Granulite facies anatexis in the Ancient Gneiss Complex, Swaziland, at 2.73 Ga: Mid-crustal metamorphic evidence for mantle heating of the Kaapvaal craton during Ventersdorp magmatism

摘要

The 3.7-3.2 Ga Ancient Gneiss Complex (AGC) of Swaziland, located S and SE of the Barberton Greenstone Belt (BGB), is a complex geological terrane where few studies have focussed on the timing and conditions of deformation and metamorphism, despite the fact that minor metasedimentary units within the AGC provide excellent opportunities for constraining metamorphic evolution. One of these, the Mkhondo Valley Metamorphic Suite consists of migmatitic metapelitic granulites, within which the peak metamorphic assemblage garnet + cordierite + biotite + plagioclase + quartz ± K-feldspar + melt is dominant. NCKFMASHTO pseudosection modelling of this anatectic assemblage constrains peak metamorphic conditions to 850-830 °C and 4.4 kbars, followed by near isobaric cooling of the terrane to 680 °C and 3.9 kbars. These conditions indicate a period of anomalously high heat flow within the crust due to the direct addition of mantle heat. U-Pb SHRIMP dating of zircons from an in situ leucosome indicates an age of ca. 2.73 Ga for peak granulite facies metamorphism, approximating the age of Ventersdorp Supergroup flood basalt volcanism on the craton and widespread intracratonic granitic magmatism. The current study suggests that substantial heat input to the Kaapvaal cratonic crust during underplating by Ventersdorp related magmas, possibly related to the presence of a ～2.7 Ga mantle plume marginal to the craton, was responsible for high-grade metamorphism in the AGC and a period associated with regional crustal anatexis. The resultant mid-crustal ductility and weakening may have initiated mobilisation of the mid-crust as gneissic domes, producing basement uplift and sedimentary recycling of auriferous reefs of the Central Rand Group of the Witwatersrand Supergroup.