Elsevier

Lithos

Volumes 266–267, December 2016, Pages 453-470
Lithos

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

https://doi.org/10.1016/j.lithos.2016.10.008Get rights and content

Highlights

  • Cenozoic MVF nephelinite-basanite-alkali olivine basalts presented in 4 groups.

  • Compositions of primitive characteristics: 10%<Mgo < 15% and 200ppm < Ni < 500ppm.

  • Are sodic, fractional crystallization limited, exhibit LILE and Nb-Ta enrichment.

  • Magmas from amphibole ± phlogopite and amphibole-rich garnet and spinel lherzolites.

  • Geochemical modelling shows mixing of FOZO and EM components.

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 < 15 wt.% and 200 ppm < Ni < 500 ppm, 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 (80–93 km deep), while the nephelinites are derived from the adiabatic asthenosphere at ~ 105 km 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.

Section snippets

1. Introduction

The Macau volcanism (Macau Volcanic Field — MVF) is the most recent magmatic activity documented in the South American platform since the widespread eruption of the Mesozoic tholeiitic and alkaline magmatic events related to the opening of the Atlantic Ocean from the Jurassic to the Cretaceous which include the Paraná Igneous Province (Cordani and Vandoros, 1967, Fodor et al., 1985, Mantovani et al., 1985, Marques et al., 1999, Peate et al., 1992, Peate and Hawkesworth, 1996, Peate, 1997,

2. Geological overview

The Borborema Province is a geological entity that was part of the West Gondwana supercontinent, which resulted from a number of diachronic collisional events related to the Brasiliano/Pan-African orogeny (e.g., Van Schmus et al., 2008). Presently, the Borborema Province preserves huge areas of (Archean to) Paleoproterozoic gneissic to migmatitic rocks, which constitute the basement for several Meso- to Neoproterozoic sedimentary and volcano-sedimentary basins, all metamorphosed and deformed in

3. Sampling strategy

Thirty-four samples have been analysed for element and isotope geochemical data for this study. Part of these results were previously reported in the unpublished PhD thesis (2002) of the co-author M.H.B.M. Hollanda, which are all the Sr–Nd isotopic data for samples tagged HD and DCO, as well as part of the major and minor element data. The others were analysed for this study. The whole dataset is reproduced in Table 2, Table 3.

Of the nearly sixty eruptive centres in the MVF, four are the

4. Analytical procedures

Thin sections were used to investigate petrographic features using a conventional optical microscope for a few samples and the major element mineral chemistry data (Table 1) were obtained using electron microprobe techniques at the Institute of Geosciences at the Federal University of Rio de Janeiro (Brazil). The mineral chemistry analyses were performed on a JXA 8230 Superprobe applying a voltage of 15 keV for the analysis of olivine, pyroxene and plagioclase and of 20 keV for oxides, and a beam

5.1. Petrography and mineral chemistry

Overall, the MVF basalts are olivine-phyric, displaying textures ranging from microlitic porphyritic in the SPaP samples to microgranular porphyritic in SPo, SeA and PiC. Occasionally, they demonstrate intergranular and seriate arrangements. The mineral assemblage includes olivine + clinopyroxene + plagioclase + Fe–Ti oxides, but sub- to millimetric xenocrysts of olivine and pyroxene as well as millimetric peridotite xenoliths are found to be common within the SPaP lavas. Samples contain

6.1. Assimilation fractional crystallization

Before using the geochemical and isotopic dataset to constrain the origin of the mantle sources for the MVF basalts, we should certify the primary nature of our samples. Except for some of the SPo samples which have Mg# ~ 0.61–0.68 (average Ni ~ 175 ppm) combined with large variation in silica contents, all other MVF basalts have low silica (≤ 50 wt.%) and high MgO (mostly > 11 wt.%, to Mg#  0.68) contents, and average Ni and Cr for each SeA, PiC and SPaP ranging from 290 to 540 and 500 to 700 ppm

7. Conclusion

Cenozoic intraplate alkaline basalts comprise the MVF located at the northeastern corner of the South American plate. These rocks display incompatible trace element patterns and Sr–Nd–Pb isotope compositions similar to FOZO and EM-type OIB magmas. DM endmember should be present as a third component but hidden by the more obvious features of FOZO and EM endmembers. The compositional variability observed between the studied eruptive centres is explained not only by fractional crystallization of

Acknowledgments

This article was written as part of the E.D. Ngonge's Ph.D. thesis. Ngonge was supported by CAPES. M.H.B.M. Hollanda and M.M. Pimentel are grateful for grants received from the Brazilian National Research Council (CNPq), during the course of this research project. The analytical tests and field trips were carried out under the auspices of CNPq through Project 473.638/2011-8.

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