Geologia e petrogênese do vulcanismo alcalino miocênico da região de Serra Preta, Pedro Avelino/RN, NE do Brasil
Cenozoic intracontinental magmatism in Rio Grande do Norte occurs as small plugs, necks, dikes and flows of basaltic composition. These rocks compose the northwesternmost tip of the Macau-Queimadas Trend, a ca. 200 km long tectonomagmatic province. Part of this volcanism also intrudes the sedimentar...
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| Formato: | Dissertação |
| Idioma: | pt_BR |
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Universidade Federal do Rio Grande do Norte
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| Endereço do item: | https://repositorio.ufrn.br/handle/123456789/43127 |
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| Resumo: | Cenozoic intracontinental magmatism in Rio Grande do Norte occurs as small plugs,
necks, dikes and flows of basaltic composition. These rocks compose the
northwesternmost tip of the Macau-Queimadas Trend, a ca. 200 km long tectonomagmatic province. Part of this volcanism also intrudes the sedimentary rocks of the
Potiguar Basin, leading to thermal contact aureoles that contribute to hydrocarbon
maturation and remobilization. In the eastern portion of Pedro Avelino/RN, three
igneous bodies with different geometries intrude Cretaceous sandstones and
carbonates of the Potiguar Basin. The aim of this research is to characterize the
petrogenesis and the emplacement mechanism of these rocks through field
relationships, petrography, and whole-rock geochemistry (major, trace and rare earth
elements). The surface expression of the larger body, namely Serra Preta, was
previously interpreted as plateau-type. However, reinterpretation of field data and of
remote sensing images suggest the emplacement of NNW-SSE and NE-SW dike sets
is tectonically controlled. In terms of petrography, these rocks are predominantly
volcanic to subvolcanic olivine basalts, as well as late differentiate nepheline
microgabbros. The groundmass is crypto- to microcrystalline and is composed of
microlithes of augite, plagioclase (~An50), opaques and interstitial glass. Whole-rock
geochemistry show that these rocks are alkaline and relatively primitive, with high Mg#
(65-79), MgO (12-19 wt. %) and Ni (>200 ppm), low SiO2 (39-43%) and have normative
nepheline + olivine ± leucite. They are chemically classified as strongly silicaundersaturated basanites, melanephelinites and olivine basalts. The distribution of the
samples on AFM and binary plots indicate the fractionation of Fo-rich olivine and Caclinopyroxene. Chondrite normalized diagrams show enrichment of light rare earth
elements (LaN/YbN ~36-18) and enriched in Large Ion Lithophile Elements (LILE), with
negative anomalies of Rb, K and Hf. Geochemical modeling indicates that these rocks
are the product of low degrees of partial melting (<10%) of a phlogopite-amphibolebearing garnet-lherzolite source enriched in LILE. Fractional crystallization of ca. 80%
is responsible for the most evolved compositions, with a cumulate composed of
clinopyroxene, nepheline, magnetite, apatite, perovskite, leucite, and Ca-plagioclase.
Miocene local stresses were responsible for the reactivation of brittle structures that
acted as conduits for those mantle-derived magmas. |
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