Characterization and origin of the alkali feldspar of granitoid rocks from the Variscan Anatectic Tormes Dome (West-Central Spain)
Moro, F. Javier López; Plaza, Miguel López; Pozas, Jοse M. Martin
European Journal of Mineralogy Volume 10 Number 3 (1998), p. 535 - 550
published: Jun 22, 1998
manuscript accepted: Dec 30, 1998
manuscript received: Nov 5, 1996
ArtNo. ESP147051003013, Price: 29.00 €
Abstract Different analytical techniques were employed to characterize alkali feldspars from the anatectic area of the Variscan Tormes Dome. The alkali feldspars from anatectic peraluminous granites are more albitic (%Or 85), and in Ba (> 2000 ppm), and their structural state is variable (with triclinicity 0 < Δ < 0.97). Both optical-structural (2Vx vs. Σt1) and chemical-structural ((%Or + Cn)/Ab vs. Σt1) correlations can be observed in the set of alkali feldspar samples analysed. The thermometric values obtained suggest: a) 730-650°C: minimum crystallization temperatures; b) 560-530°C: maximum unmixing temperatures, according to the coherent solvus; c) 420-360°C: minimum temperature, according to the coherent solvus of the granitoids of the calc-alkaline series; and d) around 300°C for the albite-low microcline association of the partial deuteric unmixing field of the most peraluminous granites. Zoning models for the alkali feldspars from the calc-alkaline series suggest a complex origin: first, a subsolvus crystallization and probable processes of magmatic mixing would have occurred and this would then have been followed by a re-equilibration with K-poor oligoclase, which could be related to superimposed metamorphic-anatectic processes. The latter suggests the partial melting of gneissic and pelitic protoliths, giving rise to strongly to moderately peraluminous magmas. Alkali feldspar would crystallize from this magma, probably close to eutectic compositions. The stability of micas in anatectic magmas could be explained by higher water fugacities, hence the ordering of the alkali feldspar of the S-type leucogranites, whereas disordered feldspar is characteristic of the calc-alkaline-affinity granitoids; the more disordered, the higher the Ba and K contents.