The Murchison greenstone belt, South Arica: accreted slivers with contrasting metamorphic conditions / Зеленокаменный пояс Мурчисон, Южная Африка. Срастания в условиях контрастного метаморфизма

This paper presents new petrological and geochronological data for the ~3.09-2.92 Ga Murchison Greenstone Belt (MGB), located in South Africa’s Kaapvaal Craton, and discusses their geotectonic implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure temperature-deformation (P-T-D) histories, and are separated from each other by relatively narrow, high strain shear zones, with a sinistral, transpressive top-to-the-south movement, consistent with the deformation patterns observed throughout the belt. These patterns are explained by a N-S compressional stress field, affecting the Murchison Belt between 2.97 and 2.92 Ga. Results of new petrological investigations indicate that ultramafic to felsic volcano-sedimentary rocks of the Murchison Unit underwent a greenschist- to lower-amphibolite-facies metamorphism at maximum P—T conditions of 5.6 hal-00691575, version 1 - 26 Apr 2012 Author manuscript, published in "Precambrian Research (2012) 22" 40 ± 0.6 kbar at 570°C, along a relatively hot, minimum apparent geotherm of ~30°C/km. In contrast, the Silwana Amphibolites and the La France Formation were metamorphosed at much higher peak metamorphic conditions of 8.7-10 kbar, 630-670°C, and 8-9 kbar, 600-650 °C respectively, and require a colder apparent geotherm of ~20°C/km. A retrograde, nearly isothermal-decompression P—T path followed by isobaric cooling is also inferred for the La France Formation <...>