Petrology of continental tholeiitic magmas forming a 350-km-long Mesozoic dyke swarm in NE Brazil: Constraints of geochemical and isotopic data

The Ceara Mirim dyke swarm (northeastern Brazil) is composed of Cretaceous tholeiites with plagioclase, clinopyroxene (+/- olivine), Fe-Ti oxides and pigeonite in their groundmass. These tholeiites have been subdivided into three groups: high-Ti olivine tholeiites, evolved high-Ti tholeiites (TiO2 >= 1.5 wt.%; Ti/Y > 360), and low-Ti tholeiites (TiO2 <= 1.5 wt%; Ti/Y <= 360), with all exhibiting distinct degrees of enrichment in incompatible elements relative to Primitive Mantle. Negative Pb anomalies are found in all three groups, while Nb-Ta abundances similar to those of OIB-type magmas are found in the olivine tholeiites, with moderate to high depletions being observed, respectively, in the evolved high-Ti and low-Ti tholeiites. The low-Ti tholeiites exhibit some contamination with crustal (felsic) materials during ascent. The initial isotopic compositions of the olivine tholeiites show uniform and unradiogenic Sr-87/Sr-86 (similar to 0.7035-0.7039) combined with (in part) radiogenic Nd-143/Nd-144 and (206)pb/Pb-204 (>19.1) ratios, which together reveal a likely contribution of FOZO (FOcalZOne) component in their genesis. The other tholeiite groups show variable Sr-Nd ratios with relatively consistent Pb-206/Pb-204 ratios clustering towards an isotopically enriched mantle (EM1) component. Taken in conjunction with the Nb, this enriched signature reflects the involvement of a subduction-modified lithospheric mantle in the source of the evolved high-Ti and low-Ti tholeiites. Thus, we propose that FOZO and EMI components coexisted (including minor mixing with E-MORB magmas) and contributed in varying extents to the generation of the Ceara-Mirim dyke swarm primary melts, which segregated at 75 to 60 km in depth around the garnet-spinel fades transition zone. The mechanism that promoted melting was most likely non-plume related. We suggest that plate boundary forces linked to the opening of the Atlantic Ocean promoted passive rifting and that the resulting as-thenospheric upwelling was responsible for the melting that generated the tholeiitic magmas. (C) 2016 Elsevier B.V. All rights reserved.