Ernesto, M.

Raposo, M.I.B.

English

Cadernos Espinosanos (E-Journal)

Bol.IG-USP, Publ.Esp., 10:85-89, 1991 PALEOMAGNETISM OF THE DYKES OF THE PONTA GROSSA ARCH INTRODUCTION M./.B.Raposo,,2 M.Ernesto' Tha Ponta Grossa Arch, on tha eastam bordar of tha Paraná Basin, correeponds to a tectonic featura which was active at least since the Devonian. However, during the Jurassic and Early Cretaceous /t was upliftted and took on /ts present configuration (ALMEIDA, 1986). The northem and southem lim/ts of the arch are marked by the Guapiara and Rio Piquiri magnetic lineaments, respectively (FERREIRA, 1982; Fig. 1). One of the most impressive Phanerozoic mafic dyke swarms of southeast Brazil occurs in the Ponta Grossa Arch (OUVEIRA & MONTES, 1984; SIAL et aI., 1987). The dykes trend preferentially NW and are parallel or sub-parallel to the arch axis. A paJeomagnetic study of the Ponta Grossa dykes is being carried out in order to establish the time relationships between these intrusive rocks and the volcanic Serra Geral Formatlon of the Paraná Basin and also to investigate the tectono-magmatic event responsible for the dykes. MAGNETIZATION DIRECTIONS About 270 samples from 84 dykes rnainly from the westem part of the arch were analyzed (RAPOSO & ERNESTO, 1989). Magnetization directions are of normal and reverse polar/ties 1 Instituto AstronOmico e Geofísico, Universidade de São Paulo, Caixa Postal 30627, 01051 São Paulo, SP, Brazil. 2Bolsista da FAPESP. ~ 48° 51° v v v \ t~: ~; ~:~32. ~lvvvvv v v v L 52" 51° I 58° 54" 50° 48° 49° v v v T 1 50· (, " \ ~ ~ " '" '<;f : (,~ O o $OK,.. -=-=-23° 24° 25° 26° 480 ' 27° Figure 1 -Generalized geologic map of the Paraná Basin. The Ponta Grossa Arch is shown in detai!: 1 = crystalline basement rocks; 2 = pre-volcanic sediments (malnly Paleozoic); 3 = basic/intermediate Serra Geral Formation flows; 4 = Palmas type acid flows; 5 = Chapec6 type acld flows; 6 = post-volcanic sediments (mainly Upper Cretaceous); 7 = arch structure; 8 = tectonic and/or magnetic lineaments. indlcatlng that at least two episodes of intrusion took place. Comparision of paleomagnetlc data trom the Serra Geral volcanic rocks (ERNESTO & PACCA, 1989) with that trom the Ponta Grossa dykes points to the concluslon that the intrusive rocks are younger than the volcanics. However. the dykes could be contemporaneous with a second phase of Paraná volcanlsm. as recognlzOO by ERNESTO & PACCA (1989). representOO by baslc and acid (Chapec6 type) flow sequences in the central region of the basln. Hence. the paleomagnetic data do not support the presumptlon that the Ponta Grossa Arch structure provldOO the feeclers for the Paraná Basln flows. as reportOO in most of the papars on the subject. with the exception of those by PICCIRILLO et ai. (1988. 1989). ERNESTO et aI.. (1989) and ERNESTO & PACCA (1989). The paleomagnetic data also suggest that the dyke activity might have been controllOO by tectonlc IIneaments within the Arch. However. in order to better Identify thls influence, data are neeclOO trom the eastem portion of the Arch. ANISOTROPY OF THE MAGNETIC SUSCEPTIBIUTY KnowlOOge as to how existing fractures are fOO to give rise to the formation of a dyke swarm provldes an important constraint for definlng the relative position of the magmatic sources. Dependlng on the location of the magma source, flow may be either vertical or horizontal within the fractures. The anisotropy of the magnetic susceptibility (AMS) refers to the variation in susceptlbllity of magnetic minerais according to their principal axes. designatOO as maximum, intermOOiate and minimum susceptibility axes. In the case of dykes. the shape and distribution of the magnetic minerais are probably controllOO by the development of flux planes or \ines during the fracture feOOlng. Thus, in vertical fluxes maxlmum susceptibility axes should be alignOO in vertical planes whereas in horizontal fluxes they would be expectOO to define horizontal planes. Prellminary data on AMS of the Ponta Grossa dykes indicate that most of the maxlmum susceptibllity axes lie within horizontal planes, although other planes also existo ACKNO~DGEMENTS 87 We wish to thank FAPESP for financiai support for this research. REFERENCES ALMEIDA, F.F.M. (1986) Distribuição regional e relações tectônicas do rnagmatismo pós-Paleozóico no Brasil. Revista Brasileira de Geociências, 16(4):325-349. ERNESTO, M.; PACCA, I.G.; HIODO, F.Y.; NARDY, AJ.R. (1989) Paleornagnetism of the Mesozoic Serra Geral Forrnation, Southern Brazil. Phys.Earth Planet.Jnt. (submitted). ERNESTO, M. & PACCA, I.G. (1989) Paleornagnetism of the Paraná Basin flood volcanics, southern Brazil. In: PICCIRILLO, E.M. & MELFI, AJ. (00.) The Mesozoic flood volcanism from the Paraná Basin: petrogenetic and geophysical aspects. São Paulo, IAG. p.229-255. FERREIRA, F.J.F. (1982) Alinhamentos estruturais e magnéticos da região centro-oriental da Bacia do Paraná e seu significado tectônico. In: INSTITUTO DE PESQUISAS TECNOLÓGICAS DO ESTADO DE SÃO PAULO. Geologia da Bacia do Paraná, São Paulo, IPT. p.143-166. OUVEIRA, E.P. & MONTES, M.L. (1984) Os enxames de dique rnáficos do Brasil. In: CONGRESSO BRASILEIRO DE GEOLOGIA, 34., Rio de Janeiro, 1984. Anais. Rio de Janeiro, SBG. V.9, p.4137-4154. PICCIRILLO, E.M.; MELFI, AJ.; COMIN-CHIARAMONTI, P.; BEWENI, G. ; ERNESTO, M.; MARQUES, L.S.; NARDY, AJ.R.; PACCA, I.G.; ROISEMBERG, A.; STOLFA, D. (1988) Continental flood volcanism from the Paraná Basin (Brazil). In: MACDOUGAL, J.D. (00.) Continental flood basalts. Dorcrecht, Kluwer Academic. p.195-238. PICCIRILLO, E.M.; COMIN-CHIARAMONTI, P.; MELFI, AJ.: STOLFA, D.; BELUENI, G.; MARQUES, LS.; NARDY, AJ.R.; PINESE, J.P.P.; RAPOSO, M.I.B.; ROISENBERG, A. (1989) Petrochemical of continental flood basalt-rhyolite suites and relatOO intrusives trom the Paraná Basin (Brazil). In: PICCIRILLO, E.M. & MELFI, AJ. (ed.) The Mesozoic 1100d volcanism from the Paraná Basin: petrogenetic and geophysical aspects. São Paulo, IAG. p. 1 07-156. 88 RAPOSO, M.I.B. & ERNESTO, M. (1989) Rochas intrusivas básicas do arco de Ponta Grossa: resultados paleomagnéticos preliminares. Revista Brasileira de Geociências, 19(3):392-400. 81AL, A.N.; OUVEIRA, E.P.; CHOUDHURI, A. (1987) Mafic dyke swarms of Brazil. In: HALL8, H.C. & FAHRIG, W.F. (ed.) Mafic dyke swarms. 8t. John's, GAC. p.467-483. (Geological Association of Canada Special Paper, 34) 89 

Paleomagnetism of the dykes of the Ponta Grossa arch

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Paleomagnetism of the dykes of the Ponta Grossa arch

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