Chlorite of hydrothermal origin formed in the Strzelin and Borów granites (Fore-Sudetic Block, Poland)

Abstract

Chlorite, a product of very low- to low-grade metamorphism, is frequently used as a geothermometer because of the fact that its structure and chemical composition can reflect the physical and chemical conditions of its formation. In the hydrothermally altered Strzelin and Borów granites (the Fore-Sudetic Block, Poland) chlorite is ubiquitous. It is found in two forms: spherulitic and post-biotite, and was formed in different ways in different parts of the Strzelin and Borów granites: as result of (1) replacement of biotite or (2) crystallisation from fluid. The chlorite formed in the Borów granite shows a higher Fe content than that in the Strzelin granite, a feature related to the content of Mg and Fe in the host rock. Temperatures of chlorite formation are the lowest for unaltered granite and then gradually increase for slightly, moderately and strongly altered granite and are the highest in hydrothermal veins. This means that the temperature of the hydrothermal fluid was higher than  that of the altered granitic bodies. Moreover, the spherulitic chlorite formed at a higher temperature than did the post-biotite chlorite, and  is usually smaller because biotite replacement lasted longer than did the crystallisation of spherulitic chlorite directly from hydrothermal fluid. Such pattern are likely to occur in other granitic bodies.