The anatectic history of Archaean metasedimentary granulites from the Ancient Gneiss Complex, Swaziland

[摘要] ENGLISH ABSTRACT: This study is an investigation of the anatectic history of high-grade paragneisses from the AncientGneiss Complex (AGC) in Swaziland. The work involved an integrated field, metamorphic,geochemical, geochronological and structural study of metasedimentary granulites from threeseparate, but spatially related areas of outcrop in south-central Swaziland, which were subjected tomultiple high-grade partial melting events throughout the Meso- to Neoarchaean. The project hasaimed to constrain the age(s) and conditions of metamorphism, so as to contribute to theunderstanding of geodynamic processes in the Barberton and AGC granite-greenstone terranes, aswell as to investigate certain physical and chemical aspects of anatexis in the migmatites. Themetamorphic record retained in these rocks, constrained by phase equilibria modelling as well aszircon and monazite SHRIMP and LA-ICP-MS geochronology, informs on the state of the mid- tolower-crust of the southeastern Kaapvaal Craton during key events associated with early lithosphereassembly and crustal differentiation. It also suggests that the region is comprised of more than onehigh-grade terrane. Two of the areas investigated experienced high-temperature metamorphism atca. 3.23-3.21 Ga, in addition to a major 830-875º C, 6.5-7.6 kbar anatectic event at ca. 3.11-3.07Ga. Intermediate and younger high-temperature events are recorded at ca. 3.18 Ga, ca. 3.16 Ga and2.99 Ga. The timing of these metamorphic events coincided with the amalgamation of the easterndomain of the proto-Craton via subduction and accretion of micro-continental fragments at ca. 3.23Ga, including the Barberton Greenstone Belt (BGB) and AGC terranes, as well as discrete episodesof crustal differentiation and potassic granitic magmatism between ca. 3.23 and 3.10 Ga. The thirdarea investigated holds no record of Mesoarchaean metamorphism, but instead experienced a 830-855 ºC, 4.4-6.4 kbar partial melting episode at ca. 2.73 Ga. This broadly coincided with theformation of a large continental flood basalt province, the ca. 2.71 Ga Ventersdorp LIP, andwidespread intracratonic granitic magmatism on the Craton towards the end of the Neoarchaean. Anexplanation for the contrast in metamorphic record in the two terranes may be that the 2.71 Gagranulites represent a much younger sedimentary succession, and that granulites from the olderterrane were left too restitic, after substantial partial melting during the Mesoarchaean, to recordsubsequent high-grade events. Finally, this study documents the details of S-type granitic magmaproduction and extraction from a typical metapelitic source. Using the 2.73 Ga granulites from theAGC as a natural field laboratory, a case is made for the selective entrainment of peritectic garnet tothe magma as a mechanism for generating relatively mafic, peraluminous S-type granitecompositions. The work demonstrates the evolution of entrained peritectic garnet in such magmas,and is in strong support of a 'peritectic phase entrainment' process by which relatively mafic granitemagmas are produced from melts which, in theory, should be highly leucocratic.

[发布机构] Stellenbosch University