Unconformity‐covering pillow lava dated at 2.14 Ga: Challenging the “stable‐shelf” Minas Supergroup of the Quadrilátero Ferrífero, Minas Gerais, Brazil

• Published in: Geological journal (Chichester, England) Vol. 57; no. 5; pp. 2046 - 2057
• Main Authors: Cabral, Alexandre Raphael, Zeh, Armin, Pires, Fernando Roberto Mendes, Silva, Jefferson Ferreira, Carmo, Vitalino Elizeu Ferreira, Tupinambá, Miguel
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.05.2022
• Subjects: Brazil; Itabira iron formation (Itabira Group); Minas Gerais; pillow lava; Quadrilátero Ferrífero
• ISSN: 0072-1050; 1099-1034
• DOI: 10.1002/gj.4397

We report on the occurrence of pillow lava in the upper part of the banded‐iron‐formation (BIF) sequence of the Quadrilátero Ferrífero, famous for hosting world‐class iron‐ore deposits in Minas Gerais, Brazil. A pillow‐lava domain is found as an outlier within the BIF sequence, known as the Itabira iron formation or the Itabira Group. The pillow lava occurs as lateritized, indurated crusts in a sequence of ochreous itabirite, the weathered expression of dolomitic itabirite, a metamorphosed BIF rock containing dolomite. About 90 zircon grains, extracted from two samples of lateritized pillow lava, an indurated pillow rim and a friable pillow core, yielded Archaean to Palaeoproterozoic ages between 3.37 and 2.14 Ga, with age clusters at 2.98, 2.72, 2.25, and 2.14 Ga, and minor clusters at 3.37, 3.26, 3.16, and 2.82 Ga. The youngest grains, all exhibiting perfectly euhedral crystals, defined a Concordia age at 2,141 ± 6 Ma, interpreted as the age of subaqueous volcanism. This young zircon age and the perfectly euhedral zircon crystals suggest that the pillow‐lava domain represents an outlier of Palaeoproterozoic volcanic rocks, which covered the unconformity between the Itabira Group and the overlying Piracicaba Group. The finding of pillow lava lends support to previous, but usually ignored works, which described metavolcanic rocks in spatial association with the Itabira Group, and provides evidence for subaqueous volcanism that challenges the prevailing view of a stable‐shelf or passive‐margin setting for the geological evolution of the Minas Supergroup.