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

Abstract

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 Light Rare Earth Elements (LREE) enrichment, concave upward or flat Heavy Rare Earth Elements (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 ocean island basalt (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-stage 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°C) 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.