Levantamentos Gamaespectrométricos em Granitos Diferenciados. II: O Exemplo do Granito Joaquim Murtinho, Complexo Granítico Cunhaporanga, Paraná
Detailed mapping at the NW corner of the large Neoproterozoic Cunhaporanga Granitic Complex (CGC), Paraná state, SE Brazil, redefined the Joaquim Murtinho Granite (JMG), a late intrusion in CGC with an exposed area of about 10 km², made up mainly by evolved"alaskites" (alkali-feldspar leuc...
| Autores: | , , , , , |
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| Tipo de recurso: | artículo |
| Estado: | Versión publicada |
| Fecha de publicación: | 2009 |
| País: | Brasil |
| Institución: | Universidade de São Paulo (USP) |
| Repositorio: | Geologia USP. Série Científica (Online) |
| Idioma: | portugués |
| OAI Identifier: | oai:revistas.usp.br:article/27456 |
| Acceso en línea: | https://revistas.usp.br/guspsc/article/view/27456 |
| Access Level: | acceso abierto |
| Palabra clave: | Cunhaporanga Granitic Complex Joaquim Murtinho Granite Alaskitic granites Gamma-spectrometric terrestrial survey U-Th minerals granites Behaviour of K-Th-U weathering Complexo Granítico Cunhaporanga Granito Joaquim Murtinho Granitos alasquíticos Minerais de U-Th granitos Levantamento gamaespectrométrico terrestre Comportamento do U-Th-K intemperismo |
| Sumario: | Detailed mapping at the NW corner of the large Neoproterozoic Cunhaporanga Granitic Complex (CGC), Paraná state, SE Brazil, redefined the Joaquim Murtinho Granite (JMG), a late intrusion in CGC with an exposed area of about 10 km², made up mainly by evolved"alaskites" (alkali-feldspar leucogranites). This unit is in tectonic contact with the Neoproterozoic-Eocambrian volcano-sedimentary Castro Group, to the W, and is intrusive into other less evolved granitic units of the CGC to the E. Petrographically, JMG shows mainly mesoperthite and quartz, with subordinate amounts of altered micas and some accessory phases, mainly zircon. The equi to inequigranular granites are usually deformed with cataclastic textures, are often brecciated, and may have miarolitic structures. Formation of late albite, sericite, carbonate and hematite was caused by deuteric and hydrothermal alteration. A gamma-ray spectrometric survey at 231 stations which measured total counts (TC), Ueq K%, eU ppm and eTh ppm was used to construct several direct and derived maps. Compared to neighboring units the JMG has significant anomalies, especially in the TC, %K, eTh and eU maps, although the differences are less obvious in some derived maps. These evolved granites are enriched in these three elements. Geochemical behavior of K, Th and U is used to analyse the results observed in maps. Enhanced weathering under a subtropical climate with moderate to high average temperatures and heavy rainfall affects mainly feldspars and biotite, and may also destabilize most U and Th-bearing accessory phases. Th is most likely retained in restite minerals in soils, being relatively immobile, while part of U may migrate as uranyl ion in oxidizing media. K is especially affected by feldspar alteration to K-free clays (mainly kaolinite), and may be completely leached. Gamma-ray spectrometric methods are valid tools to study facies in granitic rocks, especially in those that are enriched in K, Th and U. |
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