The Post-Paleozoic Volcanism of Northeast Brazil and Its Tectonic Significance

THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL AND ITS TECTONIC SIGNIFICANCE

ALCIDES NOBREGA SIAL Institute of Geosciences, UFPe, Recife, C. P. 1538

ABSTRACT a spinel-Iherzolite under pressure of at least 20 kbar, included a xenolith suíte derived from a Four main Post-Paleozoic groups of vol• depth of approximately 64 ± X km. canic rocks OCC\lrin Northeast Brazil: (1) A Mesozoic diabase dike swarm in Rio Grande do The tholeiites in the Maranhão sedimentary Easin form flows and siBs in the western border Norte which extends into the States of Ceará of the basin, and instrusive forms with dikes and and Paraiba; (2) a Tertiary group of predomi• sills in the eastern border. The Middle Triassic nantly alkali basaltic rocks, in central Rio Mosquito Basalt and the Lower Cretaceous Sar• Grande do Norte and northern Paraiba; (3) a dinha Basalt occur along the Tocantins River, Mesozoic tholeiitic flow and some instrusive forming a 50 km wide, irregular outcrop belt. forms, in the State of Maranhão, cOl).stitutingthe Flows and sills of the Sardinha Basalt are also most voluminous basaltic activity of Northeast; seen between Fortaleza dos Nogueiras and Barra and (4) a Cretaceous magmatic province of Cabo do Corda. This magma derived from a shallow Santo Agostinho in the State of Pernambuco, depth in the mantle jUdging from its low Ti02 wherein andesites, rhyolites and trachytes are contento also present. The Mesozoic extrusions in Maranhão basin The diabasic dike swarm which is composed are related to the rapid northward movement of of six main east-west dike sub-swarms in the the Gondwanaland continent on a non-spherical Pre-Cambrian basement has ages between 125 earth (Middle Triassic Mosquito Basalt) and to and 130 m. y. Coarse grained, olivine-bearing the break-up of Gondwanaland (Lower Creta• diabases predominate in the southern sub-swarms, ceous Sardinha Basalt). The emplacement of the Mesozoic diabase dike swarm of Rio Grande do while porphyritic olivine-free types are more Norte resulted from the same crustal extension frequent in the northern ones. Coarse-grained, .hypersthene-rich diabases of uncertain relation which in Africa gave rise to the Benue trough to the sub-swarms are also observed. while the Tertiary alkali basaltic suite of Rio Grande do Norte is relatedei~r to internal The Tertiary volcanic suite composed of readjustment within the South American plate aJ:\karatrites, basanites, and olivine-basalts with during its westward displacement, or to the basanitic or tholeiitic affinities, whose known Tertiary pressure release of arched zones formed ages range from 18 to 42 m. y ., formed plugs, in the Upper Mesozoic during the opening oi necks, flows and dikes cut the Pre-Cambrian the South Atlantic Oceano The Cretaceous basement and Cretaceous sediments of the Apodi magmatic province of Santo Agostinho is pro• Basin in a north~south trend. The alkali basaltic bably related to the Ascencion plume activity magma which form~ by the partial melting 01 during the late stages of continental separation.

An. Acad. bras. Ciénc., (1976), 48, (Suplemento) 300 ALCIDES NóBREGA SIAL

INTRODUCTION do Norte. This group is interpreted as Tertiary in age. Four main Post-Paleozoic groups of volcamc The fourth group af volcanic roeM which rocks are present in the Northeast Brazilian are of Cretaceous age is the magmatic province mainland. One is represented by extensive of Cabo de Santo Agostinho in the State of Per• tholeiitic basaltic flows, sills and dikes in the nambuco. This province is loeated ·south of Maranhão sedimentary basin. This basin Recife. Dikes, plugs, sills and loecoliths of whose eastern and western flanks are the andesite, rhyolite and trachyte intrude the Cabo Brazilian coastal shield and the central Bra• Formation, and basaltic flows which rest on it zilian shield respectively, lies in the states of are the main members of this suite. Fissure Maranhão and Piaui, southeast of the mouth type volcanism appears to have been the most of the Amazon River and has an area of about common mode of emplacement af magma 111 600.000 square kilometers. this province, although explosivetypes have also The basaltic roeks in this basin are con• been recorded. Due to théstrong weathering, a centrated in three major areas. The first occurs consequence of the aIternating dry and wet at Port Franco, Grajaú and Fortaleza dos No• tropical climate, the basalts in this province gueiras where flows predominate. The second have been altered to an immature red-brown concentration which oecurs at São João dos ferruginons laterite. The main basaltic flow Patos, Pastos Bons and surrounding areas is named Tiriri Basalt, is 94 m. y. in age according to Vandoros et aI. (1966). represented mostly by intrusive forms (dikes and sills). The third important occurrence is The primary purpose of this study is to situated close to Vitoria do Alto Parnaiba, elucidate the history of the drift of Northeast where intrusive bodies also predominate. Isolated Brazil away from Africa by iIÍterpreting this dikes or dike swarms trending SW-NE and ,t-fold igneous history. SE-NW are present in the central and southern portions of the basin in the State of Maranhão. MABANHAO THOLEllTIC OUTPOURING Dikes trending SE-NW and sills occur in the AS MID-PLATE TECTONIC EVENT northeastern portions in the State of Piaui (São Felix do Piaui, Beneditinos, São Miguel do Tapuio, , The regional geologic setting, petrographical Batalha, Mudança and Buriti dos Lopes). and chemical characteristics, .and the geogra• phical distribution of the tholeiites in the Ma• The second important group of basaltic ranhão Basin were discussed by Sial (1974). rocks in Northeast Brazil is a tholeiitic diabase Their age as indicated in literature (Table 1) dike swarm mostly in the State OI Rio Grande and supported by some field observations is do Norte, but which extends into the neighbouring confined to the Mesozoicperiod. Several samples regions of the states of Paraiba and Ceará. This of these basaltic rocks had their ages determined dike swarm is mainly composed of six dike in the United States or at the Geochronological Center of São Paulo, Brazil, or in both (Nunes sub-swarms in Rio Grande do Norte, mostly and others, 1973). Three af the sarnples listed trending E-W, although different trends are also in Table 1 whose ages were determined at the recorded. They appear to cut the Pre-Cambrian same time in the United States and in São basement and they extend more than 200 km. Paulo, yielded discordantresuIts. Those deter• They are interpreted as Mesozoic in age. mined in the United States' carne out syste• matically higher. This puts some restrictions on The third group occurs in central Rio the validity of these results as a whole and for Grande do Norte and the northern part of Pa• this reason Aguiar's et ai. (1969) idea for two raiba asa series of predaminantly aIkali-ba• distinct eruptive stages wiIl be observed here. saltic plugs ~isposed in a general N-S trend, The first one took place during the Middle and by less common small olivine-basaltic flows Triassic and the second one during the Lower and some narrow dikes of the same rock type. Cretaceous. The region of the main oecurrence of thése Unfortunately, the geographic distribution of plugs, smaIl flows and dikes lies in a Pre-Cambrian the tholeiitic flows, sills and dikes of the two terrain, but a few af them pierced Cretaceous magmatic events is not weIl known. Aguiar et ai. sediments of the Potiguar Basin which is located (op. cit.) indicated the oecurrence of Middle mainly in the northern portion of Rio Grande ·Triassic and Lower Cretaceous flows and sills

An. Acad. bras. CMnc., (1976), 48, (Suplemento) THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL 301

along the Tocantins River, and Lower Cretaceous the African plate on a non-spherical earth during small flows and sills between Fortaleza dos No• the Tertiary to explain the African Rift system. gueiras and Barra do Corda. However, they left It is here assumed that the Middle Triassic all the diabasic dikes and the basalts in the and Lower Cretaceous ·basaltic extrusions have east flank and southern portions of the basin without an indication of which event they something to do with the latitudinal changes and belonged to. break-up of Gondwanaland continent. TheMiddle Triassic basaltic extrusions in the Maranhão The dikes in the east flank of the basin cut basin could be ascribed to the lithospheric the Devonian sediments. They show the same cracking of Gondwanaland due to its rapid trend of the diabase dike swarms observed in northward movement during this time. 'I'he Sierra Leone, Liberia and Ivory Coast whose stresses within the plate were probably strong age is Triassic-Jurassic (May, 1971, and Dal• enough to fracture the lithosRhere. Basaltic rymple and others, 1975). This suggests that volcanism took place. duriJ:!.gthe time the lithos• the dikes of the Maranhão Basin probably belong phere was kept under tensi0!1' Due to this, to the Triassic magmatic event in this basin. cracking of the plate or a combination· of this After carefuI studies on the latitudinal mo• process with a hot-spot activity was very im• vements of African plate from the Paleozoic to portant during the Middle Triassic when a great the present day based on paleomagnetic data, amount of magma was extruded. It came to Oxburg et al., (1974) concluded that the break• rest or to a dormant period in the Upper Triassic up of Gondwanaland was preceeded by a rapid and Jurassic periods, once Gondwanaland stopped latitudinal change in the Late Paleozoic and early moving northward and the stresses within the Mesozoic, followed by northward movement of plate ceased.

~54' . '-"v52'42' <'o '~Al-4~ 44' 31' 3 • 30' a' o' I' ) 2' GUIANA .~.~ , IASIN• '. Qs, 4' ~ "'OIT" ( '- .' .." " 6'

DO NORTE30'32' 34 6' ~ 6' 10' I' ai I R A Z L

10' 54' 52' 50' a 46 44' Fig. 1 - Principal location of the four main groups of basaltic rocks of Northeast Brazil. I) Mesozoic tho• leiitic basalts. Maranhão; 11) and 111) Mesozoic diabase dike swarm and Tertiary basaltic suite, Rio Grande do Norte and Paralba; IV) Magmatlc province of Cabo de Santo Agostinho, Pernambuco. Modlfled from Hayes and Ewing (1970).

An. Acad. bras. CMnc., (1976), 48, (Suplemento) THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL .301 along the Tocantins River, and Lower Cretaceous the African plate on a non-spherical earth during small flows and sills between Fortaleza dos No• the Tertiary to explain the African Rift system. gueiras and Barra do Corda. However, they left It is here assumed that the Middle Triassic all the diabasic dikes and the basalts in the and Lower Cretaceous ·basaltic extrusions have east flank and southern portions of the basin without an indication of which event they something to do with the latitudinal changes and belonged to. break-up of Gondwanaland continent. The Middle Triassic basaltic extrusions in the Maranhão The dikes in the east fIank of the basin cut basin could be ascribed to the lithospheric the Devonian sediments. They show the same cracking of Gondwanaland due to its rapid trend of the diabase dike swarms observed in northward movement during this time, 'l'he Sierra Leone, Liberia and Ivory Coast whose stresses within the plate were probably strong age is Triassic-Jurassic (May, 1971, and Dal• enough to fracture the lithosp'here. Basaltic rymple and others, 1975). This suggests that volcanism took place. dUI'ingthe time the lithos• the dikes of the Maranhão Basin probably belong phere was kept under tensi0!1' Due to this, to the Triassic magmatic event in this basin. cracking of the plate ar a combination· af this After careful studies on the latitudinal mo• process with a hot-spat activity was very im• vements of African plate from the Paleozoic to portant during the Middle Triassic when a great the present day based on paleomagnetic data, amount of magma was extruded. It carne to Oxburg et al., (1974) concluded that the break• rest or to a dormant period in the Upper Triassic up of Gondwanaland was preceeded by a rapid and Jurassic periods, once Gondwanaland stapped latitudinal change in the Late Paleozoic and early moving northward and the stresses within the Mesozoic, followed by northward movement of plate ceased.

a' 54' 52' 3 ' 30° '0 GUIANA

IA51N 6'

° ~ o' " 0°

2°

DO NOITE30'32' 34 ~ 6° 10' " I' I R A Z L

10' 54' 52' 50' I 46 44' Fig. 1 - Principal location of the four main groups of basaltic rocks of Northeast Brazll. I) Mesozoic tho• leiltlc basalts, Maranhão; II) and III) Mesozolc diabase dike swarm and Tertiary basaltic sulte, Rio Grande do Norte and Paralba; IV) Magmatlc province of Cabo de Santo Agostinho, Pernambuco. Modified from Hayes and Ewlng (1970).

An. Acad. bras. Cillnc., (1976), 48, (Suplemento) 302 ALCIDES NúBREGA SIAL

TABLE 1 - ABSOLUTE AGE FOR SOME BASALTIC ROCKS OF THE MARANHAO BASIN , Method 192K-Ar,kmK-Ar,fromaaOeirasPlagioclasePiauiWholeofm,Coelho,rockwellinfromPiaui117115136199160196174and189HIRsample,147134175110fromsampleK-Ar,OeirasWholeK-Ar,Nunes,**±MineralrockIsaiasFloriano,Picos,Conceição54ElesbãoCantoItaueiras,Esperantina,CorebitSimplicio8Agricolândia,a6Corebit116±7Reference*,WholerockA.K-Ar,do118209154B.201Veloso,inBuriti,rockMendes,PiauidoandbetweenK-Ar,K-Ar,PiauiCanindé,PiauiothersPiauiPiauiWholeFlo- rock 260176 ** RADAM(1973). PROJECT Used Age inM.,,-"o Y. Diabase n. depthNLst-1-PLof 2.140betweenvo,Piauirianodepthn, m,TB-1-MAPiauiandwellof 1.8161.0401.816Buritim, Bra-well Floriano,n. 1Zst-1-MAPiaui RADAM168n.Rock(1973).RB-1-MATB-1-MA± 8Location* PROJECT DiabaseDiabase Diabasel>jabase DiabaseDiabase I Diabase

Remark: These rocks have been analysed at the Geochronological Center, Univ. of São Paulo, São Paulo, Brazil. Those marked with * have been analysed in USA.

An. Acad. bras. Cillnc., (1976), 48, (Suplemento) THE POST-PALEOZOIC VOLCANISlIl: OF NORTHEAST BRAZIL 303

ln the Lower Cretaceous after South Ame• seem to be in continuity with the Benue trough rica and Africa started drifting apart, the axis in Nigeria. (Figure 2). This sedimentary stresses were reactivated and some magma was trough is possibly correIative with the Potiguar extruded (Sardinha Basalt.) Probably this second . Basin in Rio Grande do Norte whose sedimentary basaltic activity in the Maranhão Basin was sequence seems to match the one observed in the concommitant to the emplacement of the diabase Benue trough, that is continental Lower Cre• dike swarm in Rio Grande do Norte and the taceous strata covered by marine Upper Cre• extrusions of the Parana Basin which took place taceous. in the Upper Jurassic and Lower Cretaceous (Amaral et aI., 1966). According to Maxwell et ai. (1970) the deep-sea drilling in the South Atlantic suggests that this is not older than 130 m. y . which THE FISSURAL THOLEIITIC DIABASE agrees with Dietz et ai. (1970) in whose model EMPLACEMENT IN RIO GRANDE DO the South Atlantic remainedclosed until 135 NORTE AS A RESULT OF m.y. This history seeIris related to events in CRUSTAL EXTENSION the mantle which led to the break-up of Gondwanaland and agrees with the average K-Ar The geographic position and pattern af the age (125 m. y .) obtained for the dolerites of Rio Grande do Norte diabase swarm strongly South Africa (Siedner et aI., 1968). This diabase suggests that its emplacement is related to the set is essentially a consequence of crustal exten• drift of Brazil away from Africa. The olivine• sion during its emplacement and probably reflects bearing faeies is located in the sauthern dikes; the chemical nature of the Upper Mantle at the to the north it disappears, being replaced by a pre-opening of the South Atlantic. Probably, quartz-normative tholeiite. The dikes are also the emplacement of the diabases in Rio Grande wider and better developed in the north which do Norte and some basalts and Lower Cretaceous reflects more intense fracturing in that region diabases in Maranhão, happened almost simulta• than in the southern one. The emplacement of neously or within a short period of time. the northern dikes was at a shallower depth resulting in a more rapid crystallization. .,' 42' I I o' May (1971) suggested the existence of a N radial pattern formed by the parallel sets of Late .04, l .04 #t Triassic to Early Cretaceous diabase dikes in , I C Eastern North America, West Africa, and north• 2' eastern South America, if the continents were reassembled into their relative positions in Triassic time. This dike swarm of Rio Grande do Norte, however, does not fit that radial .. pattern. May emphasized this fact and suggested that they are not a product af the same stress system that controlled the other dikes involved in that pattern. Burke et aI. (1971) suggested that the Niger delta in Cretaceous time was the site occupied by an RRR triple junction. According to their idea two of the arms of this junction corres• ponding to the South Atlantic and the Gulf of Guinea have continued to spread, pulling the continents apart and the third arm which cor• 10' responds to the Benue trough, after spreading • A H I A for about 30 m. y ., closed before the end of the Cretaceous. They left unexplained how the North Brazilian Ridge fits into this pattern. .•. •• If Africa and Brazil were reassembled into Fig. 2 Pattern exhibited by the basaltic dikes, their relative positions before the drift started, f1ows, and sills of the Maranhão basin in the Lower the Rio Grande do Norte dike pattern would Cretaceous period. Modified froIU Mesner et ai., (1964)

An. Acad. bras. Ciénc., (1976), 48, (Suplemento) 304 ALCIDES NóBREGA SIAL

TABLE 2 - ABSOLUTE AGE FOR SOME DIABASES OF RIO GRANDE DO NORTE, BRAZIL

Method and Mineral Rock Location Used Age in 1\1.Y. Refereoce

Diabase Lages4 ±southRioWholediketown,dike.rockroekdo 130126G.K-Ar,(?)of Norte K-Ar, Leooardos, O. H., et alo Lages Lages, Rio G. do Norte 1968, p. 137. Lages Diabase Ebert, H. et aI., 1968 p. 624

Diabase Pedra Preta Fission-traek 00 apatite 129 ± 3 8ial, A. !'l., 1974

Diabase Dike loeated K-Ar, Whole roek 131.4 ± 3.9 Cordani,: U. G., 1968 SE of Açu-RN

Olivine Canada Raneh Fission-traek on apatite 125 ± 4 8ial, A. N., 1974 Diabase Jardim de Piranhas, Rio G. do Norte Olivine Morcego Raneh Fission-traek on apatite 129 ± 2 Sial, A. N., 1974 Diabase Jardim de Piranhas, Rio G. do Norte

OlivinekmPedroaI.,et1968,ofSEkmwestroekH.K-Ar,11206238Avelinooftown,FewEbert,WholeRio±Florania±31G.36Riotown,doG.K-Ar,doWholeNorte p.624 p.624 diabase Norte Hornblende

The age obtained for the Rio Grande do Runka eropping out below Upper Cretaeeous Norte dikes is probably the same for the sediments of the Sokoto Basin. These Nigerian beginning of the Benue rifting. Due to the Cre• Mesozoie diabases have tholeiitie affinities and taeeous triple junetion situated at the Niger oUvine is present in their norm as attested by delta, the Benue rift underwent an opening and these authors in a fashion similar to what has gave rise to the Benue trough (Burke et aJ., been observed in some of the diabases of Rio 1971 and Grant, 1971). The thiek pile of sedi• Grande do Norte. Prophyritie and a nonpor• ments of about 10.000 ft fills this trough in phyritie facies have also been deseribed in the Nigeria (Wright, 1968), with deltaie, estuarine, Nigerian diabases whose Kj Ar dates indieated and marine and rare interealated alkaline basie 165 ± 3 m.y. and 121 ± 2 m.y. old, respeet• lavas. These alkaline basie lavas are younger ively. Aeeording to MeCurry (written eommu• than the tholeiitie diabases of Rio Grande do nieation). there may be pre-Albian intrusives in Norte in Brazil, and their age is supposed to be the basement beneath the Benue trough, and Middle Cretaeeous. There are some Mid-Jurassie these wO'll1dpresumably be dikes with a north• or Lower Cretaeeous basalts at Runka, Nigeria, easterly trend, possibly associated with lava deseribed by MeCurry et aI. (written eommuni• flows, though there is no evidence for their eation) and also at Gazamma, 55 km west of existence.

An. Acad. bras. Ci/lnc., (1976), 48, (Suplemento) THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL 305

== O._ic.ruot

_ •••• " •• , ••••••• 1111• •-. IhoI-ri" t1ik•• _""

SI... arouncl 110 m. y.

Flg. 3 - Inferred relative p03itions of South America anel Africa aboul 110 m.y. ago.

Oceanlc crua.

Miei .c ••••• c rid•• Seclim••••• 'y 'lIIi,.

My •••• it •• ofth. '.rlMllft~uco li ••••••• :-':. DoIe,ite di Ice I•• "" • .•••• Itie f Iows ••• eI • iJl•

S'." IM'waan 110 .ne1 9Om.y.

Fig. 4 - Inferred situation for South America and Africa in between 110 and 90 m.y. (1ast stage of the separation). Modified from Grant (1971)

Âno ÂQlld. brCJ8. OUftc., (1976). 48. (Suplemento) 306 ALCIDES NúBr.EGA SIAL

Some of the events related to the opening enough to detach the two continents which were of the Benue trough and the south Atlantic connected by a fairly narrow linkage. These mo• Ocean are discussed below and summarized in tions concentrated stresses on the Benue trough Table 2. whose sediments becarme folded as a resulto Probably a strike-slip movement in the so-called Allard et aI. (1969) refers to the distribution Pernambuco lineament, in the State of Pernam• of salt deposits on both coasts, Brazilian and buco, Brazil, which had been represented by a African, and stated that marine conditions did gravity fault thus far, took place at this time not reach the Gabon (Africa) and Sergipe generating an intense lateral sheared zone. The (Brazil) basins until Aptian time (110 m.y. time of the folding of the Benue trough sedi• ago) supporting what is being proposed here. ments given by Grant (1971)seems to be in Reyre (1966) found marine Cenomanian overlying agreement with what is here proposed. Albian sediments which are represented by con• tinental sandstones at the top close to Calabar Using paleontological data, Reyment (1969) (Nigeria). Black et aI. (1970) admitted an suggested that the final separation of South Albian age for the Benue rift valley bounded by America from Africa took place during the east-northeasterly faults. They also indicated Turonian (about 90 m. y . ). This also seems to the existence of Cenomanian-Turonian marine support the idea presented in this work. An transgression in this trough. important marine transgression which took place in the Northeast coast of Brazil, in the Cam• According to. Grant (1971) who proposed a RRF triple junction at the Niger delta, the final panian-Maestrichtian period which was recorded detachment of Brazil from Africa took about by the Gramame and Maria Farinha Formations 20 m.y. and it was dependent on the transformal (Paraiba Group) in the coasts of Pernambuco fault motion of the St. Paul and Romanche and Paraiba, and the Jandaíra Formation (Apodi fracture zones. Such movements probably hap• Group) in Rio Grande do Norte, seems to be in pened from 110 to 90 m. y. ago. This would be agreement with it.

TABLE 8 - INFERRED AGE OF TOE MAIN EVENTS IN TOE OPENING OF TOE SOUTO ATLANTIC OCEAN

North Brazilian Ridge, formation of , . 90-80 m.y. ago Folding of the sediments in the Benue trough in Africa (Santonian and Post Maestrichtian) . 110-80 m. y. ago Final continental detachment by transform fauIt motion (Romanche and st. PauI's) ; . 110-90 m. y. ago Establishment of the Benue rift as a trough . 120-110m.y. ago

Beginning of the openi~g in Africa . 125-130m.y. ago of South Atlantic (about

half way up each continent) j BeginningEmp~~~~::~:eof theo:w~:openingG~~~~~.of ~~.the~~~t.e.Benue~~~~~rift 125-130m.y. ago Beginning of the openíng of South Atlantic (southernrnost part) . 140 m.y. ago

An. Acad. bras. m6nc., (1976), 48. (Suplemento) THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL 307

THE RELATIONSHIPS BETWEEN THE westward 1l0rizontal displacement of the South TERTIARY ALKALI-BASALTlC VOLCANISM American plate. No east-west trending Cenozoic OF RIO GRANDE DO NORTE AND PARAIBA, volcanic chain has been observed in that part AND THE HORIZONTAL DISPLACEMENT of the Brazilian platform. OF THE CONTINENTAL BLOCK One explanation which could be invoked is that of a hot-spot underneath the continental The Tertiary basaItic suite which is mainly concentrated in Rio Grande do Norte with some block during. Tertiary time. The north-south occurrences in Paraiba is represented by plugs, trend would be explained by either a northward or southward motion of the plume in the mantle small flows, and less commonly by dikes. They are which would result in an alignment af volcanic mostly restricted to the PreCambrian shield vents. Latitudinal motion of Atlantic hot-spots where they occupy a N-S trending band which extends about 120 km and has a width of about has been observed by Burke et aJo (1973). 25 km, aIthough some of them are also found According to these authors, a reiâtive motion of Tristan da Cunha/Gough, Discovery seamounts cutting Cretaceous sediments of the Apodi Basin. and Bouvet hot-spots took place at an average In Rio Grande do Norte, there is an interest• of 1.8 cm/yr during the past 120 m. y. From ing change from south to north in the structural this it is possible to believe that there may forms taken by this Tertiary basaItic magma. have been one in the Rio Grande do Norte• The magma had a tendency to form flows where Paraíba region, below the continental block. it pierced the sediments of the Apodi Basin. However, the six available radiometric ages of Small flows are found near Açu, and in Macau these bodies indicated in Table 4 show no special where a thickness of 45 m was determined at connection between the geographic distribution a place named Alagamar (Kegel, 1957) in Rio of the dated rocks and their age. This does Grande do Norte. In Paraíba at Boa Vista one not support a hot-spot motion although there can also observe small flows of platy basaIt, is a possibility that this might become evident where the magma followed some E-W trending following further accurate geochronological work. fractures in its way to the surface; and at about 15 km north of Cubati where a very de• The pattern showed by the Tertiary volcanic co~posed small flow was recorded. bodies does not represent a linear trend like the Cameroun volcanic line in Africa whose basaIts Between the two areas cited above, plugs have similar chemistry to those in Northeast are the predominant form of igneous bodies. AlI Brazil. The Cameroun volcanic line was not con• the plugs have been considered as necks by sidered by Grant (1972) to be a result of «the Rolff (1965). Actually some of them show a lithosphere plate moving over a sub-lithosphere conical shape and this designation should be magma source as its igneous activity was not more appropriate. developed in episodic and sequential fashion The Tertiary basalts include ankaratrites, along its line.» There is no indication that the basanites, and olivine basalts. Some phonolites north-south rift or rifts in central Rio Grande occur ar Caruru Hill (Vandoros et aJ., 1967) and do Norte or Paraíba which parallels the trend Ancori Hill (Rao et aI., 1972) about 7 km south of shown by these volcanic bodies is anomalous. the former, not very far from Fortaleza,. State When Brazil and Africa started separating, of Ceará. The Caruru phonolite was dated as several zones of weakness were possibly formed 26 m. y. old and probably the Ancori has about in the edge of each continent as a result of the the same age. They are similar to each other drift. During the Tertiary, mainly from the petrographically and petrochemically as attested Oligocene to Miocene (36 to 12 m. y. ago) the by Rao et aJo (op. cit.). Dikes of very weathered Andean orogenic episode was taking place in the aIkali-trachyte found at Redenção (Prof. A. B. western side of the South American continent. Rao, oral communication), Ceará, show sanidine, Most of this important tectonic event was riebeckite and opaque mineraIs. There is a possi• synchronous with the Tertiary volcanic activity bility that those phonolites and alkali-trachyte in Rio Grande do Norte and Paraiba. The dikes are related to the alkali-basaltic activity changes inthe spreading rates of the Mid• in Rio Grande do Norte, although this would be Atlantic ridge observed by Maxwell et aJo (1970) difficuIt to prove at the present time. during the Miocene concommittant with the The north-south trend formed by the plugs Andean orogen could cause readjustment in the and necks of this aIkaline province represents a South American plate and give rise to this vol• puzzling problem if it is to be explained by the canism.

An. Acad. bras. Cienc.,. (1976), 48, (Suplemento) 308 ALCIDES No BREGA SIAL

TABLE 4 - ABSOLUTE AGE FOR SOME BASALTS OF RIO GRANDE DO NORTE, BRAZIL

Rock Location IM~~and_ Olivine Cabllgi Peak (neck), K-Ar, Whole rock (?) 19.7 ± 08 Cordani, U. G., 1970 in basalt Municipio de Angicos, Dantas, .T. R. A., 1974 Rio G. do Norte 0livine Logradouro Peak, Rio G. K-Ar, Whole rock 18. O ± 0.5 Ebert, H. et al., 1968, basalt do Norte p. 624 01ivinc Belo Monte Ranch, K-Ar, Whole rock 19.1 ± 1 Brito Ne.ves, B. B. basaIt Cubati, Rio G. do Norte (~itten communica• tion) AmygdaIoi-1322NorteK-Ar,19731970,K-Ar,G.p.VistaWholeH.,flows,Cruzeta,Pit-RioWholevos,FlowsAçudeSerrinhaBrejuidoBoaraibaThomazEbert,rockrockRio78742.02470664rockof(Mine,of?)? )G.Queimad:l;i,deFilho,Macau,±aI.,±(doCurrais30.4238.60?)4020CurraisK-Ar,K-Ar,13031973NorteA.Rio±±(writ-WholeNo-7.34Fl1-6.0G. Ebertrock et(?) aI., ten communication) basaItbasalt daI basal rlÚba do Norte Ohvine OlivineBasaltOlivine

Wright (1970) proposed that Africa and basaltic activity in Northeast Brazil was also South America were being wedged progressively associated with a high heat flow continental apart from the South throughout most of Creta• environment as happened in Nigeria. ceous time, but the stresses initiating the break became active from .Turassic time. As a The depth of magma generation for this consequence, a tensionaI north-south split in the Tertiary suite (Sial, 1974) calculated, using the south of the composite South America/ African solubility of Al20a in the enstatite of the peri• continental plate would cause a compressional dotitic nodules (spinel-lherzolites), included in its bulge further north along the same line. Black basaIts (MacGregor, 1974), yielded a minimum et alo (1970) invoked the pressure release of depth of 64 ± x km. The minimum depth of these arched zones as the possible mechanism magma generation obtained for the Nigerian of the generation of the magma which formed Tertiary basalts is of about 102 ± x km, using the Tertiary to Recent olivine-basalt-trachyte the sarne technique above and the Makanjuola's association in Nigeria and Cameroun. This (1970) analytical results for some nodules mechanism was originally advocated by Bailey included in those basalts. This suggests that the (1964) as a possible tectonic control of aIkaline phenomena which allowed the formation of these magmatism and could be the case of the Tertiary two magmas, in Rio Grande do Norte and Ni• basaltic Tocks of Rio Grande do Norte and Pa• geria, although similar, were completely inde• ralba in Brazil, wherein a possible compressional pendent. area was formed in the early stages of the On the other hand, a continent as huge as drift. This basaltic activity in Brazil is then South America undergoing an important displa• assumed to be an event correlated to the cement as has happened since the beginning of emplacement of the alkali basaltic province of the drift, could suffer many interna1 readjust• Nigeria and Carneron which is restricted to the ments which could generate or open pre-existing shield affected by the Pan-African thermo• deep fractures through which magma could rise. tectonic reactivation. Probably the Tertiary A similar phenomenon has been observed in the

Án. Ácad. bras. Ci~nc., (1976), 48, (Suplemento) THE POST-PALEOZOIC VOLCANISM OF NORTHEAST BRAZIL 309

African plate wherein extensive rifting yielded The chemical analyses of the basalts show basaltic magma to come up in the eastern portion low Si02 content, a very high Fe20S ratio of it. (about 2.7), and a significantly large amount of The idea of readjustment inside of the South H20-, which is locked up in the secondary mine• American plate by either pressure release oi raIs and zeolites filling cavities. The basalt arched zones or continental horizontal displa• cropping out at Engenho Boa Vista has a higher cement or both seems to be more reasonable concentration of Si02 as well as FeO. Usually than that of a «hot-spot». The type and small these basalts show a high percentage of iron oxide mineraIs in the groundmass. volume of magma poured out, which are not typical of hot-spots, seem to considerably dimi• Andesite has been found at Engenho Algo• nish this possibility if not to rule it out com• doais and Camela. The phenocrysts of plagio• pletely. clase have very well defined z~ning, indicating The difference observed in the available that the plagioclase wasfractionated during the radiometric ages suggests that the emplacement last stages of the uprise of ~hat magma. The of the Tertiary suite happened in a relatively ratio Fe20S/FeO in this rock shows an oxidating short period of time. This is reinforced by the state of its Fe-oxides. lack of E-W chains which would probably be The rhyolite of Algodoais and the granite formed assuming the westward displacement of of Gaibu are quite similar in both chemistry and the South American plate. mineralogy. They are believed to be cogenetic and their trace elements have shown a promi• THE CRETACEOUS F1SSURAL VOLCANISM nent similarity. OF THE MAGMATIC PROVINCE OF CABO DE SANTO AGOSTINHO AND ITS Andesites and related calc--alkaline rocks RELATIONSHlPS WlTH THE ASCENCION of the calc-alkaline suite are quite uncommon in PLUME ACTIVITY the eastern parte of South America. In the Plate Tectonics theory calc-alkaline suites are The magmatic province of Cabo, Sirinhaem, frequently found in regions cIose to subduction Ponte dos Carvalhos and Ipojuca has been zones, either on continental margins or in island• known for a long time, located in an area ares. This kind of picture does not seem to whose northern limit lies about 20 km south prevail in eastern South America where no evi• of Recife, Pernambuco. Dikes, plugs, sills and dence for a subduction zone has been found. laccoliths of andesite, rhyolite and trachyte Therefore, another mechanism must be invoked which appear cutting sediments of the Cabo in order to explain the emplacement of those Formation, and basaltic flows are the main volcanics. members of this suite. Dacites have not been The final continental detachment by trans• found. form fault motion happened between 110 to 90 The sediments of the Cabo Formation, re• m .y. The ages of the rocks in this province presented by conglomerate, arkose and claystone show that Brazil was very cIose to the Mid• are of probable Cretaceous age. Some of the Atlantic ridge when eruptions first started. li volcanic rocks of this province have been dated Brazil is brought back to its original position by Vandoros and others (1966) indicating a before the drift, the magmatic province of Cabo range of 85 to 100 m. y . among the basalts, de Santo Agostinho lies approximately where rhyolites, trachytes and granite. The Tiriri Ascencion Island is. This strongly suggests that basalt which constitutes the main basaltic flow Ascencion plume might have been active at is 94 m .y. in age. A similar age was deter• that time causing tensional fractures in the mined for the rhyolite of Algodoais which lithosphere above. These fractures probably appears to be slightly older (99 m .y .). The reached the mantle and as a result, tholeiitic granite of Santo Agostinho Cape in Gaibu Beach, magma began rising and was trapped in the crust for some time. a subvolcanic pluton considered as the only cratogenic granite in Brazil, with an age of The heat coming from these trapped basaltic around 90 Ín. y ., is thought to be similar to liquids plus the heat generated by radioactive those occurring in Nigeria and Cameroun (AI• decay in the crust, provided the 'heat necessary meida, 1969). The trachytes are younger as to partiaUy melt the crust. This melting gave shown by the Itapoama trachyte whose age is rise to granitic liquids which were intruded around 85 m.y. along pre-existing fractures, crystallizing

An. Acad. bra8. Ci/lnc., (1976), 48, (Suplemento) 310 ALCIDES NóBREGA SIAL rhyolites at the surface and granite at shallow of the basalt move in the direetion of an ande• depth. This kind of mechanism would provide an sitie eomposition. explanation for the earlier emplacement of some A separate origin for the rhyolites and rhyolites (99 m.y. at Algodoais). granite in eontrast to the differentiation model Morgan (1971) and Rhodes (1971) relate the outlined above eould be tested by determining Nigerian Mesozoic granites to a mantle plume Sr87/Sr86 of this material. However, as these whose present day position is at Ascencion ratios are not available, the statement above 1sland. According to Almeida (1969) there is beeomes only a supposition. a striking petrographic similarity between the granite of Cabo de Santo Agostinho and these ACKNOWLEDGEMENTS Nigerian granites. This reinforces the suppo• sition of the consanguinity of the forming This work is part of a PhI? Dissertation magmas. submitted to the University of California, Davis, The basaltic magma in the magmatic pro• USA, 1974. The author is indebted to Prof. 1an vince of Cabo de Santo Agostinho cutting sedi• MaeGregol' for his guidanee and Profs. Cordell ments whose water content varied from plaee Durrell and Eldridge Moores for the eontinuous to plaee, erystallized under a high oxygen fugaeity assistanee. This researeh was partially supported resulting in a high oxidation of the Fe-oxides by the Brazilian National Research Council (magnetite-hematite buffer). Aeeording to (T. C. 16016) and grants from the Geologieal Osborn (1959) erystallization under high P02 Soeiety of Ameriea (1832-74), Society of the in a buffered system makes the bulk eompositioiJ. Sigma Xi, and the University of California.

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An. Acad. bras. Cillnc., (1976), 48, (Suplemento)