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|Title:||Archean Crustal Evolution in the Barberton Mountain Land, South Africa: U-Pb and Nd Isotopic Constraints|
|Authors:||Kamo, Sandra L.|
|Advisor:||Davis, D. W.|
Dickin, A. P.
McNutt, R. H.
|Keywords:||Archean, crustal, Barberton, mountain, land, isotopic, constraints, evolution, magmatic|
|Abstract:||<p> New U-Pb ages from the Barberton Mountain Land document an 800 m.y. period of Archean magmatism (ca. 3540 Ma to ca. 2740 Ma) that can be divided into five distinct episodes. Magmatic activity during Episode I includes tonalite-trondhjemite gneisses such as the Steynsdorp pluton (3509 +8/-7 Ma) and a tectonically interleaved sliver (3538 +4/-2 Ma) located at the base of the Onverwacht Group. Trondhjemitic magmatism of Episode II is synchronous with volcanism and inferred D1 thrusting of the Onverwacht Group and is represented by plutons such as the Doornhoek (3448 ± 4 Ma), Theespruit (3443 +4/-3 Ma), and Stolzburg (3459 +5/-4 Ma). A quartz-feldspar porphyry dyke, related to Episode II, intrudes the Komati Formation of the Onverwacht Group and yields an age of 3467 +12/-7 Ma. D2-related thrusting, volcanism, sedimentation, and tonalitic plutonism in the north-central part of the Barberton greenstone belt (BGB) occurred during Episode III and are recorded by an ignimbrite which was deposited between the Fig Tree and Moodies Groups during regional deformation (3227 ± 1 Ma), by pre- and post-tectonic porphyries (3227 Ma), and by the emplacement of the Kaap Valley tonalite (3227 ± 1 Ma). Episode IV is characterized by the intrusion of large sheet-like potassic batholiths to the north and south of the BGB and syenitic plutonism at ca. 3107 Ma, and by D3-related deformation at the northern margin of the BGB. Gold mineralization also appears to coincide in time with this episode. The last known period of Archean magmatic activity in the BGB is represented by late phase granite magmatism at ca. 2740 Ma (Episode V).</p> <p> Epsilon Nd values range from +1.3 to -0.7 in rocks varying in composition from gabbro to granodiorite that were emplaced during Episodes I to IV. A gabbro from the Komati Formation, with a baddeleyite age of 3352 +6/-5 Ma, has an ϵNd of +1.3, which is the only value to fall on a depleted mantle growth curve. This value is similar to ϵNd data obtained by others for ca. 3450 Ma basalts from the Onverwacht Group. Three Episode IV plutons have identical ϵNd values of -0.5, and a granite pluton from Episode V has an ϵNd of -4.3, indicating a source compatible with remelting of older crust.</p> <p> In summary, the protracted evolution of the Barberton Mountain Land occurred in a series of discrete events widely separated in time. Each event is characterized by a number of geological processes (magmatism, volcanism, sedimentation) that appear to coincide in time with thrust-related deformation as a result of compressional tectonics. The duration of the main period of crustal growth occurred over ≈365 m.y. from ca. 3470 Ma, with the formation of the main Onverwacht sequence and its associated tonalitic-trondhjemitic rocks through ca. 3230 Ma, with the intrusion of the Kaap Valley tonalite and D2 thrust-related deformation, to ca. 3105 Ma, with the widespread emplacement of sheet-like potassic batholiths bordering the Barberton greenstone belt.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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