HYDROTHERMAL REPLACEMENT OF FELDSPARS IN IGNEOUS ENCLAVES OF THE VELAY GRANITE AND THE GENESIS OF MYRMEKITES

Hydrothermal circulations through the Velay granite, France, induced e xtensive 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 oli goclase, as in the host granite, to a higher anorthite content (An(44) ), demonstrating an efficient local-scale re-equilibration between flu ids and enclave feldspars. Experimental data indicate that the composi tion of aqueous chloride solutions in equilibrium with quartz and two feldspars in the range 450-600 degrees C, 1-2 kbar, is strongly depend ent on the anorthite content of the plagioclase. When a granite-derive d (sodic) fluid is fluxed into a quartz and K-feldspar bearing enclave whose plagioclase is calcic, its composition moves towards higher CaC l2 and KCI contents to maintain equilibrium with the two feldspars; th is 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 rep lacement of K-feldspar by QPI relative to the re-equilibration of calc ic plagioclase. Our observations indicate that incipient myrmekitisati on, 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 variat ions in the anorthite content of plagioclase along the fluid pathways. Some other consequences of the high feldspar reactivity in late-magma tic systems are outlined.