Boletim Da Sociedade Brasileira De Geologia

BOLETIM DA SOCIEDADE BRASILEIRA DE GEOLOGIA VOLUME 17 • DEZEMBRO DE 1968 • NuMERO 1 fNDICE Radiometric age determinations from Northern Brazil Por Fernando Flavia Marques de Almeida , Geraldo Conrado Melcher, Umberto G. Cordani, Koji Kawashita e Paul Vandoros 3 Notas sabre a morfogenese dos derrames basalticos Por Guido Guidicini e Jayme de Oliveira Campos 15 a Paleoz6ico da Bacia de Jatoba, Pernambuco Por Paulo M. C. Barretto 29 New glacial features of the upper Paleozoic Itarare Subgroup in the State of Sao Paulo, Brazil Por A. C. Roeha-Campos, J. E. S. Farjallat e R. yoshida . 47 Itarare Subgroup (Tubarao Group) in the Mococa-Casa Branca Region - Sao Paulo State Por Paulo M. Figueiredo Filho e Lawrence A. Frakes 59 oontnbutcao ao estudo da Bacia de Rezende (Rio de Janeiro) Por Alfredo Jose Simon Bjornberq, Heinz Ebert, Paulo Milton Barbosa Landim e Neide Maria Malusa 65 Sabre a aplicaQRo de eletro-resistividade na Baixada Santista - (S . Paulo) Por Nelson Ellert , .. 77 NOTAS A posicao estratigrafica da formacao Riachuelo - (Oretaceo, Sergipe) Por Karl Beurlen . .. ... .. .. .. .. .. .. .. ... 85 Nota preliminar sabre idades radiometricas em rochas da regiao da Serra dos orgaos e vizinhancas (Leste de Minas Gerais e Estado do Rio de Janeiro) Por U. G. Cordani, J. Delhal, C. B. Gomes e D. Leaent 89 The conservation of Pwnsteadia Rigby 1963 (Over Cistella Phunstead 1958) Por J. F. Rigby 93 RADIOMETRIC AGE DETERMINATIONS FROM NORTHERN BRAZIL(*) Por F. F . M. DE ALMEIDA, G. C. MELCHER, U. G. CORDANI, K. KAWASHITA, and P. VANDOROS Centro de Pesquisas Geoeronol6gicas U.S. P. ABSTRACT work is part of a joint project of age mea surements, undertaken by the University of If the hypothes is of Contine ntal Drift is vali d . Sao Paulo and the Massachusetts Institute Precambrian and ea r ly Cambrian rocks of No rthe rn of Technology, with the objective of checking Brazil sho u ld have the sam e ages as roc ks from the the possible age correlations of basement co rresponding parts of Africa. wh en th e two conti rocks from Western Africa with the corres nents are fitted back together. To test th is ponding parts of South America, if the two suppo si ti on, a joint program of age de te rminatio ns is b eing car rie d out by t he geochronologi cal continents are fitted back together. laboratories of the Universit.y of Sao P aulo and the The present report is concerned mainly Massachusetts In stitute of T echnolo,gy. About with the results of the Potassium-argon 80 K·Ar age detenninations wer e made on sa mples measurements and an attempt is made to of basement rocks fr om Am ana, two sma ll areas interpret our results in view of the available eas t of B elem, th e r eg ion aro und Siio L uis an d man y points of t he large area that covers mo st information on the geology of Northern Bra of Northea stern Brazil. Drill core sa mp les of zil . Samples we're collected at 63 points in basem ent ro cks covered by younger sedime nts in many of the exposed basement areas of the sta tes of Para. and MaranhAo also w er e dated. Northern Brazil, and drill cores obtained from 12 Rb-Sr m easurements on so me 'O f thes e sa mples were al so made. Th e r esults indicate th at 3 localities where igneous and metamorphic the area b et ween Re cif e and Sao Luis fo rm s a rocks are overlain by younger sediments. large age p ro vince 440 to 620 m .y . ol d. Som e 77 age determinations by the K-Ar method isolated samples within this province yielded old er and 12 Rb-Sr analyses were made in the ages. up to 1900 m.y.; they se em t o corresp ond to r emobilized baseme nt rock s . laboratory at Sao Paulo. Rb-Sr m easurem ents confirm the K-Ar ages. Some results of Rb-Sr whole-rock analy with th e exc ep ti on of one sample from Ceara . Th is ses on the same samples, carried out at the rock has an age of 2.000 m. y . detennined by the Massachusetts Institute of Technology, and Rb-Sr wh ol e-rock m ethod and a 480 m .y. K-Ar age.Sam pl es fr om near Siio Luis yi elde d K-Ar the correlation of the data that have been ages around 2.200 m .y. Althou gh not enough obtained so far in our cooperative program d etailed geo logica l infonnation is availa ble, so me are being reported by. Hurley et al. (1967). of the major structural features of Northern Brazil can be compared to the large structures mapped in the cor respondi ng parts of Africa. EXPER~ENTAL METHODS INTRODUCTION The K-Ar dating techniques employed at Sao Paulo have been described in detail by The purpose of this paper is to present Amaral et al (1966). Crushed and sized the results of age determinations on basement rocks from Northern and Northeastern Bra zil, carried out at the geochronology labora * P resented at the Annual Meeting of Geo logi tory of the University of Sao Paulo. This cal Soc iet y of Am erica, nov. 1966. 4 BOL. DA SOCIEDADE BRASILEIRA DE GEOLOGIA - V. 17, Ny 1, 1968 49' 46' 4 4' 42' POTASSDUM-AIRGON AGIES IF!ROM NOIRTHIIEASTEIRN BRAZDL SA SEMENT AREAS c:::::::J 2 c::==J SEDIMENTARY COVER 17 /1 MAIN STRUCTURAL TRENDS ___.,.- FAULT ZOf:l/E 182 0 SAMPLE L.OCATION WITH REFERENC E i NUMBER (I) AND KfAr AGE (1740) I) ' c 4 80 -610 AGE INTERVAL. OF GROUP OF SAMPL.ES o 100 ZOO 100 400 ~oo Km 44' 36' Figure nv 1 samples of sepa rat ed minerals, weighti ng Potassium de terminations on 0. 3 g sam around 0,2 grams, were fused by induction ple splits were made by flame photometry heating in ultra-high vacuum systems, em using a Baird-Atomic research fl ame photo ploying Cu-CuO and Titanium for gas purifi meter wit h a Li int ernal st andar d. Only on cation . Only three whole rocks were dated in some hom blendes with very low K content this w ork. The gas released was «spiked» the analy ses were made on larger samples, for isotope dilution analysis using individual up to 1 g. aliquots (about 0.7 X 10-:; ccSTP) of pure The K 1 0 con sta nts employed in calcula Ar3B prepared in groups of about 400 (Rey t ions are : nolds and Spira, 1966). Mass spectrom etry 1 0 was carried out by the static me-thod on a At ot = 0.530 X 1(}-9y - l Ak = 0.585 X 10- Y-l Reynolds type mass spectrometer. K 40 = 1.19 X 10-2 atom %. F.F.M. de ALMEIDA, G.C. MELCHER, V.G. CORDANI, K. KAWASHITA, P. VANDOROS 5 The analytical precision of the method Bahia (49 to 57), are shown on figure 1. was discussed by Amaral et al (1966) . It This map also shows the radiometric ages of was found that the K analyses are reprodu basement drill core samples in sedimentary cible to better than 1% and that replicate areas. The results of age measurements in argon measurements agree to within 2%. the large basement area of Northeastern For most calculated ages an uncertainty of Brazil are grouped into intervals on figure 1 about ± 3'/0 may be assumed, not taking and shown in detail on figure 2. into account uncertainties of the decay cons Two K-Ar measurements were made on tants. Somewhat larger . errors may result mica schists from the Serra do Navio mang from less accurate K flame photometric anese mining district (Sample locality 1). analyses of amphiboles with less than 0.3% K , The geology of the manganese deposits was and for samples in which the air correction described by Nagell (1962) but practically is large. The estimated errors of each deter no regional geologic information on the mination are indicated on table 2. basement of Amapa is available yet. The For the Rb-Sr method the chemical values of 1.820 m.y. and 1.760 m.y. suggest procedure was essentially the same as des that this area represents a southern exten cribed by Aldrich et al (1956). Isotopic ana sion of the Guyana Shield where similar ages lyses were made on a Reynolds type mass have been reported (McConnell et al, 1964; spectrometer with 4.5 in ch radius of curvatu Priem et al, 1966). re, to which a single filament solid source was The results from points 58 (2.220 m.y.) adapted (Kawashita et al, in preparation). and 59 (1970 m.y.j are the first determina A constant of A{3 = 1.47 X 10-lIy-l and iso tions on rocks from this large basement area topic ratios for normal strontium: 84/88 = and show that at least this part was not = 0.0067, 86/88 = 0.1194, and 87/88 = 0.0845 affected by the 450-640 m.y, events which were used for calculations. ocurred in many other regions of Brazil. The main sources of error are inadequate Evidently. much more work is required to spiking, uncertainty of radiogenic strontium determine if the area west of the Tocantins determinations in samples with high normal river is a southern extension of the Guiana strontium values and laboratory contamina Shield or belongs to another isolated old tion of materials with low strontium content.

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