The role of alkalis in the formation of coronitic textures in metamangerites and metaanorthosites from the Adirondack Complex, United States

The effect of potassium and sodium activities on corona-forming reactions d uring the retrograde stage of metamorphism is studied and exemplified by me tamangerites and metaanorthosites from the Adirondack Complex, New York Sta te, United States. K-feldspar rims and veinlets, which accompany garnet and amphibole coronitic textures in the orthorocks, are the mineralogic indica tors of rock interaction with fluids containing potassium and sodium salts. The calcium content of garnet in the rocks systematically increases from t he earliest generation of the mineral to the latest, which coexist with new ly formed K-feldspar. According to the reaction [GRAPHICS] an increase in the calcium content of the garnet corresponds to an increase in the alkali activity in the fluid (at constant T and P) during corona gr owth. Calculation of temperature, pressure, oxygen, and alkali activities s howed that the formation of garnet-K-feldspar-quartz coronas in metamangeri tes and metaanorthosites from the Adirondack Complex has taken place at T = 700-600 degrees C, P = 7-6 kbar under conditions of low oxygen and carbon dioxide activities and has resulted from the isobaric cooling of the rocks from metamorphic peak at high potassium and sodium activities in aqueous-ch loride reduced fluid. The determining factor in the formation of the Grt Kfs + Qtz assemblage of the rocks was low a(H2O)(fl), which was caused by t he high concentration of potassium and sodium salts (up to X-salts = 0.7) i n the fluid. Two consequent stages were distinguished in the evolution of f luid in the Adirondack Complex: (1) high-temperature (700-600 degrees C) st age of rock interaction with aqueous fluid saturated with potassium and sod ium chlorides and (2) low-temperature (<600 degrees C) stage of rock altera tion by aqueous-carbon dioxide fluids.