The geology of the Archaean granitoid-greenstone terrane in the vicinity of Three Sisters, Barberton Greenstone Belt.

[摘要] This thesis provides a comprehensive account of the geology of the Archaean granitoid-greenstone terrane centred around Three Sisters in the northeastern sector of the Barberton Mountain Land.The supracrustal succession in the region comprises a diverse variety of altered volcanic and sedimentary rock types that have been correlated with the principal lithostratigraphic units of the Barberton greenstone belt (BGB) as fellows:1) schistose basic and ultrabasic lithologies correlated with the Theespruit Formation of the Gnverwacht Group are mainly developed in a narrow unit fringing the northern margin of the BGB;2) ferruginous shale greywacke - banded iron-formation assemblages correlated with the Jheba and Belvue Road Formations constitute the dominant Fig Tree Group imks in the region. A sec «ence or silicic » .-stavolcaniclastic rocks, now altered to a variety of micaceous schists, occurs west, north ar.a northeast of Three Sisters. Viljoen and Viljoen (1970) cc rrela ted these schists ith the Theespruit Formation. In this study, the schists have been assigned'to a new lithostratigraphic unit, referred to as the Bien Venue Formation, which forms the uppermost formation of the Fig Tree Group in the northeastern part of the BGB. Isotopic studies on zircons indicate that the schists have an age of 3256 ± Ma, which is some 200 Ma younger titan the most recent age estimates for the lower portions of the Gnverwacht Group. Chemically, the silicic schists resemble calc-alkaline rocks found in modem arcs, suggesting that the Bien Venue Formation represents a period of arc-like volcanism; and3) conglomeratic and quartzitic rocks constitute the dominant lithologies within the Moodies Group, which uncoriorroably or paraconformably overlies lithologies of the Fig Tree and Onverwacht Groups.North of the BGB is a complex suite of granitoid rocks, previously investigated by Robb et al. (1983) who defined a large (—60 kn ng and —6 km wide), elongate plutonic body of tonalitic-to-trondhjemitic composition known as the Stentor pluton. It was suggested that this pluton is correlatable with the irondhjemite gneiss plutons that intrude the southwestern parts of the BGB. Field evidence indicates, however, that the Stentor plutonforms a much smaller (* .14 km long and ~ 4 km wide) ovoid body located immediately north of the village of Louw’s Creek. Furthermore, the pluton consists of equigranular-textured graiodiorite-adamcllite, totally unlike any of the trondhjemite gneisses. In terms of texture, mineralogy and chemical composition, the Stentor pluton closely resembles the Hebron and Berlin plutohs which constitute a phase of the Nelspruit batholith. Thus, it is concluded that the Stentor pluton also forms an integral part of the batholith.Three deformation phases have been identified. The regional event affected all stratigraphic units in response to a northerly oriented compressions! stress and gave rise to east-northeasterly trending, tight-.« 5soclinal, upright and north-verging folds that are bounded by southward-dipping longitudinal reverse faults. The regional deformation occurred both prior and subsequent to the emplacement of the Stentor pluton at circa 3100 Ma. Deformation associated with the diapiric intrusion of the Stentor pluton into the greenstone assemblage during the regional deformation pnase, led to the formation of large-scale folds that have modified earlier formed structures. The final deformation episode is manifested by the presence of nortnwest- to northeast-striking, oblique-slip normal faults that exhibit both left- and right-lateral strike-slip components.

[发布机构] University of the Witwatersrand