Geology, petrography and geochemistry of the acid volcanism of the Paraná Magmatic Province in the Piraju-Ourinhos region, SE Brazil

A succession of trachydacitic lava flows over 150 m thick constitutes the lowermost portion of the Serra Geral Fm. along the Paranapanema River valley in the Piraju-Ourinhos region, State of São Paulo. The flows have a well-defined zonality: the central portion is made up of grey porphyritic dacites with low glass and vesicle proportion which grade towards the contacts to strongly vesiculated varieties with glassy groundmass which, when in contact with the Botucatu Fm. basal sandstones or with larger bodies of inter-flow sandstone, show a typical chocolate-brown color. The observed compositional variation is small, and in part a reflection of secondary alteration. Independent of texture, mode of occurrence and stratigraphic position, the rocks show similar contents of SiO2 (65-67 wt%) and all other major oxides, and a ferrous (mg#= 20-27) and metaluminous A/CNK= 0.85-0.95) character; alteration associated to vesicle infilling and color change to chocolate-brown results in lower mg# and higher A/CNK. Mineral and whole rock chemical data indicate high crystallization temperatures around 1000oC (pyroxene thermometry; apatite saturation), consistent with the predominant mode of occurrence as flows.

[1]  P. Reis,et al.  Geochemistry of the eocretacic basalt magmatism in the Piraju-Ourinhos region, SE Brazil, and implications to the stratigraphy of the Serra Geral Formation , 2007 .

[2]  K. Putirka.  Igneous thermometers and barometers based on plagioclase + liquid equilibria: Tests of some existing models and new calibrations , 2005 .

[3]  Â. Min,et al.  Paleomagnetism, geochemistry and dating of the North-eastern Paraná Magmatic Province: tectonic implications , 1999 .

[4]  Tarcísio José Montanheiro.  Prospecção e Caracterização de Pozolanas na Bacia do Paraná, Estado de São Paulo , 1999 .

[5]  C. T. Correia,et al.  DEVELOPMENT OF A FUED GLASS DISC XRF FACILITY AND COMPARISON WITH THE PRESSED POWDER PELLET TECHNIQUE AT INSTITUTO DE GEOCIENCIAS, SAO PAULO UNIVERSITY , 1999 .

[6]  E. Ruberti.  Petrologia e geoquímica das suítes carbonatíticas do Mato Preto (PR) e da Barra de Itapirapuã (PR-SP) , 1998 .

[7]  C. Harris,et al.  Crustal Origin for the Paraná Rhyolites: Discussion of ‘Description and Petrogenesis of the Paraná Rhyolites, Southern Brazil’ by Garland et al. (1995) , 1997 .

[8]  K. Stewart,et al.  3-D, 40Ar? 39Ar geochronology in the Paran continental flood basalt province , 1996 .

[9]  A. Thompson.  Fertility of crustal rocks during anatexis , 1996, Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

[10]  C. Hawkesworth,et al.  Description and Petrogenesis of the Paraná Rzhyolites, Southern Brazil , 1995 .

[11]  H. Ulbrich,et al.  A correção do drift instrumental em ICP-AES com espectrômetro seqüencial e a análise de elementos maiores, menores e traços em rochas , 1995 .

[12]  Nobuo Morimoto,et al.  Nomenclature of Pyroxenes , 1988, Mineralogical Magazine.

[13]  A. Melfi,et al.  Petrogenetic Aspects of Acid and Basaltic Lavas from the Paraná Plateau (Brazil): Geological, Mineralogical and Petrochemical Relationships , 1986 .

[14]  R. W. Le Maitre,et al.  A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram , 1986 .

[15]  M. Ernesto,et al.  Flood Basalt to Rhyolite Suites in the Southern Paranà Plateau (Brazil): Palaeomagnetism, Petrogenesis and Geodynamic Implications , 1984 .

[16]  E. Watson,et al.  The behavior of apatite during crustal anatexis: Equilibrium and kinetic considerations , 1984 .

[17]  T. M. Harrison,et al.  Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types , 1983 .