High resolution tephra and U/Pb chronology of the 3.33–3.26Ga Mendon Formation, Barberton Greenstone Belt, South Africa

Abstract

a b s t r a c tThe Mendon Formation in the Barberton Greenstone Belt of South Africa marks the boundary between the Onverwacht and Fig Tree Groups. These groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation, respectively. This transition marks the end of komatiitic volcanism in the Barberton Greenstone Belt and is accompanied by numerous impact-related spherule layers. This study characterizes the upper Mendon Formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary. A suite of whole rock and handheld X-ray fluorescence analyses are presented in conjunction with textural information, stratigraphic relationships, and U/Pb ages to create a temporal and chemostratigraphic framework for the Mendon Formation. Local and regional stratigraphic variations, including absence of distinctive layers and variation in layer thickness, seen across the Mendon preclude ascription of a single stratigraphy that accurately describes the >1.2 km of section present in this formation. These variations indicate diachronous deposition of the Mendon Formation over a wide areal extent and into multiple basins or sub-basins by more than one magmatic source. 204 Pb-corrected 206 Pb/ 238 U and 207 Pb/ 235 U concordia model ages of 3279 ± 9.1 Ma and 3287.3 ± 2.9 Ma for two samples from upper portions of the Mendon Formation provide temporal context for deposition. Two samples from the basal 10 m of the Fig Tree Group, above the S2 spherule bed that marks the boundary between the Onverwacht and Fig Tree Groups, give model ages of 3267.8 ± 6.9 Ma and 3261 ± 18 Ma. These ages provide added constraints for the Onverwacht-Fig Tree boundary and confirm that the Weltevreden Formation is roughly age-correlative with the uppermost Mendon Formation.While the Mendon and Weltevreden Formations are in part age-correlative and have similar lithologies, they do not appear to be genetically related. The dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the Mendon and Weltevreden Formations indicate that the primary mode of crustal formation was likely plume-related magmatic accretion and not subduction. The relatively sharp transition within the Barberton Greenstone Belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact-related spherule layers, which suggest that major impacts may have played a role in the evolution of early tectonics to more modern, subduction-related styles.

Sedimentology of the ∼3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa

“…Several zircon U/Pb ages have also been reported from units within the Mendon Formation: 3,298 ± 3 Ma from a thin tuffaceous layer in M3c (Byerly et al, 1996), 3,287 ± 3 Ma from a felsic ash in M2c near the Swaziland border (Decker et al, 2015), and 3,280 ± 9 Ma from a tuffaceous sandstone above M4c in the Barite Syncline locality (see sample locations BS-02 and BS-03 in Fig. 2) (Decker et al, 2015).…”

“…Previous authors have suggested that this is related to diachronous onset and conclusion of upper Mendon deposition across different structural belts in the BGB related to contemporaneous rifting ( Fig. 2) and/or that local topographic highs impeded the lateral continuity of volcanic units (Lowe, 1994a;Byerly, 1999;Lowe, 1999a;Thompson Stiegler et al, 2010;Decker et al, 2015).…”

“…Several zircon U/Pb ages have also been reported from units within the Mendon Formation: 3,298 ± 3 Ma from a thin tuffaceous layer in M3c (Byerly et al, 1996), 3,287 ± 3 Ma from a felsic ash in M2c near the Swaziland border (Decker et al, 2015), and 3,280 ± 9 Ma from a tuffaceous sandstone above M4c in the Barite Syncline locality (see sample locations BS-02 and BS-03 in Fig. 2) (Decker et al, 2015). The depositional rates of the rapidly erupted komatiites were probably enormously greater than those of the interbedded fine-grained carbonaceous, siliceous, and ferruginous sediments, implying that the sedimentary lithofacies of the upper Mendon Formation represent the majority of the accumulation time recorded: up to 75 million years, but potentially somewhat less time considering that there may be unconformities within the sequence.…”

“…The sedimentary layers are now composed largely of silicified volcaniclastic sediments, black chert, banded black-and-white chert, and banded ferruginous chert (Byerly, 1999;Decker et al, 2015). This stratigraphy is complicated by the observation that most Mendon sections do not contain the full sequence of volcanic cycles.…”

“…The age of the Mendon Formation is constrained by a maximum age of 3,334 ± 3 Ma from a zircon U/Pb age in a tuffaceous band in the uppermost Kromberg Formation (Byerly et al, 1996) and a minimum age of ~3,260 Ma, based on two ages from tuffs in the base of the overlying Mapepe Formation: 3,259 ± 4 Ma (Byerly et al, 1996) and 3267 ± 7 Ma (Decker et al, 2015). Several zircon U/Pb ages have also been reported from units within the Mendon Formation: 3,298 ± 3 Ma from a thin tuffaceous layer in M3c (Byerly et al, 1996), 3,287 ± 3 Ma from a felsic ash in M2c near the Swaziland border (Decker et al, 2015), and 3,280 ± 9 Ma from a tuffaceous sandstone above M4c in the Barite Syncline locality (see sample locations BS-02 and BS-03 in Fig.…”