Petrology of the alkaline rocks of the Macau Volcanic Field, NE Brazil

Abstract

The Macau Volcanic Field (MVF) in the Borborema Province, NE Brazil, contains multiple centres of volcanic activity of Early to Late Cenozoic ages. We present element and Sr–Nd–Pb isotope geochemical data for four of the few most prominent basalt types of this volcanic field: Serrote Preto-type, Serra Aguda-type, Pico do Cabugi-type and Serra Preta-type, in order to assess their magmatic history from source to crystallization and the evolution of the mantle beneath the Borborema Province. The basalts are basically sodic nephelinitic–basanitic–alkali olivine basalts enriched in LILE and in Nb–Ta. The Serra Preta, Cabugi and Serra Aguda types demonstrate compositions close to primitive characteristics with 10%<MgO<15wt.% and 200ppm<Ni<500ppm, and experienced limited fractional crystallization of olivine–clinopyroxene–plagioclase–oxides with negligible wall-rock assimilation. Rb/Sr and Ba/Rb constraints support the generation of SiO2-undersaturated magmas from mantle melting of amphibole-bearing peridotites with minor phlogopite. The source for the basanites and alkali basalts is estimated to be a garnet-bearing domain around the lithosphere–asthenosphere boundary (80–93km deep), while the nephelinites are derived from the adiabatic asthenosphere at ~105km with temperatures of ~1480°C. Their incompatible trace element patterns and Sr–Nd–Pb isotopic compositions are similar to FOZO and EM-type OIB magmas. From the comparison of data with those of the Ceará-Mirim dyke swarm we propose that there is a ubiquitous FOZO reservoir in the SCLM beneath the Borborema Province. This FOZO signature characterized the upwelling asthenosphere during the lithospheric extension and thinning at the opening of the Equatorial Atlantic and is clearly represented in the Mesozoic olivine tholeiites of Ceará-Mirim. The upwelled asthenosphere cooled as a rigid SCLM since the Cretaceous and has preserved its FOZO signature evident in the Macau Cenozoic basalts. The EM signatures in MVF lavas probably represent a deep component derived from metasomatizing fluid–melts that have interacted with an originally old OIB-type mantle to produce metasomatic minerals (such as K-rich amphibole, mica) with highly incompatible trace elements contents which enhanced the generation of alkaline magmas through mantle melting.