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The Petrogenesis of Tholeiitic Diabases in Eastern Parnaíba Basin

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The Petrogenesis of Tholeiitic Diabases in Eastern Parnaíba Basin DOI: 10.1590/2317‑4889201720160041 ARTICLE The petrogenesis of tholeiitic diabases in eastern Parnaíba Basin: evidence for geochemical heterogeneities in the subcontinental lithospheric mantle in NE Brazil Petrogênese de diabásios toleíticos na porção oriental da Bacia do Parnaíba: evidências para heterogeneidade no manto litosférico subcontinental no NE do Brasil Adriano Guilherme da Silva1, Cícera Neysi de Almeida1, Sérgio de Castro Valente2*, Leonardo Fonseca Borghi de Almeida1 ABSTRACT: The sedimentary rocks within the Paleozoic Parnaiba RESUMO: As rochas sedimentares paleozóicas da Bacia do Parnaíba, basin in NE Brazil were intruded by voluminous tholeiitic diabase no Nordeste do Brasil, foram intrudidas por soleiras volumosas de diabá‑ sills and covered by coeval basaltic flows. This paper presents lithogeo- sio toleítico e cobertas por derrames basálticos. Este artigo apresenta da‑ chemical data of borehole samples obtained from wells located in the dos geoquímicos obtidos a partir de amostras de poços da porção leste da eastern portion of the Parnaiba basin. The diabases are subalkaline bacia sedimentar. Os diabásios são subalcalinos, toleíticos e agrupam‑se tholeiitc rocks comprising three high-TiO2 and three low-TiO2 suites em três suites de alto‑TiO2 e três de baixo‑TiO2 não relacionadas por that are unrelated by differentiation processes. Fractional crystalliza- processos de diferenciação. Os processos petrogenéticos foram investi‑ tion of olivine and augite was the predominantly evolutionary pro- gados com base em modelagem geoquímica e revelaram que as suites cesses within individual high- and low-TiO2 suites as depicted by toleíticas evoluíram por cristalização fracionada de augita e olivina à exce‑ trace element geochemical modelling, exception being made for one ção de uma, de baixo‑TiO2, evoluída por AFC em câmaras magmáticas low-TiO2 suite that evolved by AFC. Parental compositions for both pequenas na crosta superior. As composições parentais de ambas as sui‑ low- and high-TiO2 suites are related with variably enriched, spi- tes, de baixo‑TiO2 e alto‑TiO2, estão relacionadas com fontes mantélicas nel harzburgitic sources likely to represent the heterogeneous sub- harzburgíticas variavelmente enriquecidas dentro da zona de estabilidade continental lithospheric mantle underneath the sedimentary basin. espinélio representadas pelo manto litosférico subcontinental. Esta fonte The geochemical provinciality of the Parnaiba tholeiitic magmatism mantélica é lateralmente e/ou verticalmente quimicamente heterogênea seems unrelated with the Transbrasiliano Lineament but may be due e a provincialidade geoquímica em escala local parece não ter sido con‑ to lithospheric mantle amalgamation and remobilization occurred trolada pelo Lineamento Transbrasiliano, mas sim por remobilização do during previous tectonic events. manto litosférico subcontinental possivelmente amalgamado durante KEYWORDS: Transbrasiliano; Parnaiba basin; Diabases. processos orogênicos colisionais ocorridos previamente. PALAVRAS‑CHAVE: Transbrasiliano; Bacia do Parnaíba; Diabásios. 1Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro (RJ), Brazil. E‑mail: [email protected]; [email protected]; [email protected] 2Universidade Federal Rural do Rio de Janeiro – UFRRJ, Seropédica (RJ), Brazil. E‑mail: [email protected] *Corresponding author. Manuscript ID: 20160041. Received in: 03/16/2016. Approved in: 02/08/2017. 109 Brazilian Journal of Geology, 47(1): 109-126, March 2017 The petrogenesis of the Parnaiba diabases INTRODUCTION the Silurian, the Devonian and the Permian‑Carboniferous separated by well‑marked unconformities all over the basin Tholeiitic basalt flows and diabase intrusions (dykes and (Goes et al. 1990). These three sedimentary cyles are related sills) occur associated with the sedimentary rocks of the with five sedimentary supersequences from the begining of Paleozoic Parnaíba basin in northeast Brazil (Aguiar 1969). the Paleozoic to the end of the Cretaceous, including two These magmatic rocks comprise the Mosquito Formation magmatic episodes represented by the tholeiitic basalts (199 ± 2,4 Ma; 40Ar/39Ar in plagioclase; Merle et al. 2011) and diabases of the Mosquito Formation (Triassic) and the and the Sardinha Formation (c.a. 129 – 124 Ma; K‑Ar and Sardinha Formation (Cretaceous) (Aguiar 1969). 40Ar/39Ar, whole‑rock; Bellieni et al. 1990, Fodor et al. 1990). The main regional structures within the Parnaíba basin The former has been associated with the Central Atlantic are the NE‑trending, Transbrasiliano Lineament and the Magmatic Province (CAMP; Marzoli et al. 1999) and the NW‑trending, Picos‑Santa Inês Lineament (Fig. 1), which break‑up of Pangea, prior to the opening of the Central controlled the main NE and NW depositional axis from Atlantic Ocean in the Florida‑Guyana‑Liberia area (e.g., the Early Silurian up to the Early Carboniferous. Later on, Bertrand 1991) whereas the latter has been related with the the basin acquired the typical oval shape of a syneclise lea‑ Paraná‑Etendeka Continental Flood Basalt Province and the ding to the migration of the depocenter to the central parts fragmentation of Gondwana, previously to the opening of of the basin (Vaz et al. 2007). Recent small magnitude, the South Atlantic ocean (Bellieni et al. 1990). shallow depths earthquakes (< 60 km) within the northe‑ The tholeiitic dykes and sills are interbedded with the ast part of the basin (Lima Neto 2013) might be related to Paleozoic sedimentary rocks of the Pimenteiras and Cabeças the reactivation of ENE‑WSW‑trending shear zones (Médio formations (Devonian), the Poti Formation (Mississipian), Coreaú Domain) extending far below the basin eastwards and the Piauí Formation (Pensylvanian) of the Parnaíba the Borborema Province. basin (Vaz et al. 2007). The sedimentary rocks within these Gravimetric and seismic data show that the thickness formations are related with the petroleum systems identified of the continental crust in the area of the Parnaíba basin in this Paleozoic basin (Abelha 2013). The magmatic rocks does not exceed 40 km (Assumpção et al. 2013, Castro et al. of the Sardinha and Mosquito formations played their role 2014) with Moho at shallow to intermediate depths in the in the generation and migration as well as sealing and tra‑ northeast and central portions of the basin. The boreholes pping of hydrocarbons (e.g. Rodrigues 1995). of the present study are located in this area (Fig. 1). Previous works on the tholeiitic magmatism within the Parnaíba basin were done on the basis of petrographic, geo‑ chemical and geochronological data obtained from surface PREVIOUS WORK ON samples (Bellieni et al. 1990, Fodor et al. 1990, Baksi & THE PARNAÍBA BASALTS Archibald 1997, Marzoli et al. 1999, Ernesto et al. 2004, Merle et al. 2011). In this paper we present new petrographic The tholeiitic basalts of the Sardinha Formation pre‑ and lithogeochemical data obtained from borehole samples vail eastwards whereas those of the Mosquito Formation of the Parnaíba tholeiitic rocks and put constraints on the prevail westwards the Parnaíba basin (e.g., Fodor et al. geochemical heterogeneity of the subcontinental lithospheric 1990; Ernesto et al. 2004). Some authors proposed that mantle beneath the Parnaíba sedimentary basin in NE Brazil. the high‑TiO2 Sardinha and low‑TiO2 Mosquito basalts could be distinguished on the basis of the higher Ti, Ba, La, Ce, Y, Zr, and Sr contents of the former (Bellieni et al. GEOLOGICAL FRAMEWORK 1990). Nevertheless, low‑TiO2 basalts were later recognised OF THE PARNAÍBA BASIN in the Sardinha Formation (Baksi & Archibald 1997) and distinction from their counterparts within the Mosquito The Parnaíba basin is located in northeastern Brazil Formation are difficult to be made on the basis of petro‑ (Fig. 1). It extends over 6.7 x 105 km2 with approximately logical data. 3,500 m of maximum thickness of sediments (Vaz et al. Geochronological K‑Ar data obtained for the Mosquito 2007). The sedimentation of immature siliciclasts started in basalts (Fodor et al. 1990) peaked at ca. 160 Ma and ca. the Late Cambrian and Early Ordovician (Jaibaras Group) 200 Ma whilst for the Sardinha basalts ages spreaded over and took place onto grabens originated from the separa‑ ca. 130 Ma and ca. 115 Ma. Later, 40Ar/39Ar data pointed tion of Laurentia and Baltica (Bond et al. 1984, Oliveira & to ca. 200 – 190 Ma for the low‑TiO2 Mosquito basalts, Mohriak 2003). The stratigraphic record of the Parnaíba basin and ca. 129 – 124 Ma for the high‑TiO2 Sardinha basalts as can be divided into three main sedimentary cycles during well as ca. 123 Ma for their low‑TiO2 counterparts (Baksi 110 Brazilian Journal of Geology, 47(1): 109-126, March 2017 Adriano Guilherme da Silva et al. & Archibald 1997). More recent 40Ar/39Ar ages on plagio‑ It is now generally accepted by many authors that high‑ clase separates gave 196 ± 1,0 Ma, 198,0 ± 1,2 Ma and and low‑TiO2 basalts, dykes and sills occur both in the 199,2 ± 1,0 Ma for the high‑TiO2 basalts of the Mosquito Mosquito and Sardinha formations in the Parnaíba basin Formation, i.e., coeval to the low‑TiO2 ones (Merle et al. (Bellieni et al. 1990, Fodor et al. 1990, Baksi & Archibald 2011). In general, the geochronological data corroborates 1997, Marzoli et al. 1999, Ernesto et al. 2004, Merle et al. the association of the high‑ and low‑TiO2 Mosquito basalts 2011). Previously to the publication of geochronological data, with the tectonic events related with the opening of the petrogenetic
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