REASSESSMENT OF ARCHEAN CRUSTAL DEVELOPMENT IN THE BARBERTON MOUNTAINLAND, SOUTH-AFRICA, BASED ON U-PB DATING

This paper presents U-Pb ages for 23 samples from the Barberton Mounta in Land, an Archean granite-greenstone terrain in the Kaapvaal craton of South Africa. A reexamination of the geology of this area in the li ght of previous U-Pb dating and the new ages indicates that much of it s deformation can probably be accounted for by a single short-lived co mpressional event within the time span 3230-3215 Ma which was synchron ous with deposition of the Moodies Group and the upper part of the Fig Tree Group sediments. This event may have been responsible for thrust ing of much older sequences in the Onverwacht Group, including slivers of 3.54 Ga old gneiss, circa 3.47 Ga old komatiitic and tholeiitic se quences, and circa 3.45 Ga old felsic sequences. Key constraints on th e timing of deformation are previously determined 3227-3225 Ma ages fo r 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 p reviously determined 3227 +/- 3 Ma age for an undeformed porphyry in a n inferred thrust fault; 3227 +/- 1 Ma ages for both the Kaap Valley p luton and an ignimbrite in the Stolzburg syncline, both of which preda te regional upright folding; and a 3216 +/- 2 Ma age for the Dalmein p luton which appears to cut regional, upright NE-SW trending folds in t he Kromberg syncline, part of the final phase of deformation associate d with regional compression. Areally extensive granitoid sheets and sy enogranitic plutons were subsequently emplaced into the Barberton regi on at about 3105 Ma. The linear configuration of the syeno-granitic pl utons, which are also aligned with the gabbroic Usushwana complex as w ell as the geometry of the granitoid sheets, suggests that the circa 3 105 Ma magmatism was caused by incipient rifting around a NW-SE trendi ng crustal fracture. This episode was probably coeval with a late regi onal transtensional phase of deformation recorded in the greenstone be lt. Some of the above interpretations are in conflict with previous mo dels for early development of the Barberton greenstone belt. A 3352 +/ - 6 Ma age from a metagabbro which is comagmatic with a proposed ultra mafic sheeted dike complex is over 100 m.y. younger than the probable age of volcanic rocks which host the dikes. This conflicts with the su ggestion that much of the Onverwacht Group comprises an ophiolitic sec tion, obducted in an intraoceanic setting at circa 3.45 Ga, that parti ally melted to form trondhjemitic magmas which intruded along active t hrust faults. Present data suggest that, despite its antiquity and the diversity of ages and lithologies found within it, the Barberton gree nstone belt can generally be understood in terms of actualistic tecton ic processes at convergent margins.