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Petrology, isotope characteristics, and K-Ar ages of the Maranha??o, northern Brazil, Mesozoic basalt province

Contributions to Mineralogy and Petrology

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, , ,
DOI: 10.1007/BF00306664

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Abstract

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this ???700x250 km Maranha??o province (5??-8??S) has low-Ti basalts (???1.1 wt% TiO2) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4-4.4 wt% TiO2) in the eastern part about 115-122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44-33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb(n) 4.2 and 8.8, where La(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Parana??) province in southern Brazil. The Maranha??o low-Ti basalts have more radiogenic Sr and Pb and higher ??18O than the high-Ti basalts. Respectively, low- vs high-Ti: e{open}Sr26-54 vs 15-18; 206Pb/204Pb=18.25-.78 vs 18.22-.24; and ??18O 8.9-12.6 vs 6.5-8.6. Nd isotopes overlap: e{open}Nd-1.6 to -3.8 vs -2.1 to -3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranha??o and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranha??o has Pb ??7/4=4.6-11, and Pb ??8/4=72-87; Serra Geral has Pb ??7/4=10-13, and Pb ??8/4=95-125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranha??o low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting. ?? 1990 Springer-Verlag.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Petrology, isotope characteristics, and K-Ar ages of the Maranha??o, northern Brazil, Mesozoic basalt province
Series title:
Contributions to Mineralogy and Petrology
DOI:
10.1007/BF00306664
Volume
104
Issue:
5
Year Published:
1990
Language:
English
Publisher location:
Springer-Verlag
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Contributions to Mineralogy and Petrology
First page:
555
Last page:
567
Number of Pages:
13