Petrogenesis of the calc-silicate skarns from Garby Izerskie, Karkonosze-Izera block

Abstract

The exocontact skarns of the Garby Izerskie mountain group in Izera Mts (Sudetes, Southern Poland), which occurr as intercalations in hornfelsed schists, were characterized by means of the paragenetic analysis. A number of parageneses were found, consisting of hedenbergite, grossular, wollastonite, vesuvianite, albite, actinolite, calcite, epidote, quartz and fluorite. The skarn minerals formed in four stages of different physico-chemical conditions. Stage I, which took place during the prograde metamorphism in a quasi-isochemical system, resulted in crystallization of hedenbergite (ca. 500°C), wollastonite and grossular (ca. 600-650°C) at pstatic = ptotal = pfluid = 2 kbar. The latter two minerals formed at peak metamorphic temperatures, when at ca. 650°C the fluids had a CO2 concentration of 30 molar percent. Stage II began the retrograde metamorphism, also in a quasi-isochemical system, and resulted in the origin of vesuvianite, actinolite, prehnite, epidote and calcite pseudomorphs after wollastonite. Vesuvianite was the earliest retrograde mineral, which had probably started growing when the temperature decreased below 600-580°C. Water activity increased during retrograde metamorphism at the expense of the CO2 activity, whose concentration was lower than 2 molar percent during the prehnite formation. Stage III comprised mainly silicification which began at least at 410°C in an allochemical system; it could have partly overlapped stages I and II in time. Stage IV, which was mainly the fluorine metasomatosis at 360 to 110°C developed also as an allochemical process. Thus, the skarns from Garby Izerskie are high-temperature calc-silicate varieties of the pyroxene-garnet-wollastonite type with the subsequent alteration of the calcic plagioclase to the sodic one, and the silica and fluorine metasomatosis.