Archaean granulite-facies paragneisses from central Swaziland: inferences on Palaeoarchaean crustal reworking and a complex metamorphic history

We present a petrographic, petrological, geochemical, and geochronological study (U–Pb/Lu–Hf) on granulite-facies paragneisses of the Mahamba Gneiss Complex in central Swaziland, eastern Kaapvaal Craton. Our data suggest that prograde metamorphism occurred at c. 3.07 Ga. Dating of detrital zircons of a metapelite in combination with geochronological and ambiguous structural relationships with granitoid gneisses suggests two possible scenarios: (1) the time of deposition of the sedimentary protoliths is prior to c. 3.58 Ga; (2) c. 3.58 Ga granitoid crust was the basement for the sedimentary protoliths. Furthermore, enrichment in Ni and Cr in the Mahamba metasediments may originate from erosion of a greenstone terrane similar in composition to the dominantly mafic and ultramafic lithologies of the Barberton Greenstone Belt or Dwalile supracrustal belt. Evidence for an older crustal basement is provided by the oldest detrital zircons, which yield ages up to c. 3.7 Ga. Because the metamorphic conditions of metapelitic rocks have no similarity to P–T paths of modern subduction–collision tectonic settings we conclude that the metamorphic history of the Mahamba Gneiss Complex cannot be explained by this model. We propose that deformation and metamorphism occurred through intracratonic contraction and a thermal event possibly related to Mpuluzi batholith emplacement.

我们针对位于东部卡普瓦尔克拉通（Kaapvaal Craton）斯威士兰中部的马汉巴片麻岩杂岩（Mahamba Gneiss Complex）的麻粒岩相（granulite-facies）副片麻岩（paragneisses），开展了采用铀-铅/镥-铪（U–Pb/Lu–Hf）测年方法的岩相学、岩石学、地球化学与地质年代学研究。研究数据显示，其进变质作用（prograde metamorphism）发生于约3.07 Ga。对一块变泥质岩（metapelite）的碎屑锆石（detrital zircons）进行定年，并结合其与花岗质片麻岩（granitoid gneisses）的地质年代学关联与模糊构造关系，我们提出两种可行情景：（1）该沉积原岩（sedimentary protoliths）的沉积时代早于约3.58 Ga；（2）形成于约3.58 Ga的花岗质地壳为该沉积原岩提供了基底。此外，马汉巴变沉积岩中镍（Ni）与铬（Cr）的富集，可能源自与巴伯顿绿岩带（Barberton Greenstone Belt）或德瓦利尔表壳岩带（Dwalile supracrustal belt）以镁铁质-超镁铁质岩性为主的岩系成分相似的绿岩地体（greenstone terrane）的侵蚀作用。最古老的碎屑锆石（年龄可达约3.7 Ga）为存在更古老的地壳基底提供了佐证。由于变泥质岩的变质条件与现代俯冲-碰撞构造环境（subduction–collision tectonic settings）的温压路径（P–T paths）无相似性，我们认为马汉巴片麻岩杂岩的变质演化历史无法用该模型解释。我们提出，变形与变质作用通过克拉通内部收缩（intracratonic contraction）以及一场可能与姆普卢齐岩基（Mpuluzi batholith）就位相关的热事件得以发生。