Petrogenesis and Tectonics of Eocene–Oligocene Phonolites of Mecejana, Ceará, NE Brazil: the Role of the Fernando de Noronha Fracture Zone, Equatorial Atlantic

Abstract

Located on the western continental tip of the Fernando de Noronha transform fracture (FNTF), northeastern Brazil, the Eocene-Oligocene Mecejana Phonolitic Province (MPP) was subjected to a multi-methodological study, comprising field and microscopic characterization, mineral and whole rock chemistry, and Sr-Nd isotopes. These rocks intrude the Precambrian crystalline basement, forming mainly plugs oriented in the NE-SW direction. They are composed of alkali basalt, phono-tephrite, tephri-phonolite, and phonolite with phenocrysts of nepheline and sanidine, and minor diopside, aegirine-augite, aegirine, kaersutite, biotite and titanite. Chemically, they are sodic (Na2O/K2O > 1) and display LREE enrichment, concave upward or flat HREE patterns, weak to strong Eu anomaly, and high contents of Nb, Ta and Zr. They delineate two chemical groups, the less evolved one comprising basalt to tephri-phonolites and the more evolved constituted by phonolites. Oxide and trace element modeling points out to an evolution by fractional crystallization, leaving olivine-rich gabbro and foidolite / nepheline syenite cumulates, respectively. The analyzed samples have Sr-Nd isotopic ratios akin to OIB rocks (positive εNd of +3.0 to +0.9 at 30 Ma) and low to high 87Sr/86Sri (at 30 Ma) of 0.70264-0.70955. The data suggest a three stages evolution model for the MPP: (1) <5% partial melting of a metasomatized phlogopite-amphibole-garnet peridotite generates an alkali basalt melt; (2) polybaric differentiation of this melt at crustal level (~570-220 MPa, 1040-645 oC) leads firstly to phono-tephrites and tephri-phonolites and then to (3) foidolite and nepheline syenite. Petrographic, chemical, and isotopic characteristics of the MPP and similar rocks of the Fernando de Noronha islands suggest the control by the FNTF and reactivation of NE-SW trending deep-rooted Precambrian faults allowing channeling and emplacement of the alkaline magmas.

New detrital petrographic and thermochronologic constraints on the Late Cretaceous–Neogene erosional history of the equatorial margin of Brazil: Implications for the surface evolution of a complex rift margin

“…In the modern source area of the Ceará, Barrerinhas and Pará‐Maranhão basins, there are very few AFT ages that are Oligocene in age (Figures 1 and 3). Eocene and Oligocene intrusive and effusive magmatic phenomena affected the Ceará Basin and the structural high that separates it from the Potiguar Basin (Mizusaki et al, 2002; Souza et al, 2022). Thus, it remains unclear whether the 29‐Ma‐old detrital age population can be referred to local partial annealing due to high heat flow related to magmatism, a volcanic source or a source exhuming in the late Oligocene (Japsen et al, 2012; Jelinek et al, 2014; Morais Neto et al, 2009).…”

“…44 Ma) and in the Oligocene (ca. 32 Ma), volcanic intrusive and effusive activity resulted in the emplacement of alkali basaltsphonolites and microgabbros (Mizusaki et al, 2002;Souza et al, 2022). Pará-Marañhao Basin (after Soares et al, 2007); of the Barreirinhas basin (after Trosdtorf Jr. et al, 2007); and of the Ceará basin (after Condé et al, 2007…”

“…44 Ma) and in the Oligocene (ca. 32 Ma), volcanic intrusive and effusive activity resulted in the emplacement of alkali basalts–phonolites and microgabbros (Mizusaki et al, 2002; Souza et al, 2022).…”

Plug Frei Martinho: Magmatismo Cenozoico Alcalino No Norte Do Estado Da Paraíba

“…A Província Borborema (PB) é uma entidade geológica que fez parte do supercontinente Gondwana Ocidental, sua configuração atual é resultado de eventos colisionais diacrônicos relacionados à orogenia Brasiliana/ Pan-Africana (p.ex. Brito Neves et al, 2000;Van Schmus et al, 2008). O arcabouço estrutural desta província é controlado por zonas de cisalhamento de escala continental, que representam grandes fronteiras entre domínios geológicos distintos e permitiram a ascensão e colocação de magmas graníticos.…”