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

Abstract

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.

Karst-bauxite formation during the Great Oxidation Event indicated by dating of authigenic rutile and its thorium content

“…500 Myr younger than the age of the youngest detrital zircon (2666 ± 23 Ma) in the karst bauxite, defining the maximum depositional age. The bauxite-rutile age also overlaps with an age of 2141 ± 6 Ma for the subaqueous volcanism that covers the erosional unconformity between the Itabira and the Piracicaba groups 14 . Furthermore, it is within error identical to an age of 2095 ± 65 Ma suggested to date the thermal overprint during the Transamazonian gneiss-dome emplacement in the QF 15 .…”

“…In combination, stratigraphical relationships and geochronological data indicate that karstification and bauxitisation of karst-filling clastic sediments took place within the GOE, specifically at its final stage, the Lomagundi event (see below). The erosional unconformity between the Itabira and the Piracicaba groups, dated at 2.14 Ga 14 , constrains the minimum age for the uplift of the Gandarela dolomite rocks and their exposure to karst weathering, during the gneiss-dome emplacement in the QF 15 , immediately followed by metamorphism.…”

“…The quarry dolomite rocks are delimited to the north by predominantly clastic rocks of the Piracicaba Group, which disconformably overlies the Itabira Group. The disconformable contact between the two groups is an erosional surface 10 , the age of which has been constrained at 2141 ± 6 Ma by U–Pb dating of zircon grains from a deeply weathered sequence of pillow lavas 14 . This Palaeoproterozoic age is interpreted as the subaqueous volcanism that covered the erosional unconformity between the Itabira and the Piracicaba groups.…”

Oxidised gold lode stains black: the historical Encardideira (Chico Rei) deposit, Ouro Preto, Brazil

“…The itabirite sequence, the depositional age of which is estimated at 2.65 Ga (Cabral et al 2012), grades upwards into dolomitic rocks of the Gandarela Formation (Dorr 1969). The Cauê Itabirite and the Gandarela Formation make up the Itabira Group, above which predominantly clastic metasedimentary rocks of the Piracicaba Group rest on an erosional unconformity, dated at 2.14 Ga (Cabral et al 2022). The Itabira Group is underlain by the Caraça Group.…”