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

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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.
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