3230–3200 Ma post-orogenic extension and mid-crustal magmatism along the southeastern margin of the Barberton Greenstone Belt, South Africa (original) (raw)
Related papers
Tectonics, 1994
Rocks of the 3570-3080 Ma Barberton greenstone belt record a geological history of at least 490 million years (m.y.), and were affected by regional thermal perturbations for over 1500 m.y. Small tectonic wedges of 3570-3511 Ma schistose tonalite are the oldest rocks yet recognized, but little is known about their tectonothermal history. The earliest, well-recognized tectonothermal events (3490-3450 Ma) represent mid-ocean ridge-like processes. These were followed by two periods of arc-related and trench-related processes, separated by -160 m.y.; the first (3445-3416 Ma) recording an intraoceanic suprasubduction-like environment and the second, (3260-3225 Ma) intra-arc and interarc-like processes, culminating in arc amalgamation. Accretion-like convergent processes dominated between -3230 Ma and 3080 Ma, overlapping with the assembly of the Kaapvaal craton. At 3080 Ma the Barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system. Soon afterward, transtension became distributed across the entire Kaapvaal craton, heralding the onset of formation of the Witwatersrand basin by-3070 Ma.
Journal of African Earth Sciences, 2001
This study presents new single-zircon U-Pb data that constrain the timing of granitoid emplacement within and to the south of the Murchison greenstone belt, South Africa. Four main magmatic events are identified at ca 3020, 2970, 2820 and 2680 Ma for the Baderoukwe, Discovery, Willie and Mashishimale intrusions, respectively. The age of 3020 Ma, found for the Baderoukwe Pluton, which is intrusive into the Letaba, Bawa and Sable schist belts, suggests that the schist belts are older than 3020 Ma. The age of the Discovery Granite is identical to that of the rhyodacitic Rubbervale Formation, suggesting that granite magmatism was associated with volcanism in the Murchison greenstone belt at around 2970 Ma. The emplacement of the Willie Granite is contemporaneous with several other granites on the Kaapvaal Craton, suggesting that there was a major period of felsic magmatism on the craton between 2820 and 2880 Ma. The 2700 Ma Mashishimale Granite reflects, furthermore, the age of many of the late-stage potassic granite plutons on the Kaapvaal Craton, indicating that this period was also an important crustforming event.
Geology and tectonostratigraphy of the Onverwacht Suite, Barberton Greenstone Belt, South Africa
Precambrian Research, 2011
A synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern Barberton Greenstone Belt into seven units with different geological histories. Current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained. We argue here that the status of the traditional 'formations' of the Onverwacht Group of the Barberton Greenstone Belt be changed to 'complexes', and collectively these complexes be referred to as the Onverwacht Suite. The total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km, respectively. The precise age range of the rocks present in each complex is largely unknown. The original spatial relationships between the complexes can in most cases only be inferred.
Precambrian …, 2012
29 This paper presents new petrological and geochronological data for the ~3.09-2.92 Ga Murchison 30 Greenstone Belt (MGB), located in South Africa's Kaapvaal Craton, and discusses their geotectonic 31 implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the 32 Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure-33 temperature-deformation (P-T-D) histories, and are separated from each other by relatively narrow, high-34 strain shear zones, with a sinistral, transpressive top-to-the-south movement, consistent with the 35 deformation patterns observed throughout the belt. These patterns are explained by a N-S compressional 36 stress field, affecting the Murchison Belt between 2.97 and 2.92 Ga. Results of new petrological 37
Journal of African Earth Sciences, 1999
Field, petrological, geochemical, isotopic and geophysical data have been assembled to determine the nature and extent of Archssan Kraaipan granite-greenstone rocks on the western edge of the Kaapvaal Craton, southern Africa. The Kraaipan greenstone belts, consisting of metamorphosed mafic volcanic rocks and interlayered metasediments (mainly banded iron formations, jaspilites and ferruginous cherts), occur poorly exposed beneath cover sequences comprising mainly Neoarchaean Ventersdorp Supergroup volcanic rocks and a blanket of Tertiary-Recent Kalahari sediments. A variety of granitoid rocks intruded the Kraaipan greenstones, which, on the basis of whole rock Pb-Pb dating of banded iron formations, have yielded an age of 3410 +61/-64 Ma. The earliest granitic rocks, which comprise tonalites and trondhjemitic gneisses, were dated using the single grain Pb evaporation technique on zircons, and yielded minimum ages ranging from 3 162 f 8 to 3070 f 7 Ma in the study area. This, coupled with 3250-3030 Ma ages reported for gneisses in the Kimberley and other areas on the western edge of the Kaapvaal Craton, suggests a prolonged evolution for the basement gneisses which were also disturbed between 2940 and 2816 Ma ago, probably during episodes of migmatisation. Potassium-rich granitoids, also dated using the single grain Pb evaporation method, range in age from 2880 &2 to 2846*22
Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa
Nature, 2006
Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0-2.5 Gyr ago) are disputed 1,2 . The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 8C km 21 or less 3 , have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients 4 are greater than 25 8C km 21 ). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites 5,6 and circumstantial evidence for subduction processes, including possible accretion-related structures 2 , has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history 7 . Here we report garnet-albite-bearing mineral assemblages that record pressures of 1.2-1.5 GPa at temperatures of 600-650 8C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12-15 8C-similar to those found in recent subduction zones-that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt 8 . These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.