibas Group); the ample extension of pelitic and carbonatic sedimentation correlates with the pre-inversion limestones of the above geo- synclines as well as the remnants of the final molasses of this cycle. This cover, in part lightly metamorphosed, has been deformed in various areas either by reflex action of the tecto-genesis of the marginal folded zones or by the reactivation of basement faults in the inner part of the province. The Brasiliano co~;er complex developed between 1000 m.y. and the beginning of the Phanerozoic w h e n t h e molasses accumulated. They are associated with important deposits of lead, zinc, sedimentary phosphates and carbonates. The Phanerozoic cover has been preserved on the craton, in Mesozoic coastal rift-valleys formed during the opening of the ocean, attaining thick- 13 nesses of thousands of meters, as in the RecSncavo graben in the State of Bahia. In the inner part of the province there subsist remainders of extensive bu t thin continental sediment cover of Late Cretaceous to Tertiary age as well as the Quaternary sedimentation of the river floodplains. TOCANTINS P R OVINCE The Tocantins Province (Fig. 5) is situated between the Amazonian and S~o Francisco cratons and limited to the north and south, respectively, by the sedimentary basins of Parnaiba and Paran~. The province appears as a region chiefly covered by savannah, with local forests. Its relief is low with isolated ranges standing out, but it becomes elevated to about 1200 m above sea level in the centre of the State of Goi~s, in the so-called Planalto Central. The great lowlands of the Pantanal in Mato Grosso, and of the Araguaia in Goi~s belong to this province. Three structurally distinct regions can be recognized in the Tocantins Province: a central region corresponding in its major part to the median massif of Goi~s; an eastern one coinciding approximately with the Uruaqu and Brasflia fold belts; and a western region with the Paraquay--Araguaia fold belts. The oldest rocks of the province, with ages older than 2600 m.y., consti- tute the major part of the central region. They are chiefly various types of gneisses, including granulitic rocks locally migmatized and penetrated by granitoid rocks of various ages. In structural and possibly also lithological terms, two realms can be distinguished in the central region, separated at about the 16°S parallel by a deflection of the structures, called the Pirineus Megaflexure, which affected the youngest folded rocks as well as those of the older basement. South of this flexure NW--NNW-oriented structural directions dominate, whereas to the north they are more NNE--NE aligned. In the eastern part of the southern segment there occurs a large zone of granulitic rocks. All rocks of the province present the effects of polymeta- morphism, episodic fracturing of different ages and isotopic rejuvenation. Intrusive basic and ultrabasic rocks are common, they are locally mineralized with copper and nickel sulphides, and some of them are completely talcified and serpentinized. Ultramafic--alkalic massifs of Cretaceous age, with epi- genetic mineralization of nickel are known from Ipor~, Montes Claros de Goi~s and other localities. In the northern segment of the central region very ancient gneisses dominate, of granodioritic aspect and with diffuse foliation and generally wi thout banding. In someareas such rocks consti tute the basement for vol- cano-sedimentary sequences, represented by green rocks (chlorite-schists, talc-actinolite-schists and amphibolites) associated with ferruginous quart- zites, itabirites and schists. These sequences appear mineralized in gold, copper, talc and amphibolic asbestos. In the eastern part of the region there occur the great mafic--ultramafic complexes of Goian~sia--Barro Alto, 14 o k m 5 6 4 4 Fig. 5. Tocantins Province. Legend: 1 = Older basement reworked during the Upper Pre- cambrian (Goi~s Massif in the central region and part of the Guaxup4 Massif at the south- eastern corner); 2 = Uruaqu fold belt; 3 -- Brasiliano fold belts (Brasflia belt bordering the S~o Francisco Province, Paraguay--Araguaia belt bordering the Tapaj6s province); 4 -- sedimentary covers related to the adjacent Brasiliano belts; 5 = Phanerozoic sedi- mentary covers. Niquel~ndia and Cana Brava, associated with granulites, the last two com- plexes being surrounded by younger metasedimentary rocks. These com- plexes have ages older than 3000 m.y. and show mineralizations of Ni, Cu and Co; in Cana Brava the principal Brazilian chrysolitic asbestos mine is found. Granulites have also been recognized southeast of Porto Nacional and 15 more to the east a belt of greenstones has recently been identified. The Uruaqu fold belt in the east of the Tocantins Province is still a poorly defined unit, extending from Natividade (Goi~s) to the region of Arax~ (Minas Gerais) from where it continues eastward. In G o i ~ and also in Minas Gerais, the basement of the Uruaqu fold belt is composed of granulites, gneisses, amphibolites and calco~silicated rocks which consti tute the Gua- xup~ Massif. This massif has a triangular form and separates the southern part of the fold belt in two branches, oriented NW and NE, the latter limited by a important transcurrent fault. The Uruaqu fold belt is const i tuted of pelitic and psammitic metasedi- ments as well as metabasites. Its folds are of holomorphic character, verging to the S~o Francisco Craton. North of the Pirineus Megaflexure the domina- ting directions of these folds are oriented between NNE and the meridian, whereas in the south they have a general NW direction. Generally it is sup- posed that these rocks belong to the Uruaquano Cycle, the chief metamor- phism of which should have occurred about 1000 m.y. ago, although some recent authors consider the possibility of these rocks being older and belonging to the Transamazonian Cycle. Granitic rocks are rare in this fold belt, occurring in brachyanticlines the granitic nuclei of which may contain cassiterite. The Brasflia fold belt, whose length is over 1100 km, developed at the western and southern flank of the S~o Francisco Craton during the Brasi- liano Cycle. The sedimentation, of miogeosynclinai character, started with a fine detritic sedimentation, followed by psammites and pelites with inter- calated limestones and dolomites. The folds of holomorphic type verge towards the craton; they have been metamorphosed in greenschist facies, both phenomena decreasing in the same direction. The magmatic activity related to the evolution of this fold belt is restricted to andesitic volcanism in the north of the belt, and to the intrusion of a few granite plutons in the south. Thrust faults are frequent, indicating a movement towards the craton border. A molassoid sedimentation, developed at the end of the cycle, covers the fold belt as well as the craton proper. Carbonate rocks, sedimentary phosphates, lead and zinc consti tute its mineral resources. The Paraguay--Araguaia fold belt developed at the border of the Ama- zonian Craton, similarly to the relationship between the Brasflia belt and the S~o Francisco Craton. It shows an S-type curved form extending over 3200 km. Its extremes are covered by the sediments of the Parnaiba and Paran~ basins. A great part of the fold belt is covered by modern sediments of the Araguaia Plain in which the large Bananal island is found, in Goi~s State, and the Mato Grosso Pantanal. In its southern part two phases of geosynclinal evolution can be distinguished, in the first of which when later folded and metamorphosed, flysch-type sediments were deposited. During the second phase a conglomerate was deposited, indicating a marine transgression, fol- lowed by a glacial-marine deposit ion and, in angular unconformity , an upper carbonatic unit with stromatolitic structures, capped by pelites. The 16 metamorphism is in greenschist facies and an intense holomorphic folding parallel to the axis of the belt, verges towards the Amazonian Craton. A few post-tectonic granitic plutons of about 500 m.y. age occur in the middle part of the belt. The last stratigraphic unit, possibly older than the intrusions, corresponds to molassic deposits that are partly continental and chiefly accu- mulated in a foredeep which extends from the fold belt over the craton border. In the region of Corumb,'i, State of Mato Grosso, there occur arkoses followed by a sequence of alternating beds of ferruginous jaspilite, compact hematite, thick-bedded cryptomelane and intercalated clastic sediments. Enormous reserves of iron and manganese ore occur in this Jacadigo Group, the age of which is still uncertain. In the northern part of the belt the basal sequence is composed of gneisses, schists and quartzites, with conformable and unconformable amphibolites. Then follows a series of terrigenous sediments, represented by phyllites, sub- ordinated quartzites and some limestone intercalations, capped by a mag- matic--sedimentary sequence, represented by chlorite-schists, talc-actinolite- schists and metabasites. There occur numerous unconformable bodies of mafic and ultramafic rocks changed into serpentinites and magnesian schists. This magmatism is related to the Tocantins--Araguaia geosuture, developed at the eastern limit of the Amazonian Craton. The metamorphism of the belt decreases rapidly from east to west, passing from an amphiboli te facies into a greenschist facies, reducing to anchymetamorphic transformations near the border of the craton. Granitic rocks appear locally, in brachyanti- clinal nuclei surrounded by banded gneisses. The Phanerozoic sedimentation in the Tocantins Province remains limited to the filling of the Agua Bonita graben in Goi~s, with supposed Devonian beds, and to relicts of Cretaceous and Cenozoic continental covers, among which stand out those of the Pantanal of Mato Grosso (with thicknesses of up to 500 m) and of the Araguaia plain. Late Cretaceous alkalic magmatism, as well as intrusions of Early Cretaceous diabase dykes, occurred in various parts of the south of the province. MANTIQUEIRA PROVINCE The Mantiqueira Province (Fig. 6) is situated along the Atlantic coast south of latitude 15°S, extending to the frontier with Uruguay. It occupies an area of about 450,000 km 2, and is bordered at one side by the coastal basins and the continental margin and at the other side by the S~o Francisco, Tocantins and Paran~ Provinces. The Mantiqueira Province presents an almost entirely mountainous region, w i th altitudes higher than 1500 m in the coastal mountain range; the culminating point is the Pico da Bandeira which with its 2890 m constitutes the highest peak of the eastern part of the continent. In the northern part of the province is situated the southeastern fold belt, developed during the Brasiliano Cycle, and const i tuted by three fold systems 17 5 3 4 1 2 ""'":'":""."! ":" " 1 4 -~2s 3 500 km Fig. 6. Man t ique i r a Province . Legend : 1 -- Older b a s e m e n t reworked dur ing the U p p e r P r e c a m b r i a n (a -- Pe lo tas Massif, b = Joinvi l le Massif, c = pa r t o f t h e G u a x u p ~ Massif); 2 = Brasi l iano fold be l t s (A = Tijucas belt , B = Apia i bel t , C = S~'o R o q u e bel t , D = Ara- quaf be l t ) ; 3 = m e t a s e d i m e n t a r y area re la ted to t h e Ara~ua f be l t (Rio Pa rdo G r o u p ) ; 4 = molasse depos i t s ; 5 -- P h a n e r o z o i c s e d i m e n t a r y covers. Heavy l ines r ep resen t ma jo r faults . separated by two median massifs with a general NE orientation. The fold systems have been formed by geosynclinal sequences which in the bet ter studied areas exhibit a lower psammitic--pelitic sequence, with carbonate lenses near the top and an upper, chiefly pelitic, sequence of f lysch-type character. These rocks have been metamorphosed into a green- schist and amphiboli te facies under low P/T. In large areas they underwent migmatization processes• The folding was intense, polyphase and of linear type, with poorly evidence vergence. The pre-tectonic magmatism of mafic 18 nature is poorly developed, contrary to the syntectonic magmatism of acid nature represented by numerous stocks and batholiths dated at about 650 m.y. There also exist post-tectonic intrusions, less frequently forming stocks and batholiths, dated at about 540 m.y. In the median massifs the pre-Brasiliano basement is exposed as nuclei with granulites, charnockites, mafic and ultramafic rocks, and migmatites of the Middle and Early Precambrian. Numerous small basins developed at the final evolution stage of the folded region. They are located within the fold systems and its margins and filled with molassic sediments and acid to intermediate lavas with thicknesses of over 5000 m. The older molasses have been folded, whereas the younger ones have only been tilted by faulting. The cited volcanic rocks and the associated granitoid plutons hav# been dated at 450--500 m.y. Bundles of thrust faults, chiefly longitudinal, divide the southeastern fold belt in sections. The faults seem to be ancient and deep, having passed through a phase of intense reactivation at the end of the Precambrian and the beginning of the Paleozoic when they affected the metamorphites and granites formed during the Brasiliano Cycle. The area between latitudes 23 ° and 24°S is characterized by a structure of faulted blocks where the Brasiliano structures curving towards N60E are replaced by the pre-Brasiliano ones situated more to the north. The limits of this transition are still not defined, due to absence of detailed investigations. The principal mineral resources of the southeastern fold belt are the vein deposits of Pb--Zn associated with limestones, Au and Cu in the molassic basins of the south of Brazil, W and Sn in gneisses, feldspars and lithium- bearing minerals of the pegmatites, Mn and Ti associated with the older rocks. During the Paleozoic, the Ponta Grossa and Rio Grande arcs developed at the borders of the Paran~ Basin, exercising their influence strongly on the modelling of the basin and the delimitation of the province. With the Mesozoic--Cenozoic reactivation of the South American Plat- form, there occurred the intrusion of a number of diabase dykes and the formation of many alkalic plugs, some with associated carbonatites, within the Ponta Grossa and Rio Grande arcs. Reactivation of ancient faults as well as newly formed ones permitted the development of a relief and the appear- ance of some small taphrogenic basins filled with Cenozoic fluvial and lacustrine sediments. In the north of the Mantiqueira Province is othe Ara~ual fold belt, devel- oped during the Brasiliano Cycle at the borders of the S~o Francisco Craton. Near this belt, north of 19°S, the structures have a NNE orientation, but near !5°S they turn to ESE after a strong inflection, reaching the coastal plain at the northern limit of the province. In the part with NNE orientation, the belt is const i tuted by geosynclinal pelitic to psefitic sediments, with associated limestones, itabirites and mafic volcanic rocks, composing the Macaubas Group. Pebbly greywackes exist in 19 this sequence that seem to be glacial-marine sediments correlated to the deposits of glacial origin found on the S~o Francisco Craton (Jequitai Formation). The sediments present a metamorphism varying from green- schist to amphiboli te facies, with increasing intensity going away from the craton. Radiometric age determinations point to 600 m.y. for the meta- morphism and about 517 m.y. for the final granites. Holomorphic folds and large thrust faults affected these rocks, verging towards the craton. The northern part of the fold belt is still poorly known and the limits between the Mantiqueira and S~o Francisco Provinces are still not precise, except in the coastal area where they coincide with the Planalto fault. The rocks of amphibolitic facies of the Araquaf belt pass into the gneisses and migmatites, with associated schists, quartzites, gondites, calco-silicate rocks, marbles, charnockites and acid to basic granites, of older ages. The limit has not been fixed due to sparse research in the area. In the remainder of the Mantiqueira Province some lithostratigraphic units have been described whose mutual relations are still not explained. In its southern part units are recognized with metasediments of amphibolitic to granulitic facies, with an age of about 2000 m.y. (Paraiba, Amparo, Itan- homi groups, Juiz de Fora Formation) and at tr ibuted to the Paraiba do Sul fold belt developed during the Transamazonian Cycle. In the central-south- ern part gneisses are found with associated mafic and ultramafic rocks, with ages older than 2600 m.y. (Barbacena and Mantiqueira Formations), upon which the Paraiba Group should lie unconformably. The Brasiliano Cycle affected these units intensively, producing deforma- tion, blastic modifications, and intense migmatization and granitization. Thus pegmatites were formed which in the centre-north of the zone contain a district producing various types of gemstones. B O R B O R E M A P R O V I N C E The Borborema Province (Fig. 7) covers an area of about 380,000 km 2, coinciding with the Nordeste fold belt, developed during the Brasiliano Cycle. It is limited by the S~o Francisco and Parnaiba provinces as well as the coastal basins and continental margin. In the major part of its area it has a semi-arid climate with the development of large pediments. The residual erosion relief only rarely exceeds altitudes of 1000 m. The region of Brasiliano folds presents a complex arrangement, in mosaic, including different systems of linear folds mutually separated by elevated parts of the basement, either related to faults or not. The folding systems of Sergipe and Medio-Coreau are situated at the extreme southeast and northwest of the province, respectively. They have a marginal disposition in relation to the S~o Francisco and SXo Lufz cratons. In these systems similar sets of metasediments are present in which a level of carbonate rocks, thousands of meters thick, is intercalated within terri- genous clastics of various types. These sequences generally extend to the 20 5 3 4 o • 8:1: . . . . :..-:~ I 4 2 3 1 2 ~ 4 0 3 6 L 200 krn, Fig . 7. B o r b o r e m a P r o v i n c e . L e g e n d : 1 -- O l d e r b a s e m e n t r e w o r k e d du r i ng t h e U p p e r P r e c a m b r i a n (a -- P e r n a m b u c o - - A l a g o a s Mass i f , b = Te ixe i r a Mass i f , c -- R i o P i r anhas Mass i f , d = TrS ia Mass i f , e = S a n t a Qui t~ria Mass i f ) ; 2 = Bras i l i ano f o l d be l t s (A = Se rg i pan b e l t , w i t h o = o u t e r z o n e a n d i = i n n e r z o n e , B = R i a c h o d o P o n t a l be l t , C ~ P i a n c S - - A l t o Br fg ida be l t , D = P a j e f i - - P a r a ~ a be l t , E = Se r id5 be l t , F = J agua r ibe b e l t , G = C u r u - - I n d e p ~ n d ~ n c i a be l t , H = M~dio Coreaf i be l t ) ; 3 -- s e d i m e n t a r y cove r s co r r e l a t i ve o f t h e Bras i l i ano be l t s ; 4 = m o l a s s e d e p o s i t s ; 5 = P h a n e r o z o i c s e d i m e n t a r y cove r s . Heavy l ines r e p r e s e n t m a j o r fau l t s . borders of the neighbouring cratonic nuclei, with decreasing thickness and degree of deformation. The holomorphic folds are arranged fol lowing the craton borders in relation to which they verge• The fold systems farther away from the ancient cratons are constituted of 21 psammitic--pelitic metasediments of at least two cycles, those of the younger one prograding in relation to those of the older cycle and of the basement highs. An intense holomorphic folding exhibits vergences towards the inner massifs which are partially covered by geosynclinal deposits with at tenuated deformations. The metamorphism is of plurifacial character, with facies variations of greenschist to amphibolite, under low P/T. Migmatization is frequent, chiefly in the basal terrigenous sequences. The pre-folding magmatism was of basic nature and not very intense. The syntectonic magmatism was extremely active, const i tuted of stocks and batholi ths of granitoid rocks dated at about 650 m.y. The late tectonic grani- tic magmatism, the only one occurring in the marginal zones, took place about 540 m.y. ago. Molassic deposits accumulated in backdeeps and foredeeps associated with the marginal systems, extending even over the cratons with decreasing thick- ness and degree of deformation. The materials seem to have been deposited between 510 and 470 m.y. The basement highs const i tute median massifs and geanticlines, exposing gneissic--migmatitic complexes of Early to Middle Precambrian age, remo- bilized during the Brasiliano Cycle. A complex fault system of large size divides the province into sections, separating the fold systems or cutting through them. These faults seem to be very old, deep and reactivated on different occasions, with variable character. The transcurrent character noted on the maps, is only the most obvious, having occurred late during the Brasiliano Cycle. The general trend of the province's structures is arranged as a fan opening towards the northeast, diagonally to the coast. It suffered deviations and inflections due to the faulting movements; two of these faults, the lineaments of Pernambuco and Patos or Paraiba, bo th with a W--E direction, stand out in this assemblage due to their size. The principal mineral deposits of the province are Be, Ta and Li associated with the pegmatites, W and Mo in scarns, Fe, magnesite, marble and graphite. There exist remnants of the Paleozoic and Mesozoic cover associated with various small taphrogenic basins, originated by reactivation of ancient faults. These sediments are relicts of formerly more extensive cover, bound to the primitive extent of the Parnaiba and RecSncavo--Tucano--Jatob~ basins. Cretaceous diabase dykes and Tertiary alkalic--basaltic volcanic products occur locally. Remains of thin Tertiary and Quaternary sedimentary covers lie upon erosion surfaces, escalated from the coast inland. AMAZONAS PROVINCE The Amazonas Province coincides with the great sedimentary basin which developed upon the Amazonian Craton during the Paleozoic (Fig. 8). It is an extensive low-lying region of about 1,250,000 km 2 , the relief only becoming 2 2 ::i::.::....:.! ii : . • :'-. . ~ + o o :i:.!. i: :' :iil l : . . % " : . . . : - i . . " . . . ' . . : . . : ::il 2 0 _ I y. , - . : ..... : .. . - . . . I P 3 0 - - -.}.- Fig. 8. A m a z o n i a n (7 ) , Parnafba (8) and Paran~ (9) Provinces. Countour lines represent the depth o f the basement in ki lometers . Arches: A = Iquitos, B = Purus, C = Gurup~, D = Tocant ins , E = Ferrer--Urbano Santos , F = S~o Francisco, G = Goi~nia, H = Asunc ion , I = Ponta Grossa, J = Rio Grande. Basins: P = Acre , Q = Upper A m a z o n , R = Middle A m a z o n , S = Parnafba, T = Paran~. higher than 200 m in the extreme west of the province, near Peru. It is drained by the Amazon river, running in its axis, and covered by a dense tropical forest. The important Cenozoic continental sedimentation covers the Paleozoic structures which only crop out in narrow strips along the basin borders. The evolution of the Amazonas Basin during the Paleozoic has been largely affected by the presence of three transverse arched ridges, Iquitos in the west, Purus in the centre and Gurup~ in the east. The tectonic action of these arcs strongly condit ioned many of the erosion and sedimentation 23 processes which took place in the basin, as well as subdividing it in four sub- basins: Acre in the west, Upper and Middle Amazonas, and Maraj6 or Lower Amazonas, this latter already belonging to the coastal province. The Paleozoic marine transgression in the Amazonas Basin started during the Early Silurian if not during the latest Ordovician. The detritic sedimen- tat ion took place in environments varying between tidal flats and neritic bot toms. A regression and again a transgression followed during the Devo- nian when the sea remained in the basin between the Emsian and Frasnian. The thick sequence of detrital sediments then accumulated, represents environments ranging between littoral and infraneritic, to even bathyal. A long hiatus followed until the sea returned once more to occupy the basin during the Late Carboniferous and deposited detritic sediments, limestones and evaporites (in this order), with a total thickness of 3000 m in the Middle Amazonas area. The neritic sedimentation environment points to a progres- sive closure of the basin, culminating in the accumulation of up to 1200 m of evaporites, with intercalated limestones and shales. Probably the action of the Gurup~ horst provoked this closure. Continental beds identified in wells seem to indicate a Permian sedimenta- t ion in the area of greatest thicknesses of sediment in the Middle Amazonas basin. A great quant i ty of diabase dykes and sills, some of them attaining hundreds of meters in thickness, occur intercalated in the Paleozoic beds. This magmatism, that t ook place in more than one phase, developed between the end of the Carboniferous and the Early Jurassic, based on isotopic datings. The outf lows which must have existed, have been eroded before the Tertiary. In the Middle Amazonas basin continental layers of Late Cretaceous age are also known. The accumulated sediments upon the craton border, near the Andean region of Peru, extended to the extreme west of the province where they became folded and faulted. The Cenozoic sedimentation, chiefly from the Tertiary, is completely con- tinental, covering the major part of the province. It may attain a thickness of about 1500 m, in the Upper Amazonas area. The Quaternary deposits remain limited to the low floodplains of the big rivers. P A R N A I B A P R O V I N C E The Parnaiba Province (Fig. 8) is situated in the north of Brazil, with an area of about 650,000 km 2. It coincides chiefly with the sedimentary basin • of Piaui--Maranh~o, but in its northern part Precambrian basement rocks are locally exposed. The major part of the province has a tabular relief with con-centric cuestas, their altitudes generally not being higher than 600 m. The subsidence of the basin started in the Early Silurian or possibly during the Late Ordovician, after a long period of erosion of the basement rocks of the South American Platform. The basal sedimentation, of littoral and neritic origin, is composed of medium- to coarse-grained sandstones with 24 conglomeratic intercalations capped by fine sandstones, siltstones and shales attaining a thickness of 700 m. Shales follow, with sandstone intercalations resulting from the marine transgression of the Early Devonian. During the Middle Devonian there occurred a short regression phase marked by deltaic sandstones, followed by a wide transgression which, during the Late Devo- nian (Famennian), was responsible for the accumulation of a thick sequence of dark, frequently bituminous shales. Then there took place a regression of the sea, and during the Early Carboniferous the extent of the basin was reduced. The initially marine sedimentation became continental, with fluvial sandstones, sfltstones, lacustrine shales and thin coal beds. The clastic sedi- mentation continued during the Late Carboniferous, but near its top it con- tains fossiliferous limestone beds of a periodic ingression of the sea. In the Early Permian there follows a continental sedimentation of sandstones, sili- cified limestones, siltstones with beds of oolitic chert, and gypsum. After these episodes the region was uplifted and underwent a long period of erosion and non-deposition, interrupted during the Late Triassic by a limnic sedimentation of silt- and claystones, with a few sandstones. Then the first sign of basaltic magmatism appeared, becoming quite important during the Late Jurassic and Early Cretaceous, and responsible for large outflows which cover eolian sandstones, indicative of a dry climate during that period. Numerous sills and dykes were introduced within the layers of the basin. In the north of the basin there follows, after the basaltic magmatism, a large and shallow Cretaceous sedimentary basin which probably continued towards the east of the province. The sedimentation in this basin started with eolian sandstones, capped by fine clastics with banks of limestone and gypsum, proving that a marine ingression took place during the Aptian. The Cretaceous sedimentation ended in the Albian, with the deposition of sand- stones, siltstones and shales from lacustrine, fluvial, estuarine and deltaic environments. These layers possess marine equivalents in the Barreirinhas coastal basin. During the Cenozoic the province suffered erosion such that the sediments of this time now only occur near the coast. At the northern border of the Parnaiba Province, there arose during the Early Cretaceous the so~alled Ferrer--Urbano Santos arc, with orientation parallel to the coast. The rupture of the northern flank of the ridge by suc- cessive normal faults was the origin of the coastal basins of graben type of Bragan~a, S~o Luiz, and Barreirinhas. The appearance of the Ferrer--Urbano Santos arc brought to the surface Precambrian structures which crop out in isolated spots through the Meso- zoic-Cenozoic cover. They permit the recognition of two geological and geochronological realms separated by a zone of cataclastic rocks in which penetrate intrusions of maflc to intermediate character, of Cretaceous age. This strip coincides with the axis of the above arc. North of it, gneisses and migmatites with structural trends oriented to the NW and intercepted by granitic bodies, show radiometric ages of more than 1600 m.y., and their ~2 25 Fig. 9. Middle Nor the rn region of Brazil. Legend: 1 --- Trans-Amazonian basemen t and covers; 2 -- Brasiliano bel ts (A -- Gurupi belt , B = M~dio Coreafi bel t) ; 3 = sed imen ta ry covers correlat ive o f the Brasiliano belts; 4 = molasse deposi ts ; 5 = Phanerozo ic sedimen- tary covers. Heavy lines represen t major faults; d i scon t inuous line indicate the sou the rn l imit o f the S~o Lugs Craton. cover of slightly deformed arkosic psammites is dated at about 900 m.y. The eastern outcrops, situated east of S~o Luiz to the State of Cear~, present Brasiliano K/Ar ages. South of the above strip, gneisses, quartzites and phyl- lites with orientations between NNW and N--8 also show Brasiliano ages, as obtained by various methods. It is supposed that the older structures present in this region form part of a primitive cratonic nucleus (S~o Luiz) which originally continued into the Craton of West Africa, with reworking during the Brasiliano Cycle (Fig. 9). The structural region situated south of the cataclastic zone must represent an extension of the Brasiliano folding systems of the neighbouring provinces. Gold is the principal mineral product of this area with Precambrian rocks. PARANA PROVINCE The Paran~ Province (Fig. 8) corresponds to the Paran~ sedimentary basin, one of the large syneclises which became implanted upon the South American Platform from the Silurian on. It is limited by the Tocantinas and Mantiqu- eira Provinces and presents between 29 ° and 30°S a projection reaching the Atlantic Ocean. The relief of the province is const i tuted by high-lying table- land and concentric cuestas, drained by the Uruguay and Paran~ rivers. At its higher eastern border the altitudes rarely surpass 1200 m, but do attain 1808 m on the Igreja hill on the basaltic highland in Santa Catarina. 26 The form and size of the Paran~ Basin varied much during geological time, due to slow tectonic oscillations as indicated by the paleogeographical recon- structions. At present it occupies an area of about 1,200,000 km 2 in Brazil, extending into Paraguay, Uruguay and Argentina. The present basin con- figuration reflects some tectonic features which acted at its border in the Paleozoic and Mesozoic, such as the Goi~nia flexure and the arcs of Central Paraguay, Ponta Grossa and Rio Grande. The evolution of the basin during the Devonian shows an extensive marine transgression coming from the west. During the Carboniferous the marine ingressions become episodic, passing in the Permian to continental sedimen- tation which continued from then on. The oldest fossfliferous sediments dated in the Paran~ Basin are of Silurian age and are recognized in Paraguay; there is no proof that they extend into Brazil. In the Devonian, the sea remained in the basin between the Koblenzian and Frasnian, with the deposi- t ion of initially coarse detrital sediments followed by black shales, attaining a total thickness of 1000 m. During the Late Carboniferous the basin accumulated an up to 1500 m thick continental sedimentary sequence, for the greater part resulting from five advances of inland ice. During interglacial phases three marine ingressions left lithological records in the Carboniferous column, also including post- glacial sediments. The Permian is represented by sedimentary rocks which attain thicknesses of 1700 m, deposited wi thout interruption with the post-glacial Carboni- ferous units, under conditions of restricted waters passing gradually to fluvial. The pack is dominantly pelitic, reflecting a slow subsidence of the basin. From the end of the Paleozoic on, erosion processes became dominant; only one record of Late Triassic sediments is known from the south of Brazil, represented by about 200 m thick fluvial, fossiliferous sedimentary rocks. During the Late Jurassic the reactivation process of the South American Platform supervened. There occurred an extensive continental sedimentation covering an area of over 1,300,000 km 2 with eolian, locally fluvial and fluvio-lacustrine deposits which may attain a 400 m thickness.These sedi- ments have been covered by basalt outf lows which also occur intercalated. The outf lows may attain a total thickness of 1,529 m in the centre-north of the province, whereas the calculated volume exceeds 650,000 km 3. Together with this magmatism a large quant i ty of dykes was formed, some with thick- nesses of 100 m and lengths of tens of kilometers that are present chiefly at the eastern border of the basin and in the Mantiqueira Province. Sills up to 200 m thick also occur intercalated in the sedimentary rocks. Accompanying the basaltic volcanism and following it during the Late Cretaceous, an alkali magmatism took place at various sites along the basin borders, originating numerous types of ultramafic, mafic and intermediate rocks, many of them still present within the province. During the Late Cretaceous fluvial sediments were deposited initially in 27 restricted areas, afterwards extending over the entire southern half of the province. These sediments possess thicknesses of not more than 300 m. In the Tertiary the coastal area was uplifted and a thin fluvial sedimenta- tion started to invade the province related to a drainage that was not very much different from the present one. In the whole province the beds are generally slightly dipping towards the inland. Local deformations have been observed, associated with faults or igneous intrusions. The mineral wealth of the Paran~ Province is associated with sedimentary rocks (coal, pyrobi tumen and limestone) and to magmatics, chiefly those of alkalic affiliation. C O A S T A L P R O V I N C E AND C O N T I N E N T A L M A R G I N The youngest structural province considered here, developed from the Late Jurassic on at the ocean border, having contacts with almost all other provinces. The coastal plain is the physiographic feature of the emerged part of this province; it is a surface a few meters above sea level, composed of alluvial, transitional, and coastal marine deposits, locally remobilized by wind action. The plain is characteristically narrow and very elongated, even discontinuous at some sites. Its major expressions remain restricted to the Amazon river mouth {400 km) and some other river systems which supply Holocene deltas such as those of the Parnaiba, Jaguaribe, Sffo Francisco, Doce, Paraiba do Sul and other rivers. Generally the plain continues landward in the form of low tablelands until a few tens of meters above sea level, comprising Cretaceous and Tertiary continental and marine sediments which reach the crystalline rocks at the limit of the province. The Brazilian continental margin, the submerged part of the province, shows variable physiographic and sedimentary connotat ions visualized in three distinct segments. Along the northern coast, from Amap~ to Maranhffo, the shelf is up to 200 km wide, rather shallow and with a very regular surface. The gradient and the relief of the slope vary greatly; the Amazon cone is the most impor- tant feature. The surface deposits are chiefly sandy. Off the coast of the eastern part of northeastern Brazil that shows a very irregular shelf surface followed by an abrupt break to the slope, the surface deposits are mainly bioclastic, the terrigenous sediments remaining restricted to the neighbour- hood of the river mouths. Off the coast between Bahia and Rio Grande do Sui the shelf always presents variable widths and depths, is rich in important physiographical features, and has an abrupt slope. From Rio de Janeiro southward the fine terrigenous deposits become dominant , with absence of carbonate sediments. In their distinct phases, the differences in the structural patterns of the crystalline basement and in the tectonic style of the evolution o f the Brazi- 28 lian continental margin, permit a separation into three parts in relation to the coastal basins formed during the process of continental break-up. The southern part started its evolution first, during the Late Jurassic. It presents a tension tectonic style with faults more or less parallel to the basement structures. The stratigraphical column of the then formed basins is the most complete, concerning the lithological record of the successive stages of the continental separation process. The northern part from Rio Grande do Norte to the Amazon presents identical tectonic style, with faults almost diagonal to the basement structures and a neo-Cretaceous superimposed compression style. Its evolution and lithostratigraphic record dated from the Albian on, with important deflections from the initial stages. The inter- mediate part of Pernambuco and Paraiba contains the last basin to be formed (during the Late Cretaceous) with structures perpendicular to the Precam- brian basement directions, marking the border of the so-called 'transverse zone'. It presents in relative terms a smaller subsidence, although the ten- sional tectonic style has been confirmed. The principal mineral resources of these basins are petroleum, gas, eva- porites, carbonate rocks and sedimentary phosphate. CONCLUSIONS With its almost continental size, Brazil possesses very diverse geological and geotectonical features, formed during different epochs of the earth's history. The best synthesis of these features is given by a division of the terri- tory into ten structural provinces. The provinces are based on the fundamental characteristics of their base- ment rocks and younger covers, including lithology, structure and age. Finally, also spatial distribution and geographical setting have been con- sidered, so that every province represents a natural geological region. At the present state of knowledge about Brazilian geology, the limits between these provinces are chiefly arbitrary. Three provinces -- Rio Branco, Tapaj6s and S~o Francisco - - i n c l u d e ancient cratonic nuclei, composed of gneissic--migmatitic complexes, granu- lites and also greenstones, whose youngest folded belts show a Middle Pre- cambrian age. The Late Proterozoic cover occurs frequently in these cratons, presenting immature clastic and volcanogenic lithologies as well as disconti- nuous folding. The other basement provinces -- Tocantins, Borborema and Mantiqueira -- are located adjacent to the three cratonic ones; their lithological and struc- tural features were chiefly imposed during the Brasiliano Cycle (1000--500 m.y.), with evidence of a Proterozoic geosynclinal evolution. Thick molassic covers of Cambro-Ordovician age test ify to the end of this evolution. Another three provinces refer to sedimentary covers, present in three broad syneclises -- Parnalba, Paran~ and Amazon -- extending over hundreds of thousand of square kilometers, whose evolutive processes are chiefly 29 Paleozoic (post-Ordovician), showing some five thick cratonic sedimentary sequences with isopachs of up to 5000 m in the depositional centre. The youngest province is the Atlantic coast and continental margin, more than 8000 km long. The sedimentary sequences have accumulated in coastal grabens and half-grabens, as clastics and carbonates of Cretaceous and Ceno- zoic age, and they form the evidence for the successive stages during the pro- cess of continental break-up (rift) and beginning of continental drift. [Received September 4, 1980]
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