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This paper presents U‐Pb ages for 23 samples from the Barberton Mountain Land, an Archean granite‐greenstone terrain in the Kaapvaal craton of South Africa. A reexamination of the geology of this area in the light of previous U‐Pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short‐lived compressional event within the time span 3230–3215 Ma which was synchronous with deposition of the Moodies Group and the upper part of the Fig Tree Group sediments. This event may have been responsible for thrusting of much older sequences in the Onverwacht Group, including slivers of 3.54 Ga old gneiss, circa 3.47 Ga old komatiitic and tholeiitic sequences, and circa 3.45 Ga old felsic sequences. Key constraints on the timing of deformation are previously determined 3227–3225 Ma ages for detrital zircons in thrusted units of the Fig Tree Group; a 3229 +4/−3 Ma age for a deformed porphyry which is pre‐ or synthrusting; a previously determined 3227 ± 3 Ma age for an undeformed porphyry in an inferred thrust fault; 3227 ± 1 Ma ages for both the Kaap Valley pluton and an ignimbrite in the Stolzburg syncline, both of which predate regional upright folding; and a 3216 ± 2 Ma age for the Dalmein pluton which appears to cut regional, upright NE‐SW trending folds in the Kromberg syncline, part of the final phase of deformation associated with regional compression. Areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the Barberton region at about 3105 Ma. The linear configuration of the syenogranitic plutons, which are also aligned with the gabbroic Usushwana complex as well as the geometry of the granitoid sheets, suggests that the circa 3105 Ma magmatism was caused by incipient rifting around a NW‐SE trending crustal fracture. This episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt. Some of the above interpretations are in conflict with previous models for early development of the Barberton greenstone belt. A 3352 ± 6 Ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m.y. younger than the probable age of volcanic rocks which host the dikes. This conflicts with the suggestion that much of the Onverwacht Group comprises an ophiolitic section, obducted in an intraoceanic setting at circa 3.45 Ga, that partially melted to form trondhjemitic magmas which intruded along active thrust faults. Present data suggest that, despite its antiquity and the diversity of ages and lithologies found within it, the Barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins.

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We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g−1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g−1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.