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The geological significance of the Rb-Sr whole-rockisochron of hornfelsed schists from the Izerskie Garby Zone, Karkonosze-Izera block, southernwest Poland

Fila-Wójcicka E.

Acta Geologica Polonica

2004

Vol. 54, no. 3

407--411

Preliminary Rb-Sr isotope data for hornfelsed schists from the "Stanislaw" quarry in the Izerskie Garby Zone are reported. An isochron based on three points representing three whole-rock samples yields an age of 333 [plus-minus] 4 Ma (Visean) and an initial 87Sr/86Sr ratio of 0.709567. The Rb-Sr age of 333 [plus-minus] 4 Ma, indicating the date of the Rb-Sr system closure, can be interpreted as the cessation of contact metamorphism in the Izerskie Garby Zone. The age of 333 [plus-minus] 4 Ma is close to the peak of contact metamorphism in this zone. Contact metamorphism and silification in the Izerskie Garby Zone were genetically related to the activity of the Variscan Karkonosze pluton. Contact metamorphism was older than the biotite cooling at ca. 320 Ma.

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

Fila-Wójcicka E.

Acta Geologica Polonica

2000

Vol. 50, no. 2

211-222

The exocontact skarns of the Garby Izerskie mountain group in Izera Mts (Sudetes, Southern Poland), which occur 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, quarz 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 stopni C), wollastonite and grossular (ca 600-650 stopni C) at p[static]=p[total]=p[fluid]=2kbar. The latter two minerals formed at peak metamorphic temperatures, when at ca 650 stopni 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 stopni 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 stopni 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 stopni 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.