The role of alkalis in the formation of coronitic textures in metamangerites and metaanorthosites from the Adirondack Complex, United States (vol 6, pg 583, 1998)

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 CaAl2Si2O8 + 3SiO(2) + {4/3K(+) + 2/3H(2)O} = 4/3KAlSi(3)O(8) + 1/3Ca(3)Al( 2)Si(3)O(12) + 4/3H(+) An Qtz fluid Kfs Grs fluid an increase in the calcium content of the garnet corresponds to an in crease in the alkali activity in the fluid (at constant rand P) during coro na growth. Calculation of temperature, pressure, oxygen, and alkali activit ies showed that the formation of garnet-K-feldspar-quartz coronas in metama ngerites and metaanorthosites from the Adirondack Complex has taken place a t T= 700-600 degrees C, P = 7-6 kbar under conditions of low oxygen and car bon dioxide activities and has resulted from the isobaric cooling of the ro cks from metamorphic peak at high potassium and sodium activities in aqueou s-chloride reduced fluid. The determining factor in the formation of fr whi ch was caused by the high concentration of the Crt + Kfs + Qtz assemblage o f the rocks was low a(H2O)(fl) potassium and sodium salts (up to X-salts =0 .7) in the fluid. Two consequent stages were distinguished in the evolution of fluid in the Adirondack Complex: (I) high-temperature (700-600 degrees C) stage of rock interaction with aqueous fluid saturated with potassium an d sodium chlorides and (2) low-temperature (<600 degrees C) stage of rock a lteration by aqueous-carbon dioxide fluids.