Hydrothermal replacement of feldspars in igneous enclaves of the Velay granite and the genesis of myrmekites
Garcia, Daniel; Pascal, Marie-Lola; Roux, Jacques
European Journal of Mineralogy Volume 8 Number 4 (1996), p. 703 - 718
published: Jul 29, 1996
manuscript accepted: Feb 16, 1996
manuscript received: Jul 21, 1994
ArtNo. ESP147050804006, Price: 29.00 €
Abstract Hydrothermal circulations through the Velay granite, France, induced extensive mineral reactions in some of its igneous enclaves. K-feldspar is partly or completely replaced by myrmekite-like quartz-plagioclase intergrowths (QPI) and primary plagioclase by secondary, quartz-free plagioclase. Both types of secondary plagioclase have almost the same composition in a given sample; this composition varies from P-rich oligoclase, as in the host granite, to a higher anorthite content (An44), demonstrating an efficient local-scale re-equilibration between fluids and enclave feldspars. Experimental data indicate that the composition of aqueous chloride solutions in equilibrium with quartz and two feldspars in the range 450-600°C, 1-2 kbar, is strongly dependent on the anorthite content of the plagioclase. When a granite-derived (sodic) fluid is fluxed into a quartz and K-feldspar bearing enclave whose plagioclase is calcic, its composition moves towards higher CaCl2 and KC1 contents to maintain equilibrium with the two feldspars; this results in the coupled replacement of calcic plagioclase by a more sodic one, and of K-feldspar by QPI, as observed in the enclaves. Mass-balance calculations show that a low fluid chlorinity favours the replacement of K-feldspar by QPI relative to the re-equilibration of calcic plagioclase. Our observations indicate that incipient myrmekitisation, as commonly observed in rocks bearing quartz and two feldspars, is well explained by Becke's (1908) metasomatic model; it may be due to limited fluid-rock exchange of Na, K and Ca and driven by small variations in the anorthite content of plagioclase along the fluid pathways. Some other consequences of the high feldspar reactivity in late-magmatic systems are outlined.