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

Abstract

The Ceará 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 (TiO 2 ≥1.5 wt.%; Ti/Y >360), and low-Ti tholeiites (TiO 2 ≤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 87 Sr/ 86 Sr (~0.7035-0.7039) combined with (in part) radiogenic 143 Nd/ 144 Nd and 206 Pb/ 204 Pb (>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 206 Pb/ 204 Pb ratios clustering toward 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 Ceará-Mirim dyke swarm (CMDS) primary melts, which segregated at 75 to 60 km in depth around the garnet-spinel facies 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 asthenospheric upwelling was responsible for the melting that generated the tholeiitic magmas.

Stress states during the emplacement of the eastern Rio Ceará-Mirim dike swarm, Borborema Province, northeastern Brazil

“…Mizusaki et al (2002) assembled geochronological data from the RCMDS, reporting ages between 145 to 110 Ma with a magmatic peak around 130 Ma, although this compilation also considers less accurate 40K-40Ar data. Ngonge et al (2016), Souza et al (2003), and Araújo et al (2001) obtained, respectively, 126.9 ± 4 Ma, 132.2 ± 1 Ma and 143 ± 4 Ma to 110.7 ± 1 Ma 40 Ar-39 Ar mineral and whole-rock step-heating plateau ages. Paleomagnetic data register dikes emplacement from Late Jurassic to Early Cretaceous, but Jurassic dikes are not supported by recent radiometric approaches in this region (Bellieni et al 1992;Ernesto et al 1991).…”

“…The Rio Ceará-Mirim dike swarm (RCMDS) is classically described in the State of Rio Grande do Norte (RN) showing E-W striking dikes, up to southern Ceará, where it progressively shifts to NE-SW (Figure 1c). This deflection in the western RCMDS has been traditionally interpreted as influenced from the NE-SW Cariri-Potiguar trend, a basement reactivation zone that aligns some rift basins (Oliveira 1993;Bellieni et al 1992;Ngonge et al 2016;Hollanda et al 2006). At the eastern RCMDS, the focus of this study, dikes truncate the NE-SW Precambrian foliation and volcanoclastic rocks occur in the non-outcropping basal syn-rift sedimentary layers of the Potiguar Basin (Pendências Formation) (Bellieni et al 1992;Araripe and Feijó 1994;Pessoa Neto et al 2007).…”

Análise de correspondências de litogeoquímica de vulcanismos cenozoicos na porção setentrional da Província Borborema, Brasil

“…A primeira etapa do trabalho envolveu a compilação de análises geoquímicas de rochas vulcânicas da região setentrional da Província Borborema (Figura 1). Os dados geoquímicos do corpo vulcânico subaflorante em Fortaleza, que difere petrográfica e geoquimicamente do vulcanismo Messejana (Werneck e Magini, 2017), foi comparado com dados de outros vulcanismos da região existentes na literatura (Guimarães et al, 1982;Rao e Sial, 1972;Lopes, 2002;Silveira, 2006;Ngonge et al, 2016).…”

Geochemical characterization of the Equatorial Atlantic Magmatic Province and its correlation with other magmatic events related to the South Atlantic Opening

“…Defined as a 350 km-long E-trending linear dike swarm (Archanjo et al, 2000;Hollanda et al, 2006;Ngonge et al, 2016a), the extension and geometry of the RCM was recently re-examined after high-resolution airborne magnetic data of the Borborema Province available by the Brazilian Geological Survey (CPRM). These data allowed to show that the RCM drastically turns from EW towards a NE-SW direction at approximately 38ºW, which continuously propagates for, at least, 600-700 km more in length.…”

“…This work brings a significant database of geochemical (major oxides and trace elements) and Sr-Nd-Pb isotope data obtained from the aforementioned igneous rocksi.e., the dikes composing the NE-trending branch of the RCM and the intrusions that represent the Sardinha Magmatism. The data obtained for the NE-trending RCM were integrated with existing data for the dikes that constitute the E-trending branch of the RCM (Ngonge et al, 2016a) to discuss the petrogenesis of the entire dike swarm. A subordinate intrusive system named Canindé dike swarm was additionally investigated in a similar approach.…”