Chloritization and polytypism of biotite in the Łomnica granite, Karkonosze Massif, Sudetes, Poland: stable isotope evidence

Abstract

Biotites from core samples of the Łomnica granite (Karkonosze Mts., Sudetes) are chloritized. Two generations of chlorite occur: an earlier one that filled interstices, reducing granite permeability, and a later, major one that replaced biotite crystals. The degree of chloritization of biotite varies from 0% to 93%. The rate of potassium removal from biotite during the progress of chloritization is not constant. The rapid removal of K is observed at initial stages of chloritization. After the replacement of 20% of the biotite, K removal is much slower. The degree of chloritization of biotite correlates with isotopic signals: (i) the change in δD is semi-logarithmic starting at −85‰ for unaltered biotite, and achieving −50‰ in the chloritized one, while some biotitic hydrogen underwent replacement during or even prior to chloritization; and (ii) δ 18O correlates linearly, changing from about 4.9‰ to 2.8‰. Quartz observed in cold cathodoluminescence (CCL) shows internal zoning, which might be indicative of the hydrothermal remobilization. The temperature of chloritization, estimated from the isotopic fractionation of oxygen between quartz and chlorite, was about 300 °C, which agrees with chlorite geothermometer data. Water from within the chloritization environment had higher δD and lower δ 18O than magmatic water. The preservation of magmatic hydrogen and oxygen isotopic ratios in unaltered biotite indicates only limited water–rock interaction in a subsolidus environment. The polytypic composition of biotite, expressed as the percent ratio of 2M 1 polytype in a mixture with 1M (and 3T) varies between 30% and 60% of 2M 1 polytype. An isotopic effect was observed in coexisting polytypes of biotite. The 2M 1 polytype tends to concentrate the lighter isotope of oxygen relative to the 1M (and 3T) polytypes.