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7461746 
Journal Article 
The role of alkalis in the formation of coronitic textures in metamangerites and metaanorthosites from the Adirondack Complex, United States 
Safonov, OG 
1998 
1 
Petrology
ISSN: 0869-5911
EISSN: 1531-8540 
6 
6 
583-602 
English 
WOS:000077124600007 
The effect of potassium and sodium activities on corona-forming reactions during the retrograde stage of metamorphism is studied and exemplified by metamangerites and metaanorthosites from the Adirondack Complex, New York State, United States. K-feldspar rims and veinlets, which accompany garnet and amphibole coronitic textures in the orthorocks, are the mineralogic indicators of rock interaction with fluids containing potassium and sodium salts. The calcium content of garnet in the rocks systematically increases from the earliest generation of the mineral to the latest, which coexist with newly 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 growth. Calculation of temperature, pressure, oxygen, and alkali activities showed that the formation of garnet-K-feldspar-quartz coronas in metamangerites 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-chloride 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 the high concentration of 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: (1) high-temperature (700-600 degrees C) stage of rock interaction with aqueous fluid saturated with potassium and sodium chlorides and (2) low-temperature (<600 degrees C) stage of rock alteration by aqueous-carbon dioxide fluids. 
Physical Sciences - Other Topics