Showing papers by "Jan Kramers published in 1989"
TL;DR: An early to mid-Proterozoic age for the Magondi orogeny is confirmed by Rb-Sr ages of 1890 ± 260 Ma from enderbitic granulites at the Rukomeshe River causeway, east of Makuti, and of 1780 ± 280 Ma from garnet-bearing and granitic granulite from the Nyaodza region just east of lake Kariba, which can be correlated with deformation in the Kheis Belt occupying a similar position on the western margin of the Kaapvaal craton RE
35 citations
TL;DR: In this article, an open-system fractional crystallization model for formation of TTG crust through solidification from a shallow-level tonalitic magma ocean is proposed. But this model assumes that the base of a thick convecting magma layer will be undersaturated with respect to H 2 O and CO 2.
Abstract: Archean granulite terrains are generally dominated by charnockite-enderbite suites. These rocks are geochemically similar to the tonalite-trondhjemite-granodiorite (TTG) gneisses of Archean cratons. Following an open-system fractional crystallization model for formation of TTG crust through solidification from a shallow-level tonalitic magma ocean, we propose that such granulites could have formed by crystallization at depth at the base of the same magma layer, which is undergoing convection. Increasing solubilities with depth of fluids in a magma mean that the base of a thick convecting magma layer will be undersaturated with respect to H 2 O and CO 2 . This allows the formation of charnockitic assemblages. Crystallization at depth can involve the reverse of dehydration-melting reactions; e.g., producing biotite and quartz at the expense of K-feldspar orthopyroxene, and liquid. Such reactions buffer the activity of water but not of CO 2 during crystallization; hence, fluids exsolved during progressive solidification are likely to be CO 2 -rich.
17 citations
TL;DR: In this paper, the authors report on the Rb-Sr isotopic systematics of the Lochiel suite and associated tin-bearing pegmatites in northwestern Swaziland, one of the largest composite batholiths of this region.
6 citations