Acid magma generation in the Paraná Magmatic Province, Piraju-Ourinhos region, state of São Paulo: a contribution of isotopic geochemistry end of trace elements in rocks end minerals

Author(s)

Vivian Azor de Freitas

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2009-05-12

Examining board members

Valdecir de Assis Janasi; Leila Soares Marques; Eleonora Maria Gouvêa Vasconcellos

• Advisor: Valdecir de Assis Janasi

Abstract

Dacitic rocks from Piraju-Ourinhos, State of São Paulo, outcrop for ca. 60 km along the Paranapanema River valley, and constitute the northernmost expositions of the Parana Magmatic Province acid magmatism. They rest directly over the Botucatu Formation eolic sandstones and are recovered by Pitanga-type high Ti basalt flows. Basalt dykes and sills that occur in the region show different chemical relationship and can be compared to the Paranapanema, Urubici and Pitanga basalt types. Chemically classified as trachydacites, these rocks are porphyritic with 5 to 15% plagioclase, clinopyroxene, opaque minerals and apatite phenocrysts. Aphanitic to phaneritic groundmass is composed of glass, plagioclase, clinopyroxene, quartz and alkali feldspar. Glass can make up 10 to 90% of the original groundmass and is usually devitrified, being recognized by relict quenching textures. Vesicles and amygdalas are abundants in such trachydacites varieties and can achieve to 40% of rock. In this work, the age of the acid magmatism was obtained by U-Pb TIMS in baddeleyite and zircon concentrates. The value [134.4 0,9 (2\'sigma\')], is more accurate and precise compared with ages previously obtained in the trachydacites from region [133 -134 ± 6 Ma (K-Ar); 128.7 ± 1 Ma (\'ANTPOT.40 Ar\'/\'39 ANTPOT.Ar\')], and within the short age interval currently admitted for the Paraná volcanism climax. Initial \'ANTPOT.87 Sr\'/\' ANTPOT.86 Sr\' ratios of the trachydacites (0.7078 to 0.7080) are slightly more radiogenic than those of associated basalts (0.7056 to 0.7068), whereas \'épsilon\'\'Nd IND.134\' are more negative (~ -5 versus -4). These differences suggest that, although the basalts must have a genetic link with the acid magmatism of region, some crustal contribution may exist in the acid magmas. Initial \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\' obtained by LA-ICPMS show identical values for the groundmass and plagioclase and apatite phenocrysts (~0.7077), consistent with equilibrium crystallization. Most of the clinopyroxene phenocrysts have initial \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\' different from the groundmass (usually smaller; 0.7045-0.7071; only one crystal is more radiogenic, 0.7084). Together with two the plagioclase phenocrysts (with initial \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\'= 0.7083 and 0.7074), they did not crystallize in equilibrium to the groundmass, and are likely antecrysts. Geochemical modelling using major elements and both compatible and incompatible trace-elements show that it is possible to obtain the acid magmas after 60 to 80% fractional crystalization of a Pitanga-type basalt. The main obstacle for such model would be the wide compositional silic gap between acid and basic magmas.; however, this gap could result from physical limitations to crystal-liquid separation at intermediate compositions and to the extraction by filter pressing of more evolved residual liquids. The small differences in Sr-Nd isotopic signature between acid and basic rocks can be explained by an AFC model, with 60% of basaltic magma crystalization plus ~10-30% assimilation of a granitic liquid derived from the pre- Cambrian basament. On the other hand, a model of remelting of basalt underplates generates acid magmas with higher contents of compatible elements contents (Ni, Cr, V) and therefore would demand some fractionation to achieve the observed trachydacite compositions. (AU)

• FAPESP's process: 06/05917-6 - Generation of acid magmas in Paraná Magmatic Province: a contribution of isotopic geochemistry and trace elements in minerals by LA-ICPMS
• Grantee: Vivian Azor de Freitas
• Support Opportunities: Scholarships in Brazil - Master