The petrogenesis of the older (> 3.0 Ga) potassic granitoids of eastern Mpumalanga (South Africa) and Swaziland : an investigation of crustal formation processes in the early Earth

[摘要] ENGLISH ABSTRACT: Earth's oldest preserved granitoid crust dates back to the Paleoarchean and consists predominantlyof sodic tonalite-trondhjemite-granodiorite (TTG) granitoids that arose through the partial melting ofhydrated metabasalts. In contrast, granites (sensu stricto) typically postdate the TTG and appear latein the plutonic record of the old cratons.However, the existence of Hadean zircons with mineral inclusion suites that are consistent withcrystallization from peraluminous granitic magmas indicates that granitic rocks formed part of theearliest felsic crust; although we have direct evidence, this earliest felsic crust is not preserved.In this PhD I present an unusual variety of markedly CaO-poor, K2O-rich, rutile-bearing, peraluminousgranite and rhyolite that are located in the basal conglomerate of the Moodies Group (South Africa).These rocks challenge the common view of the Archean craton evolution as they were producedconcurrently with TTG magmas during three magmatic cycles in the Barberton Greenstone Belt(BGB) and were later emplaced, as clasts, in a younger conglomerate.The study of mineral inclusions located in the zircons present within the granites and rhyolites, showsthat alkali feldspar inclusions are abundant relative to plagioclase inclusions and demonstrates thatthe main characteristics of these granites, i.e. they are K-rich and Ca-poor, are a magmatic signature.The oxygen isotope signature of these zircon grains reveals that the zircons have preserved the δ18Ovalue of the magma from which the granites originated and that the source of the granites had amagmatic oxygen isotope value close to the one of the regional coeval TTG. Further study of thezircons shows that their Lu-Hf isotopic system reflects the crustal signature of the magma into whichthey grew. Sm-Nd study of the granites and rhyolites whole rock indicates that the minimum age ofthe source's protolith of the granites and rhyolites is close to 3.9 billion years, which is in agreementwith the zircons' Lu-Hf signature. Additionally I show in this thesis that the peraluminous characterof the granites and rhyolites, along with their high Sr and low Ca content associated to their Eu/Eu* ~ 1 is a consequence of phengite melting in a metagreywacke source at pressures in excess ofplagioclase stability.My work therefore illustrates that K-rich, Ca-poor peraluminous granites were generated in thePaleo and Meso Archean, alongside with the sodic TTG, through partial melting of sediments at highpressures. Not only has this process demonstrated the ability of the early Earth to recycle relativelyyoung material since 3.9 billions years ago, but it has also contributed to each episode of continentalcrustal growth through the Paleoarchean to Mesoarchean in the BGB, despite leaving no plutonicrecord at the typical mid-crustal level of exposure that the TTG plutons around the belt represent.

[发布机构] Stellenbosch University