Banca de QUALIFICAÇÃO: JOYCE LORENA OLIVEIRA

Uma banca de QUALIFICAÇÃO de MESTRADO foi cadastrada pelo programa.
STUDENT : JOYCE LORENA OLIVEIRA
DATE: 19/11/2020
TIME: 11:00
LOCAL: Videoconferencia - https://cutt.ly/6fcqwHK
TITLE:

Geology and petrogenesis of the miocenic alcaline magmatism of Pedro Avelino / RN, NE Brazil

KEY WORDS:

intraplate volcanism; alkaline magma; Miocene; dyke system; NE Brazil.

PAGES: 30
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

The most recent intracontinental volcanism in Brazil occurs as small volumes of mafic magmas intruding the Borborema Province. Part of this volcanism also affects Cretaceous sedimentary rocks, generating thermal contact aureoles and contributing for remobilization and maturation of hydrocarbons. The Serra Preta body, located to the East of Pedro Avelino/RN, is one example of this volcanism during the Miocene, crosscutting sandstones and calcareous of the Potiguar Basin. Due to its linear geometry, unlike the circular shape of other examples in the region, interpretations regarding its geometry diverge in the literature. In this way, the present work addresses the study of these rocks in detail, with emphasis on its geometry, petrography and petrogenesis. The surface expression of the Serra Preta body is explained by a tectonically controlled magmatic emplacement generating volcanic conduits as dyke systems with a main NNW-SSE direction and a NE-SW secondary one. The rocks studied show volcanic to hypabyssal features, being constituted petrographically by olivine basalts (the most representative and with olivine and clinopyroxene xenocrysts and microphencrysts) and microporphyritic nepheline diabases, the last one as late differentiates. The basaltic matrix is composed by augite, plagioclase (~An₅₀) and opaques (magnetite and ilmenite) microlithes, as well interstitial glass. Late features include amygdales and interfingered veins filled with carbonate, apatite, zeolite, feldspathoid, biotite and clinopyroxene. In terms of geochemistry, these rocks correspond to relatively primitive (#mg = 65-79) sodic alkaline magmas with low SiO₂ (39-43%) and high MgO (12-19%) contents, usually with normative nepheline + olivine ± leucite. Harker and AFM diagrams indicate fractionation controlled mainly by fractionation of forsterite-rich olivine during magma differentiation. They present enrichment in light rare earth elements (La_N/Yb_N ~ 36-18), absence of europium anomaly, and enrichment in the most incompatible lithophile elements, with negative anomalies of K and Hf. The different textural facies found suggest the final cooling of a strongly silica undersaturated alkaline basaltic magma in shallow crustal levels; generating basanites, melanephelinites and transitional basalts (olivine basalts). The emplacement mode interpreted as dykes is opposed to previous interpretations which defined part of the Serra Preta as basaltic flows, was favored by the tectonic space opened due to local tectonic stress in the Miocene, which reactivated brittle structures in the hosting rocks. 

BANKING MEMBERS:
Externo à Instituição - FRANCISCO VALDIR SILVEIRA
Interno - 2042352 - FREDERICO CASTRO JOBIM VILALVA
Presidente - 350630 - ZORANO SERGIO DE SOUZA