The metamorphic and anatectic history of Archaean metapelitic granulites from the South Marginal Zone, Limpopo Belt, South Africa.

[摘要] ENGLISH ABSTRACT: Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final productsof crustal differentiation. Therefore, the links that exists between some migmatitesand crustal-derived granites can be ambiguous.This study is an investigation of the anatectic history of a high-grade terrain: the SouthernMarginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in SouthAfrica. The work involved an integrated field, metamorphic, geochemical and geochronogicalstudy of the metasedimentary granulites from two separate quarries in the northernzone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry,where Neoarchean high-grade partial melting features can be observed.The project has aimed to address two main issues: (1) to accurately constrain the pressuretemperatureconditions and the age of the metamorphic episode in the SMZ, with implicationfor the geodynamic processes near the end of the Archean, (2) to investigatethe fluid-absent partial melting reactions that control formation of K2O-poor leucosomesand to understand the chemical relationships in the system source-leucosome-melt–S-typegranite.The P-T-t record retained in the Bandelierkop Formation metapelites, constrained byphase equilibria modelling as well as zircon LA-ICP-MS geochronology, gives an insightinto crustal differentiation processes in the lower crust. Rocks in both quarries indicatehigh-temperature metamorphism episodes with peak conditions of 840-860 oC and 9-11kbar at c. 2.71 Ga with formation of leucosomes (L1) during the prograde path. Minorleucocratic features (L2) were produced during decompression to 6-7 kbar. The end of themetamorphic event is marked by the granulites/amphibolites facies transition (< 640 oC)at c. 2.68 Ga. The maximum deposit age for the detrital zircons in the metapelites (c.2.73 Ga) indicates a rapid burial process (0.17 cm.y1). Those evidences strongly supportthat the Southern Marginal Zone contains sediments deposited in an active marginduring convergence, and that the metapelites were metamorphosed and partially meltedas a consequence of continental collision along the northern margin of the Kaapvaal Cratonat c. 2.7 Ga.The leucocratic features generated along this P-T-t path display an unusual chemistrywith low K2O and FeO+MgO content and high CaO content. The combination of fieldobservations, chemical mapping and geochemical analyses leads to the conclusion the majorpart of the leucosomes (L1) crystallized prior to syn-peak of metamorphism concurrentwith melt extraction from the source.This study documents the details of leucosomes formation using field observations inthe Southern Marginal Zone and numerical modelling. This work demonstrates that theformation of K2O-poor leucosome in the metasedimentary lower crust is controlled bythe difference in volume of equilibration and heterogeneities within the migmatites. Thepartial melting of the source coupled with melt loss and water diffusivity within the melttransfer site is a potential mechanism to explain the chemical link in the sytem residuum–melt–S-type granite.

[发布机构] Stellenbosch University