RATES AND MECHANISMS OF MAGMA EMPLACEMENT IN THE USUTU INTRUSIVE SUITE: IMPLICATIONS FOR CA. 3.2 GA ASSEMBLY OF THE KAAPVAAL CRATON

Our ability to characterize the timescales and mechanisms of synorogenic magmatism is crucial to our understanding of continental assembly, stabilization, and modification. In the southeast Kaapvaal craton of southern Africa, the ca. 3.5 to 3.2 Ga Barberton Greenstone Belt (BGB) preserves evidence for the assembly of two or more continental blocks between ca. 3230 and 3215 Ma, and is an ideal place to study Archean tectonomagmatic processes. However, very little attention has been paid to magmatic rocks south of the BGB, due in part to terminally discordant U-Pb isotopic systematics in zircons. Utilizing “chemical abrasion” techniques in our analysis of zircons, we have overcome the discordance problem, and are able to date magmatic rocks with high precision and accuracy. Here we present initial ID-TIMS U-Pb geochronological and structural data from the Usutu intrusive suite, a voluminous complex of ca. 3.2 Ga granodioritic to tonalitic rocks which provide important insights into the tectonomagmatic evolution of the southeast Kaapvaal craton.

Field mapping and U-Pb zircon geochronology show that the Usutu intrusive suite represents at least 6 distinct units with both gradational and sharp contacts, and intruded the underlying 3.6-3.45 Ga basement gneisses as subvertical to moderately-dipping sheets between ca. 3235 and 3225 Ma – coeval with the ca. 3.2 orogenic event evidenced in the BGB. Despite the relatively short duration of magmatism, the rocks preserve evidence for complex partitioning of strain both regionally and mesoscopically, with fabrics that range from mylonitic to magmatic. In addition, ductile strain in the NW margin of the intrusive suite records components of both right-lateral and normal motion, culminating in mylonitic shear-zones separating it from the BGB. Such evidence suggests that in places, magmas intruded into active shear zones, but elsewhere were only subjected to regional stresses during crystallization. A U-Pb zircon date from a cross-cutting dyke shows that, at least locally, shear strain terminated by ca. 3221 Ma. Evidence for significant transtension in the NW portion of the complex during and after intrusion is important in refining tectonic models for the assembly and exhumation of this portion of the Kaapvaal craton.